Wednesday, December 24, 2014
THE NEED FOR GROUNDING
The Institute of Electrical and Electronics Engineers (IEEE) defines grounding as a conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth or to some conducting body of relatively large extent that serves in place of the earth. It is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground current to and from the earth (or the conducting body). Based on this definition, the reasons for grounding can be identified as:
• Personnel safety by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system.
• Personnel safety and control of electrostatic discharge (ESD) by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system and the Earth.
• Fault isolation and equipment safety by providing a low-impedance fault return path to the power source to facilitate the operation of over current devices during a ground fault.
The IEEE definition makes an important distinction between ground and earth. Earth refers to mother earth, and ground refers to the equipment grounding system, which includes equipment grounding conductors, metallic raceways, cable armor, enclosures, cabinets, frames, building steel, and all other noncurrent-carrying metal parts of the electrical distribution system.
• Personnel safety by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system.
• Personnel safety and control of electrostatic discharge (ESD) by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system and the Earth.
• Fault isolation and equipment safety by providing a low-impedance fault return path to the power source to facilitate the operation of over current devices during a ground fault.
The IEEE definition makes an important distinction between ground and earth. Earth refers to mother earth, and ground refers to the equipment grounding system, which includes equipment grounding conductors, metallic raceways, cable armor, enclosures, cabinets, frames, building steel, and all other noncurrent-carrying metal parts of the electrical distribution system.
GROUNDING ELECTRODE
The process of connecting the grounding system to earth is called earthing and consists of immersing a metal electrode or system of electrodes into the earth. The conductor that connects the grounding system to earth is called the grounding electrode conductor. The function of the grounding electrode conductor is to keep the entire grounding system at earth potential (i.e., voltage equalization during lightning and other transients) rather than for conducting ground-fault current. Therefore, the NEC allows reduced sizing requirements for the grounding electrode conductor when connected to made electrodes.
The basic measure of effectiveness of an earth electrode system is called earth electrode resistance. Earth electrode resistance is the resistance, in ohms, between the point of connection and a distant point on the earth called remote earth. Remote earth, about 25 ft from the driven electrode, is the point where earth electrode resistance does not increase appreciably when this distance is increased. Earth electrode resistance consists of the sum of the resistance of the metal electrode (negligible) plus the contact resistance between the electrode and the soil (negligible) plus the soil resistance itself. Thus, for all practical purposes, earth electrode resistance equals the soil resistance. The soil resistance is nonlinear, with most of the earth resistance contained within several feet of the electrode. Furthermore, current flows only through the electrolyte portion of the soil, not the soil itself. Thus, soil resistance varies as the electrolyte content (moisture and salts) of the soil varies. Without electrolyte, soil resistance would be infinite.
Soil resistance is a function of soil resistivity. A 1-cubic-meter sample of soil with a resistivity of 1 ohm-meter will present a resistance of 1 ohm between opposite faces. A broad variation of soil resistivity occurs as a function of soil types, and soil resistivity can be estimated or measured directly. Soil resistivity is usually measured by injecting a known current into a given volume of soil and measuring the resulting voltage drop. When soil resistivity is known, the earth electrode resistance of any given configuration (single rod, multiple rods, or ground ring) can be determined by using standard equations developed by Sunde, Schwarz, and others.
Earth resistance values should be as low as practicable, but are a function of the application. The NEC approves the use of a single made electrode if the earth resistance does not exceed 25Ω. IEEE Standard 1100 reports that the very low earth resistance values specified for computer systems in the past are not necessary. Methods of reducing earth resistance values include the use of multiple electrodes in parallel, the use of ground rings, increased ground rod lengths, installation of ground rods to the permanent water level, increased area of coverage of ground rings, and the use of concrete-encased electrodes, ground wells, and electrolytic electrodes.
The basic measure of effectiveness of an earth electrode system is called earth electrode resistance. Earth electrode resistance is the resistance, in ohms, between the point of connection and a distant point on the earth called remote earth. Remote earth, about 25 ft from the driven electrode, is the point where earth electrode resistance does not increase appreciably when this distance is increased. Earth electrode resistance consists of the sum of the resistance of the metal electrode (negligible) plus the contact resistance between the electrode and the soil (negligible) plus the soil resistance itself. Thus, for all practical purposes, earth electrode resistance equals the soil resistance. The soil resistance is nonlinear, with most of the earth resistance contained within several feet of the electrode. Furthermore, current flows only through the electrolyte portion of the soil, not the soil itself. Thus, soil resistance varies as the electrolyte content (moisture and salts) of the soil varies. Without electrolyte, soil resistance would be infinite.
Soil resistance is a function of soil resistivity. A 1-cubic-meter sample of soil with a resistivity of 1 ohm-meter will present a resistance of 1 ohm between opposite faces. A broad variation of soil resistivity occurs as a function of soil types, and soil resistivity can be estimated or measured directly. Soil resistivity is usually measured by injecting a known current into a given volume of soil and measuring the resulting voltage drop. When soil resistivity is known, the earth electrode resistance of any given configuration (single rod, multiple rods, or ground ring) can be determined by using standard equations developed by Sunde, Schwarz, and others.
Earth resistance values should be as low as practicable, but are a function of the application. The NEC approves the use of a single made electrode if the earth resistance does not exceed 25Ω. IEEE Standard 1100 reports that the very low earth resistance values specified for computer systems in the past are not necessary. Methods of reducing earth resistance values include the use of multiple electrodes in parallel, the use of ground rings, increased ground rod lengths, installation of ground rods to the permanent water level, increased area of coverage of ground rings, and the use of concrete-encased electrodes, ground wells, and electrolytic electrodes.
EARTH ELECTRODE
Earth electrodes may be made electrodes, natural electrodes, or special-purpose electrodes. Made electrodes include driven rods, buried conductors, ground mats, buried plates, and ground rings. The electrode selected is a function of the type of soil and the available depth. Driven electrodes are used where bedrock is 10 ft or more below the surface. Mats or buried conductors are used for lesser depths. Buried plates are not widely used because of the higher cost when compared to rods. Ground rings employ equally spaced driven electrodes interconnected with buried conductors. Ground rings are used around large buildings, around small unit substations, and in areas having high soil resistivity.
Natural electrodes include buried water pipe electrodes and concrete-encased electrodes. The NEC lists underground metal water piping, available on the premises and not less than 10 ft in length, as part of a preferred grounding electrode system. Because the use of plastic pipe in new water systems will impair the effectiveness of water pipe electrodes, the NEC requires that metal underground water piping be supplemented by an additional approved electrode. Concrete below ground level is a good electrical conductor. Thus, metal electrodes encased in such concrete will function as excellent grounding electrodes.
Natural electrodes include buried water pipe electrodes and concrete-encased electrodes. The NEC lists underground metal water piping, available on the premises and not less than 10 ft in length, as part of a preferred grounding electrode system. Because the use of plastic pipe in new water systems will impair the effectiveness of water pipe electrodes, the NEC requires that metal underground water piping be supplemented by an additional approved electrode. Concrete below ground level is a good electrical conductor. Thus, metal electrodes encased in such concrete will function as excellent grounding electrodes.
GROUNDING ON BARE ROCK
A bare rock mountaintop location provides special challenges to the facility design engineer. There is no soil, thus there are no ground rods. Radials are the only means to develop a ground system. Install a large number of radials, laid straight, but not too taut. The portions not in contact with the rock are in air and form an inductance that will choke the surge current. Because rock is not conductive when it is dry, keep the radials short. Only a test measurement will determine how short the radials should be. A conventional earth-resistance tester will tell only half the story (besides, ground rods cannot be placed in rock for such a measurement). A dynamic ground tester offers the only way to obtain the true surge impedance of the system.
The surge impedance, measured by a dynamic ground tester, should be 25Ω or less. This upper limit number is chosen so that less stress will be placed on the equipment and its surge protectors. With an 18 kA strike to a 25Ω ground system, the entire system will rise 450 kV above the rest of the world at peak current. This voltage has the potential to jump almost 15.75 in. (0.35 in./10 kV at standard atmospheric conditions of 25°C, 30 in. of mercury and 50% relative humidity).
For non-soil conditions, tower anchor points should have their own radial systems or be encapsulated in concrete. Configure the encapsulation to provide at least 3 in. of concrete on all sides around the embedded conductor. The length will depend on the size of the embedded conductor. Rebar should extend as far as possible into the concrete. The dynamic ground impedance measurements of the anchor grounds should each be less than 25Ω.
The size of the bare conductor for each tower radial (or for an interconnecting wire) will vary, depending on soil conditions. On rock, a bare no. 1/0 or larger wire is recommended. Flat, solid-copper strap would be better, but may be blown or ripped if not covered with soil. If some amount of soil is available, no. 6 cable should be sufficient. Make the interconnecting radial wires continuous, and bury them as deep as possible; however, the first 6 to 10 in. will have the most benefit. Going below 18 in. will not be cost-effective, unless in a dry, sandy soil where the water table can be reached and ground-rod penetration is shallow. If only a small amount of soil exists, use it to cover the radials to the extent possible. It is more important to cover radials in the area near the tower than at greater distances. If, however, soil exists only at the outer distances and cannot be transported to the inner locations, use the soil to cover the outer portions of the radials.
If soil is present, install ground rods along the radial lengths. Spacing between ground rods is affected by the depth that each rod is driven; the shallower the rod, the closer the allowed spacing.
Because the ultimate depth a rod can be driven cannot always be predicted by the first rod driven, use a maximum spacing of 15 ft when selecting a location for each additional rod. Drive rods at building corners first (within 24 in. but not closer than 6 in. to a concrete footer unless that footer has an encapsulated Ufer ground), then fill in the space between the corners with additional rods.
Drive the ground rods in place; do not auger; set in place, then back-fill. The soil compactness is never as great on augured-hole rods when compared with driven rods. The only exception is when a hole is augured or blasted for a ground rod or rebar and then back-filled in concrete. Because concrete contains lime (alkali base) and is porous, it absorbs moisture readily, giving it up slowly. Electron carriers are almost always present, making the substance a good conductor.
If a Ufer ground is not being implemented, the radials may be Cad-welded to a subterranean ring, with the ring interconnected to the tower foot-pad via a minimum of three no. 1/0 wires spaced at 120° angles and Cad-welded to the radial ring.
ROCK BASED RADIAL ELEMENTS
On bare rock, a radial counterpoise will conduct and spread the surge charge over a large area. In essence, it forms a leaky capacitor with the more conductive earth on or under the mountain. The conductivity of the rock will be poor when dry, but quite good when wet. If the site experiences significant rainfall before a lightning flash, protection will be enhanced. The worst case, however, must be assumed: an early strike under dry conditions.The surge impedance, measured by a dynamic ground tester, should be 25Ω or less. This upper limit number is chosen so that less stress will be placed on the equipment and its surge protectors. With an 18 kA strike to a 25Ω ground system, the entire system will rise 450 kV above the rest of the world at peak current. This voltage has the potential to jump almost 15.75 in. (0.35 in./10 kV at standard atmospheric conditions of 25°C, 30 in. of mercury and 50% relative humidity).
For non-soil conditions, tower anchor points should have their own radial systems or be encapsulated in concrete. Configure the encapsulation to provide at least 3 in. of concrete on all sides around the embedded conductor. The length will depend on the size of the embedded conductor. Rebar should extend as far as possible into the concrete. The dynamic ground impedance measurements of the anchor grounds should each be less than 25Ω.
The size of the bare conductor for each tower radial (or for an interconnecting wire) will vary, depending on soil conditions. On rock, a bare no. 1/0 or larger wire is recommended. Flat, solid-copper strap would be better, but may be blown or ripped if not covered with soil. If some amount of soil is available, no. 6 cable should be sufficient. Make the interconnecting radial wires continuous, and bury them as deep as possible; however, the first 6 to 10 in. will have the most benefit. Going below 18 in. will not be cost-effective, unless in a dry, sandy soil where the water table can be reached and ground-rod penetration is shallow. If only a small amount of soil exists, use it to cover the radials to the extent possible. It is more important to cover radials in the area near the tower than at greater distances. If, however, soil exists only at the outer distances and cannot be transported to the inner locations, use the soil to cover the outer portions of the radials.
If soil is present, install ground rods along the radial lengths. Spacing between ground rods is affected by the depth that each rod is driven; the shallower the rod, the closer the allowed spacing.
Because the ultimate depth a rod can be driven cannot always be predicted by the first rod driven, use a maximum spacing of 15 ft when selecting a location for each additional rod. Drive rods at building corners first (within 24 in. but not closer than 6 in. to a concrete footer unless that footer has an encapsulated Ufer ground), then fill in the space between the corners with additional rods.
Drive the ground rods in place; do not auger; set in place, then back-fill. The soil compactness is never as great on augured-hole rods when compared with driven rods. The only exception is when a hole is augured or blasted for a ground rod or rebar and then back-filled in concrete. Because concrete contains lime (alkali base) and is porous, it absorbs moisture readily, giving it up slowly. Electron carriers are almost always present, making the substance a good conductor.
If a Ufer ground is not being implemented, the radials may be Cad-welded to a subterranean ring, with the ring interconnected to the tower foot-pad via a minimum of three no. 1/0 wires spaced at 120° angles and Cad-welded to the radial ring.
Tuesday, December 23, 2014
EFFECTS OF BLACKOUT
A facility that is down for even 5 min can suffer a significant loss of productivity or data that may take hours or days to rebuild. A blackout affecting a transportation or medical center could be life-threatening. Coupled with this threat is the possibility of extended power-service loss due to severe storm conditions. Many broadcast and communications relay sites are located in remote, rural areas or on mountaintops. Neither of these kinds of locations are well-known for their power reliability. It is not uncommon in mountainous areas for utility company service to be out for extended periods after a major storm. Few operators are willing to take such risks with their business. Most choose to install standby power systems at appropriate points in the equipment chain.
The cost of standby power for a facility can be substantial, and an examination of the possible alternatives should be conducted before any decision on equipment is made. Management must clearly define the direct and indirect costs and weigh them appropriately. Include the following items in the cost versus risk analysis:
• Standby power-system equipment purchase and installation cost
• Exposure of the system to utility company power failure
• Alternative operating methods available to the facility
• Direct and indirect costs of lost up-time because of blackout conditions
A distinction must be made between emergency and standby power sources. Strictly speaking, emergency systems supply circuits legally designated as being essential for safety to life and property. Standby power systems are used to protect a facility against the loss of productivity resulting from a utility company power outage.
The cost of standby power for a facility can be substantial, and an examination of the possible alternatives should be conducted before any decision on equipment is made. Management must clearly define the direct and indirect costs and weigh them appropriately. Include the following items in the cost versus risk analysis:
• Standby power-system equipment purchase and installation cost
• Exposure of the system to utility company power failure
• Alternative operating methods available to the facility
• Direct and indirect costs of lost up-time because of blackout conditions
A distinction must be made between emergency and standby power sources. Strictly speaking, emergency systems supply circuits legally designated as being essential for safety to life and property. Standby power systems are used to protect a facility against the loss of productivity resulting from a utility company power outage.
