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Reliability of a system is difficult to measure. Perhaps the best way is through evaluation of the consequences of possible consumer interruptions. Investigations have shown that the best measure of reliability is that of consumer reaction.

Five conditions that have been identified impact the value an average consumer puts on an unsupplied megawatt-hour of lost energy:

The activities affected by the curtailment and, therefore, the time of day and mix of customers
The number of interruptions
Availability of advance warning
Weather conditions and, therefore, the time of year
The duration of the interruption

Figure 9-1 shows that this reaction increases dramatically as the frequency of outages increase, as the duration of the outage increases, and with the magnitude or extent of the outage. The following function presents a means of evaluating this reaction:

R = function of {K, F, T, P, t}

Where K is an empirical coefficient proportional to the consumer’s dependence on electricity, F is the frequency of interruptions, T is equal to duration of the interruptions, P is the amount of load interrupted, and t is the time when the interruption occurs. Experience has shown that K increases with increasing consumption of electricity per customer, and t is greatest at the time of day, week, or year when people suffer the greatest hardships if service is interrupted. This criterion for reliability evaluation does not consider other curtailments of service, such as voltage or frequency reductions. These “partial” curtailments are not as important to most consumers as a complete interruption but they should also be considered.

Figure 9-1. NERC regions. Source: NERC.

A number of indices have been developed, primarily for the distribution system, to provide another measure of reliability:

SAIFI (System Average Interruption Frequency Index) measures the average frequency of sustained interruptions per customer

SAIDI (System Average Interruption Duration Index) measures the average time that all customers are interrupted

CAIDI (Customer Average Interruption Duration Index) represents the average time required to restore service to the average customer per sustained outage

MAIFI (Momentary Average Interruption Frequency Index) tracks the average frequency of momentary interruptions, typically defined as less than five minutes

COSTS OF ELECTRICAL POWER OUTAGES IN UNITED STATES

The costs of electric power outages to U.S. electric customers are generally called “socioeconomic” costs. Attempts have been made to quantify these costs but the estimates vary widely. One source reports that the costs are $26 billion each year and that they have been increasing as the electric power industry is restructured. A 2001 report from the Electric Power Research Institute (EPRI) states that power outages and problems with power quality cost the U.S. economy over $119 billion per year. Costs are due to:

Loss of life due to accidents (e.g., no street lights)
Loss of life of the ill and elderly (death rates go up)
Loss of productivity by industry
Loss of sales by business
Loss of wages of labor
Damage to equipment in industry
Fires and explosions
Riots and thefts
Increased insurance rates

CAUSES OF ELECTRICAL POWER OUTAGES

Interruptions in the supply of electricity to customers can occur at any hour of the day or night and can last from fractions of a second to many hours or even days. Interruptions can be caused by disturbances to or malfunctions of any of the three components of the power system: generation, transmission, or distribution. They can also be caused by the unavailability of adequate resources to supply the customer load. These two attributes of reliability are characterized by NERC as security and adequacy.

Data shows that over 90% of customer outages are caused by problems originating on the local distribution system. Although generation and transmission-related outages are less common than those related to the distribution system, they often have much more serious consequences because of the number of customers affected and the duration of the outage.

Disturbances can be initiated by:

1) External events such as:

Environmental factors, including wind, rain, lightening, ice, fire, floods, and earthquakes
Accidents such as cars hitting poles
Sabotage (sadly)

2) Internal events such as

Insufficient resources
Failure of equipment due to electrical or mechanical stresses
Operating errors or decisions

Lack of resources can be due to:

1) Insufficient generation caused by

Low load forecasts
Shortages of fuel due to supply disruptions or delivery/transportation problems
Opposition to the construction of required new generating capacity
Failure of equipment due to electrical or mechanical stresses poor planning
Excessive maintenance outages
Regulatory actions restricting the operation of power plants
Transmission constraints
Generation being retired because it is noncompetitive in the new competitive market

2) Insufficient transmission or distribution caused by:

Low load forecasts
Opposition to the construction of required new transmission or distribution lines
Failure of equipment due to electrical or mechanical stresses
Poor planning
Intentional outages required because of other infrastructure work, such as the widening of roads

The duration of the interruption will be affected by the severity of the disturbance, the power system facilities affected, the redundancy or reserve built into the system, and the preparedness of the involved operating entities to respond. Some interruptions are of very short duration because the disturbance is transient and the system self-corrects. Some interruptions, such as those caused by tornadoes or ice storms, damage significant portions of the system, requiring many days to restore service. When there are insufficient generation resources, the outages may be of a controlled and rotating nature. Their duration might be only during peak load hours.

The extent of the interruption will be determined by the initiating disturbances and the facilities affected. For example, cascading outages caused by a fault occurring when a system is operating above a safe level can involve many states, as can a widespread ice storm. Conversely, a distribution pole damaged by a car may affect only a few homes.

An increasingly important aspect of power system reliability is the quality of service or power quality. With the increasing importance of computers and new electronic communication procedures, imperfections in electric service become increasingly important to the customer. Such imperfections include:

Momentary interruptions
Voltages outside of acceptable limits
Voltage dips of very short duration

Protection against power quality imperfections can often be handled by the consumer. Pressure is mounting, however, for the supplier to improve quality. This raises the question of the responsibility for such improvements in a deregulated power industry with separate companies providing distribution, transmission, and power supply services

SMART GRID AND MICRO GRID

SMART GRID

Smart Grid is one of a number of names used to describe a transmission and distribution network that uses modern sensing, computational, and communication technologies to improve the reliable and economic functioning of the bulk power system. Other names include Modern Grid, Intelli grid, and Future Grid. A subset of this notion is the development of micro grids.

