Project1manu Training

7/31/2019 Project1manu Training

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Acknowledgement

The motive and objective behind the project would remain unfulfilled

without the mentioning of those who constantly provided us help and support

during the course of the project. I would sincerely like to thank the professionals of

NTPC who constantly provided us help and extended their hand whenever i

needed. I would sincerely like to thank Mr Abhijit Kumar, of the switchyard

department, who provided us with all the details and information required to

accomplish this. In the end, I would like to thank everyone of the NTPC family, for

their undue support and help.


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NTPC SINGRAULI

NTPC Limited, the largest thermal power generating company in India, was

incepted in year 1975. It is a public sector company wholly owned by Government of

India (GOI). In a span of 30 years, NTPC has emerged as a major power company of

international repute and standard. NTPCs core business includes engineering,

construction and operation of power generating stations and providing consultancy

to power utilities as well. NTPC Singrauli is a thermal power station, situated in

Singrauli(Uttar Pradesh). It is a super thermal power station which has a production

capacity of 2000 MW, which are produced by 7 units, functioning together. The 7

units comprise of 5 units of 200 MW each, and 2 units of 500 MW each. Hence, an

overall production of 2000 MW is achieved. The thermal power station plays an

important role in the lives of the people staying in and around the city. The entire

mechanism of electricity production is eco friendly and cheap, thus making it one of

the most important and the leading power producers of the country.


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Contents

1. Basic cycles of a thermal power plant.

1.1 Coal to steam cycle

1.2 Steam to mechanical power cycle

1.3 Mechanical power to electricity

2. Basic departments of NTPC Singrauli.

2.1 Coal handling plant

2.2 Demineralisation plant

2.3 Boiler maintenance and turbines

2.4 The switchyard

2.4.1 The circuit breaker

2.4.2Transformer

2.4.3The current transformer

2.4.4The voltage transformer

2.4.5Lightning arresters

2.4.6Isolators

2.4.7Shunt reactors

2.4.8Electrical bus

2.4.9Earth switch

2.4.10Fire fighting equipment

2.4.11Control relay panels

2.4.12Supporting structures for hanging buses

3. Bibliography.


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1. BASIC CYCLES OF A THERMAL POWER PLANT:

1.1) Coal to steam cycle:

Coal from the coal wagons is unloaded in the coal handling plant. This coal is

transported up to the raw coal bunkers, with the help of belt conveyers. Coal is

transported to bowl mills by the coal feeders. The coal is pulverised in the bowl mill,

where it is ground to a powder form. This crushed coal is taken away to the furnace

through coal pipes, with the help of hot and cold air mixtures from the primary air

fan (PA). The PA fan takes atmospheric air, a part of which is sent to the air pre

heaters, for heating, while a part goes directly to the mill, for temperature control.

Atmospheric air from forced draft ( FD) fan, is heated in the air heaters and sent to

the furnace as combustion air.

Water from the boiler feed pump passes through the economiser and reaches the

boiler drum. Water from the drum passes through the down comers and goes to the

water ring header. Water from the bottom ring header is divided to all the four sides

of the furnace. Due to heat and the density difference, the water rises up in the water

wall tubes. Water is partly converted into steam, as it rises up in the furnace. This

steam and water mixture is again taken to the boiler drum, where the steam isseparated from water. Water follows the same path, while the steam is sent to the

super heaters for superheating. The super heaters are located inside the furnace, and

the steam is superheated (540 degrees) and finally goes to the turbines.

Flue gases from the furnace are extracted from the induced draft (ID) fan, which

maintains a balanced draft in the furnace with FD fan.

1.2) Steam to mechanical power cycle:

From the boiler, a steam pipe conveys steam to the turbine through a stop valve

(which can be used to shut off steam in an emergency) and through control valves,

that automatically regulate the supply of steam to the turbine. Stop valves and

control valves are located in the steam chest and a governor, driven from the main

turbine shaft, operates the control valves to regulate the amount of steam used( this

depends upon the speed of the turbine and the amount of electricity required from

the generator).


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Steam from the control valves enters the high pressure cylinder of the turbine, where

it passes through a ring of stationary blades, fixed to the cylindrical walls. These act

as nozzles and direct the steam into a second ring of moving blades mounted on a

disc, secured to a turbine shaft. This second ring turns the shaft as a result of the

force of the steam. The stationary and moving blades together constitute a stage of

the turbine. The steam passes through each stage in turn until it reaches the end of

the high pressure cylinder and in its passage; some of its heat energy is changed into

mechanical energy. The steam leaving the high pressure cylinder goes back to the

boiler for reheating and returns by a further pipe to the intermediate pressure

cylinder. Here it passes through another series of stationary and moving blades.

1.3) Mechanical power to electricity:

The turbine shaft is mechanically coupled to the generator rotor shaft through thrust

bearings. The steam rotates the turbine at 3000 rpm thus the rotor of the generator

also rotates at 3000 rpm. This speed is necessary to generate electricity at a frequency

of 50 Hz with a two pole turbo- generator.

