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GROUNDING

Grounding is one of the most important aspects of an electrical distribution system but often the least understood. Your Electrical Code sets out the legal requirements in your jurisdiction for safety standards in electrical installations. For instance, the Code may specify requirements in the following areas:

(a) The protection of life from the danger of electric shock, and property from damage by bonding to ground non-current carrying metal systems;

(b) The limiting of voltage on a circuit when exposed to higher voltages than that for which it is designed;

(c) The limiting of ac circuit voltage-to-ground to a fixed level on interior wiring systems;

(d) Instructions for facilitating the operation of electrical apparatus

(e) Limits to the voltage on a circuit that is exposed to lightning.

In order to serve Code requirements, effective grounding that systematically connects the electrical system and its loads to earth is required.

Connecting to earth provides protection to the electrical system and equipment from superimposed voltages from lightning and contact with higher voltage systems. Limiting over voltage with respect to the earth during system faults and upsets provides for a more predictable and safer electrical system. The earth ground also helps prevent the build-up of potentially dangerous static charge in a facility.

The grounding electrode is most commonly a continuous electrically conductive underground water pipe running from the premises. Where this is not available the Electrical Codes describe other acceptable grounding electrodes.

Grounding resistances as low as reasonably achievable will reduce voltage rise during system upsets and therefore provide improved protection to personnel that may be in the vicinity.

Connection of the electrical distribution system to the grounding electrode occurs at the service entrance. The neutral of the distribution system is connected to ground at the service entrance. The neutral and ground are also connected together at the secondary of transformers in the distribution system. Connection of the neutral and ground wires at any other points in the system, either intentionally or unintentionally, is both unsafe (i.e., it is an Electrical Code violation) and a power quality problem.

EQUIPMENT BONDING

Equipment bonding effectively interconnects all non-current carrying conductive surfaces such as equipment enclosures, raceways and conduits to the system ground. The purpose of equipment bonding is:

1) To minimize voltages on electrical equipment, thus providing protection from shock and electrocution to personnel that may contact the equipment.

2) To provide a low impedance path of ample current-carrying capability to ensure the rapid operation of over-current devices under fault conditions.

If the equipment were properly bonded and grounded the equipment enclosure would present no shock hazard and the ground fault current would effectively operate the over-current device.

TOP 5 POWER QUALITY MYTHS

1) OLD GUIDELINES ARE NOT THE BEST GUIDELINES

Guidelines like the Computer Business Equipment Manufacturers Association Curve (CBEMA, now called the ITIC Curve) and the Federal Information Processing Standards Pub94 (FIPS Pub94) are still frequently cited as being modern power quality guidelines.

The ITIC curve is a generic guideline for characterizing how electronic loads typically respond to power disturbances, while FIPS Pub94 was a standard for powering large mainframe computers.

Contrary to popular belief, the ITIC curve is not used by equipment or power supply designers, and was actually never intended for design purposes. As for the FIPS Pub94, it was last released in 1983, was never revised, and ultimately was withdrawn as a U.S. government standards publication in November 1997. While some of the information in FIPS Pub94 is still relevant, most of it is not and should therefore not be referenced without expert assistance.

2) POWER FACTOR CORRECTION DOES NOT SOLVE ALL POWER QUALITY PROBLEMS

Power factor correction reduces utility demand charges for apparent power (measured as kVA, when it is metered) and lowers magnetizing current to the service entrance. It is not directly related to the solution of power quality problems.

There are however many cases where improperly maintained capacitor banks, old PF correction schemes or poorly designed units have caused significant power quality interactions in buildings.

The best advice for power factor correction is the same as the advice for solving power quality issues; properly understand your problem first. A common solution to power factor problems is to install capacitors; however, the optimum solution can only be found when the root causes for the power factor problems are properly diagnosed. Simply installing capacitors can often magnify problems or introduce new power quality problems to a facility.

Power factor correction is an important part of reducing electrical costs and assisting the utility in providing a more efficient electrical system. If power factor correction is not well designed and maintained, other power quality problems may occur. The electrical system of any facility is not static. Proper monitoring and compatible design will lead to peak efficiency and good power quality.

3) SMALL NEUTRAL TO GROUND VOLTAGES DO NOT INDICATE A POWER QUALITY PROBLEM

Some people confuse the term “common mode noise” with the measurement of a voltage between the neutral and ground wires of their power plug. A small voltage between neutral to ground on a working circuit indicates normal impedance in the wire carrying the neutral current back to the source. In most situations, passive “line isolation” devices and “line conditioners” are not necessary to deal with Neutral to Ground voltages.

4) LOW EARTH RESISTANCE IS NOT MANDATORY FOR ELECTRONIC DEVICES

Many control and measurement device manufacturers recommend independent or isolated grounding rods or systems in order to provide a “low reference earth resistance”. Such recommendations are often contrary to Electrical Codes and do not make operational sense. Bear in mind that a solid connection to earth is not needed for advanced avionics or nautical electronics!

5) UN-INTERRUPT-ABLE POWER SUPPLIES (UPS) DO NOT PROVIDE COMPLETE POWER QUALITY PROTECTION

Not all UPS technologies are the same and not all UPS technologies provide the same level of power quality protection.

