Electrical Maintaineance Division 1

8/6/2019 Electrical Maintaineance Division 1

http://slidepdf.com/reader/full/electrical-maintaineance-division-1 1/29

ELECTRICAL MAINTAINEANCE DIVISION 1

1)Electrostatic precipitator

A device used to remove liquid droplets or solid particles from a gas in which they aresuspended. The process depends on two steps. In the first step the suspension passes through

an electric discharge (corona discharge) area where ionization of the gas occurs. The ions

produced collide with the suspended particles and confer on them an electric charge. The

charged particles drift toward an electrode of opposite sign and are deposited on the electrode

where their electric charge is neutralized. The phenomenon would be more correctly

designated as electrodeposition from the gas phase.

The use of electrostatic precipitators has become common in numerous industrial

applications. Among the advantages of the electrostatic precipitator are its ability to handle

large volumes of gas, at elevated temperatures if necessary, with a reasonably small pressure

drop, and the removal of particles in the micrometer range. Some of the usual applications

are:

(1) removal of dirt from flue gases in steam plants;

(2) cleaning of air to remove fungi and bacteria in establishments producing antibiotics and

other drugs, and in operating rooms;

(3) cleaning of air in ventilation and air conditioning systems;

(4) removal of oil mists in machine shops and acid mists in chemical process plants;

(5) cleaning of blast furnace gases;

(6) recovery of valuable materials such as oxides of copper, lead, and tin; and

(7) separation of rutile from zirconium sand.



External view of electrostatic precipitator



2)COAL HANDLING PLANT (C.H.P)

The old coal handling plant caters to the need of units 2,3,4,5 and 1 whereas the latter

supplies coal to units 4 and V.O.C.H.P. supplies coal to second and third stages in the advent

coal to usable form to (crushed) form its raw form and send it to bunkers, from where it is

send to furnace.

Major Components:-

a. Wagon Tippler: - Wagons from the coal yard come to the tippler and are emptied here.

The process is performed by a slip ±ring motor of rating: 55 KW, 415V, 1480 RPM. This

motor turns the wagon by 135 degrees and coal falls directly on the conveyor through

vibrators. Tippler has raised lower system which enables is to switch off motor when required

till is wagon back to its original position. It is titled by weight balancing principle. The motor

lowers the hanging balancing weights, which in turn tilts the conveyor. Estimate of the

weight of the conveyor is made through hydraulic weighing machine.

Wagon Tripler at Badarpur Thermal Power Station, New Delhi



b. Conveyor: - There are 14 conveyors in the plant. They are numbered so that their function

can be easily demarcated. Conveyors are made of rubber and more with a speed of 250-

300m/min. Motors employed for conveyors has a capacity of 150 HP. Conveyors have a

capacity of carrying coal at the rate of 400 tons per hour. Few conveyors are double belt, this

is done for imp. Conveyors so that if a belt develops any problem the process is not stalled.

The conveyor belt has a switch after every 25-30 m on both sides so stop the belt in case of

emergency. The conveyors are 1m wide, 3 cm thick and made of chemically treated

vulcanized rubber. The max angular elevation of conveyor is designed such as never to

exceed half of the angle of response and comes out to be around 20 degrees.

Conveyer Belts

c. Zero Speed Switch:-It is safety device for motors, i.e., if belt is not moving and the motor

is on the motor may burn. So to protect this switch checks the speed of the belt and switches

off the motor when speed is zero.

d. Metal Separators: - As the belt takes coal to the crusher, No metal pieces should go along

with coal. To achieve this objective, we use metal separators. When coal is dropped to the

crusher hoots, the separator drops metal pieces ahead of coal. It has a magnet and a belt and

the belt is moving, the pieces are thrown away. The capacity of this device is around 50 kg.



The CHP is supposed to transfer 600 tons of coal/hr, but practically only 300-400 tons coal is

transfer.

e. Crusher: - Both the plants use TATA crushers powered by BHEL. Motors. The crusher is

of ring type and motor ratings are 400 HP, 606 KV. Crusher is designed to crush the pieces to

20 mm size i.e. practically considered as the optimum size of transfer via conveyor.

f. Rotatory Breaker: - OCHP employs mesh type of filters and allows particles of 20mm

size to go directly to RC bunker, larger particles are sent to crushes. This leads to frequent

clogging. NCHP uses a technique that crushes the larger of harder substance like metal

impurities easing the load on the magnetic separators.

g.Milling System

y RC Bunker: - Raw coal is fed directly to these bunkers. These are 3 in no. per boiler.