ADVANCED SYSTEM PROTECTION
A more sophisticated power-control system is shown in Figure 1, where a dual feeder supply is coupled with a motor-generator set to provide clean, undisturbed ac power to the load. The m-g set will smooth over the transition from the main utility feed to the standby, often making a commercial power failure unnoticed by on-site personnel. A conventional m-g set typically will give up to 0.5 s of power fail ride-through, more than enough to accomplish a transfer from one utility feed to the other. This standby power system is further refined in the application illustrated in Figure 2, where a diesel generator has been added to the system. With the automatic overlap transfer switch shown at the generator output, this arrangement also can be used for peak demand power shaving.
Figure 3 shows a simplified schematic diagram of a 220 kW UPS system utilizing dual utility company feed lines, a 750 kVA gas-engine generator, and five dc-driven motor-generator sets with a 20- min battery supply at full load. The five m-g sets operate in parallel. Each is rated for 100 kW output. Only three are needed to power the load, but four are on-line at any given time. The fifth machine provides redundancy in the event of a failure or for scheduled maintenance work. The batteries are always on-line under a slight charge across the 270 V dc bus. Two separate natural-gas lines, buried along different land routes, supply the gas engine. Local gas storage capacity also is provided.
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Figure 1: A dual feeder standby power system using a
motor-generator set to provide power fails ride-through and transient-disturbance
protection. Switching circuits allow the m-g set to be bypassed, if necessary.
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| Figure 2: A premium power-supply backup and conditioning system using dual utility company feeds, a diesel generator, and a motor-generator set. An arrangement such as this would be used for critical loads that require a steady supply of clean ac. |
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| Figure 3: Simplified installation diagram of a high-reliability power system incorporating dual utility feeds, a standby gas-engine generator, and five battery-backed dc m-g sets. |
CHOOSING A SPECIFIC SIZE OF GENERATOR
Engine-generator sets are available for power levels ranging from less than 1 kVA to several thousand kVA or more. Machines also can be paralleled to provide greater capacity. Engine-generator sets typically are classified by the type of power plant used:
• Diesel:
• Diesel:
Advantages: rugged and dependable, low fuel costs, low fire or explosion hazard.
Disadvantages: somewhat more costly than other engines, heavier in smaller sizes.
• Natural and liquefied petroleum gas:
• Natural and liquefied petroleum gas:
Advantages: quick starting after long shutdown periods, long life, low maintenance.
Disadvantage: availability of natural gas during area wide power failure subject to question.
• Gasoline:
• Gasoline:
Advantages: rapid starting, low initial cost.
Disadvantages: greater hazard associated with storing and handling gasoline, generally shorter mean time between overhaul.
• Gas turbine:
• Gas turbine:
Advantages: smaller and lighter than piston engines of comparable horsepower, rooftop installations practical, rapid response to load changes.
Disadvantages: longer time required to start and reach operating speed, sensitive to high input air temperature.
The type of power plant chosen usually is determined primarily by the environment in which the system will be operated and by the cost of ownership. For example, a standby generator located in an urban area office complex may be best suited to the use of an engine powered by natural gas, because of the problems inherent in storing large amounts of fuel. State or local building codes can place expensive restrictions on fuel-storage tanks and make the use of a gasoline- or diesel-powered engine impractical. The use of propane usually is restricted to rural areas. The availability of propane during periods of bad weather (when most power failures occur) also must be considered.
The generator rating for a standby power system should be chosen carefully and should take into consideration the anticipated future growth of the plant. It is good practice to install a standby power system rated for at least 25% greater outputs than the current peak facility load. This headroom gives a margin of safety for the standby equipment and allows for future expansion of the facility without over loading the system.
An engine-driven standby generator typically incorporates automatic starting controls, a battery charger, and automatic transfer switch. (Refer Figure) Control circuits monitor the utility supply and start the engine when there is a failure or a sustained voltage drop on the ac supply. The switch transfers the load as soon as the generator reaches operating voltage and frequency. Upon restoration of the utility supply, the switch returns the load and initiates engine shutdown. The automatic transfer switch must meet demanding requirements, including:
• Carrying the full rated current continuously
• Withstanding fault currents without contact separation
• Handling high inrush currents
• Withstanding many interruptions at full load without damage
The nature of most power outages requires a sophisticated monitoring system for the engine-generator set. Most power failures occur during periods of bad weather. Most standby generators are unattended. More often than not, the standby system will start, run, and shut down without any human intervention or supervision. For reliable operation, the monitoring system must check the status of the machine continually to ensure that all parameters are within normal limits. Time-delay periods usually are provided by the controller that requires an outage to last from 5 to 10 s before the generator is started and the load is transferred. This prevents false starts that needlessly exercise the system. A time delay of 5 to 30 min usually is allowed between the restoration of utility power and return of the load. This delay permits the utility ac lines to stabilize before the load is reapplied.
The type of power plant chosen usually is determined primarily by the environment in which the system will be operated and by the cost of ownership. For example, a standby generator located in an urban area office complex may be best suited to the use of an engine powered by natural gas, because of the problems inherent in storing large amounts of fuel. State or local building codes can place expensive restrictions on fuel-storage tanks and make the use of a gasoline- or diesel-powered engine impractical. The use of propane usually is restricted to rural areas. The availability of propane during periods of bad weather (when most power failures occur) also must be considered.
The generator rating for a standby power system should be chosen carefully and should take into consideration the anticipated future growth of the plant. It is good practice to install a standby power system rated for at least 25% greater outputs than the current peak facility load. This headroom gives a margin of safety for the standby equipment and allows for future expansion of the facility without over loading the system.
An engine-driven standby generator typically incorporates automatic starting controls, a battery charger, and automatic transfer switch. (Refer Figure) Control circuits monitor the utility supply and start the engine when there is a failure or a sustained voltage drop on the ac supply. The switch transfers the load as soon as the generator reaches operating voltage and frequency. Upon restoration of the utility supply, the switch returns the load and initiates engine shutdown. The automatic transfer switch must meet demanding requirements, including:
• Carrying the full rated current continuously
• Withstanding fault currents without contact separation
• Handling high inrush currents
• Withstanding many interruptions at full load without damage
The nature of most power outages requires a sophisticated monitoring system for the engine-generator set. Most power failures occur during periods of bad weather. Most standby generators are unattended. More often than not, the standby system will start, run, and shut down without any human intervention or supervision. For reliable operation, the monitoring system must check the status of the machine continually to ensure that all parameters are within normal limits. Time-delay periods usually are provided by the controller that requires an outage to last from 5 to 10 s before the generator is started and the load is transferred. This prevents false starts that needlessly exercise the system. A time delay of 5 to 30 min usually is allowed between the restoration of utility power and return of the load. This delay permits the utility ac lines to stabilize before the load is reapplied.
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| Figure: Typical configuration of an engine-generator set. |
The transfer of motor loads may require special consideration, depending upon the size and type of motors used at a plant. If the residual voltage of the motor is out of phase with the power source to which the motor is being transferred, serious damage can result to the motor. Excessive current draw also may trip over current protective devices. Motors above 50 hp with relatively high load inertia in relation to torque requirements, such as flywheels and fans, may require special controls. Restart time delays are a common solution.
Automatic starting and synchronizing controls are used for multiple-engine-generator installations. The output of two or three smaller units can be combined to feed the load. This capability offers additional protection for the facility in the event of a failure in any one machine. As the load at the facility increases, additional engine-generator systems can be installed on the standby power bus.
Automatic starting and synchronizing controls are used for multiple-engine-generator installations. The output of two or three smaller units can be combined to feed the load. This capability offers additional protection for the facility in the event of a failure in any one machine. As the load at the facility increases, additional engine-generator systems can be installed on the standby power bus.
STANDBY GENERATOR TYPES
Generators for standby power applications can be induction or synchronous machines. Most engine-generator systems in use today are of the synchronous type because of the versatility, reliability, and capability of operating independently that this approach provides. Most modern synchronous generators are of the revolving field alternator design. Essentially, this means that the armature windings are held stationary and the field is rotated. Therefore, generated power can be taken directly from the stationary armature windings. Revolving armature alternators are less popular because the generated output power must be derived via slip rings and brushes.
The exact value of the ac voltage produced by a synchronous machine is controlled by varying the current in the dc field windings, whereas frequency is controlled by the speed of rotation. Power output is controlled by the torque applied to the generator shaft by the driving engine. In this manner, the synchronous generator offers precise control over the power it can produce.
Practically all modern synchronous generators use a brushless exciter. The exciter is a small ac generator on the main shaft; the ac voltage produced is rectified by a three-phase rotating rectifier assembly also on the shaft. The dc voltage thus obtained is applied to the main generator field, which is also on the main shaft. A voltage regulator is provided to control the exciter field current, and in this manner, the field voltage can be precisely controlled, resulting in a stable output voltage.
The frequency of the ac current produced is dependent on two factors: the number of poles built into the machine, and the speed of rotation (rpm). Because the output frequency must normally be maintained within strict limits (60 or 50 Hz), control of the generator speed is essential. This is accomplished by providing precise rpm control of the prime mover, which is performed by a governor.
There are many types of governors; however, for auxiliary power applications, the isochronous governor is normally selected. The isochronous governor controls the speed of the engine so that it remains constant from no-load to full load, assuring a constant ac power output frequency from the generator. A modern system consists of two primary components: an electronic speed control and an actuator that adjusts the speed of the engine. The electronic speed control senses the speed of the machine and provides a feedback signal to the mechanical/hydraulic actuator, which in turn positions the engine throttle or fuel control to maintain accurate engine rpm.
The National Electrical Code provides guidance for safe and proper installation of on-site engine generator systems. Local codes may vary and must be reviewed during early design stages.
The exact value of the ac voltage produced by a synchronous machine is controlled by varying the current in the dc field windings, whereas frequency is controlled by the speed of rotation. Power output is controlled by the torque applied to the generator shaft by the driving engine. In this manner, the synchronous generator offers precise control over the power it can produce.
Practically all modern synchronous generators use a brushless exciter. The exciter is a small ac generator on the main shaft; the ac voltage produced is rectified by a three-phase rotating rectifier assembly also on the shaft. The dc voltage thus obtained is applied to the main generator field, which is also on the main shaft. A voltage regulator is provided to control the exciter field current, and in this manner, the field voltage can be precisely controlled, resulting in a stable output voltage.
The frequency of the ac current produced is dependent on two factors: the number of poles built into the machine, and the speed of rotation (rpm). Because the output frequency must normally be maintained within strict limits (60 or 50 Hz), control of the generator speed is essential. This is accomplished by providing precise rpm control of the prime mover, which is performed by a governor.
There are many types of governors; however, for auxiliary power applications, the isochronous governor is normally selected. The isochronous governor controls the speed of the engine so that it remains constant from no-load to full load, assuring a constant ac power output frequency from the generator. A modern system consists of two primary components: an electronic speed control and an actuator that adjusts the speed of the engine. The electronic speed control senses the speed of the machine and provides a feedback signal to the mechanical/hydraulic actuator, which in turn positions the engine throttle or fuel control to maintain accurate engine rpm.
The National Electrical Code provides guidance for safe and proper installation of on-site engine generator systems. Local codes may vary and must be reviewed during early design stages.
UPS SYSTEMS
An uninterruptible power system is an elegant solution to power outage concerns. The output of the UPS inverter can be a sine wave or pseudo sine wave. When shopping for a UPS system, consider the following:
• Power reserve capacity for future growth of the facility.
• Inverter current surge capability (if the system will be driving inductive loads, such as motors).
• Output voltage and frequency stability over time and with varying loads.
• Required battery supply voltage and current. Battery costs vary greatly, depending upon the type of units needed.
• Type of UPS system (forward-transfer type or reverse-transfer type) required by the particular application. Some sensitive loads may not tolerate even brief interruptions of the ac power source.
• Inverter efficiency at typical load levels. Some inverters have good efficiency ratings when loaded at 90% of capacity, but poor efficiency when lightly loaded.
• Size and environmental requirements of the UPS system. High-power UPS equipment requires a large amount of space for the inverter/control equipment and batteries. Battery banks often require special ventilation and ambient temperature control.
• Power reserve capacity for future growth of the facility.
• Inverter current surge capability (if the system will be driving inductive loads, such as motors).
• Output voltage and frequency stability over time and with varying loads.
• Required battery supply voltage and current. Battery costs vary greatly, depending upon the type of units needed.
• Type of UPS system (forward-transfer type or reverse-transfer type) required by the particular application. Some sensitive loads may not tolerate even brief interruptions of the ac power source.
• Inverter efficiency at typical load levels. Some inverters have good efficiency ratings when loaded at 90% of capacity, but poor efficiency when lightly loaded.
• Size and environmental requirements of the UPS system. High-power UPS equipment requires a large amount of space for the inverter/control equipment and batteries. Battery banks often require special ventilation and ambient temperature control.
STANDBY POWER SYSTEM NOISE
Noise produced by backup power systems can be a serious problem if not addressed properly. Standby generators, motor-generator sets, and UPS systems produce noise that can disturb building occupants and irritate neighbors or landlords.
The noise associated with electrical generation usually is related to the drive mechanism, most commonly an internal combustion engine. The amplitude of the noise produced is directly related to the size of the engine-generator set. First, consider whether noise reduction is a necessity. Many building owners have elected to tolerate the noise produced by a standby power generator because its use is limited to emergency situations. During a crisis, when the normal source of power is unavailable, most people will tolerate noise associated with a standby generator.
If the decision is made that building occupants can live with the noise of the generator, care must be taken in scheduling the required testing and exercising of the unit. Whether testing occurs monthly or weekly, it should be done on a regular schedule.
If it has been determined that the noise should be controlled, or at least minimized, the easiest way to achieve this objective is to physically separate the machine from occupied areas. This may be easier said than done. Because engine noise is predominantly low-frequency in character, walls and floor/ceiling construction used to contain the noise must be massive. Lightweight construction, even though it may involve several layers of resiliently mounted drywall, is ineffective in reducing low-frequency noise. Exhaust noise is a major component of engine noise but, fortunately, it is easier to control. When selecting an engine-generator set, select the highest-quality exhaust muffler available. Such units often are identified as hospital-grade mufflers.
Engine-generator sets also produce significant vibration. The machine should be mounted securely to a slab-on-grade or an isolated basement floor, or it should be installed on vibration isolation mounts. Such mounts usually are specified by the manufacturer.
Because a UPS system or motor-generator set is a source of continuous power, it must run continuously. Noise must be adequately controlled. Physical separation is the easiest and most effective method of shielding occupied areas from noise. Enclosure of UPS equipment usually is required, but noise control is significantly easier than for an engine-generator because of the lower noise levels involved. Nevertheless, the low-frequency 120 Hz fundamental of a UPS system is difficult to contain adequately; massive constructions may be necessary. Vibration control also is required for most UPS and m-g gear.
The noise associated with electrical generation usually is related to the drive mechanism, most commonly an internal combustion engine. The amplitude of the noise produced is directly related to the size of the engine-generator set. First, consider whether noise reduction is a necessity. Many building owners have elected to tolerate the noise produced by a standby power generator because its use is limited to emergency situations. During a crisis, when the normal source of power is unavailable, most people will tolerate noise associated with a standby generator.