In fact, many lower priced UPS systems do not provide any power quality improvement or conditioning at all; they are merely back-up power devices. If you require power quality protection like voltage regulation or surge protection from your UPS, then make sure that the technology is built in to the device.

MAJOR FACTORS CONTRIBUTING TO POWER QUALITY ISSUES

The three major factors contributing to the problems associated with power quality are:

USE OF SENSITIVE ELECTRONIC LOADS

The electric utility system is designed to provide reliable, efficient, bulk power that is suitable for the very large majority of electrical equipment. However, devices like computers and digital controllers have been widely adopted by electrical end-users. Some of these devices can be susceptible to power line disturbances or interactions with other nearby equipment.

THE PROXIMITY OF DISTURBANCE-PRODUCING EQUIPMENT

Higher power loads that produce disturbances – equipment using solid state switching semiconductors, arc furnaces, welders and electric variable speed drives – may cause local power quality problems for sensitive loads.

SOURCE OF SUPPLY

Increasing energy costs, price volatility and electricity related reliability issues are expected to continue for the foreseeable future. Businesses, institutions and consumers are becoming more demanding and expect a more reliable and robust electrical supply, particularly with the installation of diverse electrical devices. Compatibility issues may become more complex as new energy sources and programs, which may be sources of power quality problems, become part of the supply solution. These include distributed generation, renewable energy solutions, and demand response programs Utilities are regulated and responsible for the delivery of energy to the service entrance, i.e., the utility meter. The supply must be within published and approved tolerances as approved by the regulator. Power quality issues on the “customer side of the meter” are the responsibility of the customer. It is important therefore, to understand the source of power quality problems, and then address viable solutions.

POWER QUALITY

The Institute of Electrical and Electronic Engineers (IEEE) defines power quality as: “The concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment.”

Making sure that power and equipment are suitable for each other also means that there must be compatibility between the electrical system and the equipment it powers. There should also be compatibility between devices that share the electrical distribution space. This concept is called Electromagnetic Compatibility (“EMC”) and is defined as: “The ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.”

The best measure of power quality is the ability of electrical equipment to operate in a satisfactory manner, given proper care and maintenance and without adversely affecting the operation of other electrical equipment connected to the system.

Power quality difficulties can produce significant problems in situations that include:

• Important business applications (banking, inventory control, process control)
• Critical industrial processes (programmable process controls, safety systems, monitoring devices)
• Essential public services (paramedics, hospitals, police, air traffic control)

Power quality problems in an electrical system can also quite frequently be indicative of safety issues that may need immediate corrective action. This is especially true in the case of wiring, grounding and bonding errors.

POWER QUALITY

By Engr. Aneel Kumar November 08, 2014

The term power quality (PQ) is generally applied to a wide variety of electromagnetic phenomena occurring within a power system network. The ability of the power systems to deliver undistorted voltage, current and frequency signals is termed as quality of power supply. Unexpected variation of the voltage or current from normal characteristics can damage or shut down the critical electrical equipment designed for specific purpose. Such variations happen in electrical networks with a great frequency due to a competitive environment and continuous change of power supply. In a highly evolved electrical system PQ sensitive demands can be classified as

Digital economy (such as banking, share market and railways),
Continuous process manufacturing industries, and
Fabrication and essential services.

Cost incurred to operate all the above types of loads vary from 3 to120 per kVA per event. This is huge and greatly affects economic operation of power industries. To mitigate PQ issues; customers are also equipped with some back-up instruments apart from grid supply. According to IEEE standard 1159-1995, the PQ disturbances include a wide range of PQ phenomena namely transient (impulsive and oscillatory), short duration variations (interruption, sag and swell), frequency variations, long duration variations (sustained under voltages and sustained over voltages) and steady state variations (harmonics, notch and flicker) with a time scale which ranges from tens of nanoseconds to steady state.

POWER QUALITY MEASURABLE QUANTITIES

By Engr. Aneel Kumar November 08, 2014

VOLTAGE DIP

is a reduction in the RMS voltage in the range of 0.1 to 0.9 pu (retained) for duration greater than hall a mains cycle and less than 1 minute, Often referred to as a ‘sag’, Caused by faults, increased load demand and transitional events such as large motor starting.

VOLTAGE SWELL

is an increase in the RMS voltage in the range of 1.1 to 1.8 pu for a duration greater than half a mains cycle and less than 1 minute, Caused by system faults, load switching and capacitor switching.

TRANSIENT

is an undesirable momentary deviation of the supply voltage or load current. Transients are generally classified into two categories: impulsive and oscillatory.

HARMONICS

are periodic sinusoidal distortions of the supply voltage or load current caused by non-linear loads. Harmonics are measured in integer multiples of the fundamental supply frequency. Using Fourier series analysis the individual frequency components of the distorted waveform can be described in terms of the harmonic order, magnitude and phase of each component.

INTER HARMONICS

Distorted voltage or current wave-forms containing periodic distortions of a sinusoidal nature that are not integer multiples of the fundamental supply frequencies are termed inter harmonics.

FLICKER

is a term used to describe the visual effect of small voltage variations on electrical lighting equipment (particularly tungsten filament lamps). The frequency range of disturbances affecting lighting appliances, which are detectable by the human eye, is 1-30 Hz.