4 & ½ tons of coal are fed in 1 hr. the depth of bunkers is 10m.

y RC Feeder: - It transports pre crust coal from raw coal bunker to mill. The quantity

of raw coal fed in mill can be controlled by speed control of aviator drive controlling

damper and aviator change.

y Ball Mill: - The ball mill crushes the raw coal to a certain height and then allows it to

fall down. Due to impact of ball on coal and attraction as per the particles move over

each other as well as over the Armor lines, the coal gets crushed. Large particles are

broken by impact and full grinding is done by attraction. The Drying and grinding

option takes place simultaneously inside the mill.

y Classifier:- It is an equipment which serves separation of fine pulverized coal

particles medium from coarse medium. The pulverized coal along with the carrying

medium strikes the impact plate through the lower part. Large particles are then

transferred to the ball mill.

y Cyclone Separators: - It separates the pulverized coal from carrying medium. The

mixture of pulverized coal vapour caters the cyclone separators.



y The Turniket: - It serves to transport pulverized coal from cyclone separators to

pulverized coal bunker or to worm conveyors. There are 4 turnikets per boiler.

h. Worm Conveyor: - It is equipment used to distribute the pulverized coal from bunker of

one system to bunker of other system. It can be operated in both directions.

i. Mills Fans: - It is of 3 types:

Six in all and are running condition all the time.

y ID Fans: - Located between electrostatic precipitator and chimney.

Type-radical

Speed-1490 rpm

Rating-300 KW

Voltage-6.6 KV

Lubrication-by oil

y FD Fans: - Designed to handle secondary air for boiler. 2 in number and provide

ignition of coal.

Type-axial

Speed-990 rpm

Rating-440 KW

Voltage-6.6 KV

y Primary Air Fans: - Designed for handling the atmospheric air up to 50 degrees

Celsius, 2 in number.

Type-Double suction radial

Rating-300 KW

Voltage-6.6 KV

Lubrication-by oil

Type of operation-continuous



f. Bowl Mill: - One of the most advanced designs of coal pulverizes presently manufactured.

Motor specification ±squirrel cage induction motor

Rating-340 KW

Voltage-6600KV

Curreen-41.7A

Speed-980 rpm

Frequency-50 Hz

No-load current-15-16 A

Single line diagram of CHP

SINGLE LINE DIAGRAM OF OCHP

C-1B

WT-1 WT-2

CR.B CR. A

C-1A

C-2B C-2A

C-3A

C-3B

C-10A

C-10B

BUNKERS BUNKERS

COAL YARD

C-9

C-4

C-5

C-6

C-7C-8ILMS

ILMS

MD MD



2. NEW COAL HANDLING PLANT (N.C.H.P)

1. Wagon Tippler:-

Motor Specification

(i) H.P 75 HP

(ii) Voltage 415, 3 phase

(iii) Speed 1480 rpm

(iv) Frequency 50 Hz

(v) Current rating 102 A

2. Coal feed to plant:-

Feeder motor specification

(i) Horse power 15 HP

(ii) Voltage 415V,3 phase

(iii) Speed 1480 rpm

(iv) Frequency 50 Hz

3. Conveyors:-

10A, 10B

11A, 11B

12A, 12B

13A, 13B

14A, 14B

15A, 15B

16A, 16B

17A, 17B

18A, 18B

4. Transfer Point 6

5. Breaker House



6. Rejection House

7. Reclaim House

8. Transfer Point 7

9. Crusher House

10. Exit

The coal arrives in wagons via railways and is tippled by the wagon tipplers into the hoppers.

If coal is oversized (>400 mm sq) then it is broken manually so that it passes the hopper

mesh. From the hopper mesh it is taken to the transfer point TP6 by conveyor 12A ,12B

which takes the coal to the breaker house , which renders the coal size to be 100mm sq. the

stones which are not able to pass through the 100mm sq of hammer are rejected via

conveyors 18A,18B to the rejection house . Extra coal is to sent to the reclaim hopper via

conveyor 16. From breaker house coal is taken to the TP7 via Conveyor 13A, 13B. Conveyor

17A, 17B also supplies coal from reclaim hopper, From TP7 coal is taken by conveyors 14A,

14B to crusher house whose function is to render the size of coal to 20mm sq. now the

conveyor labors are present whose function is to recognize and remove any stones moving in

the conveyors . In crusher before it enters the crusher. After being crushed, if any metal is

still present it is taken care of by metal detectors employed in conveyor 10.