If the decision is made that building occupants can live with the noise of the generator, care must be taken in scheduling the required testing and exercising of the unit. Whether testing occurs monthly or weekly, it should be done on a regular schedule.
If it has been determined that the noise should be controlled, or at least minimized, the easiest way to achieve this objective is to physically separate the machine from occupied areas. This may be easier said than done. Because engine noise is predominantly low-frequency in character, walls and floor/ceiling construction used to contain the noise must be massive. Lightweight construction, even though it may involve several layers of resiliently mounted drywall, is ineffective in reducing low-frequency noise. Exhaust noise is a major component of engine noise but, fortunately, it is easier to control. When selecting an engine-generator set, select the highest-quality exhaust muffler available. Such units often are identified as hospital-grade mufflers.
Engine-generator sets also produce significant vibration. The machine should be mounted securely to a slab-on-grade or an isolated basement floor, or it should be installed on vibration isolation mounts. Such mounts usually are specified by the manufacturer.
Because a UPS system or motor-generator set is a source of continuous power, it must run continuously. Noise must be adequately controlled. Physical separation is the easiest and most effective method of shielding occupied areas from noise. Enclosure of UPS equipment usually is required, but noise control is significantly easier than for an engine-generator because of the lower noise levels involved. Nevertheless, the low-frequency 120 Hz fundamental of a UPS system is difficult to contain adequately; massive constructions may be necessary. Vibration control also is required for most UPS and m-g gear.
SEALED LEAD ACID BATTERY
The lead-acid battery is a commonly used chemistry. The flooded version is found in automobiles and large UPS battery banks. Most smaller, portable systems use the sealed version, also referred to as gel-cell or SLA.
The lead-acid chemistry is commonly used when high power is required, weight is not critical, and cost must be kept low. The typical current range of a medium-sized SLA device is 2 Ah to 50 Ah. Because of its minimal maintenance requirements and predictable storage characteristics, the SLA has found wide acceptance in the UPS industry, especially for point-of-application systems.
The SLA is not subject to memory. No harm is done by leaving the battery on float charge for a prolonged time. On the negative side, the SLA does not lend itself well to fast charging. Typical charge times are 8 to 16 hours. The SLA must always be stored in a charged state because a discharged SLA will sulphate. If left discharged, a recharge may be difficult or even impossible.
Unlike the common NiCd, the SLA prefers a shallow discharge. A full discharge reduces the number of times the battery can be recharged, similar to a mechanical device that wears down when placed under stress. In fact, each discharge-charge cycle reduces (slightly) the storage capacity of the battery. This wear down characteristic also applies to other chemistries, including the NiMH.
The charge algorithm of the SLA differs from that of other batteries in that a voltage-limit rather than current-limit is used. Typically, a multistage charger applies three charge stages consisting of a constant- current charge, topping-charge, and float-charge. (Reffer Figure) During the constant-current stage, the battery charges to 70% in about 5 hours; the remaining 30% is completed by the topping charge. The slow topping-charge, lasting another 5 hours, is essential for the performance of the battery. If not provided, the SLA eventually loses the ability to accept a full charge, and the storage capacity of the battery is reduced. The third stage is the float-charge that compensates for self-discharge after the battery has been fully charged.
During the constant-current charge, the SLA battery is charged at a high current, limited by the charger itself. After the voltage limit is reached, the topping charge begins and the current starts to gradually decrease. Full-charge is reached when the current drops to a preset level or reaches a low-end plateau.
The proper setting of the cell voltage limit is critical and is related to the conditions under which the battery is charged. A typical voltage limit range is from 2.30 to 2.45 V. If a slow charge is acceptable, or if the room temperature can exceed 30°C (86°F), the recommended voltage limit is 2.35 V/cell. If a faster charge is required and the room temperature remains below 30°C, 2.40 or 2.45 V/cell can be used. Table compares the advantages and disadvantages of the different voltage settings.
The lead-acid chemistry is commonly used when high power is required, weight is not critical, and cost must be kept low. The typical current range of a medium-sized SLA device is 2 Ah to 50 Ah. Because of its minimal maintenance requirements and predictable storage characteristics, the SLA has found wide acceptance in the UPS industry, especially for point-of-application systems.
The SLA is not subject to memory. No harm is done by leaving the battery on float charge for a prolonged time. On the negative side, the SLA does not lend itself well to fast charging. Typical charge times are 8 to 16 hours. The SLA must always be stored in a charged state because a discharged SLA will sulphate. If left discharged, a recharge may be difficult or even impossible.
Unlike the common NiCd, the SLA prefers a shallow discharge. A full discharge reduces the number of times the battery can be recharged, similar to a mechanical device that wears down when placed under stress. In fact, each discharge-charge cycle reduces (slightly) the storage capacity of the battery. This wear down characteristic also applies to other chemistries, including the NiMH.
The charge algorithm of the SLA differs from that of other batteries in that a voltage-limit rather than current-limit is used. Typically, a multistage charger applies three charge stages consisting of a constant- current charge, topping-charge, and float-charge. (Reffer Figure) During the constant-current stage, the battery charges to 70% in about 5 hours; the remaining 30% is completed by the topping charge. The slow topping-charge, lasting another 5 hours, is essential for the performance of the battery. If not provided, the SLA eventually loses the ability to accept a full charge, and the storage capacity of the battery is reduced. The third stage is the float-charge that compensates for self-discharge after the battery has been fully charged.
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Figure: The charge states of an SLA battery.
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The proper setting of the cell voltage limit is critical and is related to the conditions under which the battery is charged. A typical voltage limit range is from 2.30 to 2.45 V. If a slow charge is acceptable, or if the room temperature can exceed 30°C (86°F), the recommended voltage limit is 2.35 V/cell. If a faster charge is required and the room temperature remains below 30°C, 2.40 or 2.45 V/cell can be used. Table compares the advantages and disadvantages of the different voltage settings.
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| Table: Recommended Charge Voltage Limit for the SLA Battery |
GROUND SYSTEM MAINTENANCE
Out of sight, out of mind does not or, at least, should not apply to a facility ground system. Grounding is a crucial element in achieving reliable operation of electronic equipment. If a ground system has been buried for 10 years or more, it is due for an inspection. Soil conditions vary widely, but few areas have soil that permits a radial- or screen-based ground system to last much more than 15 years.
The method of construction and bonding of the ground network also can play a significant role in the ultimate life expectancy of the system. For example, ground conductors secured only by mechanical means (screws and bolts, crimping, and rivets) can quickly break down when exposed to even mild soil conditions. Unless silver-soldered or bonded using an exothermic method, such connections soon will be useless for all practical purposes.
The inspection process involves uncovering portions of the ground system to check for evidence of failure. Pay particular attention to interconnection points, where the greatest potential for problems exists. In some cases, a good metal detector will help identify portions of the ground system. It will not, however, identify breaks in the system. Portions of the ground system still will need to be uncovered to complete the inspection. Accurate documentation of the placement of ground-system components will aid the inspection effort greatly.
Check any buried mechanical connections carefully. Bolts that have been buried for many years may be severely deteriorated. Carefully remove several bolts, and inspect their condition. If a bolt is severely oxidized, it may twist off as it is removed. After uncovering representative portions of the ground system, document the condition of the ground through notes and photographs. These will serve as a reference point for future observation. The photos in Figure illustrate some of the problems that can occur with an aging ground system. Note that many of the problems experienced with the system shown in the photographs resulted from improper installation of components in the first place.
The method of construction and bonding of the ground network also can play a significant role in the ultimate life expectancy of the system. For example, ground conductors secured only by mechanical means (screws and bolts, crimping, and rivets) can quickly break down when exposed to even mild soil conditions. Unless silver-soldered or bonded using an exothermic method, such connections soon will be useless for all practical purposes.
The inspection process involves uncovering portions of the ground system to check for evidence of failure. Pay particular attention to interconnection points, where the greatest potential for problems exists. In some cases, a good metal detector will help identify portions of the ground system. It will not, however, identify breaks in the system. Portions of the ground system still will need to be uncovered to complete the inspection. Accurate documentation of the placement of ground-system components will aid the inspection effort greatly.
Check any buried mechanical connections carefully. Bolts that have been buried for many years may be severely deteriorated. Carefully remove several bolts, and inspect their condition. If a bolt is severely oxidized, it may twist off as it is removed. After uncovering representative portions of the ground system, document the condition of the ground through notes and photographs. These will serve as a reference point for future observation. The photos in Figure illustrate some of the problems that can occur with an aging ground system. Note that many of the problems experienced with the system shown in the photographs resulted from improper installation of components in the first place.
(a)
(b)
(c)
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| Figure: Ground system inspection: (a) Even though a buried copper strap may appear undamaged, give it a pull to be sure. This strap came apart with little effort. (b) Acidic soil conditions created holes in this ground screen. (c) Small pieces of copper strap were used in this ground system to attach radials to the ground screen around the base of a tower. Proper installation procedures would have incorporated a solid piece of strap around the perimeter of the screen for such connections. |
SWITCHGEAR MAINTENANCE
All too often, ac power switchgear is installed at a facility and forgotten until a problem occurs. A careless approach to regular inspection and cleaning of switchgear has resulted in numerous failures, including destructive fires. The most serious fault in any switchgear assembly is arcing involving the main power bus. Protective devices may fail to open, or open only after a considerable delay. The arcing damage to bus bars and enclosures can be significant. Fire often ensues, compounding the damage.
Moisture, combined with dust and dirt, is the greatest deteriorating factor insofar as insulation is concerned. Dust or moisture are thought to account for as much as half of switchgear failures. Initial leakage paths across the surface of bus supports result in flashover and sustained arcing. Contact overheating is another common cause of switchgear failure. Improper circuit-breaker installation or loose connections can result in localized overheating and arcing.
An arcing fault is destructive because of the high temperatures present (more than 6000°F). An arc is not a stationary event. Because of the ionization of gases and the presence of vaporized metal, an arc can travel along bare bus bars, spreading the damage and sometimes bypassing open circuit breakers. It has been observed that most faults in three-phase systems involve all phases. The initial fault that triggers the event may involve only one phase, but because of the traveling nature of an arc, damage quickly spreads to the other lines.
Preventing switchgear failure is a complicated discipline, but consider the following general guidelines:
• Install insulated bus bars for both medium-voltage and low-voltage switchgear. Each phase of the bus and all connections should be enclosed completely by insulation with electrical, mechanical, thermal, and flame-retardant characteristics suitable for the application.
• Establish a comprehensive preventive maintenance program for the facility. Keep all switchboard hardware clean from dust and dirt. Periodically check connection points for physical integrity.
• Maintain control over environmental conditions. Switchgear exposed to contaminants, corrosive gases, moist air, or high ambient temperatures may be subject to catastrophic failure. Conditions favorable to moisture condensation are particularly perilous, especially when dust and dirt are present.
• Accurately select over current trip settings, and check them on a regular basis. Adjust the trip points of protection devices to be as low as possible, consistent with reliable operation.
• Divide switchgear into compartments that isolate different circuit elements. Consider adding vertical barriers to bus compartments to prevent the spread of arcing and fire.
• Install ground-fault protection devices at appropriate points in the power-distribution system.
• Adhere to all applicable building codes.
Moisture, combined with dust and dirt, is the greatest deteriorating factor insofar as insulation is concerned. Dust or moisture are thought to account for as much as half of switchgear failures. Initial leakage paths across the surface of bus supports result in flashover and sustained arcing. Contact overheating is another common cause of switchgear failure. Improper circuit-breaker installation or loose connections can result in localized overheating and arcing.
An arcing fault is destructive because of the high temperatures present (more than 6000°F). An arc is not a stationary event. Because of the ionization of gases and the presence of vaporized metal, an arc can travel along bare bus bars, spreading the damage and sometimes bypassing open circuit breakers. It has been observed that most faults in three-phase systems involve all phases. The initial fault that triggers the event may involve only one phase, but because of the traveling nature of an arc, damage quickly spreads to the other lines.
Preventing switchgear failure is a complicated discipline, but consider the following general guidelines:
• Install insulated bus bars for both medium-voltage and low-voltage switchgear. Each phase of the bus and all connections should be enclosed completely by insulation with electrical, mechanical, thermal, and flame-retardant characteristics suitable for the application.
• Establish a comprehensive preventive maintenance program for the facility. Keep all switchboard hardware clean from dust and dirt. Periodically check connection points for physical integrity.
• Maintain control over environmental conditions. Switchgear exposed to contaminants, corrosive gases, moist air, or high ambient temperatures may be subject to catastrophic failure. Conditions favorable to moisture condensation are particularly perilous, especially when dust and dirt are present.
• Accurately select over current trip settings, and check them on a regular basis. Adjust the trip points of protection devices to be as low as possible, consistent with reliable operation.
• Divide switchgear into compartments that isolate different circuit elements. Consider adding vertical barriers to bus compartments to prevent the spread of arcing and fire.
• Install ground-fault protection devices at appropriate points in the power-distribution system.
• Adhere to all applicable building codes.
PLANT MAINTENANCE
Maintenance of the facility electrical system is a key part of any serious energy-management effort. Perform the following steps on a regular basis:
• Measure the current drawn on distribution cables. Document the measurements so that a history of power demand can be compiled.
• Check terminal and splice connections to make sure they are tight.
• Check power-system cables for excessive heating.
• Check cables for insulation problems.
• Clean switchboard and circuit-breaker panels.
• Measure the phase-to-phase load balance at the utility service entrance. Load imbalance can result in inefficient use of ac power.
• Measure and chart the power factor of the load. Develop and post a simplified one-line schematic of the entire power network as well as other building systems, including heating, air conditioning, security, and alarm functions. A mimic board is helpful in this process. Construct the mimic board control panel so that it depicts the entire ac power-distribution system.
The board should have active indicators that show what loads or circuit breakers are turned on or off, what functions have been disabled, and key operating parameters, including input voltage, load current, and total kVA demand. Safety considerations require that machinery not be activated from the mimic board. Permit machinery to be energized only at the apparatus. As an alternative, remote control of machines can be provided, if a remote/local control switch is provided at the apparatus.
Environmental control systems should be monitored closely. Air-conditioning, heating, and ventilation systems often represent a significant portion of the power load of a facility. Computer-based data logging equipment with process control capability can be of considerable help in monitoring the condition of the equipment. The logger can be programmed to record all pertinent values periodically and to report abnormal conditions.
• Measure the current drawn on distribution cables. Document the measurements so that a history of power demand can be compiled.
• Check terminal and splice connections to make sure they are tight.
• Check power-system cables for excessive heating.
• Check cables for insulation problems.
• Clean switchboard and circuit-breaker panels.
• Measure the phase-to-phase load balance at the utility service entrance. Load imbalance can result in inefficient use of ac power.
• Measure and chart the power factor of the load. Develop and post a simplified one-line schematic of the entire power network as well as other building systems, including heating, air conditioning, security, and alarm functions. A mimic board is helpful in this process. Construct the mimic board control panel so that it depicts the entire ac power-distribution system.
The board should have active indicators that show what loads or circuit breakers are turned on or off, what functions have been disabled, and key operating parameters, including input voltage, load current, and total kVA demand. Safety considerations require that machinery not be activated from the mimic board. Permit machinery to be energized only at the apparatus. As an alternative, remote control of machines can be provided, if a remote/local control switch is provided at the apparatus.