The Smart Grid can perhaps be described as any activity that might improve the performance and efficiency of the electric power system. Those applications being discussed seem to fit into two broad categories:

1. Those impacting the operation and control of the bulk power system

2. Those impacting the distribution system, especially activities that would involve interaction with the customer and the customer’s electric devices

Besides addressing technical issues, substantive public policy issues also need attention, especially for those technologies that would alter or change the usage pattern of customers. Besides developing the technical tools to optimize the operation of the power grid, efforts to adjust customer demand need to be carefully reviewed to ensure that the rights of citizens/consumers are not compromised.

MICRO GRID

Micro grids are a subset of the effort to develop Smart Grid technology. They are entities that coordinate distributed energy resources in a decentralized way so as to reduce the need for control from a centralized location; for example, small commercial areas managed as one entity. These distributed resources can include various forms of distributed generation, heat and electricity storage, and controllable customer loads. The micro grid may or may not be connected to the local grid. Besides technical issues, there are also regulation/policy issues that must be addressed when implementing this technology.

ELECTRICAL ENERGY STORAGE CONCEPTS AND TECHNOLOGIES

There are a large range of possible approaches and concepts for storing energy in electric utility systems. These are discussed in the following subsections.

1) MECHANICAL SYSTEMS

HYDRO PUMPED STORAGE: In hydro pumped storage, water is pumped from a lower to a higher elevation. The water at the higher elevation can be stored and used to generate electricity for later utility use when it flows down through a hydro turbine to drive an electric generator. Pumping and generation may also be accomplished with a reversible pump–turbine connected to a motor generator. The reservoirs needed for the pumped storage operation may be natural bodies of water, reservoirs of existing hydro plants or of water storage systems, especially constructed surface reservoirs, underground caverns, or a combination of these. Typical efficiency of this process is about 70%, with 30% used in the pumping generating cycle.

The major barriers to widespread use of conventional pumped storage are siting, geological factors, environmental and space constraints because of the large size of commercially feasible installations, and long construction times. The uncertainty and potential high costs of underground construction costs are believed to be the reason that no underground projects have been pursued to actual construction.

In underground pumped storage, the lower reservoir and power plant are located in deep underground caverns and the upper reservoir is at the surface. By being free of surface topographical restrictions, the siting of these underground plants should be considerably easier than the siting of conventional pumped storage facilities. The underground reservoir and power plant could use naturally occurring caverns, abandoned mines, or a mined-out cavern consisting of a tunnel labyrinth excavated specifically for the pumped storage reservoir. In existing mine sites, firsthand knowledge of the subsurface rock formations is available, and existing shafts can be used. With the elevation difference (head) between the upper and lower reservoirs now a variable parameter not limited by surface topography, the major design restrictions are equipment capability and rock conditions. If very high heads are used, there may be cost penalties associated with very deep mining; nevertheless, mining costs per unit of energy storage capacity should decrease with depth because of proportional reductions in the volume of the reservoir.

Underground construction and mining technologies are available and can be adapted for this system. The largest uncertainty is construction of the underground reservoir: its cost, durability with pressure cycling, and the rate of water leakage into the lower reservoir. Costs are heavily dependent upon suitability of the site and local labor conditions. The economics of scale in pumped hydro dictates sizes in the range of 1000 to 2000 MW.

COMPRESSED AIR ENERGY STORAGE: Compressed air storage uses a modified combustion turbine (split Brayton cycle), uncoupling the compressor and turbine so that they can operate at different times and incorporating the intermediate storage of compressed air. During off-peak load periods, the turbine is disengaged and the compressor is driven by the generator, which is now used as a motor and takes its power from other generating units through the system’s interconnections. The stored compressed air is subsequently used during peak load periods when it is mixed with fuel in the combustion chamber, burned, and expanded through the turbine. During that period, the compressor is disengaged and the entire output of the turbine is used to drive the generator.

Since in normal operation the compressor consumes about two-thirds of the power output of the turbine, the rating of the combustion turbine operating from the stored compressed air is increased roughly by a factor of three. This permits redesign of the compressors, the combustion process, and the combustion turbine, free from the aerodynamic and thermodynamic restrictions inherent in designs of conventional combustion turbines. Current estimates for the heat rate of the combustion turbine operating from stored compressed air are in the range of 4000 Btu/kW-hr. A compression/generation energy ratio in the range of 0.65 to 0.75 should be readily available. The variable maintenance cost should not be any greater than that for a conventional combustion turbine.

The compressed air may be stored in naturally occurring reservoirs (caverns, porous ground reservoirs, and depleted gas or oil fields) or manmade caverns (dissolved-out salt caverns, abandoned mines, or mined hard-rock caverns). Air storage may be accomplished at variable pressure or, through the use of a hydrostatic leg, at constant pressure. Each approach has its advantages and all are applicable to different underground geologies and reservoir designs. Designs of plants in the 50 to 250 MW range and larger have been explored.

Research is required to investigate:

(a) Geological conditions for underground storage,
(b) New approaches to underground cavern construction,
(c) Energy losses storing and moving air,
(d) Alternative concepts of air storage, and
(e) Corrosion effects on turbines from air contamination.

The major uncertainties include the cost of the air storage facilities, the performance and durability of the storage facility with pressure and thermal cycling, and leakage from the storage reservoir. Also, additional geological survey work to identify the availability and number of possible sites is necessary before the future role of compressed air storage may be assessed. With the high costs of fuel and the needs of intermittent renewable resources, renewed interest in compressed air storage has developed and a concept that uses high-pressure storage in buried pipes for smaller installations is being explored.