VOLTAGE IMBALANCE

is defined as a deviation in the magnitude and/or phase of one or more of the phases, of a three-phase supply, with respect to the magnitude of the other phases and the normal phase angle (l20⁰).

FREQUENCY DEVIATION

is a variation in frequency from the nominal supply frequency above/below a predetermined level, normally plus minus 0.1 percent.

TRANSIENT INTERRUPTION

is defined as a reduction in the supply voltage, or load current, to a level less than 0.1 pu for a time of not more than 1 minute. Interruptions can be caused by system faults, system equipment failures or control and protection malfunctions. Interruptions are considered to be measurable events coining under the field of ‘quality of supply’.

OUTAGE

is defined as an interruption that has duration lasting in excess of one minute.

BATTERY FAQ

1. What is the “end of useful life”?

The IEEE defines “end of useful life” for a UPS battery as being the point when it can no longer supply 80 percent of its rated capacity in ampere-hours. When your battery reaches 80 percent of its rated capacity, the aging process accelerates and the battery should be replaced.

2. How can I ensure that my UPS batteries are maintained and serviced properly?

With proper maintenance, battery life can be predicted and replacements scheduled without interrupting your operations.

These are IEEE and OEM recommendations for general maintenance:

• Comprehensive maintenance programs with regular inspections
• Re-torque all connections, as required
• Load testing
• Cleaning the battery area, as required

3. Do I have to replace my UPS batteries with the same brand of batteries?

Eaton recommends that if you use brand X and need to replace one or two batteries in the string, you should use the same brand because it will have the same characteristics. If you need to replace the whole string, then you can change brands with fewer risks.

4. Are maintenance-free batteries maintenance free?

Though sealed batteries are sometimes referred to as maintenance-free, they still require scheduled maintenance and service. The term maintenance-free refers to the fact that they don’t require fluid. Preventive maintenance is the key to maximizing your UPS battery service life.

5. What about battery disposal?

It’s imperative that your service technicians adhere to EPA guidelines for the disposal of all UPS batteries. Remember, it’s the owner’s responsibility to make sure these guidelines are followed.

6. Is there any difference between the batteries used by smaller UPSs, from 250 VA to 3 kVA, and the ones used by larger UPSs?

While basic battery technology, and the risks to battery life, remains the same regardless of UPS size, there are some inherent differences between small and large applications. Smaller UPSs typically have only one VRLA battery that supports the load and needs maintenance. As systems get larger, increasing battery capacity to support the load gets more complicated. Larger systems may require multiple strings of batteries, introducing complexity to battery maintenance and support. Individual batteries must be monitored to prevent a single bad battery from taking down an entire string and putting the load at risk. Also, as systems get larger, wet-cell batteries become much more common. The differences in battery maintenance between VRLA and wet-cell batteries discussed earlier apply.

7. Our facility was damaged by a flood and our batteries were partially submerged in water. What should we do?

The first concern in this situation is safety. Containing any contamination is critical to preventing hazards to workers and the environment.

8. My UPS has been in storage for over a year. Are the batteries still good?

As batteries sit unused, with no charging regimen, their battery life will decrease. Due to the self-discharge characteristics of lead-acid batteries, it’s imperative that they’re charged periodically during storage or permanent loss of capacity will occur. To prolong shelf life without charging, store batteries at 10°C (50°F) or less.

9. What is thermal runaway?

Thermal runaway occurs when the heat generated in a lead acid cell exceeds its ability to dissipate it, which can lead to an explosion, especially in sealed cells. The heat generated in the cell may occur without any warning signs and may be caused by overcharging, excessive charging, internal physical damage, internal short circuit or a hot environment.

10. Is it safe to transport sealed batteries?

VRLA batteries marked as “non-spillable” are safe and approved for all transportation methods.

11. What is the difference between hot-swappable and user-replaceable batteries?

Hot-swappable batteries can be changed out while the UPS is running. User-replaceable batteries are usually found in smaller UPSs and require no special tools or training to replace. Batteries can be both hot-swappable and user-replaceable. Please check your user’s guide for details on your UPS batteries.

12. How is battery runtime affected if I reduce the load on the UPS?

The battery runtime will increase if the load is reduced. As a general rule, if you reduce the load by half, you triple the runtime.

13. If I add more batteries to a UPS can I add more load?

Adding more batteries to a UPS can increase the battery runtime to support the load, but it doesn’t increase the UPS capacity. Be sure your UPS is adequately sized for your load, then add batteries to fit your runtime needs.

14. What is the average lifespan of UPS batteries?

The standard lifespan for VRLA batteries is three to five years; for wet-cell batteries it’s up to 20 years. However, expected life can vary greatly due to environmental conditions, number and depth of discharge cycles, and adequate maintenance.

Having a regular schedule of battery maintenance and monitoring will ensure you know when your batteries are reaching their end-of-life.

15. Why are batteries disconnected on small, single-phase UPSs when they’re shipped?

This is so that they’re in compliance with Department of Transportation regulations.

16. If I have the serial number from the Eaton UPS or battery cabinet, can I find out how old the batteries are?

Every Eaton battery has a manufacturer date code that indicates when it was made. The battery or battery cabinet will also feature a sticker for each time the batteries have been recharged while in storage. Stored batteries require charging periodically during storage to avoid loss of capacity. Recharging stored batteries doesn’t affect battery warranty.