Environmental control systems should be monitored closely. Air-conditioning, heating, and ventilation systems often represent a significant portion of the power load of a facility. Computer-based data logging equipment with process control capability can be of considerable help in monitoring the condition of the equipment. The logger can be programmed to record all pertinent values periodically and to report abnormal conditions.
PEAK ELECTRICAL POWER DEMAND
Conserving energy is a big part of the power bill reduction equation, but it is not the whole story. The peak demand of the customer load is an important criterion in the utility company's calculation of rate structures. The peak demand figure is a measure of the maximum load placed on the utility company system by a customer during a predetermined billing cycle. The measured quantities may be kilowatts, kilovolt- amperes, or both. Time intervals used for this measurement range from 15 to 60 min. billing cycles may be annual or semiannual. Figure 1 shows an example of varying peak demand.
If a facility operated at basically the same power consumption level from one hour to the next and one day to the next, the utility company could predict accurately the demand of the load, and then size its equipment (including the allocation of energy reserves) for only the amount of power actually needed.
For the example shown in the figure, however, the utility company must size its equipment (including allocated energy reserves) for the peak demand. The area between the peak demand and the actual usage is the margin of inefficiency that the customer forces upon the utility. The peak demand factor is a method used by utility companies to assess penalties for such operation, thereby encouraging the customer to approach a more efficient state of operation (from the utility's viewpoint).
Load shedding is a term used to describe the practice of trimming peak power demand to reduce high-demand penalties. The goal of load shedding is to schedule the operation of nonessential equipment so as to provide a uniform power load to the utility company and, thereby, a better kWh rate. Nearly any operation has certain electric loads that can be rescheduled on a permanent basis or deferred as power demand increases during the day. Figure 2 illustrates the results of a load-shedding program. This more efficient operation has a lower overall peak demand and a higher average demand.
Peak demand reduction efforts can cover a wide range of possibilities. It would be unwise from an energy standpoint, for example, to test high-power standby equipment on a summer afternoon, when air-conditioning units may be in full operation. Morning or evening hours would be a better choice, when the air-conditioning is off and the demand of office equipment is reduced. Each operation is unique and requires an individual assessment of load-shedding options.
An automated power-demand controller provides an effective method of managing peak demand. A controller can analyze the options available and switch loads as needed to maintain a relatively constant power demand from the utility company. Such systems are programmed to recognize which loads have priority and which loads are nonessential. Power demand then is automatically adjusted by the system, based upon the rate schedule of the utility company. Many computerized demand control systems also provide the customer a printout of the demand profile of the plant, further helping managers analyze and reduce power costs. Note that both energy demand and the costs for that energy are provided.
If a facility operated at basically the same power consumption level from one hour to the next and one day to the next, the utility company could predict accurately the demand of the load, and then size its equipment (including the allocation of energy reserves) for only the amount of power actually needed.
Load shedding is a term used to describe the practice of trimming peak power demand to reduce high-demand penalties. The goal of load shedding is to schedule the operation of nonessential equipment so as to provide a uniform power load to the utility company and, thereby, a better kWh rate. Nearly any operation has certain electric loads that can be rescheduled on a permanent basis or deferred as power demand increases during the day. Figure 2 illustrates the results of a load-shedding program. This more efficient operation has a lower overall peak demand and a higher average demand.
An automated power-demand controller provides an effective method of managing peak demand. A controller can analyze the options available and switch loads as needed to maintain a relatively constant power demand from the utility company. Such systems are programmed to recognize which loads have priority and which loads are nonessential. Power demand then is automatically adjusted by the system, based upon the rate schedule of the utility company. Many computerized demand control systems also provide the customer a printout of the demand profile of the plant, further helping managers analyze and reduce power costs. Note that both energy demand and the costs for that energy are provided.
ELECTRICAL ENERGY USAGE
The kilowatt-hour (kWh) usage of a facility can be reduced by turning off loads such as heating and air conditioning systems, lights, and office equipment when they are not needed. The installation of timers, photocells, or sophisticated computer-controlled energy-management systems can make substantial reductions in facility kWh demand each month. Common sense will dictate the conservation measures applicable to a particular situation. Obvious items include reducing the length of time high-power equipment is in operation, setting heating and cooling thermostats to reasonable levels, keeping office equipment turned off during the night, and avoiding excessive amounts of indoor or outdoor lighting.
Although energy conservation measures should be taken in every area of facility operation, the greatest savings generally can be found where the largest energy users are located. Transmitter plants, large machinery, and process drying equipment consume a huge amount of power, so particular attention should be given to such hardware. Consider the following:
• Use the waste heat from equipment at the site for other purposes, if practical. In the case of high power RF generators or transmitters, room heating can be accomplished with a logic-controlled power amplifier exhaust-air recycling system.
• Have a knowledgeable consultant plan the air-conditioning and heating system at the facility for efficient operation.
• Check thermostat settings on a regular basis, and consider installing time-controlled thermostats.
• Inspect outdoor-lighting photocells regularly for proper operation.
• Examine carefully the efficiency of high-power equipment used at the facility. New designs may offer substantial savings in energy costs.
The efficiency of large power loads, such as mainframe computers, transmitters, or industrial RF heaters, is an item of critical importance to energy conservation efforts. Most systems available today are significantly more efficient than their counterparts of just 10 years ago. Plant management often can find economic justification for updating or replacing an older system on the power savings alone. In virtually any facility, energy conservation can best be accomplished through careful selection of equipment, thoughtful system design, and conscientious maintenance practices.
Although energy conservation measures should be taken in every area of facility operation, the greatest savings generally can be found where the largest energy users are located. Transmitter plants, large machinery, and process drying equipment consume a huge amount of power, so particular attention should be given to such hardware. Consider the following:
• Use the waste heat from equipment at the site for other purposes, if practical. In the case of high power RF generators or transmitters, room heating can be accomplished with a logic-controlled power amplifier exhaust-air recycling system.
• Have a knowledgeable consultant plan the air-conditioning and heating system at the facility for efficient operation.
• Check thermostat settings on a regular basis, and consider installing time-controlled thermostats.
• Inspect outdoor-lighting photocells regularly for proper operation.
• Examine carefully the efficiency of high-power equipment used at the facility. New designs may offer substantial savings in energy costs.
The efficiency of large power loads, such as mainframe computers, transmitters, or industrial RF heaters, is an item of critical importance to energy conservation efforts. Most systems available today are significantly more efficient than their counterparts of just 10 years ago. Plant management often can find economic justification for updating or replacing an older system on the power savings alone. In virtually any facility, energy conservation can best be accomplished through careful selection of equipment, thoughtful system design, and conscientious maintenance practices.
FIRST AID PROCEDURES AFTER GETTING SHOCK
Be familiar with first aid treatment for electric shock and burns. Always keep a first aid kit on hand at the facility. Figure illustrates the basic treatment for electric shock victims. Copy the information, and post it in a prominent location. Better yet, obtain more detailed information from your local heart association or Red Cross chapter. Personalized instruction on first aid usually is available locally. Table lists basic first aid procedures for burns.
For electric shock, the best first aid is prevention. In the event that an individual has sustained or is sustaining an electric shock at the work place, several guidelines are suggested, as detailed next.
If such equipment is available, hot sticks used in conjunction with lineman’s gloves may be applied to push or pull the victim away from the electrical source. Pulling the hot stick normally provides the greatest control over the victim’s motion and is the safest action for the rescuer. After the electrical source has been turned off, or the victim can be reached safely, immediate first aid procedures should be implemented.
Check for possible bone fractures if the victim was violently thrown away from the electrical source and possibly impacted objects in the vicinity. Apply splints as required if suitable materials are available and you have appropriate training. Cover the victim with a coat or blanket if the environmental temperature is below room temperature, or the victim complains of feeling cold.
If the victim is unconscious, call 911 or the appropriate plant-site paramedic team immediately. In the interim, check to see if the victim is breathing and if a pulse can be felt at either the inside of a wrist above the thumb joint (radial pulse) or in the neck above and to either side of the Adam’s apple (carotid pulse). It is usually easier to feel the pulse in the neck as opposed to the wrist pulse, which may be weak.
The index and middle finger should be used to sense the pulse, and not the thumb. Many individuals have an apparent thumb pulse that can be mistaken for the victim’s pulse. If a pulse can be detected but the victim is not breathing, begin mouth-to-mouth respiration if you know how to do so. If no pulse can be detected (presumably the victim will not be breathing), carefully move the victim to a firm surface and begin cardiopulmonary resuscitation if you have been trained in the use of CPR. Respiratory arrest and cardiac arrest are crisis situations. Because of loss of the oxygen supply to the brain, permanent brain damage can occur after several minutes even if the victim is successfully resuscitated.
Ironically, the treatment for cardiac arrest induced by an electric shock is a massive counter shock, which causes the entire heart muscle to contract. The random and uncoordinated ventricular fibrillation contractions (if present) are thus stilled. Under ideal conditions, normal heart rhythm is restored once the shock current ceases. The counter shock is generated by a cardiac defibrillator, various portable models of which are available for use by emergency medical technicians and other trained personnel.
Although portable defibrillators may be available at industrial sites where there is a high risk of electrical shock to plant personnel, they should be used only by trained personnel. Application of a defibrillator to an unconscious subject whose heart is beating can induce cardiac standstill or ventricular fibrillation, just the conditions that the defibrillator was designed to correct.
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| Figure: Basic first aid treatment for electric shock. |
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| Table: Basic First Aid Procedures |
a) SHOCK IN PROGRESS
For the case when a co-worker is receiving an electric shock and cannot let go of the electrical source, the safest action is to trip the circuit breaker that energizes the circuit involved, or to pull the power-line plug on the equipment involved if the latter can be accomplished safely. Under no circumstances should the rescuer touch the individual who is being shocked, because the rescuer’s body may then also be in the dangerous current path. If the circuit breaker or equipment plug cannot be located, then an attempt can be made to separate the victim from the electrical source through the use of a non-conducting object such as a wooden stool or a wooden broom handle. Use only an insulating object and nothing that contains metal or other electrically conductive material. The rescuer must be very careful not to touch the victim or the electrical source and thus become a second victim.If such equipment is available, hot sticks used in conjunction with lineman’s gloves may be applied to push or pull the victim away from the electrical source. Pulling the hot stick normally provides the greatest control over the victim’s motion and is the safest action for the rescuer. After the electrical source has been turned off, or the victim can be reached safely, immediate first aid procedures should be implemented.
b) SHOCK NO LONGER IN PROGRESS
If the victim is conscious and moving about, have the victim sit down or lie down. Sometimes there is a delayed reaction to an electrical shock that causes the victim to collapse. Call 911 or the appropriate plant-site paramedic team immediately. If there is a delay in the arrival of medical personnel, check for electrical burns. In the case of severe shock, there will normally be burns at a minimum of two sites: the entry point for the current and the exit point(s). Cover the burns with dry (and sterile, preferably) dressings.Check for possible bone fractures if the victim was violently thrown away from the electrical source and possibly impacted objects in the vicinity. Apply splints as required if suitable materials are available and you have appropriate training. Cover the victim with a coat or blanket if the environmental temperature is below room temperature, or the victim complains of feeling cold.
If the victim is unconscious, call 911 or the appropriate plant-site paramedic team immediately. In the interim, check to see if the victim is breathing and if a pulse can be felt at either the inside of a wrist above the thumb joint (radial pulse) or in the neck above and to either side of the Adam’s apple (carotid pulse). It is usually easier to feel the pulse in the neck as opposed to the wrist pulse, which may be weak.
The index and middle finger should be used to sense the pulse, and not the thumb. Many individuals have an apparent thumb pulse that can be mistaken for the victim’s pulse. If a pulse can be detected but the victim is not breathing, begin mouth-to-mouth respiration if you know how to do so. If no pulse can be detected (presumably the victim will not be breathing), carefully move the victim to a firm surface and begin cardiopulmonary resuscitation if you have been trained in the use of CPR. Respiratory arrest and cardiac arrest are crisis situations. Because of loss of the oxygen supply to the brain, permanent brain damage can occur after several minutes even if the victim is successfully resuscitated.
Ironically, the treatment for cardiac arrest induced by an electric shock is a massive counter shock, which causes the entire heart muscle to contract. The random and uncoordinated ventricular fibrillation contractions (if present) are thus stilled. Under ideal conditions, normal heart rhythm is restored once the shock current ceases. The counter shock is generated by a cardiac defibrillator, various portable models of which are available for use by emergency medical technicians and other trained personnel.
Although portable defibrillators may be available at industrial sites where there is a high risk of electrical shock to plant personnel, they should be used only by trained personnel. Application of a defibrillator to an unconscious subject whose heart is beating can induce cardiac standstill or ventricular fibrillation, just the conditions that the defibrillator was designed to correct.
ADVANTAGES OF SOLID AND LIQUID FUELS
Advantages of liquid fuels over the solid fuels
The following are the advantages of liquid fuels over the solid fuels:(i) The handling of liquid fuels is easier and they require less storage space.
(ii) The combustion of liquid fuels is uniform.
(iii) The solid fuels have higher percentage of moisture and consequently they burn with great difficulty. However, liquid fuels can be burnt with a fair degree of ease and attain high temperature very quickly compared to solid fuels.
(iv) The waste product of solid fuels is a large quantity of ash and its disposal becomes a problem. However, liquid fuels leave no or very little ash after burning.
(v) The firing of liquid fuels can be easily controlled. This permits to meet the variation in load demand easily.
Advantages of solid fuels over the liquid fuels
The following are the advantages of solid fuels over the liquid fuels:(i) In case of liquid fuels, there is a danger of explosion.
(ii) Liquids fuels are costlier as compared to solid fuels.
(iii) Sometimes liquid fuels give unpleasant odours during burning.
(iv) Liquid fuels require special types of burners for burning. (v) Liquid fuels pose problems in cold climates since the oil stored in the tanks is to be heated in order to avoid the stoppage of oil flow.
Monday, December 22, 2014
THERMAL AND VOLTAGE CONSIDERATIONS OF TRANSFORMER
THERMAL CONSIDERATIONS
The losses in the windings and the core cause temperature rises in the materials. This is another important area in which the temperatures must be limited to the long-term capability of the insulating materials. Refined paper is still used as the primary solid insulation in power transformers. Highly refined mineral oil is still used as the cooling and insulating medium in power transformers. Gases and vapors have been introduced in a limited number of special designs. The temperatures must be limited to the thermal capability of these materials. Again, this subject is quite broad and involved. It includes the calculation of the temperature rise of the cooling medium, the average and hottest-spot rise of the conductors and leads, and accurate specification of the heat-exchanger equipment.
VOLTAGE CONSIDERATIONS
A transformer must withstand a number of different normal and abnormal voltage stresses over its expected life. These voltages include:
- Operating voltages at the rated frequency
- Rated-frequency over voltages
- Natural lightning impulses that strike the transformer or transmission lines
- Switching surges that result from opening and closing of breakers and switches
- Combinations of the above voltages
- Transient voltages generated due to resonance between the transformer and the network
- Fast transient voltages generated by vacuum-switch operations or by the operation of disconnect switches in a gas insulated bus-bar system
This is a very specialized field in which the resulting voltage stresses must be calculated in the windings, and withstand criteria must be established for the different voltages and combinations of voltages. The designer must design the insulation system to withstand all of these stresses.