FLY WHEELS: Fly wheels store energy in the form of the kinetic energy of a rotating mass and have been used since the beginning of the industrial age. In recent years, the commercial application of flywheels to power quality and interruptible power supplies has become a commercial reality. Technological advances in rotating machinery and high-strength materials achieved since then hold promise for longer periods and greater capacity of energy storage, which raises the possibility of new applications.

Utility system applications have been restricted to special purpose uses for smoothing pulsed power needs or for short-duration power quality needs. Although advanced composite materials have been experimented with in test facilities and proposed for commercial application, the metal flywheel has been the one approach that is in general use. Proposed super flywheel designs deal primarily with the wheel itself, without treating the full energy storage system in sufficient detail. The large wheels once proposed for utility applications appear to be outside the size of current, state-of-the-art, cost-effective designs. Commercial applications have used steel wheels and various electromechanical machinery designs, including variable frequency converters.

2) THERMAL ENERGY STORAGE

Thermal energy storage may be defined as storage of energy in the form of (a) sensible heat and (b) the latent heat associated with phase changes such as the formation of ice. The major technical parameters for thermal energy storage include the storage medium, the operating temperature range, and the mode of heat exchange between the storage subsystem and the heat source/sink. Any practical system must include not only the thermal energy storage and transfer subsystems but also provisions for control and insulation. Thermal energy storage can be useful in a wide spectrum of applications, including

(a) Hot water heating,
(b) Heating and air conditioning of buildings using off-peak (or solar) energy,
(c) low-temperature process steam storage,
(d) Central station thermal storage (especially for solar–thermal power plants), and
(e) Industrial process heat storage and district heating systems.

Depending largely on the temperature of the storage medium, these uses may be grouped into applications of low-grade (relatively low-temperature) and high-grade (high temperature) heat.

STORAGE OF LOW-GRADE HEAT: Storage of sensible heat in hot water reservoirs is an established commercial practice in off-peak water heating and in chilled water applications. Heat storage in the ceramic bricks of storage heaters gained commercial acceptance in

Europe several decades ago. Among the key tasks in making storage heaters practical were the design and refinement of control methods. Development resulted in improved ceramic materials of high specific heat capacity and good thermal cycling ability, new approaches to high quality thermal insulation, and extension of the concept from individual room units to central systems.

The operation of air conditioning systems with off-peak power requires coolness storage. Although coolness storage has found only limited applications, it would be a useful option if more widely applied in summer peaking electric utilities. Storage of relatively low-temperature heat will be a key requirement for the residential and commercial utilization of solar energy, and appropriate approaches have been explored experimentally. Thermal energy storage systems based on storage of latent (phase change) heat are attractive in principle because of their high specific storage capacity. However, despite their greater volume, sensible heat storage in liquids is almost certainly more practical and economical because they avoid the problems and costs associated with heat exchangers. Storage of waste heat from power plants or industrial processes for later use is another possible application of low-grade heat storage.

STORAGE OF HIGH-GRADE HEAT: Storage of high-temperature, high pressure steam/water mixtures is a prime example of thermal energy storage but is largely only of historical interest as a step in the development of the modern steam power plant. While the basic technologies associated with thermal storage via hot water and steam is straight forward, the large size and high costs of the storage vessels has largely ruled out this technology as part of conventional power plants. The benefit of energy storage is in conjunction with solar thermal power plants. The use of storage of sensible heat in a high-temperature oil or liquid molten salt can extend the operating hours of the power generation portion of the solar thermal power plant and result in a higher capacity factor for the plant than would be possible if the plant could only generate power when the sun was available.

3) CHEMICAL ENERGY STORAGE

Chemical energy storage is the storage of energy as chemicals (usually two different chemicals that can be gases, liquids, or solids) that can be made to react with a net release of energy. Storage of energy in chemical form has two inherent advantages. The high energy density of a chemical system results in compact, generally low-cost storage and ready transportability of energy, and chemical energy is readily converted into other useful energy forms by a variety of methods and devices. These advantages are responsible for the almost exclusive use of conventional fuels for energy storage and mobility applications.

The chemical energy storage methods and systems relevant to electric power (excluding conventional fuel storage) are largely secondary battery energy storage and hydrogen storage. In general, the reactant systems containing the stored energy must be reformed readily from their reacted (discharged) state upon addition of energy in a suitable form. Although many other schemes have been proposed, they remain only of research interest at the moment.

4) BATTERIES

In “storage” batteries, the conversion from electrical to chemical energy (charging) and the reverse process (discharging) are performed by electrochemical reactions. The electric form of input and output energy, compactness, and the modular characteristics common to electrochemical devices make batteries potentially the most useful among advanced energy storage methods.

Many different electrochemical systems have been developed, or offer prospects for development, into practical storage batteries. For more than thirty years, efforts have been underway to develop and commercialize various battery systems for use by electric utilities at the scale of distributed energy storage in the size range of megawatts to tens of megawatts; several applications of lead acid batteries have actually been operated for extended periods and more advanced systems proposed and demonstrated.

Today, great interest and very substantial funding is being invested in lithium battery systems that operate at ambient temperatures and are targeted primarily for mobility and portable power, including especially the plug-in hybrid vehicle. If this current effort is successful and the promise of lower costs in large volume production is realized, practical battery energy storage may become a practical reality on a large scale, although each individual installation may be only the tens of kilowatt hours needed for the personal vehicle applications.