The rotor carries the field winding over it. This field winding is excited by a DC

excitation system. The supply to the excitation system is tapped from the unit

auxiliary transformer. The flux generated by this field current cuts the armature coil.

The armature coil is star- star connected and is induced with three phase emf. The

emf is tapped with the help of slip rings and brushes. This emf is carried over to the

generator transformer through a bus duct. The bus duct is voltage transformer

grounded.

The generator transformer has delta connection in the primary side and star

connection in the secondary side. The generator bus supplies electric power per

phase to the three-phase transformer or bank of three single-phase transformers.

These transformers transmit electric power to the switchyard for further

transmission. These transformers also supply the unit auxiliary transformersrequired for the working of various electric motors, pumps and other equipments


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installed in the unit.

2. BASIC DEPARTMENTS OF NTPC SINGRAULI:

The thermal power plant at Singrauli can be categorised according to the methods

involved and the requirements. The essence of a thermal power plant is to generate

power, at a continuous basis, without disturbing the eco friendly balance of the

nature. Hence the basic departments can be defined as follows:

1. CHP( Coal handling plant)2. De-mineralisation plant3. Boilers maintenance and turbines4. Switchyard

The sections aforementioned play an important role in the overall power production.

The same would now be dealt with, in detail and the importance of the same would

be appreciated through detailed analysis.

2.1 COAL HANDLING PLANT:

The coal handling plant is another important section of any thermal power plant. As

we all know, that coal is an important constituent for the production of power, this


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section helps and performs the prerequisites necessary before the coal is fed in the

plant. This is a complex process, which involves sorting, crushing the coal, and

storing it for use. The coal needs to be crushed, to particles of diameter 20 mm, and

then it is fed to the boiler as and when necessary. The same is done using crushers.

Coal is sent up to the crushers using the conveyor belts, and then the same

operations are carried out. The coal handling plant faces various difficulties during

the rainy season, when it becomes difficult to handle the coal particles, because the

same tend to coagulate.

2.2 DEMINERALISATION PLANT:

The de mineralisation plant is also one of the most important sections of a power

plant. The basic need of the same arises when we have to inject water in the boiler,

for its conversion to steam. As we know, that the steam thus produced is used for

driving the turbine blades, the same water should be devoid of hardness, and other

minerals which, if not removed, may lead to the corrosion and pitting of the turbine

blades, which is detrimental for the turbines, and hence for the plant. Therefore, theneed to supply de mineralised water, hence arises. The DM plant is meant to fulfil

this purpose. Its basic function is to purify the water and to demineralise it using

various methods, which involve addition of alum and resins. The DM water thus

produced is stored in huge storage tanks and is supplied to the boilers continuously.

2.3 BOILER MAINTENANCE AND TURBINES:

2.3.1Boiler:

2.3.1(i)What is a boiler?

A boiler is used for the production of steam, from water, where the heat required is

obtained by the combustion of coal.

2.3.1(ii)Types of Boilers:


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Boilers are classified as water tube and fire tube boilers. Fire tube boilers are

generally not used in the power plants and it is the former, water tube boilers, which

are mostly put to use.

The basic functioning of a boiler can be understood as follows. The boilers are fed

with coal( in the crushed form, which is done using crushers, at the CHP or the coal

handling plant), and the same is burnt. The calorific value of coal is very high, and

hence, a large amount of heat is generated. Along with this, de mineralised water is

fed, which comes from the DM( de mineralising plant). The same is thus heated

using the heat produced by the combustion of coal. This leads to the formation of

steam, which is then heated, using super heaters, to form superheated steam. This

completes the basic working of a boiler.

Boiler auxiliaries:

Boiler auxiliaries are various equipments, which aid in the working of the boiler.

They can be categorised as follows:

1) Coal bunker2) Feeder3) Mill4) P A fan5) Air Preheater6) Burner7) F D fan8) Wind box9) Scanner fan10) Ignitor fan11) ESP

2.3.2 TURBINES AND THEIR FUNCTIONS:

2.3.2(i)What is a turbine?

A steam turbine is a mechanical device that extracts the thermal energy from

pressurized steam, and converts it into useful mechanical work.

2.3.2(ii)Types of steam turbines:
http://en.wikipedia.org/wiki/Thermal_energyhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Thermal_energy


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There are basically two types of turbines. They are:

Impulse turbines:An impulse turbine uses the impact force of the steam jet on the blades to turn the

shaft. Steam expands as it passes through the nozzles, where its pressure drops and

its velocity increases. As the steam flows through the moving blades, its pressure

remains the same, but its velocity decreases. The steam does not expand as it flows

through the moving blades.

Reaction turbines:In the reaction turbine, the rotor blades themselves are arranged to form convergent

nozzles. This type of turbine makes use of the reaction force produced as the steam

accelerates through the nozzles formed by the rotor.