LOAD LOSSES IN TRANSFORMER
The term load losses represents the losses in the transformer that result from the flow of load current in the win dings. Load losses are composed of the following elements.
Ideally, each conductor element should occupy ever y possible position in the array of strands such that all elements have the same resistance and the same induced EMF. Conductor transposition, however, involves some sacrifice of winding space. If the winding depth is small, one transposition halfway through the winding is sufficient; or in the case of a two-layer winding, the transposition can be located at the junction of the layers. Windings of greater depth need three or more transpositions. CTC cables are manufactured using transposing machines and are usually paper-insulated as part of the transposing operation.
Stray losses can be a constraint on high-reactance designs. Losses can be controlled by using a combination of magnetic shunts and/or conducting shields to channel the flow of leakage flux external to the windings into low-loss paths.
- Resistance losses as the current flows through the resistance of the conductors and leads.
- Eddy losses caused by the leakage field. These are a function of the second power of the leakage field density and the second power of the conductor dimensions normal to the field.
- Stray losses: The leakage field exists in parts of the core, steel structural members, and tank walls. Losses and heating result in these steel parts.
Stray losses can be a constraint on high-reactance designs. Losses can be controlled by using a combination of magnetic shunts and/or conducting shields to channel the flow of leakage flux external to the windings into low-loss paths.
CATHODIC PROTECTION
Cathodic protection is the responsibility of the corrosion engineer or metallurgist. The subject is fundamentally reasonably simple to understand but can be extremely mathematical in its application. Direct current is arranged to flow out from the impressed anodes into the surrounding electrolyte, which is the sea water for offshore structures or the damp ground for onshore structures. The current returns through the structure itself and then back to the negative terminal of the impressed current source. The direction of current as described prevents the loss of metal from the structure into the electrolyte. This is opposite in direction to the natural current present due to corrosion action.
The electrical engineer is not usually involved in the chemistry of the system; his work is mainly associated with sizing the AC and DC cables, accounting for the power requirements and ensuring that the equipment satisfies any hazardous area requirements that may exist.
Impressed current systems require low-voltage high-current DC power. The voltages are typically 12, 25 and 50 volts. The currents are typically 100 to 800 amperes from one unit. The power is supplied by transformer rectifier units in which the transformer coils and the power rectifier are usually immersed in insulating oil to improve heat removal. The AC supply is usually three phase at LV voltage, e.g. 380 to 440 volts, and the supply power factor is about 0.75 lagging.
The output voltage is adjustable between +33% and −25% to take care of local site variations. The correct setting is determined at site during commissioning. Adjustments are often made periodically as the site conditions vary or if the installation is modified.
The anodes are made of various materials and the choice is determined by the physical conditions, the electric field pattern, current densities, and cost and anode corrosion. Anode current densities vary between 10 amperes per meter squared for silicon iron to more than 1000 amperes per meter squared for platinised and lead alloys. The electrical engineer needs to size AC and DC cables and to choose them to suit the physical environment.
The electrical engineer is not usually involved in the chemistry of the system; his work is mainly associated with sizing the AC and DC cables, accounting for the power requirements and ensuring that the equipment satisfies any hazardous area requirements that may exist.
Impressed current systems require low-voltage high-current DC power. The voltages are typically 12, 25 and 50 volts. The currents are typically 100 to 800 amperes from one unit. The power is supplied by transformer rectifier units in which the transformer coils and the power rectifier are usually immersed in insulating oil to improve heat removal. The AC supply is usually three phase at LV voltage, e.g. 380 to 440 volts, and the supply power factor is about 0.75 lagging.
The output voltage is adjustable between +33% and −25% to take care of local site variations. The correct setting is determined at site during commissioning. Adjustments are often made periodically as the site conditions vary or if the installation is modified.
The anodes are made of various materials and the choice is determined by the physical conditions, the electric field pattern, current densities, and cost and anode corrosion. Anode current densities vary between 10 amperes per meter squared for silicon iron to more than 1000 amperes per meter squared for platinised and lead alloys. The electrical engineer needs to size AC and DC cables and to choose them to suit the physical environment.
WHAT IS RELAYING?
In order to understand the function of protective relaying systems, one must be familiar with the nature and the modes of operation of an electric power system. Electric energy is one of the fundamental resources of modern industrial society. Electric power is available to the user instantly, at the correct voltage and frequency, and exactly in the amount that is needed. This remarkable performance is achieved through careful planning, design, installation and operation of a very complex network of generators, transformers, and transmission and distribution lines. To the user of electricity, the power system appears to be in a steady state: imperturbable, constant and infinite in capacity. Yet, the power system is subject to constant disturbances created by random load changes, by faults created by natural causes and sometimes as a result of equipment or operator failure. In spite of these constant perturbations, the power system maintains its quasi steady state because of two basic factors: the large size of the power system in relation to the size of individual loads or generators, and correct and quick remedial action taken by the protective relaying equipment.
Relaying is the branch of electric power engineering concerned with the principles of design and operation of equipment (called ‘relays’ or ‘protective relays’) that detects abnormal power system conditions, and initiates corrective action as quickly as possible in order to return the power system to its normal state. The quickness of response is an essential element of protective relaying systems response times of the order of a few milliseconds are often required. Consequently, human intervention in the protection system operation is not possible. The response must be automatic, quick and should cause a minimum amount of disruption to the power system.
As the principles of protective relaying are developed in this book, the reader will perceive that the entire subject is governed by these general requirements: correct diagnosis of trouble, quickness of response and minimum disturbance to the power system. To accomplish these goals, we must examine all possible types of fault or abnormal conditions which may occur in the power system. We must analyze the required response to each of these events, and design protective equipment which will provide such a response. We must further examine the possibility that protective relaying equipment itself may fail to operate correctly, and provide for a backup protective function. It should be clear that extensive and sophisticated equipment is needed to accomplish these tasks.
Relaying is the branch of electric power engineering concerned with the principles of design and operation of equipment (called ‘relays’ or ‘protective relays’) that detects abnormal power system conditions, and initiates corrective action as quickly as possible in order to return the power system to its normal state. The quickness of response is an essential element of protective relaying systems response times of the order of a few milliseconds are often required. Consequently, human intervention in the protection system operation is not possible. The response must be automatic, quick and should cause a minimum amount of disruption to the power system.
As the principles of protective relaying are developed in this book, the reader will perceive that the entire subject is governed by these general requirements: correct diagnosis of trouble, quickness of response and minimum disturbance to the power system. To accomplish these goals, we must examine all possible types of fault or abnormal conditions which may occur in the power system. We must analyze the required response to each of these events, and design protective equipment which will provide such a response. We must further examine the possibility that protective relaying equipment itself may fail to operate correctly, and provide for a backup protective function. It should be clear that extensive and sophisticated equipment is needed to accomplish these tasks.
Saturday, December 20, 2014
NAVIGATION AIDS
Navigation aids consist of the following equipment:
1. Flashing marker lights.
2. Fog horns.
3. Platform nameplates.
4. Aircraft hazard lights.
5. Helideck landing facilities.
6. Radio communications and beacons.
7. Radar.
8. Echo-sounding and sonar.
1. White and red lights flashing the Morse letters ‘U’ every 15 seconds as follows:
2. Fog signals sounding the ‘U’ every 30 seconds as follows:
3. Illuminated identification panels.
4. Navigation buoys.
a) MAIN LIGHTS
The main white lights should have a ‘nominal’ range of 15 miles and be visible in every direction of approach; there should normally be a minimum of two and a maximum of four main white lights.
b) SUBSIDIARY LIGHTS
Subsidiary red lights of 3 miles ‘nominal’ range should be positioned to mark the horizontal extremities of the structure, in positions not occupied by white lights, to indicate any irregular projections of the complex.
c) SECONDARY LIGHTS
Secondary white lights of 10 miles ‘nominal’ range and visible in every direction of approach should automatically come into operation in the event of failure of the 15 mile main white lights; these are normally mounted in similar location to the main white lights.
d) OPERATION AND CONTROL OF LIGHTING SYSTEMS
Navigation lighting systems can be fitted with a device to automatically switch on 15 minutes before sunset until sunrise or whenever the visibility is less than 2 sea miles. There can also be a manual override device to enable the navigation aids to be switched on during unusual conditions or for maintenance and testing etc.
Failure of any of the navigation lights can be indicated in the central control room and in the radio room.
In the event of failure of the main white lights control equipment, control should automatically be transferred to the secondary system, which would cause the secondary and the main lights to flash in synchronism, and generate an alarm in the central control room and the radio room.
All subsidiary lights should operate in synchronism.
The secondary and subsidiary lights can be equipped with an automatic lamp changer or multiple filament bulbs. This provides a minimum of one standby lamp or filament which will be automatically activated in the event of a filament failure. Filament failure should produce an alarm in the central control room and the radio room until a defective bulb is replaced.
On long narrow structures or structures linked by bridges where lights may otherwise be several hundred metres apart, intermediary 3 mile red lights should be mounted in positions to deter vessel from colliding with the central sections of the structure of bridges.
The secondary and subsidiary lights should be capable of operating for 96 hours from a battery power source which is independent of the main supply. The equipment would normally operate on the main AC supply, with automatic switching to an alternative AC supply in the event of main supply failure, and automatic switching to battery supplies when no AC supply is available.
In the event of main supply failure the hazard lighting would be supplied from an emergency generator or battery supply.
No form of lighting on the structure should be capable of creating a hazard to helicopters by night-blinding the pilot due to dazzle or glare.
A high frequency radio beacon with a minimum range of 30 miles can be provided for the guidance of approaching helicopters, and VHF/AM radio would be provided for communication with pilots to comply with the appropriate standards, for the location.
The structure would also be equipped with suitable devices for ascertaining the wind speed and direction, air temperature, barometric pressure, visibility and cloud cover.
All of the equipment and interconnecting cables should be located in a safe area. The transmitters and aerials should not be located near telecommunication equipment, electronic instruments and similar equipment which could suffer interference or damage due to high energy radio frequency radiation. The aerials must be positioned to prevent the creation of high energy radio frequency fields in hazardous areas where they could cause ignition.
The aerials should be installed in such a manner and location as to allow reasonable safe access for at least two people for servicing and maintenance, whilst preventing access to un-authorized personnel.
Emergency stop switches could be provided in a safe position, adjacent to the aerials, to switch off the scanners and transmitters.
• The equipment should be located in the radio room.
• The aerials and feeder cables should be located in a safe area as close as possible to the radio room.
• An emergency power supply should provide a minimum of 6 hours duration, and minimum of 3 hours of this supply should be from batteries. The batteries, charger and supply cables should be in a safe area as close as possible to the radio room.
1. Flashing marker lights.
2. Fog horns.
3. Platform nameplates.
4. Aircraft hazard lights.
5. Helideck landing facilities.
6. Radio communications and beacons.
7. Radar.
8. Echo-sounding and sonar.
1) FLASHING MARKER LIGHTS
A typical requirement is that recommended by the British Department of Trade document ‘Standard Making Schedule for Offshore Installations’,1. White and red lights flashing the Morse letters ‘U’ every 15 seconds as follows:
Eclipse 1.00 s
Flash 1.00 s
Eclipse 1.00 s
Flash 3.00 s
Eclipse 8.00 s
Total Period 15.00 s
2. Fog signals sounding the ‘U’ every 30 seconds as follows:
Blast 0.75 s
Silent 1.00 s
Blast 0.75 s
Silent 1.00 s
Blast 2.50 s
Silent 24.00 s
Total Period 30.00 s
3. Illuminated identification panels.
4. Navigation buoys.
2) WHITE AND RED FLASHING LIGHTS
The ‘normal’ range and ‘apparent intensity’ of these flashing lights should be in accordance with the local requirements, e.g. for UK waters, IALA publication ‘Recommendations for the Notation of Luminous Intensity and Range of Lights’.a) MAIN LIGHTS
The main white lights should have a ‘nominal’ range of 15 miles and be visible in every direction of approach; there should normally be a minimum of two and a maximum of four main white lights.
b) SUBSIDIARY LIGHTS
Subsidiary red lights of 3 miles ‘nominal’ range should be positioned to mark the horizontal extremities of the structure, in positions not occupied by white lights, to indicate any irregular projections of the complex.
c) SECONDARY LIGHTS
Secondary white lights of 10 miles ‘nominal’ range and visible in every direction of approach should automatically come into operation in the event of failure of the 15 mile main white lights; these are normally mounted in similar location to the main white lights.
d) OPERATION AND CONTROL OF LIGHTING SYSTEMS
Navigation lighting systems can be fitted with a device to automatically switch on 15 minutes before sunset until sunrise or whenever the visibility is less than 2 sea miles. There can also be a manual override device to enable the navigation aids to be switched on during unusual conditions or for maintenance and testing etc.
Failure of any of the navigation lights can be indicated in the central control room and in the radio room.
In the event of failure of the main white lights control equipment, control should automatically be transferred to the secondary system, which would cause the secondary and the main lights to flash in synchronism, and generate an alarm in the central control room and the radio room.
All subsidiary lights should operate in synchronism.
The secondary and subsidiary lights can be equipped with an automatic lamp changer or multiple filament bulbs. This provides a minimum of one standby lamp or filament which will be automatically activated in the event of a filament failure. Filament failure should produce an alarm in the central control room and the radio room until a defective bulb is replaced.
On long narrow structures or structures linked by bridges where lights may otherwise be several hundred metres apart, intermediary 3 mile red lights should be mounted in positions to deter vessel from colliding with the central sections of the structure of bridges.
The secondary and subsidiary lights should be capable of operating for 96 hours from a battery power source which is independent of the main supply. The equipment would normally operate on the main AC supply, with automatic switching to an alternative AC supply in the event of main supply failure, and automatic switching to battery supplies when no AC supply is available.
3) NAVIGATION BUOYS
Navigation marker buoys can be wave or solar powered or alternatively fitted with batteries. They would be retained in a position to facilitate quick manual launching, and provision should be made for ready inspection and maintenance of batteries.4) IDENTIFICATION PANELS
The structure identification panels usually consist of black letter and figures one metre high on a yellow background with illumination or be on a retro-reflective background.5) AIRCRAFT HAZARD LIGHTING
Hazard lighting should be provided on all projections from the structure which could present a danger to helicopters approaching the platform. Positions where it would be impractical to fit red lights due to the possibility of damage or difficulty of maintenance caused by high temperature, such as flare towers and exhaust stacks, would be flood lit from convenient locations.In the event of main supply failure the hazard lighting would be supplied from an emergency generator or battery supply.
No form of lighting on the structure should be capable of creating a hazard to helicopters by night-blinding the pilot due to dazzle or glare.
6) HELICOPTER LANDING FACILITIES
Helideck markings and illumination should be in accordance with appropriate standards.A high frequency radio beacon with a minimum range of 30 miles can be provided for the guidance of approaching helicopters, and VHF/AM radio would be provided for communication with pilots to comply with the appropriate standards, for the location.