5) HYDROGEN ENERGY STORAGE SYSTEMS

Hydrogen energy storage represents the best known example of advanced chemical energy storage. Several approaches have been proposed and explored for each of the required sub-systems hydrogen generation, storage, and reconversion which can be combined in various ways into overall energy conversion and storage systems. For hydrogen generation from water, electrolysis is the only established industrial process. Current electrolysis technology is handicapped by high capital costs, but considerable potential appears to exist for development of more efficient, lower-cost electrolyzers. At present there is little commercial incentive to develop such technology.

Closed-cycle thermochemical processes are being proposed for hydrogen production via water splitting, but current work is still in the conceptual and early laboratory stages. The incentive to develop such processes derives from the potential for efficiencies and economics that might be superior to those offered by electrolysis, particularly if sources of fairly high-temperature heat, such as high-temperature, gas-cooled reactors or, perhaps, focused solar heat, become available. Integration of these processes with nuclear heat sources and commercialization of the entire hydrogen production system are likely to require many years and large capital investments.

Hydrogen storage, the second major subsystem of hydrogen energy storage systems, can take several different forms. Storage of compressed hydrogen is technically feasible now, as is storing hydrogen in more concentrated forms as a cryogenic liquid or chemically bound in metal hydrides, and logistically attractive. However, cryogenic storage of hydrogen carries a significant efficiency penalty that is unacceptable for large-scale energy storage on utility systems, and capital cost, logistics, and safety are likely to present problems for mobile applications. The outlook is better for metal hydride storage, but development efforts are still required to establish the technical and economic characteristics of this method for hydrogen storage. Reconversion of hydrogen to electric energy can be done in fuel cells or in combustion devices (gas fired boilers or gas turbines). The fuel cell approach offers potential for high efficiency, with 60% as a target for pure hydrogen fuel.

Hydrogen has unique potential for utilization of primary energy sources and flexible use of the stored energy. However, unless major advances result from current research and development on hydrogen production, storage, and conversion technologies, relatively low efficiency and high capital costs will be major barriers to the introduction of hydrogen energy storage systems.

6) ELECTRICAL STORAGE

Capacitors and Super capacitors. The capacitor is an essential and elementary device in electrical circuits that stores electrical charge, typically between two metallic plates separated by dielectric, a material that is not particularly good at conducting current.

The charge separation creates an electrical potential or voltage between the two plates. Capacitors are routinely used in power systems to compensate for the inductance of the electrical wires or conductors. Properly designed capacitors can withstand high voltages and are well suited to serve as static devices in high-voltage applications.

In the last two decades, a new class of capacitors, super capacitors have been developed and commercialized that operate on a somewhat different principal; these are electrochemical double layer capacitors. They have significantly higher charge densities than ordinary capacitors. Their properties are between those of a conventional capacitor and a battery. These devices are finding uses in situations in which a larger capacitance is needed and fast response time is desirable, but the full storage capacity of a battery is not needed. They have also been proposed as devices to be combined with batteries in some applications. The material properties of the super capacitors are very different from those of conventional capacitors and they cannot withstand high voltages, but can be placed in series like batteries to operate at modest voltages.

7) SUPERCONDUCTING MAGNETIC ENERGY STORAGE

Superconductors have the apparently near magical property of having no resistance to direct current flow (no electrical losses) and, hence, the current in a closed loop of superconductor can persist indefinitely under ideal conditions. This property underlies the concept of superconducting magnetic energy storage originally proposed as a competitor to pumped storage in the early 1970s. Very large superconducting magnets have been designed with the potential to store energy on the scale only achieved in practice by hydro pumped storage. While no large system has ever been built, the concept appears feasible and a very detailed design for a modest system capable of storing some 20 mWh was engineered for the U.S. Department of Defense as part of potential future weapon systems in the 1980s and early 1990s. At a substantially smaller scale, the technology has been commercialized and used in power quality applications at the level of megawatts and discharge applications of seconds. The economics of these systems has limited their market to rather special applications and today they are not being widely used.

BENEFITS OF ENERGY STORAGE TO TRANSMISSION AND DISTRIBUTION

By Engr. Aneel Kumar August 08, 2014 No comments:

Energy storage applications offer potential benefits to the transmission and distribution system because of the ability of modern power electronics, and some electro-chemistries, to change from full discharge to full charge, or vice versa, extremely rapidly.

These characteristics enable energy storage to be considered as a means of improving transmission grid reliability or increasing effective transmission capacity. At the distribution level, energy storage can be used in substation applications to improve system power factors and economics and can also be used as a reliability enhancement tool and a way to defer capital expansion by accommodating peak load conditions.

Energy storage can also be used to alleviate diurnal or other congestion patters and, in effect, store energy until the transmission system is capable of delivering the energy to the location where it is needed.

Other technical applications of electric energy storage include:

Grid stabilization
Grid frequency support
Grid reserves
Grid voltage support
Black start

HVDC

By Engr. Aneel Kumar August 07, 2014 No comments:

An alternate means of transmitting electricity is to use high-voltage direct current (HVDC) technology. As the name implies, HVDC uses direct current to transmit power. Direct current facilities are connected to HVAC systems by means of rectifiers, which convert alternating current to direct current, and inverters, which convert direct current to alternating current. Early applications used mercury arc valves for the rectifiers and inverters but, starting in the 1970s, thyristors became the valve type of choice.

Thyristors, also called silicon-controlled rectifier (SCRs), are controllable semiconductors that conduct when their gates receive a current pulse. They can carry very high currents and can block very high voltages. They are connected is in series to form a thyristor valve, which allows electricity to flow during the positive half of the alternating current voltage cycle but not during the negative half. Since all three phases of the HVAC system are connected to the valves, the resultant voltage is unidirectional but with some residual oscillation. Smoothing reactors are provided to dampen this oscillation.