SWITCH GEAR

It makes or breaks an electrical circuit.

1. Isolation: - A device which breaks an electrical circuit when circuit is switched on to no

load. Isolation is normally used in various ways for purpose of isolating a certain portion

when required for maintenance.

2. Switching Isolation: - It is capable of doing things like interrupting transformer

magnetized current, interrupting line charging current and even perform load transfer

switching. The main application of switching isolation is in connection with transformer

feeders as unit makes it possible to switch out one transformer while other is still on load.

3. Circuit Breakers: - One which can make or break the circuit on load and even on faults is

referred to as circuit breakers. This equipment is the most important and is heavy duty

equipment mainly utilized for protection of various circuits and operations on load. Normally

circuit breakers installed are accompanied by isolators

4. Load Break Switches: - These are those interrupting devices which can make or break

circuits. These are normally on same circuit, which are backed by circuit breakers.

5. Earth Switches: - Devices which are used normally to earth a particular system, to avoid

any accident happening due to induction on account of live adjoining circuits. These

equipments do not handle any appreciable current at all. Apart from this equipment there are

a number of relays etc. which are used in switchgear.

LT Switchgear

It is classified in following ways:-

1. Main Switch:- Main switch is control equipment which controls or disconnects the main

supply. The main switch for 3 phase supply is available for tha range 32A, 63A, 100A, 200Q,

300A at 500V grade.



2. Fuses: - With Avery high generating capacity of the modern power stations extremely

heavy carnets would flow in the fault and the fuse clearing the fault would be required to

withstand extremely heavy stress in process.

It is used for supplying power to auxiliaries with backup fuse protection. Rotary switch up to

25A. With fuses, quick break, quick make and double break switch fuses for 63A and 100A,

switch fuses for 200A, 400A, 600A, 800A and 1000A are used.

3. Contractors: - AC Contractors are 3 poles suitable for D.O.L Starting of motors and

protecting the connected motors.

4. Overload Relay: - For overload protection, thermal over relay are best suited for this

purpose. They operate due to the action of heat generated by passage of current through relay

element.

5. Air Circuit Breakers: - It is seen that use of oil in circuit breaker may cause a fire. So in

all circuits breakers at large capacity air at high pressure is used which is maximum at the

time of quick tripping of contacts. This reduces the possibility of sparking. The pressure may

vary from 50-60 kg/cm^2 for high and medium capacity circuit breakers.

HT SWITCH GEAR

1. Minimum oil Circuit Breaker: - These use oil as quenching medium. It comprises of

simple dead tank row pursuing projection from it. The moving contracts are carried on an

iron arm lifted by a long insulating tension rod and are closed simultaneously pneumatic

operating mechanism by means of tensions but throw off spring to be provided at mouth of

the control the main current within the controlled device.

Type-HKH 12/1000c

� Rated Voltage-66 KV

� Normal Current-1250A

� Frequency-5Hz

� Breaking Capacity-3.4+KA Symmetrical



� 3.4+KA Asymmetrical

� 360 MVA Symmetrical

� Operating Coils-CC 220 V/DC

§ FC 220V/DC

� Motor Voltage-220 V/DC

2. Air Circuit Breaker: - In this the compressed air pressure around 15 kg per cm^2 is used

for extinction of arc caused by flow of air around the moving circuit . The breaker is closed

by applying pressure at lower opening and opened by applying pressure at upper opening.

When contacts operate, the cold air rushes around the movable contacts and blown the arc.

It has the following advantages over OCB:-

i. Fire hazard due to oil are eliminated.

ii. Operation takes place quickly.

iii. There is less burning of contacts since the duration is short and consistent.

iv. Facility for frequent operation since the cooling medium is replaced constantly.

Rated Voltage-6.6 KV

Current-630 A

Auxiliary current-220 V/DC

3. SF6 Circuit Breaker: - This type of circuit breaker is of construction to dead tank bulk oil

to circuit breaker but the principle of current interruption is similar o that of air blast circuit

breaker. It simply employs the arc extinguishing medium namely SF6. the performance of

gas . When it is broken down under an electrical stress.