The structure would also be equipped with suitable devices for ascertaining the wind speed and direction, air temperature, barometric pressure, visibility and cloud cover.
7) RADAR
Radar is not used on all offshore platforms. Its use is determined by the nature of the platform and the frequency and type of local sea traffic. When surveillance radar is installed precautions should be adopted to ensure the minimum of danger to personnel from high energy radiation and dangers associated with rotating aerial scanners, interference with electronic instruments and communication, and the elimination of ignition in hazardous atmosphere in accordance with the standards e.g. BS3192 and 4992.All of the equipment and interconnecting cables should be located in a safe area. The transmitters and aerials should not be located near telecommunication equipment, electronic instruments and similar equipment which could suffer interference or damage due to high energy radio frequency radiation. The aerials must be positioned to prevent the creation of high energy radio frequency fields in hazardous areas where they could cause ignition.
The aerials should be installed in such a manner and location as to allow reasonable safe access for at least two people for servicing and maintenance, whilst preventing access to un-authorized personnel.
Emergency stop switches could be provided in a safe position, adjacent to the aerials, to switch off the scanners and transmitters.
8) RADIO DIRECTION-FINDER
Platforms that are permanently manned would require equipment for obtained bearings on radio navigation beacons and survival craft transmitting on international distress frequencies. If the equipment is of a type approved by the British Department of Trade (or similar national standard) in accordance with SOLAS (1974) Regulation 12, then the SOLAS requirements could also be supplemented as follows:• The equipment should be located in the radio room.
• The aerials and feeder cables should be located in a safe area as close as possible to the radio room.
• An emergency power supply should provide a minimum of 6 hours duration, and minimum of 3 hours of this supply should be from batteries. The batteries, charger and supply cables should be in a safe area as close as possible to the radio room.
9) SONAR DEVICES
If echo-sounding equipment is required then it should be of a type approved by the Department of Trade, or similar national authority appropriate to the location, in accordance with SOLAS (1974). The installation of sonar devices should be in accordance with appropriate standards, and particular regard should be directed towards the dangers that high-powered underwater sonar transmissions may present during diving operations.LIGHTING SYSTEMS FOR ILLUMINATION
Normal lighting should provide approximately 75% of the total illumination an area of a plant that is densely filled with processing equipment and buildings. Sparsely filled areas such as road ways and perimeter fences can be fully illuminated with normal lighting, unless emergency escape routes exist in these areas.
Emergency lighting should therefore provide between 25% and 30% of the illumination in processing areas. These criteria generally apply to both outdoor and indoor locations, and to onshore and offshore installations. Emergency lighting should be supplied by power from emergency diesel generators, except for lighting that illuminates escape routes. Escape route lighting requires a source of battery power that should last for at least one hour from a loss of all other power sources. The battery may be integral with the lighting fitting or a common battery and local distribution panel for a room or group of rooms, access ways, corridors and the like. The lighting level for escape lighting does not need to be high, a typical value is 20 lUX for indoor areas is adequate. Individual oil companies have their own recommendations on these subjects.
Offshore and marine installations are by nature very compact and therefore some additional requirements are generally required, especially with regard to escape routes. Escape lighting should be provided for exit doorways, sleeping cabins in the living quarters, stairways, walkways, corridors, lounges, recreation rooms, dining rooms and gallies. It is essential to illuminate embarkation stairways, helideck, helideck offices, survival craft stations, waiting room, and areas that are associated with personnel having to leave the facility in an organized manner. If in doubt provide more than is a minimum requirement.
Emergency lighting has some separate requirements to escape lighting. For example the personnel operating the plant need to be able to see and operate control panels, visual display units, start-up emergency generators and systems, carry out switching operations, test for hazardous gas, test certain equipment and generally manage an emergency situation. They require a minimum amount of emergency lighting. Consequently the following areas and functions need to be properly illuminated.
• Plant main control room and radio room.
• Emergency generator room or module.
• Main switch room.
• Main generator room or module.
• All areas in the living quarters.
• All workshops, stores, cranes and utility areas.
• Offshore installation manager (OIM) offices.
• Obstructed areas within the plant.
• Vent stacks and flare booms.
• Perimeter areas.
During an emergency the personnel should be able to access portable lamps and torches. These should be located adjacent to exit doors, in operational rooms, plant rooms, emergency accommodation areas, OIM’s offices, central control room and muster areas. They should be provided with charger units and be suitable for zone 1 hazardous areas, and be capable of operating for at least five hours.
Where possible the control of lighting fittings should be from a non-hazardous area, i.e. one adjacent to the hazardous area, using double pole switches. The supply neutral should be switchable.
In rare situations this may not be practical in which case a switchboard or distribution board suitable for the hazardous area and the environmental conditions will need to be installed e.g. Zone 1, IP55 or 56, with a suitable gas group and temperature class.
It is often a good practical consideration to use only lighting fittings in a plant that are suitable for Zone 1 areas that are also exposed to wet weather conditions e.g. IP66 enclosures of at least Ex (e) hazardous area types, unless of course they are installed indoors in areas where water sprays are not needed. Indoor process areas such as gas compressor modules require water-based fire-fighting deluge systems. Such locations require waterproof electrical fittings of all types, e.g. lighting, junction boxes, local control stations, local control panel. Locations such as control rooms, computer rooms, electronic equipment rooms, accommodation areas and offices do not require such hazardous area fittings, and good quality domestic or light industrial fittings are usually suitable and aesthetically acceptable.
Some areas are suitable for floodlighting and high-pressure sodium fittings can be used.
The incoming three-phase supply to the lighting distribution panels should be provided with four pole switches or circuit breakers, to ensure that the neutral is opened when the panel is de-energized for maintaining sub-circuits in hazardous areas. The sub-circuit loading should be arranged to give a balanced load on the incoming supply. Each sub-circuit will be a single-phase consumer, for which the single-phase two-wire supply can be taken between one phase and neutral of a four wire system, or a single-phase two-winding step down transformer can be used. The use of a small transformer will ensure that the voltage required for the light fittings is well matched. Occasionally a 440 V three-phase supply is used throughout a plant, for which the line-to-neutral voltage is 254 V.
A single- phase nominal voltage of 254 V is out of range for the products of some manufacturers of lighting fittings. A choice of 415 V/240 V, 400 V/230 V or 380 V/220 V would enable a wider choice of standard equipment to be used.
Fluorescent lamps should be chosen and located carefully where they illuminate rotating shafts, so as to avoid a stroboscopic effect that shows the shaft to appear stationary even though it is in fact rotating at a high speed.
Lighting schemes within modules and compact plant areas should be divided into at least two groups so that a supply failure does not put the whole area into darkness. This consideration applies to both normal and emergency schemes.
When designing a lighting circuit it is customary practice to size the cables so that the farthest lamp from the supply receives no less than 95% of its nominal voltage. In addition it is assumed that all the lighting fittings are energized when this design calculation is made.
Emergency lighting should therefore provide between 25% and 30% of the illumination in processing areas. These criteria generally apply to both outdoor and indoor locations, and to onshore and offshore installations. Emergency lighting should be supplied by power from emergency diesel generators, except for lighting that illuminates escape routes. Escape route lighting requires a source of battery power that should last for at least one hour from a loss of all other power sources. The battery may be integral with the lighting fitting or a common battery and local distribution panel for a room or group of rooms, access ways, corridors and the like. The lighting level for escape lighting does not need to be high, a typical value is 20 lUX for indoor areas is adequate. Individual oil companies have their own recommendations on these subjects.
Offshore and marine installations are by nature very compact and therefore some additional requirements are generally required, especially with regard to escape routes. Escape lighting should be provided for exit doorways, sleeping cabins in the living quarters, stairways, walkways, corridors, lounges, recreation rooms, dining rooms and gallies. It is essential to illuminate embarkation stairways, helideck, helideck offices, survival craft stations, waiting room, and areas that are associated with personnel having to leave the facility in an organized manner. If in doubt provide more than is a minimum requirement.
Emergency lighting has some separate requirements to escape lighting. For example the personnel operating the plant need to be able to see and operate control panels, visual display units, start-up emergency generators and systems, carry out switching operations, test for hazardous gas, test certain equipment and generally manage an emergency situation. They require a minimum amount of emergency lighting. Consequently the following areas and functions need to be properly illuminated.
• Plant main control room and radio room.
• Emergency generator room or module.
• Main switch room.
• Main generator room or module.
• All areas in the living quarters.
• All workshops, stores, cranes and utility areas.
• Offshore installation manager (OIM) offices.
• Obstructed areas within the plant.
• Vent stacks and flare booms.
• Perimeter areas.
During an emergency the personnel should be able to access portable lamps and torches. These should be located adjacent to exit doors, in operational rooms, plant rooms, emergency accommodation areas, OIM’s offices, central control room and muster areas. They should be provided with charger units and be suitable for zone 1 hazardous areas, and be capable of operating for at least five hours.
Where possible the control of lighting fittings should be from a non-hazardous area, i.e. one adjacent to the hazardous area, using double pole switches. The supply neutral should be switchable.
In rare situations this may not be practical in which case a switchboard or distribution board suitable for the hazardous area and the environmental conditions will need to be installed e.g. Zone 1, IP55 or 56, with a suitable gas group and temperature class.
It is often a good practical consideration to use only lighting fittings in a plant that are suitable for Zone 1 areas that are also exposed to wet weather conditions e.g. IP66 enclosures of at least Ex (e) hazardous area types, unless of course they are installed indoors in areas where water sprays are not needed. Indoor process areas such as gas compressor modules require water-based fire-fighting deluge systems. Such locations require waterproof electrical fittings of all types, e.g. lighting, junction boxes, local control stations, local control panel. Locations such as control rooms, computer rooms, electronic equipment rooms, accommodation areas and offices do not require such hazardous area fittings, and good quality domestic or light industrial fittings are usually suitable and aesthetically acceptable.
Some areas are suitable for floodlighting and high-pressure sodium fittings can be used.
The incoming three-phase supply to the lighting distribution panels should be provided with four pole switches or circuit breakers, to ensure that the neutral is opened when the panel is de-energized for maintaining sub-circuits in hazardous areas. The sub-circuit loading should be arranged to give a balanced load on the incoming supply. Each sub-circuit will be a single-phase consumer, for which the single-phase two-wire supply can be taken between one phase and neutral of a four wire system, or a single-phase two-winding step down transformer can be used. The use of a small transformer will ensure that the voltage required for the light fittings is well matched. Occasionally a 440 V three-phase supply is used throughout a plant, for which the line-to-neutral voltage is 254 V.
A single- phase nominal voltage of 254 V is out of range for the products of some manufacturers of lighting fittings. A choice of 415 V/240 V, 400 V/230 V or 380 V/220 V would enable a wider choice of standard equipment to be used.
Fluorescent lamps should be chosen and located carefully where they illuminate rotating shafts, so as to avoid a stroboscopic effect that shows the shaft to appear stationary even though it is in fact rotating at a high speed.
Lighting schemes within modules and compact plant areas should be divided into at least two groups so that a supply failure does not put the whole area into darkness. This consideration applies to both normal and emergency schemes.
When designing a lighting circuit it is customary practice to size the cables so that the farthest lamp from the supply receives no less than 95% of its nominal voltage. In addition it is assumed that all the lighting fittings are energized when this design calculation is made.
INSPECTION AND TESTING OF EQUIPMENTS
Inspection and testing of the purchased equipment is one of the most important tasks in the engineering of a project. Its importance is sometimes underestimated. The first serious tests that the purchaser will witness are those in the factory where the equipment is assembled. These tests will also include a physical inspection of the equipment.
It is therefore important to state clearly in the specification what inspection and testing will be required and, where appropriate, what are the acceptable limits of the results. Most tests required in the oil industry are covered in international specifications and these can be used as references.
However, not all those in the reference documents need to be carried out in all cases. It is therefore prudent to state the requirements in the project specification in one or more of the following methods:
• Write a detailed description of exactly what is required, including the limits that are acceptable and the form in which the results should be reported. This method ensures a ‘self-contained’ approach that is very beneficial during the actual testing operation. Often time is limited to perform tests and to have all the requirements to hand without having to search through related documents enables the work to be completed very efficiently.
• Quote the exact clause numbers and sub-section headings in the reference documents for the particular tests to be performed. This may be less efficient when the time of the tests becomes due, especially if the reference documents are not easily to hand. If a statement is made such as ‘the switch gear shall be tested in accordance with the XYZ-123 international standard’ and no other clarification is included, then many debates can arise at the time of testing.
Whichever method is used it should be carefully checked by a quality assurance department before the specification is approved for purchasing the equipment.
Some types of equipment require ‘production tests’, ‘type tests’, ‘performance tests’, ‘routine tests’, ‘abbreviated tests’ or ‘special tests’, or a combination of these tests. The subtitles are sometimes used with different meanings. Production tests are required for complex equipment such as high voltage generators and motors, and these tests are performed in the factory before the complete unit is assembled. For example the rotors are balanced without the stator, air-to-water heat exchanges can be tested to withstand hydraulic pressure, winding insulation and individual coil insulation can be tested.
Type tests are performed on one from a group of identical units. These tests are comprehensive and some of which are usually only performed once in the life span of the equipment.
If the equipment is a standard product of the manufacturer for which existing certificates can show that a type test has previously been carried out, then the purchaser may wish to accept the certificate without repeating the test. This is largely a matter of choice than necessity.
Routine and abbreviated tests are generally the same form of tests. These are applied to those units in a group that have not been type tested. If only one unit is to be purchased and a type test has been waived then a routine test is usually performed and the results compared to those of a previous type test. The number of different tests included in the routine tests is less than that of the type tests.
Some of the tests may be identical in each category. Routine tests are usually witnessed by the owner or purchaser.
Performance tests are those tests that need to be carried out on combined equipment such as a gas-turbine driven generator or a pump driven by a high-voltage motor. In such cases the dynamic relationship between the various equipment's is of interest. For example, rotor vibration, critical speeds, run-up time to full speed, starting up and shutting down sequences, full-load and over-load performances, heat dissipation and cooling medium performance. Occasionally ‘special tests’ may be required. These may be due to the need to operate the unit in an unusual mode or to test special control systems that may involve associated equipment such as a power management system or a control panel. Special tests may be needed to verify the operation of protective devices in the equipment rather than the equipment itself, but which require the device to be in its fully functional position on its host equipment. The owner or the purchaser usually witnesses performance and special tests.
Routine tests usually include a thorough inspection of the equipment both before and after the testing is complete. Routine testing should not be confused with sample testing. For example a switchboard may consist of many panels of essentially the same type, e.g. motor starters, transformer feeders. The testing schedule should state whether samples of similar types could be tested in lieu of testing all the units. In either case a full routine test is generally required. Functional testing of mechanical operation should be applied to all the units, e.g. open and close contactors, rack in and out circuit breakers, operate switches and controls.
It is therefore important to state clearly in the specification what inspection and testing will be required and, where appropriate, what are the acceptable limits of the results. Most tests required in the oil industry are covered in international specifications and these can be used as references.