Recently, insulated gate bipolar transistors (IGBTs), using pulse-width modulation (PWM), have been used as valves. This technology was initially developed to be used with underwater HVDC cable installations when one of the terminals is connected to a weak electrical source such as an offshore wind farm.

An IGBT is basically a bipolar junction transistor (BJT) with a semiconductor gate structure in which the gate is controlled using voltage instead of current.

HVDC transmission lines can either be single pole or bipolar, although most are bipolar, that is, they use two conductors operating at different polarities such as ± 500 kV. HVDC submarine cables are either of the solid type with oil-impregnated paper insulation or of the self-contained oil-filled type. New applications also use cables with extruded insulation made of cross-linked polyethylene.

SUBSTATION EQUIPMENT

By Engr. Aneel Kumar August 07, 2014 No comments:

There are a number of designs used for substations. However, there are elements common to all:

>> A BUS: is the physical structure to which all lines and transformers are connected. Buses are of two generic types: open air and enclosed. Enclosed buses are used when substations are located in buildings or outdoors where space is at a premium. They involve the use of an insulating gas such as sulfur hexafluoride (SF6) to allow reduced spacing between energized phases. Bus structures are designed to withstand the large mechanical forces that can result from fields produced by high short-circuit currents. These forces vary with the third power of the current. A bus section is the part of a bus to which a single line or transformer is connected.

>> PROTECTIVE RELAYS: are devices that continuously monitor the voltages and currents associated with the line and its terminals to detect failures or malfunctions in the line or equipment. Such failures are called faults and involve contact between phases or between one or more phases and ground. The relays actuate circuit breakers.

>> CIRCUIT BREAKERS: are devices that are capable of interrupting the flow of electricity to isolate either a line or a transformer. They do so by opening the circuit and extinguishing the arc that forms, using a variety of technologies such as oil, vacuum, air blast, or sulfur hexafluoride (SF6). Breakers may be in series with the line or transformer or may be installed on both sides of the bus section where the line connects. They allow individual lines or transformers to be removed from service (de-energized) automatically when equipment (protective relays) detects operating conditions outside a safe range. They must be capable of interrupting the very high currents that occur during fault conditions and are rated by the amount of current they can interrupt. These fault current levels can be 20 or 30 times larger than the current flow under normal operating conditions, that is, thousands of amperes.

To minimize the impact of electrical shocks to the transmission system, minimizing the total time for the relay to detect the condition and the circuit breaker to open the circuit is a critical design issue. Circuit breakers also allow lines or transformers to be removed from service for maintenance. Circuit breakers normally interrupt all three phases simultaneously, although in certain special applications, single-phase circuit breakers can be employed that will open only the phase with a problem.

>> TRANSFORMERS: are devices that are used to connect facilities operating at two different voltage levels. The transformer connects to all three phases of the bus. Physically, the transformers can include all three phases within one tank or there can be three separate tanks, one per phase. Larger capacity units may have three separate tanks because their size and weight may be a limiting factor because of transportation issues.

Transformers can be designed with two mechanisms to adjust the voltage ratio. One mechanism is the provision of more than one fixed tap position on one side of the transformer. For example, a transformer might have a nominal turns ratio of 345/138, with fixed taps on the 345 kV winding of 327.8, 336.7, 345, 353.6, and 362.3. The transformer must be de-energized to adjust the fixed tap ratio. Another mechanism is called tap changing under load (TCUL). In this mechanism, the ratio can be adjusted while the transformer is energized, providing greater operating flexibility. Some transformers have both types of mechanisms, with a fixed tap adjustment in the high voltage winding and the TCUL adjustment in the low voltage winding.

Another type transformer is an autotransformer, which is used when facilities at nearly the same voltage are to be connected, such as 138 kV to 115 kV. Rather than having two separate paths for the electricity, connected only by the magnetic flux through the transformer as in a conventional unit, the winding of autotransformer involves a tap on the higher voltage winding which supplies the lower voltage.

All larger transformers have mechanisms to remove the heat generated within the tank, involving some manner of circulating the transformer insulating/cooling oil through an external heat exchanger with fins mounted on the side of the transformer and fans to circulate air across the fins to maximize heat dissipation.

>> DISCONNECT SWITCHES: are used to open a circuit when only charging current is present. These would be used primarily to connect or disconnect circuit breakers or transformers which are not carrying load current. They are also used in conjunction with circuit breakers to provide another level of safety for workers by inserting a second opening between station equipment out of service for work and the still energized section of line or bus.

>> LIGHTNING ARRESTERS: are used to protect transformers and switchgear from the effects of high voltage due to lightning strikes or switching operations. They are designed to flashover when the voltage at the transformer exceeds a preselected level that is chosen by the station design engineers to coordinate with the basic insulation level of the transformer (BIL).

>> METERING EQUIPMENT: is provided to measure line and transformer loadings and bus voltages so operating personnel can ensure that these facilities are within acceptable limits. Metering equipment also is provided at some locations to measure the flow of energy for the billing that is required for sales and purchases of energy between various participants in the electric energy market.

>> SCADA: is an acronym for system control and data acquisition. It covers the measurement, telecommunications, and computing technologies that allow more and more automation of substation operations.

Depending on the electrical characteristics of a particular part of the transmission system, other types of equipment that may be located at a substation are:

>> SHUNT REACTORS: (reactors connected from the energized bus to ground) are installed to control high voltages that occur especially at night due to the capacitive effect of lightly loaded transmission lines. These reactors can be energized always or they can be energized only at specific times. Shunt reactors are also used to reduce or control the high voltages that can occur when a sudden loss of a block of customer load occurs. The windings, insulation, and the external tank are similar to those used for transformers.