� Circuit Breakers-HPA

� Standard-1 EC 56


� Insulation Level-28/75 KV

� Rated Frequency-50 Hz

� Breaking Current-40 KA

� Rated Current-1600 A

� Making Capacity-110 KA



� Rated Short Time Current 1/3s -40 A

� Mass Approximation-185 KG

� Auxiliary Voltage

§ Closing Coil-220 V/DC

§ Opening Coil-220 V/DC

� Motor-220 V/DC

� SF6 Pressure at 20 Degree Celsius-0.25 KG

� SF6 Gas Per pole-0.25 KG

4. Vacuum Circuit Breaker: - It works on the principle that vacuum is used to save the

purpose of insulation .It regards of insulation and strength, vacuum is superior dielectric

medium and is better that all other medium except air and sulphur which are generally used at

high pressure.

� Rated frequency-50 Hz

� Rated making Current-10 Peak KA


� Supply Voltage Closing-220 V/DC

� Rated Current-1250 A

� Supply Voltage Tripping-220 V/DC

� Insulation Level-IMP 75 KVP

� Rated Short Time Current-40 KA (3 SEC)

� Weight of Breaker-8 KG



Electrical Maintenance Division -2

Generators

The transformation of mechanical energy into electrical energy is carried out by theGenerator. This Chapter seeks to provide basic understanding about the working principles

and development of Generator.

Working Principle

The A.C. Generator or alternator is based upon the principle of electromagnetic induction and

consists generally of a stationary part called stator and a rotating part called rotor. The stator

housed the armature windings. The rotor houses the field windings. D.C. voltage is applied to

the field windings through slip rings. When the rotor is rotated, the lines of magnetic flux (viz

magnetic field) cut through the stator windings. This induces an electromagnetic force

(e.m.f.) in the stator windings. The magnitude of this e.m.f. is given by the following

expression.

E = 4.44 /O FN volts

0 = Strength of magnetic field in Weber¶s.

F = Frequency in cycles per second or Hertz.

N = Number of turns in a coil of stator winding

F = Frequency = Pn/120

Where P = Number of poles

n = revolutions per second of rotor.

From the expression it is clear that for the same frequency, number of poles increases with

decrease in speed and vice versa. Therefore, low speed hydro turbine drives generators have

14 to 20 poles where as high speed steam turbine driven generators have generally 2 poles.

Pole rotors are used in low speed generators, because the cost advantage as well as easier

construction.



Development

The first A.C. Generator concept was enunciated by Michael Faraday in 1831. In 1889 Sir

Charles A. Parsons developed the first AC turbo-generator. Although slow speed AC

generators have been built for some time, it was not long before that the high-speed

generators made its impact.

Development contained until, in 1922, the increased use of solid forgings and improved

techniques permitted an increase in generator rating to 20MW at 300rpm. Up to the out break

of second world war, in 1939, most large generator;- were of the order of 30 to 50 MW at

3000 rpm.

During the war, the development and installation of power plants was delayed and in order to

catch up with the delay in plant installation, a large number of 30 MW and 60 MW at 3000

rpm units were constructed during the years immediately following the war. The changes in

design in this period were relatively small.

In any development programme the. Costs of material and labour involved in manufacturing

and erection must be a basic consideration. Coupled very closely with these considerations is

the restriction is size and weight imposed by transport limitations.

Development of suitable insulating materials for large turbo-generators is one of the most

important tasks and need continues watch as size and ratings of machines increase. The

present trend is the use only class "B" and higher grade materials and extensive work has

gone into compositions of mica; glass and asbestos with appropriate bonding material. An

insulation to meet the stresses in generator slots must follow very closely the thermal

expansion of the insulated conductor without cracking or any plastic deformation. Insulation

for rotor is subjected to lower dielectric stress but must withstand high dynamic stresses and

the newly developed epoxy resins, glass and/or asbestos molded in resin and other synthetic

resins are finding wide applications.



Generator component

This Chapter deals with the two main components of the Generator viz. Rotor, its winding & balancing and stator, its frame, core & windings.