However, not all those in the reference documents need to be carried out in all cases. It is therefore prudent to state the requirements in the project specification in one or more of the following methods:
• Write a detailed description of exactly what is required, including the limits that are acceptable and the form in which the results should be reported. This method ensures a ‘self-contained’ approach that is very beneficial during the actual testing operation. Often time is limited to perform tests and to have all the requirements to hand without having to search through related documents enables the work to be completed very efficiently.
• Quote the exact clause numbers and sub-section headings in the reference documents for the particular tests to be performed. This may be less efficient when the time of the tests becomes due, especially if the reference documents are not easily to hand. If a statement is made such as ‘the switch gear shall be tested in accordance with the XYZ-123 international standard’ and no other clarification is included, then many debates can arise at the time of testing.
Whichever method is used it should be carefully checked by a quality assurance department before the specification is approved for purchasing the equipment.
Some types of equipment require ‘production tests’, ‘type tests’, ‘performance tests’, ‘routine tests’, ‘abbreviated tests’ or ‘special tests’, or a combination of these tests. The subtitles are sometimes used with different meanings. Production tests are required for complex equipment such as high voltage generators and motors, and these tests are performed in the factory before the complete unit is assembled. For example the rotors are balanced without the stator, air-to-water heat exchanges can be tested to withstand hydraulic pressure, winding insulation and individual coil insulation can be tested.
Type tests are performed on one from a group of identical units. These tests are comprehensive and some of which are usually only performed once in the life span of the equipment.
If the equipment is a standard product of the manufacturer for which existing certificates can show that a type test has previously been carried out, then the purchaser may wish to accept the certificate without repeating the test. This is largely a matter of choice than necessity.
Routine and abbreviated tests are generally the same form of tests. These are applied to those units in a group that have not been type tested. If only one unit is to be purchased and a type test has been waived then a routine test is usually performed and the results compared to those of a previous type test. The number of different tests included in the routine tests is less than that of the type tests.
Some of the tests may be identical in each category. Routine tests are usually witnessed by the owner or purchaser.
Performance tests are those tests that need to be carried out on combined equipment such as a gas-turbine driven generator or a pump driven by a high-voltage motor. In such cases the dynamic relationship between the various equipment's is of interest. For example, rotor vibration, critical speeds, run-up time to full speed, starting up and shutting down sequences, full-load and over-load performances, heat dissipation and cooling medium performance. Occasionally ‘special tests’ may be required. These may be due to the need to operate the unit in an unusual mode or to test special control systems that may involve associated equipment such as a power management system or a control panel. Special tests may be needed to verify the operation of protective devices in the equipment rather than the equipment itself, but which require the device to be in its fully functional position on its host equipment. The owner or the purchaser usually witnesses performance and special tests.
Routine tests usually include a thorough inspection of the equipment both before and after the testing is complete. Routine testing should not be confused with sample testing. For example a switchboard may consist of many panels of essentially the same type, e.g. motor starters, transformer feeders. The testing schedule should state whether samples of similar types could be tested in lieu of testing all the units. In either case a full routine test is generally required. Functional testing of mechanical operation should be applied to all the units, e.g. open and close contactors, rack in and out circuit breakers, operate switches and controls.
REQUIREMENTS OF EQUIPMENTS DESIGN AND CONSTRUCTION
Oil industry equipment tends to be more robust than normal industrial equipment due to the often harsh and hostile environments in which it is expected to function without trouble for long periods of time. The indirect cost of equipment failures and outages is high and reliability is of paramount importance.
An essential requirement is the definition of the degree of protection of the enclosure for the environment, which may be either outdoor or indoor, and hazardous or non-hazardous. The international standards most often used are IEC60529 and NEMA-ICS1-110 for the degree of protection against liquids and particles. These references are applied for the hazardous area protection.
Wound components such as motor and transformer windings need to have their insulation specified to withstand the surface temperature of the copper conductors. IEC60085 and ANSI/NEMA describe the different classes of insulation that are normally available. Where IEC60085 or ANSI/NEMA is the reference, the two most common are Class B and Class F. These state the maximum temperature rise in degrees Celsius above the conductor temperature when the temperature of the cooling medium for the equipment is no greater than 40◦C.
For most equipment ratings used in the oil industry the temperature rise limits are 80◦C for Class B and 100◦C for Class F (Class H allows 125◦C). It is common practice to specify Class F insulating materials but to restrict the actual temperature rise to that of Class B. These stem from the recommendation in IEC60085 that for ratings equal and above 5000 kVA or if the iron core length is equal and above one metre, that this combination of classes should be used.
The owner may have particular requirements for the materials to be used for insulation and their impregnation. This may be due to their experience with marine and highly humid environments.
Other aspects that should be included are protective devices, measurement detectors, terminal blocks, segregation of circuits and terminals, voltage surge suppression, skid construction, floor frames, lifting eyes, jacking points, earthing bosses, indicating devices, control switches, automatic voltage regulators, exciters, detachable panels and doors, forced cooling, shaft bearings and seals, lubrication systems, anti-condensation heaters, noise levels, labelling and nameplates, painting etc. Some of these may be efficiently included in the data sheet.
An essential requirement is the definition of the degree of protection of the enclosure for the environment, which may be either outdoor or indoor, and hazardous or non-hazardous. The international standards most often used are IEC60529 and NEMA-ICS1-110 for the degree of protection against liquids and particles. These references are applied for the hazardous area protection.
Wound components such as motor and transformer windings need to have their insulation specified to withstand the surface temperature of the copper conductors. IEC60085 and ANSI/NEMA describe the different classes of insulation that are normally available. Where IEC60085 or ANSI/NEMA is the reference, the two most common are Class B and Class F. These state the maximum temperature rise in degrees Celsius above the conductor temperature when the temperature of the cooling medium for the equipment is no greater than 40◦C.
For most equipment ratings used in the oil industry the temperature rise limits are 80◦C for Class B and 100◦C for Class F (Class H allows 125◦C). It is common practice to specify Class F insulating materials but to restrict the actual temperature rise to that of Class B. These stem from the recommendation in IEC60085 that for ratings equal and above 5000 kVA or if the iron core length is equal and above one metre, that this combination of classes should be used.
The owner may have particular requirements for the materials to be used for insulation and their impregnation. This may be due to their experience with marine and highly humid environments.
Other aspects that should be included are protective devices, measurement detectors, terminal blocks, segregation of circuits and terminals, voltage surge suppression, skid construction, floor frames, lifting eyes, jacking points, earthing bosses, indicating devices, control switches, automatic voltage regulators, exciters, detachable panels and doors, forced cooling, shaft bearings and seals, lubrication systems, anti-condensation heaters, noise levels, labelling and nameplates, painting etc. Some of these may be efficiently included in the data sheet.
A TYPICAL FORMAT FOR A EQUIPMENTS SPECIFICATION
The following format is reasonably typical of an equipment specification. Owners and purchasers, of course, have their particular style and preferences as to the order in which the paragraphs and clauses are placed in the specification document.
Where appropriate it is prudent to describe or list what is not included in the scope of supply. This will minimise misunderstandings at a later stage when quotations are being compared, e.g. for the above example, gearbox, prime mover, base frame or skid assembly.
It is recommended that particularly important words, phrases, terms and abbreviations are defined in the specification itself; especially if they differ in use from say those given in an IEC specification. (An example that regularly appears is the difference in meaning between ‘shall’ and ‘should’.)
Some of the material in this section could equally well be placed at the end of the document as an appendix.
If equipment is to be specified for use in hazardous areas, e.g. Zone 1, Zone 2, then the equipment as purchased should not have been modified in any manner that could invalidate its hazardous area certification. Components that can be vulnerable to modification are terminal boxes, gland plates and threaded entries.
The basic requirements for performance can be categorized as follows:
Starting up.
Normal continuous operation.
Permissible but limited overloading.
Short-circuit withstand.
Shutting down.
It will be useful to the recipient to have an understanding of the power system or network into which the equipment will belong. This is especially important when specifying the high-voltage generation and distribution equipment, and some of the main low-voltage equipment such as switchgear.
The modes of operation of the power system may have some bearing upon the design of the equipment being specified, e.g. method of earthing neutrals, minimum and maximum fault currents, dips in system voltage and frequency, normal and abnormal switching configuration.
The owner may have some restriction on how to start up and shut down equipment, e.g. limits on starting currents of motors, voltage dip limits at switchgear, duration of start up or shut down, purging with safe air or inert gas, interlocking schemes, manual or automatic sequences.
For some equipment, especially generators and their prime-movers, the normal or rated duty may need to be emphasised so that the correct rating for the prime-mover is chosen, and an adequate margin for short-term permissible overloading exists. Emergency generators used offshore may need to be allowed to run in overloaded conditions until they run out of fuel or actually fail. International specifications should be referred to for the description of full-load duty for particular types of equipment, for example IEC60034 for generators and motors and for switchgear see sub-section 7.1.
If equipment needs to function continuously in high ambient temperatures, e.g. 40◦C or higher, then the derating of the manufacturer’s standard equipment should be quoted and explained by the manufacturer. This is especially important with switchgear busbars and circuit breakers. Some manufacturers may not wish to quote for high ambient conditions, and many of the international standards use 40◦C as their upper limit.
The short-circuit withstand performance may be important with certain types of equipment, e.g. generators, high-voltage motors, switchgear, power transformers. This should be described or stated in the data sheet. The rms and peak values of short-circuit currents may need to be described.
Some equipment may be sensitive to unbalanced loading, unbalanced supply voltages or the harmonic content of the supply.
The indirect cost of equipment failures and outages is high and reliability is of paramount importance.
An essential requirement is the definition of the degree of protection of the enclosure for the environment, which may be either outdoor or indoor, and hazardous or non-hazardous. The international standards most often used are IEC60529 and NEMA-ICS1-110 for the degree of protection against liquids and particles. These references are applied for the hazardous area protection.
Wound components such as motor and transformer windings need to have their insulation specified to withstand the surface temperature of the copper conductors. IEC60085 and ANSI/NEMA describe the different classes of insulation that are normally available. Where IEC60085 or ANSI/NEMA is the reference, the two most common are Class B and Class F. These state the maximum temperature rise in degrees Celsius above the conductor temperature when the temperature of the cooling medium for the equipment is no greater than 40◦C.
For most equipment ratings used in the oil industry the temperature rise limits are 80◦C for Class B and 100◦C for Class F (Class H allows 125◦C). It is common practice to specify Class F insulating materials but to restrict the actual temperature rise to that of Class B. These stem from the recommendation in IEC60085 that for ratings equal and above 5000 kVA or if the iron core length is equal and above one metre, that this combination of classes should be used.
Various IEC standards for switchgear refer to IEC60694 sub-section 4.4.1 for the requirements of rated current and sub-section 4.4.2 for temperature rise of enclosed components such as bare terminals, busbars and risers, panel surfaces, and built-in apparatus. It also refers to IEC60085 for the classes of insulation. Busbars and risers can be bare or insulated and so it is not practical to state a requirement for their temperature rise in the project specification.
The owner may have particular requirements for the materials to be used for insulation and their impregnation. This may be due to their experience with marine and highly humid environments.
Other aspects that should be included are protective devices, measurement detectors, terminal blocks, segregation of circuits and terminals, voltage surge suppression, skid construction, floor frames, lifting eyes, jacking points, earthing bosses, indicating devices, control switches, automatic voltage regulators, exciters, detachable panels and doors, forced cooling, shaft bearings and seals, lubrication systems, anti-condensation heaters, noise levels, labelling and nameplates, painting etc. Some of these may be efficiently included in the data sheet.
It is therefore important to state clearly in the specification what inspection and testing will be required and, where appropriate, what are the acceptable limits of the results. Most tests required in the oil industry are covered in international specifications and these can be used as references.
However, not all those in the reference documents need to be carried out in all cases. It is therefore prudent to state the requirements in the project specification in one or more of the following methods:
• Write a detailed description of exactly what is required, including the limits that are acceptable and the form in which the results should be reported. This method ensures a ‘self-contained’ approach that is very beneficial during the actual testing operation. Often time is limited to perform tests and to have all the requirements to hand without having to search through related documents enables the work to be completed very efficiently.
• Quote the exact clause numbers and sub-section headings in the reference documents for the particular tests to be performed. This may be less efficient when the time of the tests becomes due, especially if the reference documents are not easily to hand. If a statement is made such as ‘the switchgear shall be tested in accordance with the XYZ-123 international standard’ and no other clarification is included, then many debates can arise at the time of testing.
Whichever method is used it should be carefully checked by a quality assurance department before the specification is approved for purchasing the equipment.
Some types of equipment require ‘production tests’, ‘type tests’, ‘performance tests’, ‘routine tests’, ‘abbreviated tests’ or ‘special tests’, or a combination of these tests. The subtitles are sometimes used with different meanings. Production tests are required for complex equipment such as high voltage generators and motors, and these tests are performed in the factory before the complete unit is assembled. For example the rotors are balanced without the stator, air-to-water heat exchanges can be tested to withstand hydraulic pressure, winding insulation and individual coil insulation can be tested.
Type tests are performed on one from a group of identical units. These tests are comprehensive and some of which are usually only performed once in the life span of the equipment.
If the equipment is a standard product of the manufacturer for which existing certificates can show that a type test has previously been carried out, then the purchaser may wish to accept the certificate without repeating the test. This is largely a matter of choice than necessity.
Routine and abbreviated tests are generally the same form of tests. These are applied to those units in a group that have not been type tested. If only one unit is to be purchased and a type test has been waived then a routine test is usually performed and the results compared to those of a previous type test. The number of different tests included in the routine tests is less than that of the type tests.
Some of the tests may be identical in each category. Routine tests are usually witnessed by the owner or purchaser.
Performance tests are those tests that need to be carried out on combined equipment such as a gas-turbine driven generator or a pump driven by a high-voltage motor. In such cases the dynamic relationship between the various equipments is of interest. For example, rotor vibration, critical speeds, run-up time to full speed, starting up and shutting down sequences, full-load and over-load performances, heat dissipation and cooling medium performance. Occasionally ‘special tests’ may be required. These may be due to the need to operate the unit in an unusual mode or to test special control systems that may involve associated equipment such as a power management system or a control panel. Special tests may be needed to verify the operation of protective devices in the equipment rather than the equipment itself, but which require the device to be in its fully functional position on its host equipment. The owner or the purchaser usually witnesses performance and special tests.
Routine tests usually include a thorough inspection of the equipment both before and after the testing is complete. Routine testing should not be confused with sample testing. For example a switchboard may consist of many panels of essentially the same type, e.g. motor starters, transformer feeders. The testing schedule should state whether samples of similar types could be tested in lieu of testing all the units. In either case a full routine test is generally required. Functional testing of mechanical operation should be applied to all the units, e.g. open and close contactors, rack in and out circuit breakers, operate switches and controls.
• Tender documentation.
• Purchase order documentation.
• At the time of delivery of the equipment.
Documentation can be divided into drawings and documents.
- Introduction.
- Scope of supply.
- Service and environmental conditions.
- Compliant international standards.
- Definition of technical and non-technical terms.
- Performance (or functional) requirements.
- Design and construction details.
- Inspection and testing.
- Spare parts.
- Documentation.
- Packing and transportation.
- Appendices, if necessary.