Schematic diagram of the steam turbine

2.4 THE SWITCHYARD:

The next feature of a power plant is referred to as the switchyard. The voltage thus

produced, must be stepped up to a value of 400 kV. This is necessary because the

same has to be transmitted over to transmission lines for long distances. Thus if the

voltage is stepped up, the transmission losses can be reduced to a great extent.

Again, various faults needs to be detected, so that the faulty current thus produced


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does not hamper the working of the instruments. Again, adequate protection

schemes for various electrical instruments, like the transformer and generator are

required. Again, the frequency of the working instruments should stay constant at 50

Hz. Thus the switchyard comprises of basic electrical instruments, which take care of

the aforementioned points, and hence increases the efficiency of the system.

Components of the switchyard:

The switchyard basically comprises of the following electrical equipments, which, in

an overall manner, help improve the efficiency of the system. They are as follows:

1. Circuit breakers

2.Transformer

3. Current transformers

4. Capacitor voltage transfer bus

5. Lightning arresters

6. Isolators

7. Electrical bus

8. Wave trap

9. Interconnecting transformers


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10. Shunt reactors

11.Supporting structures for the hanging buses

12.Earth switch

13.Control Room and control relay panel

14.Fire fighting equipments

15.Power cables & control cables

2.4.1 THE CIRCUIT BREAKER:

2.4.1(i)What is a circuit breaker?

A circuit breaker can either make or break a circuit either automatically or manually,

like no load, full load and short circuit conditions. The characteristic feature of a

circuit breaker has made it a very useful electrical device in electrical power system.

2.4.1(ii)Types of circuit breakers:

Circuit breakers are classified under the following heads:

(a)Oil circuit breakers(b)Air blast circuit breakers(c)Sulphur hexafluoride circuit breakers(d)Vacuum circuit breakers

The mechanism and working principle of all the circuit breakers vary from each

other, but their basic objective is the same. Each type of circuit breaker has its

advantage and disadvantage. The same would now be discussed in detail.

(a)Oil circuit breakers:

As the name suggests, the principle arc quenching medium in this case is oil.

Whenever the fixed contacts are separated from the moving contacts, an arc is struck

in between the contacts, which cannot be allowed in a circuit. The same needs to be

extinguished. Therefore, as soon as the arc is struck, the arcing medium is

completely surrounded by oil, and the heat of the arc, thus produced helps in

producing hydrogen gas, which helps in quenching the arc.

Advantages of oil circuit breaker:Oil circuit breakers have the following advantages:


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1. The hydrogen gas thus produced has excellent cooling properties. Hence itcools the arc.

2. The volume of hydrogen gas is about 1000 times, that of air. Thus it causesturbulence in the arcing region, thus eliminating the arcing products.

Disadvantages of oil circuit breakers:The oil circuit breakers also have a few limitations. They are:

1. The use of oil as the arc quenching medium increases the risk of fire.2. Reuse of the oil leads to carbonisation and degradation of the oil.

(b)Air blast circuit breakers:

The air blast circuit breakers are another category of circuit breakers which use air as

the arc quenching medium. They, in a similar manner, remove the arcing productsfrom the arcing region, thus extinguishing the arc.

Advantages of air blast circuit breakers:1. The risk of fire, as in the oil circuit breakers is removed.2. The problem of carbonisation is also removed, because reuse of air doesnt

degrade its quality.

Disadvantages of air blast circuit breakers:1. Air blast circuit breakers have inferior arc quenching properties.2. Regular maintenance of the compressor plant is required, which supplies the

air blast.

(c)Sulphur hexa-fluoride (SF6) circuit breakers:

The Sulphur hexafluoride circuit breakers are the most widely used circuit breakers.

They involve the use of sulphur hexafluoride gas, which acts as an arc quenching

medium.

Advantages of the sulphur hexafluoride circuit breakers:1. Due to their superior arc quenching properties, they have very low arcing

time.

2. Since the dielectric strength of the gas is very high, they can interrupt highvalues of current.

Disadvantages of sulphur hexafluoride circuit breakers:


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1. Sulphur hexafluoride gas is expensive.2. Regular conditioning of the gas is required, especially after use.

2.4.2TRANSFORMER

First transformer was designed by the turn of nineteenth century. The capacity was

200 KW. Over the years, transformer developments have been greatly based on

advances in the core material-winding insulation. Tress boards di-electric medium.

Two decisive innovations during the first decades of twenty century, were the

invention of silicon electrical steel & adoption of material insulating oil for insulation

& cooling. This has resulted in steady increase in transformers rating and higher

transmission voltages.

During the early periods of twentieth century transformer of 10MVA rating was too

large for three-phase application & three single-phase transformers were used

during that time the transmission voltages increased to 70kV transformer bushings

has created several problems for this voltage.

The shortage of raw material during the First World War impaired technical

developments.

The basic design principles for transformers were fixed during 1920s & these are

still being followed.