Rotor

The electrical rotor is the most difficult part of the generator to design. It revolves in most

modern generators at a speed of 3,000 revolutions per minute. The problem of guaranteeing

the dynamic strength and operating stability of such a rotor is complicated by the fact that a

massive non-uniform shaft subjected to a multiplicity of differential stresses must operate in

oil lubricated sleeve bearings supported by a structure mounted on foundations all of which

possess complex dynamic be behavior peculiar to themselves. It is also an electromagnet and

to give it the necessary magnetic strength the windings must carry a fairly high current. The

passage of the current through the windings generates heat but the temperature must not beallowed to become so high, otherwise difficulties will be experienced with insulation. To

keep the temperature down, the cross section of the conductor could not be increased but this

would introduce another problems. In order to make room for the large conductors, body and

this would cause mechanical weakness. The problem is really to get the maximum amount of

copper into the windings without reducing the mechanical strength. With good design and

great care in construction this can be achieved. The rotor is a cast steel ingot, and it is further

forged and machined. Very often a hole is bored through the centre of the rotor axially from

one end of the other for inspection. Slots are then machined for windings and ventilation.

Rotor winding

Silver bearing copper is used for the winding with mica as the insulation between conductors.

A mechanically strong insulator such as micanite is used for lining the slots. Later designs of

windings for large rotor incorporate combination of hollow conductors with slots or holes

arranged to provide for circulation of the cooling gas through the actual conductors. When

rotating at high speed. Centrifugal force tries to lift the windings out of the slots and they are

contained by wedges. The end rings are secured to a turned recess in the rotor body, by

shrinking or screwing and supported at the other end by fittings carried by the rotor body.

The two ends of windings are connected to slip rings, usually made of forged steel, and

mounted on insulated sleeves.



Rotor balancing

When completed the rotor must be tested for mechanical balance, which means that a check

is made to see if it will run up to normal speed without vibration. To do this it would have to be uniform about its central axis and it is most unlikely that this will be so to the degree

necessary for perfect balance. Arrangements are therefore made in all designs to fix

adjustable balance weights around the circumference at each end.

Stator

Stator frame: The stator is the heaviest load to be transported. The major part of this load is

the stator core. This comprises an inner frame and outer frame. The outer frame is a rigid

fabricated structure of welded steel plates, within this shell is a fixed cage of girder built

circular and axial ribs. The ribs divide the yoke in the compartments through which hydrogen

flows into radial ducts in the stator core and circulate through the gas coolers housed in the

frame. The inner cage is usually fixed in to the yoke by an arrangement of springs to dampen

the double frequency vibrations inherent in 2 pole generators. The end shields of hydrogen

cooled generators must be strong enough to carry shaft seals. In large generators the frame is

constructed as two separate parts. The fabricated inner cage is inserted in the outer frame

after the stator core has been constructed and the winding completed. Stator core: The stator

core is built up from a large number of 'punching" or sections of thin steel plates. The use of

cold rolled grain-oriented steel can contribute to reduction in the weight of stator core for two

main reasons:

a) There is an increase in core stacking factor with improvement in lamination cold Rolling

and in cold buildings techniques.

b) The advantage can be taken of the high magnetic permeance of grain-oriented steels of work the stator core at comparatively high magnetic saturation without fear or excessive iron

loss of two heavy a demand for excitation ampere turns from the generator rotor.



Stator Windings

Each stator conductor must be capable of carrying the rated current without overheating. The

insulation must be sufficient to prevent leakage currents flowing between the phases to earth.

Windings for the stator are made up from copper strips wound with insulated tape which isimpregnated with varnish, dried under vacuum and hot pressed to form a solid insulation bar.

These bars are then place in the stator slots and held in with wedges to form the complete

winding which is connected together at each end of the core forming the end turns. These end

turns are rigidly braced and packed with blocks of insulation material to withstand the heavy

forces which might result from a short circuit or other fault conditions. The generator

terminals are usually arranged below the stator. On recent generators (210 MW) the windings

are made up from copper tubes instead of strips through which water is circulated for cooling

purposes. The water is fed to the windings through plastic tubes.

Generator Cooling System

The 200/210 MW Generator is provided with an efficient cooling system to avoid excessive

heating and consequent wear and tear of its main components during operation. This Chapter

deals with the rotor-hydrogen cooling system and stator water cooling system along with the

shaft sealing and bearing cooling systems.

Rotor Cooling System

The rotor is cooled by means of gap pick-up cooling, wherein the hydrogen gas in the air gap

is sucked through the scoops on the rotor wedges and is directed to flow along the ventilating

canals milled on the sides of the rotor coil, to the bottom of the slot where it takes a turn and

comes out on the similar canal milled on the other side of the rotor coil to the hot zone of the

rotor. Due to the rotation of the rotor, a positive suction as well as discharge is created due to

which a certain quantity of gas flows and cools the rotor. This method of cooling gives

uniform distribution of temperature. Also, this method has an inherent advantage of

eliminating the deformation of copper due to varying temperatures.