17. Will Eaton replace batteries for other manufacturers’ UPSs?

Yes. Eaton batteries works on nearly all other manufacturers’ UPSs. In addition, we have extensive knowledge of Best Power, Deltec, IPM and Exide Electronics models because these product lines were purchased by Eaton.

18. What are the risks associated with a lack of battery maintenance?

The primary risks of improperly maintained batteries are: load loss, fire, property damage and personal injury.

19. Who are the major battery manufacturers?

There are many battery manufacturers, but the major ones are: C&D, Enersys, CSB, Yuasa, Panasonic and GS – to name a few.

20. If I have one bad battery, should I only replace that faulty battery, or replace the entire battery string?

Having one faulty battery doesn’t mean you have to replace the entire battery string, which can be very costly. You can replace the bad battery with a fully charged unit but you also need to test the health of the entire string to the cell level to identify if additional strain from the faulty battery damaged other units.

All it takes is one bad battery to ruin an entire string and bring your systems down during a power outage or other interruption.

There is no precise way to predict battery failure. Continuous battery monitoring and scheduled maintenance are the most effective way to identify bad batteries early enough for spot replacement.

21. Why do batteries fail?

Batteries can fail for a multitude of reasons, but common reasons are:

• High or uneven temperatures

• Inaccurate float charge voltage

• Loose inter-cell links or connections

• Loss of electrolyte due to drying out or damaged case

• Lack of maintenance, aging

22. What is the importance of power density when talking about batteries?

Batteries differ markedly in the number of watts per cell. A higher density battery provides more runtime for the footprint. You may even find you can reach your runtime requirements with fewer battery cabinets, which reduces upfront and lifetime costs of battery preventive maintenance.

23. How is battery performance generally measured?

Batteries are generally rated for 100+ discharges and recharges, but many show a marked decline in charging capacity after as few as 10 discharges. The lower the charge the battery can accept the less runtime it can deliver. Look for batteries with a high-rate design that sustains stable performance for a long service term.

24. When are 10-year design life SVRLA batteries typically replaced in standard UPS applications?

UPS battery life depends on a number of factors, including operating temperature, number and duration of discharges, and if regular preventive maintenance is performed. While it’s theoretically possible for SVRLA batteries to last 10 years under optimum conditions, the industry typically recommends full replacement between years four and five for reliability purposes in UPS applications.

25. How can I determine the age of a VRLA battery?

Batteries shipped prior to December 31, 1999 have a three-digit shipping code with the first digit as the year and the following two as the month in which the battery was shipped from the factory. For example, a code of 910 would be interpreted as 1999, October. Batteries shipped on or after January 1, 2000 have a four-digit shipping code with the first two digits as the year and the following two as the month in which the battery was shipped from the factory. For example, a code of 0010 would be interpreted as 2000, October.

MAINTENANCE OF BATTERIES FOR EXTENDING ITS LIFE

Quantifying the combined effect of the four factors that affect battery life discussed in the previous page is difficult. You need a way to determine when a battery is near the end of its useful life so you can replace it while it still works, before the critical load is left unprotected. The only sure way to determine battery capacity is to perform a battery run-down test. The module is taken off line, connected to a load bank and operated at rated power until the specified runtime elapses or the unit shuts down due to low battery voltage. If battery capacity is less than 80 percent of its rated capacity, the battery should be replaced.

Thermal scanning of battery connections during the battery run-down test identifies loose connections. This test gives you the chance to see the battery during an extended, high-current discharge. Scanning should take place during discharge and recharge cycles.

An effective UPS battery maintenance program must include regular inspections, adjustments and testing, with thorough records kept of all readings. Trained technicians should:

• Inspect batteries and racks for signs of corrosion or leakage.
• Measure and record the float voltage and current of the entire bank.
• Record the voltage and electrolyte density of selected battery cells.
• Check the electrolyte level in each cell.
• Log the ambient temperature.

BATTERY ARRANGEMENT IN UPS

In most UPSs, you don’t use just one cell at a time. They’re normally grouped together serially to form higher voltages, or in parallel to form higher currents. In a serial arrangement, the voltages add up. In a parallel arrangement, the currents add up.

However, batteries are not quite as linear as the two graphics to the right depict. For example, all batteries have a maximum current they can produce; a 500 milliamp-hour battery can’t produce 30,000 milliamps for one second, because there’s no way for its chemical reactions to happen that quickly. It is also important to realize that at higher current levels, batteries can produce a lot of heat, which wastes some of their power.

Like all batteries, UPS batteries are electrochemical devices. A UPS uses a lead-acid storage battery in which the electrodes are grids of lead containing lead oxides that change in composition during charging and discharging, and the electrolyte is dilute sulfuric acid. In other words, they contain components that react with each other to create DC electrical current. These components are:

Electrolyte: The medium that provides the ion transport mechanism between the positive and negative electrodes of a cell, immobilized in VRLA batteries, and in liquid form in flooded-cell batteries

Grid: A perforated or corrugated lead or lead alloy plate used as a conductor and support for the active material

Anode: The terminal where the current flows in

Cathode: The terminal where the current flows out

Valve: (used in VRLA batteries) Used to vent the build-up of gas that goes beyond pre-determined levels

Separator: A device used for the physical separation and electrical isolation of electrodes of opposing polarities

Jar: The container holding the battery components

In Series Connection: Connecting of the positive terminal of a cell/battery to the negative terminal of the next cell/battery increases the voltage of the battery network while keeping the capacity constant. Voltage becomes double, Capacity remains same (ah).