The period between 1930-1980 was the emergence of voltage of 132 KV & 220 KV &

the largest transformer was 120MVA for 220kV systems. In the last 50s, ASEA has

supplied 400kV 1000MVA transformer for single-phase construction to the Swedish

state power board.

Experience of ASEAs 400kV transformer was subsequently used in the United

States & Canada. The transformer for Canadas 735kV system was supplied by

ASEA in 1960s.

2.4.2(i)TRANSFORMER FAULTS:

The protection package used should disconnect the device during an emergency

situation, as quickly as possible. Small, low rating transformers can be easily


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protected with the help of fuses, but high rating transformers need special protection

schemes:

Classification of transformer faults:The transformer faults can be classified as:

(a) Winding and terminal faults.(b) Core faults

(c) Tank and transformer accessory faults.

(d) On load tap changer faults.

(e) Abnormal operating conditions.

(f) Sustained or un-cleared external faults.

(a)Winding faults:Winding faults can be controlled using the following factors:

1) Source impedance.

2) Neutral earthing impedance.

3) Transformer leakage reactance.

4) Fault voltage.

5) Winding connections.

Faults in star connected windings:1) In star connected windings with earthed neutral

a) In this case, the fault current depends on the earthing resistance. The fault also

depends on its distance from the neutral. The fault voltage is directly proportional to

this distance.

b) For faults occurring in transformer secondary winding, the corresponding

primary current will depend on the transformation ratio.

2) Star connected windings with neutral point solidly earthed:

a) In this case, it is the leakage reactance, which controls the fault current. For faults

very close to the neutral end of the winding, the leakage reactance is low and hence

results in higher fault currents.


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Faults in delta connected windings:No part of a delta connected winding operates with a voltage to earth less than 50%

of the phase voltage. Hence the fault current may not be more than the rated current,

or may be even less.

Phase to phase faults:Phase to phase faults are very rare and if they occur, they lead to a very high fault

current, which is comparable to the earth fault current.

Inter-turn faults:Inter-turn faults occur because of lightning and surge voltages, whose magnitude

may be several times of the rated system voltage.

(b)Core faults

The eddy currents produced in a laminated core may lead to excessive overheating.

If the temperature becomes excessive, it may damage the windings.

(c)Tank faults:

If loss of oil occurs from the transformer oil tank, it leads to increased overheating,

because of decreased cooling. Overheating may also occur because of prolonged

overloading or blocking of cooling ducts.

(d)Abnormal faults:

Some abnormal faults in a system include the following:


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1) Overloads:

Overloads lead to increased copper loses and temperature rise. They can be carried

on for limited periods only.

2) System faults:

System faults also lead to increase in the rate of heating of the transformers. They

also lead to the increase in the copper loss.

3) Overvoltages:

Overvoltage conditions are of two types:

a) Transient surge voltages:

They are produced because of switching and lightning disturbances. They can be

prevented using surge diverters, which extinguishes the flow of power current after

discharging the surge. Hence the isolation of the transformer can be avoided.

b) Power frequency overvoltages:

This leads to increase in insulation stresses and an increase in the working flux. The

increase in the flux leads to an increase in the magnetizing current. The flux may

thus be diverted to the structural steel parts, which leads to temperature rise and canhence destroy the insulation.

4) Reduced system frequency:

A reduced system frequency will lead to an increase in the flux density, which will

have similar consequences as illustrated in the previous paragraph.

2.4.2(ii)PROTECTION SCHEMES FOR TRANSFORMERS:

The protection for transformers can be achieved according to the faults. They can be

categorized as under:

Fault type: Protection used:

Primary winding(phase phase) Differential (overcurrent)

Primary winding(phase-earth) Differential (overcurrent)

Secondary winding(phase-phase) Differential

Secondary winding(phase- earth) Differential(restricted earth fault)


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Inter turn fault Differential buckholz

Core fault Differential buckholz

Tank fault Differential buckholz

Overfluxing Overfluxing

Overheating Thermal

Transformer overcurrent protection:1) Fuses:

Fuses can be used as protection devices for low KVA or 1 MVA distribution

transformers. But they fail to work at higher ratings. In fact there are fuses which

really work well with high currents, but fail to stop currents which are less than the

rated currents. Thus fuses serve only limited purposes.

2) Over-current relays:

They are used for the protection of distribution transformers. They are also used on

large transformers having standard circuit breaker control. It serves two advantages:

a) The time delay characteristics feature of a fuse is removed, because the relay

works instantaneously.

b) An earth fault tripping element is provided in addition to the relay.

3) Restricted earth fault protection:

The over-current relays with earth fault protection fail to provide adequate

protection for transformer windings. This is highly improved using the restricted

earth fault protection.

4) Differential Protection:

A differential system can be arranged to cover the complete transformer; this is

possible because of the high efficiency of transformer operation, and the close

equivalence of ampere-turns developed on the primary and secondary windings.