>> SERIES REACTORS: are installed in a transmission line to increase the impedance of the line, to decrease current levels in the event of short circuits, or to reduce its loading under various operating conditions.

>> SHUNT CAPACITORS: are installed to provide mVArs to the system to help support voltage levels.

>> SERIES CAPACITORS: are installed to reduce the effective impedance of a transmission line. These would be installed in very long transmission lines to effectively reduce the electrical angle between the sending and the receiving parts of the system, enabling more power to flow over the line and increasing stability limits.

>> PHASE ANGLE REGULATING TRANSFORMERS: are installed to control power flow through a transmission line, causing more or less power to flow over desired lines. They use a variant on the design of a normal transformer, in which, due to the specialized way they are wound, they electrically inject an angular phase shift into the line. The angle can be made to either increase or decrease power flow on the line. Since they are expensive, they are usually used only on cable systems where, because of the cost and limited capacity of cables, maximum utilization of all parallel cable capacity is essential. In recent years, many of them are being installed in transmission lines to control parallel path flow, when power flows over paths in other systems not involved in transactions or which do not have adequate capacity.

>> FACTS (FLEXIBLE AC TRANSMISSION SYSTEMS): is a generic name used for a variety of devices intended to dynamically control voltage, impedance, or phase angle of HVAC lines.

These devices mirror and extend the benefits of the fixed series and shunt inductors and capacitors previously discussed in that the FACTS devices allow rapid and precise adjustments. Depending on the device, they provide a number of benefits: increased power transfer capability, rapid voltage control, improved system stability, and mitigation of sub-synchronous resonance. There are many devices made by many manufacturers, some of which are in the development stage and a few of which are in service. The names of the devices vary somewhat, depending on the manufacturer. The following lists some of the devices:

>> STATIC VAR COMPENSATORS (SVCS): These devices employ fixed banks of capacitors, controlled with thyristors, which can switch them on and off rapidly. In many instances, there are also thyristor-switched inductors to prevent system resonance.

>> THYRISTOR CONTROLLED SERIES COMPENSATORS (OR SERIES CAPACITORS) (TCSCS): A thyristor controlled reactor is placed in parallel with a series capacitor, allowing for a continuous and rapidly variable series compensation system.

>> STATIC COMPENSATORS (STATCOMS): These are gate turn-off type thyristor (GTO)-based SVCs. They are solid-state synchronous voltage generators that consist of a multi-pulsed, voltage-sourced inverter connected in shunt with a transmission line. They do not require capacitor banks and shunt reactors but rely on electronic processing of voltage and current waveforms to provide inductive or capacitive reactive power. They have the added advantage of output that is not seriously impacted by low system voltage.

>> UNIFIED POWER FLOW CONTROLLER (UPFC): This device has a shunt-connected STATCOM with an additional series branch in the transmission line supplied by the STATCOM’s DC circuit. The device is comparable to a phase shifting transformer. It can control all three basic power transfer parameters: voltage, impedance, and phase angle.

>> SVC LIGHT STATCOM: This is based on voltage source converter technology equipped with insulated gate bipolar transistors (IGBTs), which are power switching components. It provides reactive power as well as absorption purely by means of electronic processing of voltage and current waveforms.

SUBSTATIONS

Substations are locations where transmission lines, transformers, and generators are connected. They fulfill a number of functions:

Allow power from different generating stations to be fed into the main transmission corridors.
Provide a terminus for interconnections with other systems.
Provide a location where transformers can be connected to feed power into the sub transmission or distribution systems.
Allow transmission lines to be segmented to provide a degree of redundancy in the transmission paths.
Provide a location where compensation devices such as shunt or series reactors or capacitors can be connected to the transmission system.
Provide a location where transmission lines can be de-energized, either for maintenance or because of an electrical malfunction involving the line.
Provide protection, control, and metering equipment/

PRIMARY COMPONENTS OF AN OVERHEAD TRANSMISSION LINE

The primary components of an overhead transmission line are:

Conductors
Ground or shield wires
Insulators
Support structures
Land or right-of-way (R-O-W)

1) CONDUCTORS: are the wires through which the electricity passes. Transmission wires are usually of the aluminum conductor steel reinforced (ACSR) type, made of stranded aluminum woven around a core of stranded steel that provides structural strength. When there are two or more of these wires per phase, they are called bundled conductors.

2) GROUND OR SHIELD WIRES: are wires strung from the top of one transmission tower to the next, over the transmission line. Their function is to shield the transmission line from lightning strikes.

3) INSULATORS: are made of materials which do not permit the flow of electricity. They are used to attach the energized conductors to the supporting structures, which are grounded. The higher the voltage at which the line operates, the longer the insulator strings. In recent years, polymer insulators have become popular in place of the older, porcelain variety. They have the advantage of not shattering if struck by a projectile.

4) SUPPORT STRUCTURE: The most common form of support structure for transmission lines is a steel lattice tower, although wood H frames (so named because of their shape) and steel poles are also used. In recent years, as concern about the visual impact of these structures has increased, tubular steel towers also have come into use. The primary purpose of the support structure is to maintain the electricity-carrying conductors at a safe distance from the ground and from each other. Higher voltage transmission lines require greater distances between phases and from the conductors to ground than lower voltage lines and, therefore, they require bigger towers. The clearance from ground of the transmission line is usually determined at the midpoint between two successive towers, at the low point of the centenary formed by the line.