In Parallel Connection: Connecting all the positive or negative poles of several batteries increases the capacity of a battery network while maintaining a constant voltage. Voltage remains same capacity becomes double (ah).

FACTORS AFFECTING BATTERY PERFORMANCE

Batteries have limited life, usually showing a slow degradation of capacity until they reach 80 percent of their initial rating, followed by a comparatively rapid failure. Regardless of how or where a UPS is deployed, and what size it is, there are four primary factors that affect battery life: ambient temperature, battery chemistry, cycling and service.

1) AMBIENT TEMPERATURE

The rated capacity of a battery is based on an ambient temperature of 25°C (77°F). It’s important to realize that any variation from this operating temperature can alter the battery’s performance and shorten its expected life. To help determine battery life in relation to temperature, remember that for every 8.3°C (15°F) average annual temperature above 25°C (77°F), the life of the battery is reduced by 50 percent.

2) BATTERY CHEMISTRY

UPS batteries are electro-chemical devices whose ability to store and deliver power slowly decreases over time. Even if you follow all the guidelines for proper storage, usage and maintenance, batteries still require replacement after a certain period of time.

3) CYCLING

During a utility power failure, a UPS operates on battery power. Once utility power is restored, or a switch to generator power is complete, the battery is recharged for future use. This is called a discharge cycle. At installation, the battery is at 100 percent of rated capacity. Each discharge and subsequent recharge reduces its relative capacity by a small percentage. The length of the discharge cycle determines the reduction in battery capacity. Lead-acid chemistry, like others used in rechargeable batteries, can only undergo a maximum number of discharge/recharge cycles before the chemistry is depleted. Once the chemistry is depleted, the cells fail and the battery must be replaced.

4) MAINTENANCE

Battery service and maintenance are critical to UPS reliability. A gradual decrease in battery life can be monitored and evaluated through voltage checks, load testing or monitoring. Periodic preventive maintenance extends battery string life by preventing loose connections, removing corrosion and identifying bad batteries before they can affect the rest of the string. Even though sealed batteries are sometimes referred to as maintenance-free, they still require scheduled maintenance and service. Maintenance-free simply refers to the fact that they don’t require fluid. Without regular maintenance, your UPS battery may experience heat-generating resistance at the terminals, improper loading, reduced protection and premature failure. With proper maintenance, the end of battery life can be accurately estimated and replacements scheduled without unexpected downtime or loss of backup power.

What can go wrong with batteries?

Plate separation: Due to repeated cycling (charging and discharging), damage during handling and shipping, and overcharging.
Grid corrosion: Due to normal aging, operating in an acidic environment and high temperatures.
Internal short circuit: Due to heat (plates expand causing shorts), separator failure, handling and shipping, and grid corrosion.
External short circuit: Due to human error (shorting terminals) and leaks.
Sulfation of plates: Due to sitting discharged for an extended period, not on charge or being undercharged.
Excessive gassing: Due to often due to high temperatures or overcharging.
Drying out: Due to excessive gassing, high temperatures or overcharging.

HYDRO ELECTRIC POWER STATION

A generating station which utilizes the potential energy of water at a high level for the generation of electrical energy is known as a hydroelectric power station. Hydro-electric power stations are generally located in hilly areas where dams can be built conveniently and large water reservoirs can be obtained. In a hydro-electric power station, water head is created by constructing a dam across a river or lake. From the dam, water is led to a water turbine.

The water turbine captures the energy in the falling water and changes the hydraulic energy (i.e., product of head and flow of water) into mechanical energy at the turbine shaft. The turbine drives the alternator which converts mechanical energy into electrical energy. Hydro-electric power stations are becoming very popular because the reserves of fuels (i.e., coal and oil) are depleting day by day. They have the added importance for flood control, storage of water for irrigation and water for drinking purposes.

ADVANTAGES OF HYDRO ELECTRIC POWER STATION

(i) It requires no fuel as water is used for the generation of electrical energy.
(ii) It is quite neat and clean as no smoke or ash is produced.
(iii) It requires very small running charges because water is the source of energy which is available free of cost.
(iv) It is comparatively simple in construction and requires less maintenance.
(v) It does not require a long starting time like a steam power station. In fact, such plants can be put into service instantly.
(vi) It is robust and has a longer life.
(vii) Such plants serve many purposes. In addition to the generation of electrical energy, they also help in irrigation and controlling floods.
(viii) Although such plants require the attention of highly skilled persons at the time of construction, yet for operation, a few experienced persons may do the job well.

DISADVANTAGES OF HYDRO ELECTRIC POWER STATION

(i) It involves high capital cost due to construction of dam.
(ii) There is uncertainty about the availability of huge amount of water due to dependence on weather conditions.
(iii) Skilled and experienced hands are required to build the plant.
(iv) It requires high cost of transmission lines as the plant is located in hilly areas which are quite away from the consumers.