Basic Considerations for Transformer Differential ProtectionIn applying the principles of differential protection to transformers, a variety of

considerations have to be taken into account. These include:


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1. Correction for possible phase shift across the transformer windings (phasecorrection)

2. The effects of the variety of earthing and winding arrangements (filtering ofzero sequence currents)

3. Correction for possible unbalance of signals from current transformers oneither side of the windings (ratio correction)

4. The effect of magnetising inrush during initial energization.5. The possible occurrence of overfluxing.

2.4.2.(iii)MAINTAINENCE & PROTECTION OF TRANSFORMERS

o INTRODUCTIONGenerating station generates ac power at high voltages (usually at 15.75kV or 21 kV).

The power so generated is then transmitted at 400 kV to reduce the amount of

conducting material and reduce the transmission losses. However the distribution of

power should be carried out at low voltage (usually 440 V). Hence generated power

is transformed twice, thrice or more before it is utilized. Such AC transformation is

carried out by transformer. The transformer is a static electrical machine by means of

which electric power in one circuit is transformed to electric power of the same

frequency in another circuit.

In all power generating plants, transformers are one of the most critical equipment

which are being used at different stages depending on requirement. A power

transformer in a generating-station is not only one of the costliest equipment but

is also one of the most important links of the power system. If the power

transformer is required to give a trouble free service it should receive proper

attention for its protection and maintenance.

o MAINTENANCE OF TRANSFORMERS


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As compared to most electrical equipments a transformer requires relatively less

attention. Maintenance consists of regular inspections testing and reconditioning

whenever necessary. The principal object of maintenance is to maintain the

insulation in good condition.

o MAINTENANCE PRACTICES:Preventive maintenance of transformer: Preventive maintenance is the check and

maintenance of the equipment at pre-determined intervals to find out, if any

abnormality is about to occur which is rectified to prevent any breakdown.

Breakdown maintenance of transformer: The maintenance which is required to be

carried out on transformer after causing breakdown. For such type of maintenance

the transformer must be isolated from the supply and the system to which it is

connected.

Before carrying out any maintenance work ensure proper safety procedures as per

utility practice and ensure the following:

a) Obtain a Permit To Work (PTW) / Restoration of Motive Power(ROMP) / sanction for carrying out tests

b) The transformer and the associated equipment should be taken,

out of service/ isolated and properly earthed.

Following are the factors affecting the life of transformer:

1. Moisture-Due to higher affinity of water, the transformer oil and the insulation paper absorb

moisture from the air, in turn results into decrease of dielectric strength. Therefore

blocking of all openings for free access of air in storage and frequent reactivation of

breather is a must.

2. Oxygen-


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Oxygen may be present inside the transformer oil due to air pockets trapped in the

windings. The oxygen reacts on the cellulose of insulation and decomposes it, which

result in sludge formation, blockage of free circulation of oil.

3. Solid-Impurities-

Dielectric strength of oil diminishes appreciably by moisture quantities and by solidimpurities. It is, therefore a good practice to filter the oil after it has been in service

for a reasonable time.

General maintenance, which is normally required to be done on transformers, is as

under:

(a) Physical Inspection:

Transformer tank cover and other parts should be inspected periodically for any oil

leakages, peeling of paint or rust formation. Rusted portion should be properly

cleaned and painted. Clamping, bolts on gasket joints should be tightened properly

and if required necessary gaskets should be replaced. Welding should stop leaks

through welded joints.

(b) Core & Winding:

It is recommended that the core and windings should be removed from the tank for

visual inspection as per time schedule. The windings should be examined to ensure

that no sludge has been deposited blocking the oil ducts.

(c) Tap Changer:

Off load tap changers and on load tap changers are an integral part of the

transformer. Care should be taken to operate the switch, unlock the operating handle

by removing the locking, strip/pin move the handle to the required position and re-

lock.

(d) Conservator & Oil Level Indicator:


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Oil level should be maintained at filling level mark. Oil gauge glass should be kept

clean so that the level of oil is clearly visible. The mechanism of the float type oil

gauge should be inspected and the Float should be checked for presence of oil. The

function of alarm and trip contacts should also to be checked.

(e) Silica Gel and Dehydrating Breather:

The frequency of Breather inspection depends on local climate and operating

conditions. More frequent inspections are needed when the climate is humid and

when transformer is subjected to fluctuating load. It is recommended to replace the

silica gel when half to two third of it has saturated and become pink in color. Failure

to comply this will result in decreasing the drying efficiency of the breather.

(f) Buchholz Relay:


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Routine operation and mechanical inspection tests should be carried out at one or

two yearly intervals respectively. The operations tested by injecting air through

lower level petcock of a double float Buchholz relay. After inspection any air which

has accumulated in the gas chamber must be released at the upper level petcock

leaving the chamber full of oil.

