INDEX

I. Introduction

II. Construction and Parts of Power transformers

(a)Construction of Power Transformer

(b).Parts of power transformer

2.l. Transformer tank

2.2. Core Assembly

2.3. Winding Construction

2.4. Bushings

2.5. Transformer Oil

2.6. Radiators and Cooler Fans

2.7. Conservator Tank.

2.8. Silica gel Breather.

2.9. On Load Tap Changer (OLTC)

III. Protection of a transformer

(a)Gas operated relays

3. l . Buchholz relays

3.2. Oil surge relays / OLTC Buchholz relay

3.3. Pressure Relief Valve

(b)Oil| & Winding Temperature Indicator

(c)Differentia| protection (against internal faults)

(d) Over current and earth fault protection (against through faults)

IV. Various tests on a power transformer

4.1. Insulation Resistance test

4.2. Transformer Ratio test

4.3 Magnetizing current test

4.4 .Magnetic balance test

4.5 .Short circuit test

4.6. Vector Group test

4.7. Winding Resistance test

4.8. Tan delta test

V. Causes for transformer failures and its reduction techniques

5.1 Reasons for Failure

5.2 Failure Reduction Technique

CHAPTER I

INTRODUCTION

Transformers transfers electrical energy between circuits completely insulated from each

other. During this transfer of energy voltage level may be stepped up/down (depending on the

tums) with consequent decrease/increase currents from primary to secondary windings, thus

maintaining the power transferred constant. Therefore when voltage is stepped up the current is

stepped down. This makes it possible to use very high voltages for transmission lines resulting in

a lower current. Higher voltage and lower current reduce the conductor size, transmission line

losses and improves the voltage regulation as well. Transformers have made possible economic

delivery of electric power over long distances.

In our country the electrical energy is usually generated at 11 or 15.75 or 2l kv, stepped up

to l32;220;400 or 765 kV with the help of step up transformers for transmission and then stepped

down the voltage to ll kV for feeding distributing transformers stepping down the voltage further

to 400/230 volts for the consumer uses.

PRINCIPLE OF TRANSFORMER:

The physical basis of the transformer involves the Faradays laws of electro magnetism in

which an alternating flux induces voltage in the coil and Lenz Law which states that the effect

produced will oppose the cause. It consists of two inductive coils which are electrically separated

but magnetically linked through a path of low reluctance by mutual induction. If one coil is

connected to the source of alternating voltage an alternating flux is set up in the laminated core

most of which is linked with the other coil produces mutually induced emf. If the second coil

circuit is closed, a current flows in it and so electric energy is transferred from the primary to

secondary coil.

CHAPTER ll

CONSTRUCTION and PARTS OF TRANSFORMER

2(a) Transformer construction:

The transformer is simple in construction and consists of magnetic circuit linking with two

windings known as primary and secondary windings. Besides magnetic circuit and windings it

consists of a suitable container for the assembled core and windings such as a tank, a suitable

medium for insulating the core and the windings from its container such as transformer oil, suitable

bushings (either of porcelain , oil filled or condenser type) for insulating and bringing out terminals

of the windings from the transformer tank, temperature gauge for measurement of temperature of

hot oil or hottest spot temperature and oil gauge to indicate the oil level inside the tank.

Transformers are also provided with the conservator tank in order to slow down deterioration of

oil and keep the main tank full of oil.

Fig.2.1: Cut View of a Power Transformer

2(b) Parts of a transformer:

2.1. Transformer tank:

Small capacity tanks are fabricated from welded sheet steel, while larger ones are assembled from

plain boiler plates or cast aluminum parts, usually mounted on a shallow fabricated steel base. The

lids of these transformer tanks can be of cast iron, a water proof gasket being used at the joints.

For cooling purpose, cooling tubes are welded with the tank, but in case of radiators. Separate

radiators are individually welded and then bolted-on to the transformer. A tank must withstand the

stresses developed by jacking and lifting and shall be no longer than necessary to accommodate

the core, windings and internal connections with appropriate electrical clearance.

2.2. Core Assembly:

Core Assembly forms the magnetic circuit linking the two windings of the transformer.