EQUIPMENT OF STEAM POWER STATION

A modern steam power station is highly complex and has numerous equipment and auxiliaries. However, the most important constituents of a steam power station are:

1. Steam generating equipment
2. Condenser
3. Prime mover
4. Water treatment plant
5. Electrical equipment.

1. STEAM GENERATING EQUIPMENT:

This is an important part of steam power station. It is concerned with the generation of superheated steam and includes such items as boiler, boiler furnace, super heater, economizer, air pre-heater and other heat reclaiming devices.

(I) BOILER: A boiler is closed vessel in which water is converted into steam by utilizing the heat of coal combustion. Steam boilers are broadly classified into the following two types:

(a) Water tube boilers (b) Fire tube boilers

In a water tube boiler, water flows through the tubes and the hot gases of combustion flow over these tubes. On the other hand, in a fire tube boiler, the hot products of combustion pass through the tubes surrounded by water. Water tube boilers have a number of advantages over fire tube boilers viz., require less space, smaller size of tubes and drum, high working pressure due to small drum, less liable to explosion etc. Therefore, the use of water tube boilers has become universal in large capacity steam power stations.

(II) BOILER FURNACE: A boiler furnace is a chamber in which fuel is burnt to liberate the heat energy. In addition, it provides support and enclosure for the combustion equipment i.e., burners.

The boiler furnace walls are made of refractory materials such as fire clay, silica, kaolin etc. These materials have the property to resist change of shape, weight or physical properties at high temperatures. There are following three types of construction of furnace walls:

(a) Plain refractory walls
(b) Hollow refractory walls with an arrangement for air cooling
(c) Water walls.

The plain refractory walls are suitable for small plants where the furnace temperature may not be high. However, in large plants, the furnace temperature is quite high and consequently, the refractory material may get damaged. In such cases, refractory walls are made hollow and air is circulated through hollow space to keep the temperature of the furnace walls low. The recent development is to use water walls. These consist of plain tubes arranged side by side and on the inner face of the refractory walls. The tubes are connected to the upper and lower headers of the boiler. The boiler water is made to circulate through these tubes. The water walls absorb the radiant heat in the furnace which would otherwise heat up the furnace walls.

(III) SUPER HEATER: A super heater is a device which super heats the steam i.e., it raises the temperature of steam above boiling point of water. This increases the overall efficiency of the plant. A super heater consists of a group of tubes made of special alloy steels such as chromium-molybdenum. These tubes are heated by the heat of flue gases during their journey from the furnace to the chimney.

The steam produced in the boiler is led through the super heater where it is superheated by the heat of flue gases. Super heaters are mainly classified into two types according to the system of heat transfer from flue gases to steam viz.

(a) Radiant super heater (b) Convection super heater

The radiant super heater is placed in the furnace between the water walls and receives heat from the burning fuel through radiation process. It has two main disadvantages. Firstly, due to high furnace temperature, it may get overheated and, therefore, requires a careful design. Secondly, the temperature of super heater falls with increase in steam output. Due to these limitations, radiant super heater is not finding favor these days. On the other hand, a convection super heater is placed in the boiler tube bank and receives heat from flue gases entirely through the convection process. It has the advantage that temperature of super heater increases with the increase in steam output. For this reason, this type of super heater is commonly used these days.

(IV) ECONOMIZER: It is a device which heats the feed water on its way to boiler by deriving heat from the flue gases. This results in raising boiler efficiency, saving in fuel and reduced stresses in the boiler due to higher temperature of feed water. An Economizer consists of a large number of closely spaced parallel steel tubes connected by headers of drums. The feed water flows through these tubes and the flue gases flow outside. A part of the heat of flue gases is transferred to feed water, thus raising the temperature of the latter.

(V) AIR PRE-HEATER: Super heaters and Economizers generally cannot fully extract the heat from flue gases. Therefore, pre-heaters are employed which recover some of the heat in the escaping gases. The function of an air pre-heater is to extract heat from the flue gases and give it to the air being supplied to furnace for coal combustion. This raises the furnace temperature and increases the thermal efficiency of the plant. Depending upon the method of transfer of heat from flue gases to air, air pre-heaters are divided into the following two classes:

(a) Recuperative type (b) Regenerative type

The recuperative type air-heater consists of a group of steel tubes. The flue gases are passed through the tubes while the air flows externally to the tubes. Thus heat of flue gases is transferred to air. The regenerative type air pre-heater consists of slowly moving drum made of corrugated metal plates. The flue gases flow continuously on one side of the drum and air on the other side. This action permits the transference of heat of flue gases to the air being supplied to the furnace for coal combustion.

2. Condensers:

A condenser is a device which condenses the steam at the exhaust of turbine. It serves two important functions. Firstly, it creates a very low *pressure at the exhaust of turbine, thus permitting expansion of the steam in the prime mover to a very low pressure. This helps in converting heat energy of steam into mechanical energy in the prime mover. Secondly, the condensed steam can be used as feed water to the boiler. There are two types of condensers, namely:

(i) Jet condenser (ii) Surface condenser

In a jet condenser, cooling water and exhausted steam are mixed together. Therefore, the temperature of cooling water and condensate is the same when leaving the condenser. Advantages of this type of condenser are: low initial cost, less floor area required, less cooling water required and low maintenance charges. However, its disadvantages are: condensate is wasted and high power is required for pumping water.