The use of gas operated relay as protection for oil-immersed transformers is based

on the fact that faults as flashover, short-circuit and local overheating normally

result in gas-generation. The gas-bubbles gathering in the gas-operated relay affect a

flat-controlled contact that gives an alarm signal.

(g) Explosion Vent:

The diaphragm which is fitted at the exposed end of the vent should be inspected atfrequent intervals and replaced, if damaged. Failure to replace the diaphragm

quickly may allow the ingress of moisture in the transformer. Whenever bottom

diaphragm ruptures, oil rises inside the explosion vent pipe and is visible in the level

indicator on explosion vent.


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(h) Temperature Indicators:

The level of oil in the pockets holding the thermometer bulbs should be checked and

the oil should be replenished, if required. Temperature indicators found reading

incorrect should be calibrated with standard thermometer immersed in hot water

bath.

(i) Bushing:

Porcelain insulators and connectors should be cleaned at convenient intervals and

minutely examined for any cracks or other defects.

(j) External Connections including Earthing:

All electrical connections should be reasonably tight. It they appear blanked or

corroded, unbolt the connection and clean down to the bright metal finish with

emery paper, remake the connections and give a heavy coating of grease. It is

particularly important that heavy current carrying connections should be properly

maintained.

(k) Connectors:

To avoid prohibited temperature rise in the electrical connection of the transformer,

all screw joints should be checked and retightened. Use of thermo vision camera

should be made for locating any hot-spots in the joints.

(l) Cooling System:


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Cleaning is normally done by water flushing at high pressure.

(m) Drying Out of Transformers:

Since the transformer winding is highly hygroscopic and has a tendency to absorb

moisture from atmosphere when exposed for a long time, (during erection or repair)

the drying out of transformer to remove moisture before putting into service is

essential. There are many methods of dry out being adopted by different agencies.

Some of them are :

Hot oil circulation with high vacuum filter machine.

Vacuum and nitrogen cycle with additional heating arrangement.

Induction heating method.

Short circuit method.

Hot oil spray under vacuum.

(n) Dissolved Gas Analysis (DGA):

This has been considered as one of the important tools by all the power utilities for

detecting any incipient fault in the transformers. Any abnormal or electrical stress in

the transformers causes decomposition of the oil and / or paper insulation, thereby

producing certain gases. These gases come out and get collected in the Buchholz

relay when the quantity is more. However, these gases dissolved in the oil if the

quantity is less. The composition and the quantity of gases generated are dependent

on the severity of the fault. As such regular monitoring of these gases gives usefulinformation about the healthiness of the transformers and prior information about

the type of fault can be had by observing the trend of the various gas content. The

gases which are of interest are hydrogen, methane, ethane, ethylene, acetylene,

carbon mono-oxide, carbon di-oxide, nitrogen and oxygen. The equipment used for

determining the content of these gases in oil is vacuum gas extraction apparatus and

Gas Chromatograph.

2.4.3THE CURRENT TRANSFORMER:


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These transformers are used with low range ammeters to measure currents in high

voltage alternating current circuits. In addition to insulating the instrument from the

high voltage line, they step-down the current in a known ratio. The current or series

transformer has a primary coil of one or more turns of thick wire connected in series

with the line whose current is to be measured. The secondary consists of large

number of turns of line wire and connected across the ammeter terminals. It should

be noted that, since the ammeter resistance is very low, the current transformer

normally works short-circuited. If for any reason the ammeter is taken out of the

secondary winding, then this winding must be short-circuited. If this is not done,

then due to the absence of counter amp-turns of the secondary, the unopposed

primary m.m.f. will set up an abnormally high flux in the core, which will produce

excessive core loss with subsequent heating and a high voltage across the secondary

terminals. Hence the secondary of a current transformer should never be left open

under any circumstances.

2.4.4THE VOLTAGE TRANSFORMER:

The voltage transformer performs its task, similar to a current transformer. But

unlike the former, it is used to step down the voltage. The voltage transformer is also

used for the calibration purpose, and the same is also employed for the protection of

various devices.

These transformers are extremely accurate-ratio step-down transformers and are

used in conjunction with standard low-range voltmeters whose deflection when

divided by transformation ratio, gives the true voltage on the high-voltage side. Ingeneral, they are of the shell-type and do not differ much from the ordinary two-

winding transformers. For safety, the secondary should be completely insulated

from the high-voltage primary and should be, in addition, grounded for affording

protection to the operator.

Capacitor voltage transformers:Voltage transformers can work effectively and properly up to a voltage of 33 kV.

But above that voltage, insulation protection becomes a major factor of


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consideration, and therefore we apply a capacitor, as shown below. The arrangement

thus illustrated is referred to as a capacitor voltage transformer (CVT). The same

circuitry basically helps to step down the secondary voltage to a great extent, hence

keeping the insulation level to a considerable safe level.