Those parts of the, magnetic circuit which carry the transformer windings, are called the limbs or

legs, and those parts which connect the legs and serve for closing the magnetic circuit are termed

yokes.

The core material used in its construction should satisfy:

Maximum flux is created with minimum magnetizing current

Minimum core loss.

The use of steel in magnetic circuit introduces iron or core loss but ensures a high

permeability of the magnetic circuit. Because of the high permeability the magnitude of exciting

current necessary to create the required flux in the core is small. The presence of steel core causes

100% of the magnetic flux created by the primary o be linked with secondary. The magnetic frame

of the transformer is built up of laminated hot rolled or cold rolled oriented steel consisting of

3.5% silicon. The higher content of silicon increases the resistivity of the core, thereby reducing

the eddy current core loss.

As the flux in the core is pulsating one ,it becomes necessary that the transformer cores are

laminated and the laminations should be insulated and made as thin as possible in order to

minimize the eddy current loss.

2.3. Winding Assembly:

The most important features that the windings of a transformer should possess are:

a) The winding should be economical both as regards initial cost, with a view to the market

availability of copper

b) The efficiency of the transformer in Service

c) The heating conditions of the windings should meet standard requirements

d) The winding should be mechanically stable in respect to the forces appearing when sudden short

circuit of the transformer occurs

e) The winding should have the necessary electrical strength in respect to the over voltages.

Transformer windings are made of solid or stranded copper or aluminum strip conductors.

Heavy current capacity needs conductors of large cross section to reduce eddy current losses in the

conductors, several small wires or parallel straps are preferred to one large strap. This gives rise

to unequal reactance components the conductor which can be eliminated by transposition of

conductors.

2.4 Bushings:

Bushings are incorporated to bring the Extra High voltage winding terminations through

the cover of the transformer tank. Variety of bushings used for various voltage classes were

mentioned below:

Voltage Type of bushing

Class

<33KV Porcelain bushing

<l32KV Oil filled bushings

>l32KV Oil Impregnated

Paper (OlP) Condenser

Bushing

The oil filled bushing consists of hollow porcelain cylinder with a conductor through its

center. The space between the conductor and the porcelain is filled with oil, the dielectric strength

of which is greater than that of air

The OIP Condenser bushing is constructed of thick layers of bakelized paper alternating

with thin graded layer of tin foil. The result is a series of capacitors formed by the conductor and

the first tin-foil layer, the first and second tin foil layers ...and so on. The bakelized paper and the

tin foil are arranged in such a way that the capacitances of the capacitors and hence dielectric stress

across each capacitor are uniform throughout the radial depth of the insulator.

The last tin foil of the insulating medium of bushing is terminated to Voltage tap called as

Tan (δ) tap of bushing used to measure the Capacitance and dissipation factor of the bushing.

l. Oil impregnated core

2. Center metal tube

3. Fixing flange

4. Porcelain upper

5. Porcelain lower

6. Gaskets

7. Springs

8. Expansion bowl

9. Cable bolt

10. Air releasing screw

11. Test tap

12. Top terminal

13. Oil filling plug

l4. Oil sight glass

15. Base plate/stress shield

16. Upper arcing horn

17. Lower arcing horn

18. Nitrogen filling plug

NOTE: The Tan (δ) tap must always be earthed by the screwed on cap and should never be

removed during operation

2.5. Transformer oil:

Oil in the transformer construction serves the double purpose of cooling and insulating.

The heat is produced from the metal of the transformer passes through the insulation and raises the

temperature of oil and is then conducted either through the radiators of the tank to the surrounding

air by means of cooler fans.

Transformer oil has to fulfill certain specifications:

1. High dielectric strength:

As per IS Standard, the breakdown strength of new transformer oil when treated must be

at least 50kV rms when measured with the help of 2 spherical electrodes of l2.5mm dia.

and with a gap spacing of 2.5 mm.

2. Low viscosity:

It is to provide the good heat transfer. A high viscosity is an obvious disadvantage

because of the sluggish flow through the many small orifices in the windings.