1) INTRODUCTION
In this introductory section there should be a brief description of where the equipment is to be located, what type of installations will use the equipment and whether the environment is hazardous or non-hazardous (or both).2) SCOPE OF SUPPLY
A summary listing should indicate all the main components that constitute the equipment, e.g. AC generator, coupling, exciters, AVR, terminal boxes, lubrication system, stator cooling system, heat exchangers.Where appropriate it is prudent to describe or list what is not included in the scope of supply. This will minimise misunderstandings at a later stage when quotations are being compared, e.g. for the above example, gearbox, prime mover, base frame or skid assembly.
3) SERVICE AND ENVIRONMENTAL CONDITIONS
Here should be explained the range of environmental (ambient) temperatures, humidity, winds, and available cooling water conditions. The design ambient temperature should be stated. The type of weather throughout the year may have an influence on the design of the equipment, e.g. dust-laden wind, heavy storms, corrosive rain, air contaminated with chemicals. Outdoor and indoor conditions should be described if appropriate.4) COMPLIANT INTERNATIONAL STANDARDS
A list of only the most appropriate international standards should be included. The title, identification number and latest revision number should be given. If too many standards for the type of equipment are listed, then much confusion can arise at a later date when the quality assurance checks are made. Some standards have similar titles but have subtle differences and applications. (Mixing European and US standards can give rise to misinterpretations of their definitions and suitability as they are not necessarily identically equivalent to each other, such as in the case with some BSI and IEC standards that meet the CENELEC harmonization norms.)5) DEFINITION OF TECHNICAL AND NON-TECHNICAL TERMS
When it is proposed to issue an enquiry for the purchase of equipment on an international basis, it should be borne in mind that the interpretation of words and phrases, which may not be in regular use by the recipient, can suffer through translation. Some of the international standards, e.g. IEC60034, 60050, 60079, include sub-sections or clauses for defining words, phrases and terms. Sometimes these definitions are not easy to grasp.It is recommended that particularly important words, phrases, terms and abbreviations are defined in the specification itself; especially if they differ in use from say those given in an IEC specification. (An example that regularly appears is the difference in meaning between ‘shall’ and ‘should’.)
Some of the material in this section could equally well be placed at the end of the document as an appendix.
6) PERFORMANCE OR FUNCTIONAL REQUIREMENTS
Somewhere in the specification, or the data sheet, should be stated the expected life duration of the equipment, e.g. 25 years, and a reasonable duration of continuous service between major maintenance operations, e.g. 3, 4 or 5 years. These durations will depend upon the type of equipment, but for major items such as large generators, large high-voltage motors, switchboards, motor control centers, power transformers, these durations can be regarded as typical for the oil industry.If equipment is to be specified for use in hazardous areas, e.g. Zone 1, Zone 2, then the equipment as purchased should not have been modified in any manner that could invalidate its hazardous area certification. Components that can be vulnerable to modification are terminal boxes, gland plates and threaded entries.
The basic requirements for performance can be categorized as follows:
Starting up.
Normal continuous operation.
Permissible but limited overloading.
Short-circuit withstand.
Shutting down.
It will be useful to the recipient to have an understanding of the power system or network into which the equipment will belong. This is especially important when specifying the high-voltage generation and distribution equipment, and some of the main low-voltage equipment such as switchgear.
The modes of operation of the power system may have some bearing upon the design of the equipment being specified, e.g. method of earthing neutrals, minimum and maximum fault currents, dips in system voltage and frequency, normal and abnormal switching configuration.
The owner may have some restriction on how to start up and shut down equipment, e.g. limits on starting currents of motors, voltage dip limits at switchgear, duration of start up or shut down, purging with safe air or inert gas, interlocking schemes, manual or automatic sequences.
For some equipment, especially generators and their prime-movers, the normal or rated duty may need to be emphasised so that the correct rating for the prime-mover is chosen, and an adequate margin for short-term permissible overloading exists. Emergency generators used offshore may need to be allowed to run in overloaded conditions until they run out of fuel or actually fail. International specifications should be referred to for the description of full-load duty for particular types of equipment, for example IEC60034 for generators and motors and for switchgear see sub-section 7.1.
If equipment needs to function continuously in high ambient temperatures, e.g. 40◦C or higher, then the derating of the manufacturer’s standard equipment should be quoted and explained by the manufacturer. This is especially important with switchgear busbars and circuit breakers. Some manufacturers may not wish to quote for high ambient conditions, and many of the international standards use 40◦C as their upper limit.
The short-circuit withstand performance may be important with certain types of equipment, e.g. generators, high-voltage motors, switchgear, power transformers. This should be described or stated in the data sheet. The rms and peak values of short-circuit currents may need to be described.
Some equipment may be sensitive to unbalanced loading, unbalanced supply voltages or the harmonic content of the supply.
7) DESIGN AND CONSTRUCTION REQUIREMENTS
Oil industry equipment tends to be more robust than normal industrial equipment due to the often harsh and hostile environments in which it is expected to function without trouble for long periods of time.The indirect cost of equipment failures and outages is high and reliability is of paramount importance.
An essential requirement is the definition of the degree of protection of the enclosure for the environment, which may be either outdoor or indoor, and hazardous or non-hazardous. The international standards most often used are IEC60529 and NEMA-ICS1-110 for the degree of protection against liquids and particles. These references are applied for the hazardous area protection.
Wound components such as motor and transformer windings need to have their insulation specified to withstand the surface temperature of the copper conductors. IEC60085 and ANSI/NEMA describe the different classes of insulation that are normally available. Where IEC60085 or ANSI/NEMA is the reference, the two most common are Class B and Class F. These state the maximum temperature rise in degrees Celsius above the conductor temperature when the temperature of the cooling medium for the equipment is no greater than 40◦C.
For most equipment ratings used in the oil industry the temperature rise limits are 80◦C for Class B and 100◦C for Class F (Class H allows 125◦C). It is common practice to specify Class F insulating materials but to restrict the actual temperature rise to that of Class B. These stem from the recommendation in IEC60085 that for ratings equal and above 5000 kVA or if the iron core length is equal and above one metre, that this combination of classes should be used.
Various IEC standards for switchgear refer to IEC60694 sub-section 4.4.1 for the requirements of rated current and sub-section 4.4.2 for temperature rise of enclosed components such as bare terminals, busbars and risers, panel surfaces, and built-in apparatus. It also refers to IEC60085 for the classes of insulation. Busbars and risers can be bare or insulated and so it is not practical to state a requirement for their temperature rise in the project specification.
The owner may have particular requirements for the materials to be used for insulation and their impregnation. This may be due to their experience with marine and highly humid environments.
Other aspects that should be included are protective devices, measurement detectors, terminal blocks, segregation of circuits and terminals, voltage surge suppression, skid construction, floor frames, lifting eyes, jacking points, earthing bosses, indicating devices, control switches, automatic voltage regulators, exciters, detachable panels and doors, forced cooling, shaft bearings and seals, lubrication systems, anti-condensation heaters, noise levels, labelling and nameplates, painting etc. Some of these may be efficiently included in the data sheet.
8) INSPECTION AND TESTING
Inspection and testing of the purchased equipment is one of the most important tasks in the engineering of a project. Its importance is sometimes underestimated. The first serious tests that the purchaser will witness are those in the factory where the equipment is assembled. These tests will also include a physical inspection of the equipment.It is therefore important to state clearly in the specification what inspection and testing will be required and, where appropriate, what are the acceptable limits of the results. Most tests required in the oil industry are covered in international specifications and these can be used as references.
However, not all those in the reference documents need to be carried out in all cases. It is therefore prudent to state the requirements in the project specification in one or more of the following methods:
• Write a detailed description of exactly what is required, including the limits that are acceptable and the form in which the results should be reported. This method ensures a ‘self-contained’ approach that is very beneficial during the actual testing operation. Often time is limited to perform tests and to have all the requirements to hand without having to search through related documents enables the work to be completed very efficiently.
• Quote the exact clause numbers and sub-section headings in the reference documents for the particular tests to be performed. This may be less efficient when the time of the tests becomes due, especially if the reference documents are not easily to hand. If a statement is made such as ‘the switchgear shall be tested in accordance with the XYZ-123 international standard’ and no other clarification is included, then many debates can arise at the time of testing.
Whichever method is used it should be carefully checked by a quality assurance department before the specification is approved for purchasing the equipment.
Some types of equipment require ‘production tests’, ‘type tests’, ‘performance tests’, ‘routine tests’, ‘abbreviated tests’ or ‘special tests’, or a combination of these tests. The subtitles are sometimes used with different meanings. Production tests are required for complex equipment such as high voltage generators and motors, and these tests are performed in the factory before the complete unit is assembled. For example the rotors are balanced without the stator, air-to-water heat exchanges can be tested to withstand hydraulic pressure, winding insulation and individual coil insulation can be tested.
Type tests are performed on one from a group of identical units. These tests are comprehensive and some of which are usually only performed once in the life span of the equipment.
If the equipment is a standard product of the manufacturer for which existing certificates can show that a type test has previously been carried out, then the purchaser may wish to accept the certificate without repeating the test. This is largely a matter of choice than necessity.
Routine and abbreviated tests are generally the same form of tests. These are applied to those units in a group that have not been type tested. If only one unit is to be purchased and a type test has been waived then a routine test is usually performed and the results compared to those of a previous type test. The number of different tests included in the routine tests is less than that of the type tests.
Some of the tests may be identical in each category. Routine tests are usually witnessed by the owner or purchaser.
Performance tests are those tests that need to be carried out on combined equipment such as a gas-turbine driven generator or a pump driven by a high-voltage motor. In such cases the dynamic relationship between the various equipments is of interest. For example, rotor vibration, critical speeds, run-up time to full speed, starting up and shutting down sequences, full-load and over-load performances, heat dissipation and cooling medium performance. Occasionally ‘special tests’ may be required. These may be due to the need to operate the unit in an unusual mode or to test special control systems that may involve associated equipment such as a power management system or a control panel. Special tests may be needed to verify the operation of protective devices in the equipment rather than the equipment itself, but which require the device to be in its fully functional position on its host equipment. The owner or the purchaser usually witnesses performance and special tests.
Routine tests usually include a thorough inspection of the equipment both before and after the testing is complete. Routine testing should not be confused with sample testing. For example a switchboard may consist of many panels of essentially the same type, e.g. motor starters, transformer feeders. The testing schedule should state whether samples of similar types could be tested in lieu of testing all the units. In either case a full routine test is generally required. Functional testing of mechanical operation should be applied to all the units, e.g. open and close contactors, rack in and out circuit breakers, operate switches and controls.
9) SPARE PARTS
At the inquiry stage it is common practice to ask the manufacturer to list or describe what spare parts are needed for commissioning purposes and for normal use of the equipment.10) DOCUMENTATION
For equipment such as generators and switchgear the documentation can be extensive. Some of it is needed by the project design engineers as soon as possible after the purchase order is placed. The delivery of documentation can be made at the following basic stages:-• Tender documentation.
• Purchase order documentation.
• At the time of delivery of the equipment.
Documentation can be divided into drawings and documents.
a) TENDER DOCUMENTATION
The following dimensional drawings would normally be required at the tendering stage of a project, so that comparison can be made between the various tendering manufacturers,
• Plans and elevations of the main structure.
• Base frame or skid dimensions.
• Attached equipment such as heat exchangers and ducting.
• Location of fitting eyes and jacking points.
• Cable box positions.
• Cable gland plate positions.
• Nameplate details.
• One-line diagrams.
• Typical schematic diagrams.
• Control and logic diagrams.
In addition, the following written documents would normally be required,
• Completed data sheets.
• Quality assurance plan and procedures.
• Inspection and testing plan and procedures.
• Detailed list of performance, type, routine and special tests.
• Hazardous area certificates and certificates of conformity.
• Spare parts list.
• List of attached equipment, e.g. anti-condensation heaters, temperature detectors.
• Heat dissipation of units.
• Weight of each major component, e.g. heat exchangers, rotors, stators.
• Copies of existing type tests certificates.
• Reliability data e.g. mean time before failure.
b) PURCHASE ORDER DOCUMENTATION
After the tendering process has been completed and an order is about to be placed the following documents would be required as soon as possible,
• Revised versions of the documents submitted at the tender stage.
• Completed data sheets.
• Foundation loading details.
• Lubrication system details.
• Rotor removal and replacement procedure.
• Full details of all cable termination, gland plates and boxes.
• Lay-down area adjacent to the equipment.
• Detailed list of spare parts.
• One-line diagrams, schematic diagrams, block diagrams etc., for the specific equipment being purchased.
• Functional narrative descriptions of start up, normal operation and shut down.
• Interconnection diagrams.
• Schedule of controls, alarms and event messages.
c) AT THE TIME OF DELIVERY
Before the equipment is delivered to the site it will normally undergo the type and routine tests in the factory. These tests are often referred to as the factory acceptance tests (FAT). Some documents are required before the FAT and others afterwards. Those required before are usually the inspection reports as part of the quality assurance plan, instruction manuals for transportation, storage, installation and commissioning routine maintenance.
After the FAT is complete the purchase would normally require the testing report and a set of revised drawings.
• Plans and elevations of the main structure.
• Base frame or skid dimensions.
• Attached equipment such as heat exchangers and ducting.
• Location of fitting eyes and jacking points.
• Cable box positions.
• Cable gland plate positions.
• Nameplate details.
• One-line diagrams.
• Typical schematic diagrams.
• Control and logic diagrams.
In addition, the following written documents would normally be required,
• Completed data sheets.
• Quality assurance plan and procedures.
• Inspection and testing plan and procedures.
• Detailed list of performance, type, routine and special tests.
• Hazardous area certificates and certificates of conformity.
• Spare parts list.
• List of attached equipment, e.g. anti-condensation heaters, temperature detectors.
• Heat dissipation of units.
• Weight of each major component, e.g. heat exchangers, rotors, stators.
• Copies of existing type tests certificates.
• Reliability data e.g. mean time before failure.
b) PURCHASE ORDER DOCUMENTATION
After the tendering process has been completed and an order is about to be placed the following documents would be required as soon as possible,
• Revised versions of the documents submitted at the tender stage.
• Completed data sheets.
• Foundation loading details.
• Lubrication system details.
• Rotor removal and replacement procedure.
• Full details of all cable termination, gland plates and boxes.
• Lay-down area adjacent to the equipment.
• Detailed list of spare parts.
• One-line diagrams, schematic diagrams, block diagrams etc., for the specific equipment being purchased.
• Functional narrative descriptions of start up, normal operation and shut down.
• Interconnection diagrams.
• Schedule of controls, alarms and event messages.
c) AT THE TIME OF DELIVERY
Before the equipment is delivered to the site it will normally undergo the type and routine tests in the factory. These tests are often referred to as the factory acceptance tests (FAT). Some documents are required before the FAT and others afterwards. Those required before are usually the inspection reports as part of the quality assurance plan, instruction manuals for transportation, storage, installation and commissioning routine maintenance.
After the FAT is complete the purchase would normally require the testing report and a set of revised drawings.
11) APPENDICES
Appendices may be needed to give particular details, e.g. hazardous area applications, testing data, special tests, bearings and lubrication requirements, noise information, protective relay data, interlocking requirements, switchgear cubicle contents, control panel requirements, and copies of partially completed data sheets.
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