2.4.5LIGHTNING ARRESTERS:

Electrical equipments, which are situated in the switchyard, are open to the

atmosphere, and hence may face situations of high voltages (also referred to as surge

voltages), or lightning. This leads to a major problem, because the magnitude andintensity of the same is about 4-5 times the normal voltage, and hence is detrimental

for the electrical circuitry situated there. Therefore, in order to prevent that, we have

electrical equipments, referred to as lightning arresters, which are situated in the

switchyard. Whenever such a dangerous situation arises, the same current is forced

to flow through the lightning arresters and hence it goes to the ground. Hence, the

lightning arresters play an important role in the protection of the electrical

equipments, actually situated in the switchyard. They actually provide the least

resistance path during a situation of enormous voltage flow.


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2.4.6ISOLATORS:

The isolators serve a similar function as the circuit breaker. The only difference is

that a circuit breaker is used to interrupt the fault current when the circuit is on load,

or line-line. But the isolator is used to isolate the circuit, when the circuit is off load,

or dead-line. The isolators are classified as:

1) Bus isolator(or pantograph isolator)2) Sequential isolator

2.4.7SHUNT REACTORS:

The need for large shunt reactors appeared when long power transmission lines for

system voltage 220 kV & higher were built. The characteristic parameters of a line

are the series inductance (due to the magnetic field around the conductors) & the

shunt capacitance (due to the electrostatic field to earth). An equivalent diagram for

a line is show in the figure below

Both the inductance & the capacitance are distributed along the length of the line. So

are the series resistance and the admittance to earth. When the line is loaded, there is

a voltage drop along the line due to the series inductance and the series resistance.

When the line is energized but not loaded or only loaded with a small current, there

is a voltage rise along the line (the Ferranti-effect)

In this situation, the capacitance to earth draws a current through the line, which

may be capacitive. When a capacitive current flows through the line inductance

there will be a voltage rise along the line.

To stabilize the line voltage the line inductance can be compensated by means of


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series capacitors and the line capacitance to earth by shunt reactors. Series capacitors

are placed at different places along the line while shunt reactors are often installed in

the stations at the ends of line. In this way, the voltage difference between the ends

of the line is reduced both in amplitude and in phase angle.

Shunt reactors may also be connected to the power system at junctures where

several lines meet or to tertiary windings of transformers.

Transmission cables have much higher capacitance to earth than overhead lines.

Long submarine cables for system voltages of 100 KV and more need shunt reactors.

The same goes for large urban networks to prevent excessive voltage rise when a

high load suddenly falls out due to a failure.

Shunt reactors contain the same components as power transformers, like windings,

core, tank, bushings and insulating oil and are suitable for manufacturing in

transformer factories. The main difference is the reactor core limbs, which have non-

magnetic gaps inserted between packets of core steel.

Figure shows a design of a single-phase shunt reactor. The half to the right is a

picture of the magnetic field. The winding encloses the mid-limb with the non-

magnetic gaps. A frame of core steel encloses the winding and provides the return

path for the magnetic field.

3-phase reactors can also be made. These may have 3- or -5-limbed cores. In a 3-

limbed core there is strong magnetic coupling between the three phases, while in a 5-

limbed core the phases are magnetically independent due to the enclosing magnetic

frame formed by the two yokes and the two unwound side-limbs.

The neutral of shunt reactor may be directly earthed, earthed through an Earthing-

reactor or unearthed.


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When the reactor neutral is directly earthed, the winding are normally designed

with graded insulation in the earthed end. The main terminal is at the middle of the

limb height, & the winding consists of two parallel-connected halves, one below &

one above the main terminal. The insulation distance to the yokes can then be made

relatively small. Sometimes a small extra winding for local electricity supply is

inserted between the main winding & yoke.

When energized the gaps are exposed to large pulsation compressive forced with a

frequency of twice the frequency of the system voltage. The peak value of these

forces may easily amount to 106 N/m2 (100 ton /m2). For this reason the design of

the core must be very solid, & the modulus of elasticity of the non-magnetic (& non-

metallic) material used in gaps must be high (small compression) in order to avoid

large vibration amplitudes with high sound level consequently. The material in the

gaps must also be stable to avoid escalating vibration amplitudes in the end.

Testing of reactors requires capacitive power in the test field equal to the nominal

power of the reactor while a transformer can be tested with a reactive power equal to

10 20% of the transformer power rating by feeding the transformer with nominal

current in short circuit condition.

The loss in the various parts of the reactor (12R, iron loss & additional loss) cannot

be separated by measurement. It is thus preferable, in order to avoid corrections to

reference temperature, to perform the loss measurement when the average


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temperature of the winding is practically equal to the reference temperature.

When specifying shunt reactors for enquiry, the following data should be given:

Reactive power, Q Rated voltage, U Maximum continuous operating voltage Insulation level LI, SI Frequency, Hz AC test voltages Permissible temperature rise for oil & winding Sound level & linearity criteria, if required Type of cooling, fan, pump, radiators Peripheral features, if required Safety & monitoring equipment Loss capitalization

2.4.8ELECTRICAL BUS:

In electrical power distribution, a busbar is a strip of copper or aluminium that

conducts electricity within a switchboard, distribution board, substation or other

electrical apparatus.