Hydrogen Cooling System

Hydrogen is used as a cooling medium in large capacity generator in view of its high heat

carrying capacity and low density. But in view of its forming an explosive mixture with

oxygen, proper arrangement for filling, purging and maintaining its purity inside thegenerator have to be made. Also, in order to prevent escape of hydrogen from the generator

casing, shaft sealing system is used to provide oil sealing.

The hydrogen cooling system mainly comprises of a gas control stand, a drier, an liquid level

indicator, hydrogen control panel, gas purity measuring and indicating instruments.

The system is capable of performing the following functions :

� Filling in and purging of hydrogen safely without bringing in contact with air.

� Maintaining the gas pressure inside the machine at the desired value at all the

times.

� Provide indication to the operator about the condition of the gas inside the

machine i.e. its pressure, temperature and purity.

� Continuous circulation of gas inside the machine through a drier in order to

remove any water vapour that may be present in it.

� Indication of liquid level in the generator and alarm in case of high level.

Stator Cooling System

The stator winding is cooled by distillate. Which is fed from one end of the machine by

Teflon tube and flows through the upper bar and returns back through the lower bar of

another slot.Turbo generators require water cooling arrangement over and above the usual

hydrogen cooling arrangement. The stator winding is cooled in this system by circulating

demineralised water (DM water) through hollow conductors. The cooling water used for

cooling stator winding calls for the use of very high quality of cooling water. For this purpose

DM water of proper specific resistance is selected. Generator is to be loaded within a very

short period if the specific resistance of the cooling DM water goes beyond certain preset



values. The system is designed to maintain a constant rate of cooling water flow to the stator

winding at a nominal inlet water temperature of 40 deg.C.

A 95MW TurbineGenerator atBadarpur Thermal Power Station,New Delhi

Rating of 95 MW Generator

Manufacture by Bharat heavy electrical Limited (BHEL)

Capacity - 117500 KVA

Voltage - 10500V

Speed - 3000 rpm



Hydrogen - 2.5 Kg/cm2

Power factor - 0.85 (lagging)

Stator current - 6475 A

Frequency - 50 Hz

Stator wdg connection - 3 phase

Rating of 210 MW Generator

Capacity - 247000 KVA

Voltage (stator) - 15750 V

Current (stator) - 9050 A

Voltage (rotor) - 310 V

Current (rotor) - 2600 V

Speed - 3000 rpm

Power factor - 0.85

Frequency - 50 Hz

Hydrogen - 3.5 Kg/cm2

Stator wdg connection - 3 phase star connection

Insulation class - B



TRANFORMER

A transformer is a device that transfers electrical energy from one circuit to another by

magnetic coupling with out requiring relative motion between its parts. It usually comprises

two or more coupled windings, and in most cases, a core to concentrate magnetic flux. An

alternating voltage applied to one winding creates a time-varying magnetic flux in the core,

which includes a voltage in the other windings. Varying the relative number of turns between

primary and secondary windings determines the ratio of the input and output voltages, thus

transforming the voltage by stepping it up or down between circuits. By transforming

electrical power to a high-voltage,_low-current form and back again, the transformer greatly

reduces energy losses and so enables the economic transmission of power over long

distances. It has thus shape the electricity supply industry, permitting generation to be located

remotely from point of demand. All but a fraction of the world¶s electrical power has passedtrough a series of transformer by the time it reaches the consumer.

Basic principles

The principles of the transformer are illustrated by consideration of a hypothetical ideal

transformer consisting of two windings of zero resistance around a core of negligible

reluctance. A voltage applied to the primary winding causes a current, which develops a

magneto motive force (MMF) in the core. The current required to create the MMF is termed

the magnetizing current; in the ideal transformer it is considered to be negligible, although its

presence is still required to drive flux around the magnetic circuit of the core. An

electromotive force (MMF) is induced across each winding, an effect known as mutual

inductance. In accordance with faraday¶s law of induction, the EMFs are proportional to the

rate of change of flux. The primary EMF, acting as it does in opposition to the primary

voltage, is sometimes termed the back EMF´. Energy losses An ideal transformer would have

no energy losses and would have no energy losses, and would therefore be 100% efficient.