In a surface condenser, there is no direct contact between cooling water and exhausted steam. It consists of a bank of horizontal tubes enclosed in a cast iron shell. The cooling water flows through the tubes and exhausted steam over the surface of the tubes. The steam gives up its heat to water and is itself condensed. Advantages of this type of condenser are: condensate can be used as feed water, less pumping power required and creation of better vacuum at the turbine exhaust. However, disadvantages of this type of condenser are: high initial cost, requires large floor area and high maintenance charges.

3. PRIME MOVERS:

The prime mover converts steam energy into mechanical energy. There are two types of steam prime movers viz., steam engines and steam turbines. A steam turbine has several advantages over a steam engine as a prime mover viz., high efficiency, simple construction, higher speed, less floor area requirement and low maintenance cost. Therefore, all modern steam power stations employ steam turbines as prime movers.

Steam turbines are generally classified into two types according to the action of steam on moving blades viz.

(i) Impulse turbines (ii) Reactions turbines

In an impulse turbine, the steam expands completely in the stationary nozzles (or fixed blades), the pressure over the moving blades remaining constant. In doing so, the steam attains a high velocity and impinges against the moving blades. This results in the impulsive force on the moving blades which sets the rotor rotating. In a reaction turbine, the steam is partially expanded in the stationary nozzles, the remaining expansion takes place during its flow over the moving blades. The result is that the momentum of the steam causes a reaction force on the moving blades which sets the rotor in motion.

4. WATER TREATMENT PLANT:

Boilers require clean and soft water for longer life and better efficiency. However, the source of boiler feed water is generally a river or lake which may contain suspended and dissolved impurities, dissolved gases etc. Therefore, it is very important that water is first purified and softened by chemical treatment and then delivered to the boiler.

The water from the source of supply is stored in storage tanks. The suspended impurities are removed through sedimentation, coagulation and filtration. Dissolved gases are removed by aeration and de-gasification. The water is then ‘softened’ by removing temporary and permanent hardness through different chemical processes. The pure and soft water thus available is fed to the boiler for steam generation.

5. ELECTRICAL EQUIPMENT:

A modern power station contains numerous electrical equipment. However, the most important items are:

(I) ALTERNATORS: Each alternator is coupled to a steam turbine and converts mechanical energy of the turbine into electrical energy. The alternator may be hydrogen or air cooled. The necessary excitation is provided by means of main and pilot exciters directly coupled to the alternator shaft.

(II) TRANSFORMERS: A generating station has different types of transformers, viz.,

(a) Main step-up transformers which step-up the generation voltage for transmission of power.

(b) Station transformers which are used for general service (e.g., lighting) in the power station.

(c) Auxiliary transformers which supply to individual unit-auxiliaries.

(III) SWITCH GEAR: It houses such equipment which locates the fault on the system and isolates the faulty part from the healthy section. It contains circuit breakers, relays, switches and other control devices.

CHOICE OF SITE FOR STEAM POWER STATIONS

In order to achieve overall economy, the following points should be considered while selecting a site for a steam power station:

(I) SUPPLY OF FUEL:

The steam power station should be located near the coal mines so that transportation cost of fuel is minimum. However, if such a plant is to be installed at a place where coal is not available, then care should be taken that adequate facilities exist for the transportation of coal.

(II) AVAILABILITY OF WATER:

As huge amount of water is required for the condenser, therefore, such a plant should be located at the bank of a river or near a canal to ensure the continuous supply of water.

(III) TRANSPORTATION FACILITIES:

A modern steam power station often requires the transportation of material and machinery. Therefore, adequate transportation facilities must exist i.e., the plant should be well connected to other parts of the country by rail, road, etc.

(IV) COST AND TYPE OF LAND:

The steam power station should be located at a place where land is cheap and further extension, if necessary, is possible. Moreover, the bearing capacity of the ground should be adequate so that heavy equipment could be installed.

(V) NEARNESS TO LOAD CENTERS:

In order to reduce the transmission cost, the plant should be located near the center of the load. This is particularly important if DC supply system is adopted. However, if AC supply system is adopted, this factor becomes relatively less important. It is because AC power can be transmitted at high voltages with consequent reduced transmission cost. Therefore, it is possible to install the plant away from the load centers, provided other conditions are favorable.

(VI) DISTANCE FROM POPULATED AREA:

As huge amount of coal is burnt in a steam power station, therefore, smoke and fumes pollute the surrounding area. This necessitates that the plant should be located at a considerable distance from the populated areas.

Conclusion: It is clear that all the above factors cannot be favorable at one place. However, keeping in view the fact that now-a-days the supply system is AC and more importance is being given to generation than transmission, a site away from the towns may be selected. In particular, a site by river side where sufficient water is available, no pollution of atmosphere occurs and fuel can be transported economically, may perhaps be an ideal choice.