The size of the busbar determines the maximum amount of current that can be safely

carried. Busbars can have a cross-sectional area of as little as 10 mm but electrical

substations may use metal tubes of 50 mm in diameter (1,963 mm) or more as

busbars, and analuminum smelterwill have very large busbars used to carry tens of

thousands of amperes to the electrochemical cells that produce aluminum from

molten salts.

Busbars are typically either flat strips or hollow tubes as these shapes allow heatto

dissipate more efficiently due to their high surface areatocross-sectionalarea ratio.

The skin effect makes 5060 Hz ACbusbars more than about 8 mm (1/3 in) thick

inefficient, so hollow or flat shapes are prevalent in higher current applications. A

hollow section has higher stiffness than a solid rod of equivalent current-carrying

capacity, which allows a greater span between busbar supports in outdoor

switchyards.
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A busbar may either be supported on insulators, or else insulation may completely

surround it. Busbars are protected from accidental contact either by a metal earthed

enclosure or by elevation out of normal reach. Neutral busbars may also be

insulated.Earthbusbars are typically bolted directly onto any metal chassis of their

enclosure. Busbars may be enclosed in a metal housing, in the form of bus duct or

busway, segregated-phase bus, orisolated-phase bus.

Busbars may be connected to each other and to electrical apparatus by bolted or

clamp connections. Often joints between high-current bus sections have matching

surfaces that are silver-plated to reduce the contact resistance. At extra-high voltages

(more than 300 kV) in outdoor buses, corona around the connections becomes a

source of radio-frequency interference and power loss, so connection fittings

designed for these voltages are used.

Busbars are typically contained inside of either a distribution board or busway.

Distribution boards

Distribution boards split the electrical supply into separate circuits at one location.

Two hot busbars are visible in this distribution board, traveling vertically from the

main circuit breaker at top to feed the rows of breakers below it.

Bus ducts
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Busways, or bus ducts, are long busbars with a protective cover. Rather than

branching the main supply at one location, they allow new circuits to branch off

anywhere along the route of the busway.

The busbars contained within are visible in this opened busway, above the arrows at

left and traveling horizontally at right. This busway section was used in afire testof

afirestopsystem, achieving a 2 hourfire-resistance rating.

Bus duct penetration, awaitingfirestop.
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Electrical conduit and bus duct in a building at Texaco Nanticoke refinery in

Nanticoke, Ontario, 1980s.

2.4.9EARTH SWITCH:

The earthing resistance of the sub-stations is generally kept of the order of 0.5 ohms,

whereas for the transmission line the earthing resistance varies from 10 ohms to 20

ohms. Earthing of the sub-station is normally done by laying mat in and around the

switchyard area. Normally , ungalvanised mild steel flats are used for earthing and

for risers. The size of the flats depends upon the fault current. Separate earthing

electrodes are provide for earthing the lightening arresters whereas the remaining

equipment are earthed by connecting their earth leads to the risers of the ground

mat.

2.4.10FIRE FIGHTING EQUIPMENT:

Soak pits are provided in respect of all transformers where the quantity of oil exeeds

2000 litres. Besides , portable type of fire fighting equipment such as dry powdertype , chemical foam type are also provided in adequate quantities for protection of

the electrical equipment .For large sub-stations , automatic type of mulsifire system

have also been used for the protection of the transformer against fire.

2.4.11CONTROL RELAY PANELS:

The control room building for the sub-station includes the panels , PLCC equipment

, DC battery , LT Board Etc. The control cables are of armoured type and arenormally laid in trenches covered with RCC covers or steel covers . These trenches

are also housing the power cables.
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2.4.12SUPPORTING STRUCTURES FOR HANGING BUSES:

The supporting structures are normally of bolted steel lattice type . Angle sections

are bolted together to form a square base lattice structure . The structures are

provided with cross arms through which insulator strings are hung for supporting

the conductors . The base of these towers may vary from 2.5 meters to 6 meters . The

maximum distance from the ground is normally maintained in accordance with the

stipulations made in The Indian Electricity Rules. These clearances are as follows:

66 KV line 18 ft.

132KV line 28ft.

220KV line 23ft.

400KV line 28ft.

Switchyard Failures & Causes:Bus faults in switchyard have taken place due to

Loose contact in isolators. While bus switching of feeders.

Attributes to improper setting or auxiliary contacts or isolates.

Loose sheets from boiler roof and other high rise structures near to switchyard flying

in dust storm and land on the live posts in the switchyard .

An uncleared fault on a live bus is quiet serious as it has potential to cause grid

collapse and in order to avoid such an occurrence , in addition to following proper

and adequate maintenance practices , it is essential not to bypass the bus bar

protection provided for the purpose.


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