Despite the transformer being amongst the most efficient of electrical machines with ex the

most efficient of electrical machines with experimental models using superconducting

windings achieving efficiency of 99.85%, energy is dissipated in the windings, core, and

surrounding structures. Larger transformers are generally more efficient, and those rated for

electricity distribution usually perform better than 95%. A small transformer such as plug-in

³power brick´ used for low-power consumer electronics may be less than 85% efficient.

Transformer losses are attributable to several causes and may be differentiated between those



originated in the windings, some times termed copper loss,and those arising from the

magnetic circuit, sometimes termed iron loss. The losses vary with load current, and may

furthermore be expressed as ³no load´ or ³full load´ loss, or at an intermediate loading.

Winding resistance dominates load losses contribute to over 99% of the no-load loss can be

significant, meaning that even an idle transformer constitutes a drain on an electrical supply,

and lending impetus to development of low-loss transformers. Losses in the transformer arise

from: Winding resistance Current flowing trough the windings causes resistive heating of the

conductors. At higher frequencies, skin effect and proximity effect create additional winding

resistance and losses. Hysteresis losses Each time the magnetic field is reversed, a small

amount of energy is lost due to hysteresis within the core. For a given core material, the loss

is proportional to the frequency, and is a function of the peak flux density to which it is

subjected. Eddy current Ferromagnetic materials are also good conductors, and a solid core

made from such a material also constitutes a single short-circuited turn trough out its entire

length. Eddy currents therefore circulate with in a core in a plane normal to the flux, and are

responsible for resistive heating of the core material. The eddy current loss is a complex

function of the square of supply frequency and inverse square of the material thickness.

Magnetostriction Magnetic flux in a ferromagnetic material, such as the core, causes it to

physically expand and contract slightly with each cycle of the magnetic field, an effect

known as magnetostriction. This produces the buzzing sound commonly associated with

transformers, and in turn causes losses due to frictional heating in susceptible cores.

Mechanical losses In addition to magnetostriction, the alternating magnetic field causes

fluctuating electromagnetic field between primary and secondary windings. These incite

vibration with in near by metal work, adding to the buzzing noise, and consuming a small

amount of power. Stray losses Leakage inductance is by itself loss less, since energy supplied

to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage

flux that intercepts nearby conductive material such as the transformers support structure will

give rise to eddy currents and be converted to heat. Cooling system Large power transformers

may be equipped with cooling fans, oil pumps or water-cooler heat exchangers design to

remove heat. Power used to operate the cooling system is typically considered part of the

losses of the transformer.



Rating of transformer

Manufactured by Bharat heavy electrical limited

No load voltage (hv) - 229 KV

No load Voltage (lv) -10.5 KV

Line current (hv) - 315.2 A

Line current (lv) - 873.2 A

Temp rise - 45 Celsius

Oil quantity -40180 lit

Weight of oil -34985 Kg

Total weight - 147725 Kg

Core & winding - 84325 Kg

Phase - 3

Frequency - 50 Hz



PROTECTIONS FOR TRANSFORMER

The following protections are provided normally for a transformer.

Over current Relays

Over current protection is based on a very simple premise that in most instances of a fault, the

level of fault current dramatically increases from the pre-fault value. If one establishes a

threshold well above the nominal load current, as soon as the current exceeds the threshold, it

may be assumed that a fault has occurred and a trip signal may be issued. The relay based on

this principle is called an instantaneous over current relay, and it is widely used for protection

of radial low voltage distributing lines, ground protection of high voltage transmission lines,

and protection of machines (motors and generators). The main issue in applying this relaying

principle is to understand the behaviour of the fault current well, in particular when compared

to the variation in the load current caused by significant changes in the connected load. A

typical example where it may become difficult to distinguish the fault levels from the normal

operating levels is the over current protection of distribution lines with heavy fluctuations of

the load. To accommodate the mentioned difficulty, a variety of over current protection

applications are developed using the basic principle as described previously combined with a

time delay. One approach is to provide a fixed time delay, and in some instances, the time

delay is proportional to the current level. One possible relationship is an inverse one where

the time delay is small for high currents and long for smaller ones. Relays with such

characteristics are called Inverse Definite Minimum Time (IDMT) relays. The standard

characteristic curve of such a relay is usually represented on a logarithmic scale and gives the

operating time at different multiples (Plug Setting Multiplier) of setting current (Is), for the

maximum ³Time Multiplier Setting´ (TMS). The TMS is continuously adjustable giving a

range of time/current characteristic.