SCHEMATIC ARRANGEMENT OF STEAM POWER STATION

Steam power station simply involves the conversion of heat of coal combustion into electrical energy, it embraces many arrangements for proper working and efficiency. The schematic arrangement of a modern steam power station is shown in Figure. The whole arrangement can be divided into the following stages for the sake of simplicity:

1. Coal and ash handling arrangement
2. Steam generating plant
3. Steam turbine
4. Alternator
5. Feed water
6. Cooling arrangement

1. COAL AND ASH HANDLING PLANT:

The coal is transported to the power station by road or rail and is stored in the coal storage plant. Storage of coal is primarily a matter of protection against coal strikes, failure of transportation system and general coal shortages. From the coal storage plant, coal is delivered to the coal handling plant where it is pulverized (i.e., crushed into small pieces) in order to increase its surface exposure, thus promoting rapid combustion without using large quantity of excess air. The pulverized coal is fed to the boiler by belt conveyors. The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal. The removal of the ash from the boiler furnace is necessary for proper burning of coal.

It is worthwhile to give a passing reference to the amount of coal burnt and ash produced in a modern thermal power station. A 100 MW station operating at 50% load factor may burn about 20,000 tons of coal per month and ash produced may be to the tune of 10% to 15% of coal fired i.e., 2,000 to 3,000 tons. In fact, in a thermal station, about 50% to 60% of the total operating cost consists of fuel purchasing and its handling.

2. STEAM GENERATING PLANT:

The steam generating plant consists of a boiler for the production of steam and other auxiliary equipment for the utilization of flue gases.

(I) BOILER: The heat of combustion of coal in the boiler is utilized to convert water into steam at high temperature and pressure. The flue gases from the boiler make their journey through superheated, economizer, air pre-heater and are finally exhausted to atmosphere through the chimney.

(II) SUPER HEATER: The steam produced in the boiler is wet and is passed through a superheater where it is dried and superheated (i.e., steam temperature increased above that of boiling point of water) by the flue gases on their way to chimney. Superheating provides two principal benefits.

Firstly, the overall efficiency is increased. Secondly, too much condensation in the last stages of turbine (which would cause blade corrosion) is avoided. The superheated steam from the super heater is fed to steam turbine through the main valve.

(III) ECONOMISER: An economizer is essentially a feed water heater and derives heat from the flue gases for this purpose. The feed water is fed to the economizer before supplying to the boiler. The economizer extracts a part of heat of flue gases to increase the feed water temperature.

(IV) AIR PRE-HEATER: An air pre-heater increases the temperature of the air supplied for coal burning by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draught fan and is passed through air preheater before supplying to the boiler furnace. The air preheater extracts heat from flue gases and increases the temperature of air used for coal combustion. The principal benefits of preheating the air are: increased thermal efficiency and increased steam capacity per square meter of boiler surface.

3. STEAM TURBINE:

The dry and superheated steam from the super heater is fed to the steam turbine through main valve. The heat energy of steam when passing over the blades of turbine is converted into mechanical energy. After giving heat energy to the turbine, the steam is exhausted to the condenser which condenses the exhausted steam by means of cold water circulation.

4. ALTERNATOR:

The steam turbine is coupled to an alternator. The alternator converts mechanical energy of turbine into electrical energy. The electrical output from the alternator is delivered to the bus bars through transformer, circuit breakers and isolators.

5. FEED WATER:

The condensate from the condenser is used as feed water to the boiler. Some water may be lost in the cycle which is suitably made up from external source. The feed water on its way to the boiler is heated by water heaters and economizer. This helps in raising the overall efficiency of the plant.

6. COOLING ARRANGEMENT:

In order to improve the efficiency of the plant, the steam exhausted from the turbine is condensed by means of a condenser. Water is drawn from a natural source of supply such as a river, canal or lake and is circulated through the condenser. The circulating water takes up the heat of the exhausted steam and itself becomes hot. This hot water coming out from the condenser is discharged at a suitable location down the river. In case the availability of water from the source of supply is not assured throughout the year, cooling towers are used. During the scarcity of water in the river, hot water from the condenser is passed on to the cooling towers where it is cooled. The cold water from the cooling tower is reused in the condenser.

STEAM POWER STATION OR THERMAL POWER STATION

A generating station which converts heat energy of coal combustion into electrical energy is known as a steam power station. A steam power station basically works on the Rankine cycle. Steam is produced in the boiler by utilizing the heat of coal combustion. The steam is then expanded in the prime mover (i.e., steam turbine) and is condensed in a condenser to be fed into the boiler again. The steam turbine drives the alternator which converts mechanical energy of the turbine into electrical energy. This type of power station is suitable where coal and water are available in abundance and a large amount of electric power is to be generated.

ADVANTAGES of STEAM POWER STATION

(i) The fuel (i.e., coal) used is quite cheap.
(ii) Less initial cost as compared to other generating stations.
(iii) It can be installed at any place irrespective of the existence of coal. The coal can be transported to the site of the plant by rail or road.
(iv) It requires less space as compared to the hydroelectric power station.
(v) The cost of generation is lesser than that of the diesel power station.

DISADVANTAGES of STEAM POWER STATION

(i) It pollutes the atmosphere due to the production of large amount of smoke and fumes.
(ii) It is costlier in running cost as compared to hydroelectric plant.

GENERATING STATIONS