5) LAND OR RIGHT-OF-WAY: The land that the tower line transverses is called the right-of way (R-O-W). To maintain adequate clearances, as the transmission voltage increases, R-O-W widths also increase. In areas where it is difficult to obtain R-O-Ws, utilities design their towers to carry multiple circuits. In many areas of the country, it is not uncommon to see a structure supporting two transmission lines and one or more sub transmission or distribution lines. There are different philosophies on the selection of R-O-Ws.

AUTOMATIC GENERATION CONTROLS (AGC)

Disturbances may also change the voltage at the generator’s terminals. In response, the generator’s automatic voltage regulating system will sense the change and adjust the generator’s field excitation, either up or down, to compensate.

Transient stability or instability considers that period immediately after a disturbance, usually before the generator’s governor and other control systems have a chance to operate. In all cases, the disturbance causes the generator angles to change automatically as they adjust to find a new stable operating point with respect to one another. In an unstable case, the angular separation between one generator or group of generators and another group keeps increasing. This type of instability happens so quickly, in a few seconds, that operator corrective action is impossible.

If stable conditions exist, the generator’s speed governor system, sensing the beginning of change in speed, will then react to either admit more mechanical energy into the rotor to regain its speed or to reduce the energy input to reduce the speed. Directives may also be received by the generator from the company or balancing area control center to adjust its scheduled output.

In addition to the measures noted to improve steady-state stability, other design measures available for selected disturbances to mitigate this type instability are:

Increasing the speed by which relays detect the fault and the speed by which circuit breakers operate to disconnect the faulted equipment sooner
Using dynamic braking resistors which, in the event of a fault, are automatically connected to the system near generators to reduce export from the generators
Fast valving systems on turbines allowing rapid reduction in the mechanical energy input to the turbine generator
Automatic generator tripping
Automatic load disconnection using under frequency or under voltage protective systems
Special transmission line tripping schemes

Dynamic instability refers to a condition in which the control systems of generators interact in such a way as to produce oscillations between generators or groups of generators that increase in magnitude and result in instability, that is, there is insufficient damping of the oscillations. These conditions can occur either in normal operation or after a disturbance.

YAW CONTROL AND SPEED CONTROL OF WIND TURBINE

YAW CONTROL

The yaw control continuously orients the rotor in the direction of the wind. It can be as simple as the tail vane, or more complex on modern towers. Theoretical considerations dictate free yaw as much as possible. However, rotating blades with large moments of inertia produce high gyroscopic torque during yaw, often resulting in loud noise. Too rapid yaw may generate noise exceeding the local ordinance limit. Hence, a controlled yaw is often required and is used.

SPEED CONTROL

The wind turbine technology has changed significantly in the last 25 years. Large wind turbines being installed today tend to be of variable speed design, incorporating the pitch control and the power electronics. Small machines on the other hand must have simple, low cost power and speed control. The speed control methods fall into the following categories:

No speed control whatsoever. In this method, the turbine, the electrical generator, and the entire system is designed to withstand the extreme speed under gusty wind.
Yaw and tilt control, in which the rotor axis is shifted out of the wind direction when the wind speed exceeds the design limit.
Pitch control, which changes the pitch of the blade with the changing wind speed to regulate the rotor speed.
Stall control. In this method of speed control, when the wind speed exceeds the safe limit on the system, the blades are shifted into a position such that they stall. The turbine has to be restarted after the gust has gone.

FIGURE 5.7 Speed control methods used in small to medium size turbines.

EFFECTS OF SHORT CIRCUITS ON TRANSFORMERS

Transformers are susceptible to damage by secondary short-circuit currents having magnitudes that can be many times rated load current. The damage results from the following effects:

>> The I²R losses in the winding conductors are increased by the square of the current. This increases the temperature rise of the windings.

>> Because protective devices limit the duration of short circuits (as opposed to overloads), the temperature rise of the winding can be calculated by dividing the total energy released by the I²R losses by the thermal capacity of the conductor.

>> The short-circuit currents exclude flux in the core and increase stray flux around the core. This stray flux induces currents in metallic parts other than the winding conductors, which can be damaged thermally.

>> A short circuit applied to the secondary circuit of an auto-transformer can substantially increase the voltage across the series winding and across the common winding through induction. This not only presents the possibility of damaging the winding insulation by over voltage, but will also drive the core into saturation and significantly increase core losses with potential damaging effects from temperature.

>> Bushings and tap changers have current ratings that are usually only marginally greater than the rated load of the transformer. Since fault currents are many times rated currents and these components have short thermal time constants, they can be seriously overloaded and thermally damaged.

>> Stray flux in the vicinity of current-carrying conductors produces mechanical forces on the conductors. When a short circuit is applied to a transformer, there is a significant increase in stray flux, resulting in greater mechanical forces on the windings, leads, bushings, and all other current-carrying components. These components, especially the windings, must be braced to withstand these forces.

A good transformer design must take all of the above effects into account to minimize the risk of damage and assure a long service life.

NEW VERSUS EXISTING SUBSTATIONS

The design of new substations has the advantage of starting with a blank sheet of paper. The new substation will typically have many IEDs for different functions, and the majority of operational data for the SCADA system will come from these IEDs. The IEDs will be integrated with digital two-way communications. The small amount of direct input/output (hardwired) can be acquired using programmable logic controllers (PLC). Typically, there are no conventional remote terminal units (RTU) in new substations. The RTU functionality is addressed using IEDs and PLCs and an integration network using digital communications.

FIGURE 7.1 SA system functional architecture diagram.