Earth fault directional protection

The directional earth-fault unit measures the neutral current ³I0´, the residual voltage ³U0´

from open delta PT and the phase angle between residual voltage(U0) and neutral current(I0).

An earth-fault stage starts if all of the three criteria mentioned below are fulfilled at the same

time:

1. The residual voltage U0 exceeds the threshold or set level



2. The neutral current I0 exceeds the set value

3. If the phase angle between residual voltage and neutral current falls within the operation

area

Overvoltage and Under voltage Relays

Overvoltage relays are used for protection against over voltages. These relays operate if the

system voltage exceeds a preset value. Overvoltage relays are used for tripping shunt

capacitor banks when the system voltage improves to a desired level. On the other hand

Under-voltage relays operate when the system voltage decreases below the set value.

Surge protection or Lightning Arrestor

Lightning, is a form of visible discharge of electricity between rain clouds or between a rain

cloud and the earth. The electric discharge is seen in the form of a brilliant arc, sometimes

several kilometres long, stretching between the discharge points. How thunderclouds become

charged is not fully understood, but most thunderclouds are negatively charged at the base

and positively charged at the top. However formed, the negative charge at the base of the

cloud induces a positive charge on the earth beneath it, which acts as the second plate of a

huge capacitor.

When the electrical potential between two clouds or between a cloud and the earth reaches a

sufficiently high value (about 10,000 V per cm or about 25,000 V per in), the air becomes



ionized along a narrow path and a lightning flash results. Many meteorologists believe that

this is how a negative charge is carried to the ground and the total negative charge of the

surface of the Earth is maintained. The possibility of discharge is high on tall trees and

buildings rather than to ground. Buildings are protected from lightning by metallic lightning

rods extending to the ground from a point above the highest part of the roof. The conductor

has a pointed edge on one side and the other side is connected to a long thick copper strip

which runs down the building. The lower end of the strip is properly earthed. When lightning

strikes it hits the rod and current flows down through the copper strip. These rods form a low-

resistance path for the lightning discharge and prevent it from travelling through the structure

itself.

Differential Relaying

Differential protection is one of the most reliable and popular techniques in power system

protection. Differential protection compares the currents that enter with the currents that

leave a zone. If the net sum of the currents that enter and the currents that leave a protection

zone is essentially zero, it is concluded that there is no fault in the protection zone. However,

if the net sum is not zero, the differential protection concludes that a fault exists in the zone

and takes steps to isolate the zone from the rest of the system. The conductors bringing the

current from the current transformers to the differential relay are in some situations called

pilot wires.

Differential protection can also be used to protect the windings of a transformer by

comparing the current in the power supply's neutral wire with the current in the phase wire. If

the currents are equal then the differential protection relay will not operate. If there is a

current imbalance then the differential protection relay operates.

Distance Relay

The most common form of protection on high voltage transmission systems is distance relay

protection. Power lines have set impedance per kilometre and using this value and comparing

voltage and current the distance to a fault can be determined. The ANSI standard device

number for a distance relay is 21. The main types of distance relay protection schemes are:-



y Three step distance protection

y Switched distance protection

y Accelerated or permissive intertrip protection

y Blocked distance protection

Buchholz relay

In the field of electric power distribution and transmission, a Buchholz relay, also called a gas

relay or a sudden pressure relay, is a safety device mounted on some oil-filled power

transformers and reactors, equipped with an external overhead oil reservoir called a

conservator.

The relay has two different detection modes. On a slow accumulation of gas, due perhaps to

slight overload, gas produced by decomposition of insulating oil accumulates in the top of the

relay and forces the oil level down. A float operated switch in the relay is used to initiate an

alarm signal. This same switch will also operate on low oil level, such as a slow oil leak.

If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This

flow of oil operates a switch attached to a vane located in the path of the moving oil. This

switch normally will operate a circuit breaker to isolate the apparatus before the fault causes

additional damage. Buchholz relays have a test port to allow the accumulated gas to be

withdrawn for testing. Flammable gas found in the relay indicates some internal fault such as

overheating or arcing, whereas air found in the relay may only indicate low oil level or a leak.



References

We have reffered the following books:-

1.Transformers,Published by Tata Mcgraw Hill

2.Electrical Machines by P.S. Bhimbra

3.Data collected from NTPC

Links:-

www.google. com

Documents

Electrical Maintaineance Division 1