In existing substations there are several alternative approaches, depending on whether the substation has a conventional RTU installed. The utility has three choices for their existing conventional substation RTUs: integrate RTU with IEDs; integrate RTU as another substation IED; and retire RTU and use IEDs and PLCs, as with a new substation. First, many utilities have integrated IEDs with existing conventional RTUs, provided the RTUs support communications with downstream devices and support IED communication protocols. This integration approach works well for the operational data path, but it does not support the nonoperational and remote access data paths. The latter two data paths must be done outside of the conventional RTU. Second, if the utility desires to keep their conventional RTU, the preferred approach is to integrate the RTU in the substation integration architecture as another IED. In this way, the RTU can be easily retired when the RTU hardwired direct input/output transitions to come primarily from the IEDs. Third, the RTUs may be old and difficult to support, and the substation automation project might be a good time to retire these older RTUs. The hardwired direct input/output from these RTUs would then come from the IEDs and PLCs, as with a new substation.

WIND POWER SYSTEM DESIGN FEATURES

The following additional design trade-offs are available to the system engineer:

NUMBER OF BLADES

This is the first determination the design engineer must make. Wind machines have been built with the number of blades ranging from 2 to 40 or more. The high number of blades was used in old low, tip-speed ratio rotors for water pumping, and the application which needs high starting torque.

The modern high, tip-speeds ratio rotors for generating electrical power have two or three blades, many of them with just two. The major factors involved in deciding the number of blades are as follows:

The effect on power coefficient.
The design tip-speeds ratio.
The cost.
The nacelle weight.
The structural dynamics.
The means of limiting yaw rate to reduce gyroscopic fatigue.

Compared to the two-blade design, the three-blade machine has smoother power output and balanced gyroscopic force. There is no need to teeter the rotor, allowing the use of simple rigid hub. Adding the third blade increases the power coefficient only by about 5 percent, thus giving a diminished rate of return for the 50 percent more weight and cost. The two-blade rotor is also simpler to erect, since it can be assembled on the ground and lifted to the shaft without complicated maneuvers during the lift. The number of blades is often viewed as the blade solidity. Higher solidity ratio gives higher starting torque and operates at low speed. For electrical power generation, the turbine must run at high speed since the electrical generator weighs less and operates more efficiently at high speed. That is why all large-scale wind turbines have low solidity ratio, with just two or three blades.

ROTOR UPWIND OR DOWNWIND

Operating the rotor upwind of the tower produces higher power as it eliminates the tower shadow on the blades. This also results in lower noise, lower blade fatigue, and smoother power output. The downwind blades, on the other hand, allow the use of free yaw system. It also allows the blades to deflect away from the tower when loaded. Both types are used at present with no clear trend.

HORIZONTAL AXIS VERSUS VERTICAL AXIS

Most wind turbines built at present have a horizontal axis. The vertical axis Darrieus machine has several advantages. First of all, it is omnidirectional and requires no yaw mechanism to continuously orient itself toward the wind direction. Secondly, its vertical drive shaft simplifies the installation of the gearbox and the electrical generator on the ground, making the structure much simpler. On the negative side, it normally requires guy wires attached to the top for support. This could limit its applications, particularly for the offshore sites. Overall, the vertical axis machine has not been widely used because its output power cannot be easily controlled in high winds simply by changing the blade pitch. With modern low-cost, variable-speed power electronics emerging in the wind power industry, the Darrieus configuration may revive, particularly for large capacity applications.

SPACING OF THE TOWERS

When installing a cluster of machines in a wind farm, certain spacing between the wind towers must be maintained to optimize the power cropping. The spacing depends on the terrain, the wind direction, the speed, and the turbine size. The optimum spacing is found in rows 8 to 12-rotor diameters apart in the wind direction, and 1.5 to 3-rotor diameters apart in the crosswind direction (Figure 5-12). A wind farm consisting of 20 towers rated at 500 kW each need 1 to 2 square kilometers of land area. Of this, only a couple of percent would actually occupy the tower and the access roads. The remaining land could continue its original use (Figure 5-13). The average number of machines in wind farms varies greatly, ranging from several to hundreds depending on the required power capacity.

FIGURE 5-12 Optimum tower spacing in wind farms in flat terrain.

FIGURE 5-13 Original land use continues in a wind farm in.

When the land area is limited or is at a premium price, one optimization study that must be conducted in an early stage of the wind farm design is to determine the number of turbines, their size, and the spacing for extracting the maximum energy from the farm. The trades in such a study are as follows:

Larger turbines cost less per MW capacity and occupy less land area.
Fewer large machines can reduce the MWh energy crop per year, as downtime of one machine would have larger impact on the energy output.
The wind power fluctuations and electrical transients on fewer large machines would cost more in electrical filtering of the power and voltage fluctuations, or would degrade the quality of power, inviting penalty from the grid.

WIND POWER SYSTEM COMPONENTS

The wind power system is comprised of one or more units, operating electrically in parallel, having the following components:

The tower.
The wind turbine with two or three blades.
The yaw mechanism such as the tail vane.
The mechanical gear.
The electrical generator.
The speed sensors and control.

The modern system often has the following additional components:

The power electronics.
The control electronics, usually incorporating a computer.
The battery for improving the load availability in stand-alone mode.
The transmission link connecting to the area grid.

Because of The large moment of inertia of the rotor, the design challenges include the starting, the speed control during the power producing operation, and stopping the turbine when required. The eddy current or other type of brake is used to halt the turbine when needed for emergency or for routine maintenance. In The multiple tower wind farm, each turbine must have its own control system for operational and safety functions from a remote location.

SOCIETAL BENEFITS OF ELECTRICITY