3. Purity:

The oil must not contain impurities such as acid, alkali and sulphur or its compounds to

prevent the corrosion of the metal parts and insulation.

4. High flash point:

The temperature at which oil vapor ignites spontaneously is called the flash point. The

flash point of transformer oil should not be less than 135 degrees.

5. Free from sludging under normal operating conditions:

Sludging means slow formation of semi-solid hydrocarbon owing to heating and oxidation.

The sludge deposits itself on the windings, tank walls and in cooling ducts. Sludge being bad

conductor of heat greatly reduces the heat transfer from the windings to the oil and so increases

the temperature of windings Sometimes for preventing sludging certain chemicals called the

inhibitors are added to the transformer oil.

2.6 Radiators and cooler fans:

Radiators are incorporated to cool the low density hot oil at top of the transformer to lower

temperatures on its way to the bottom of the tank by means of Natural air cooling(AN)/Forced Air

cooling(AF) through cooler fans.

Depending on the quantity of oil used in the transformer tank, the number of radiators

required and the type of cooling required were decided by the manufacturer

The various types of cooling techniques used in a transformer were mentioned below:

Type of Description

Cooling

ON Natural on cooling

AN Natural Air Cooling

ONAN Oil Natural with Natural air cooling

ONAF Oil Natural with Forced air cooling

By Cooling Fans/Blowers.

OFAF Forced Oil cooling by Oil pump

Accompanied by cooler fans.

Figure: radiators and cooler fans

2.7 Conservator:

The oil level of a transformer changes with the changes in the temperature of the oil which

in turn depends upon the load on the transformer. The oil expands with the increase in load and

contracts when the load decreases. Large transformers are also liable to overloads which may

overheat the oil and consequently there is a sludge formation if air is present. This causes the

vaporization of a part of the oil. The oil vapours forms explosive mixture with air that ignites and

may cause a considerable damage. For these reasons it is necessary to prevent the oil from having

contact with air as well as the moisture. For this purpose conservators are employed

Conservator is a small auxiliary oil tank that may be mounted above the transformer and

connected to the main tank by a pipe. Its function is to keep the main tank of the transformer

completely filled with oil in all circumstances despite expansion or contraction of oil with the

changes in the temperature. Conservator is partly filled with oil and absorbs the expansion and

contraction of oil and keeps the main tank full of oil. It also reduces the rate of oxidation of oil,

partly because less oil surface is exposed to air and partly because of the reduced temperature of

the oil exposed to air. Thus the sludge formation is considerably reduced.

Normally the capacity of conservator should be approximately 10-l2% of the oil volume

of the main tank. The conservator tank is usually insulated on the low voltage side of the

transformer tank above the level of the transformer cover on supporting frame. A small pipe

connection between the gas space in the expansion tank and the cover of the transformer tank

permits the gas above the oil in the transformer to pass into the expansion tank so that the

transformer main tank will be completely filled with the oil

2.8 Silica gel breather:

Whenever there is change in the ambient temperature or in the load on oil immersed

transformers there is a change in the volume of oil in tank and conservator. This change forces the

air above the oil level in the conservator to be either pushed out or breathe in from outside

atmosphere. Whenever the air is breathed in there is a possibility of moisture and dust from the

atmosphere to be sucked in. This is dangerous to the insulating properties of oil Silica gel breathers

are provided to prevent this and to ensure that dry and clean air is breathed in and out through the

breather.

Construction & Operation:

Conventional silica gel breather consists of easing, silica gel crystals, and an oil seal

arrangement at the lower end of the casing.

Dry silica gel crystals which are dark blue in color have a very good capacity to absorb

moisture. When the air from outside is breathed in conservator it passes through the crystals and

moisture in the air is absorbed. Thus we air that reaches the conservator is dry. Dust particles are

partly trapped in the oil seal and partly tapped by the crystals of the silica gel. Silica gel crystals

change their color from dark blue to pink depending upon absorption of certain quantity of

moisture. On turning pink, the efficiency of absorption of moisture becomes Very low. The crystals

can be reactivated by heating them. The moisture absorbed then gets evaporated and the crystals

again turn dark blue and are ready for re-use.

Figure: construction of power transformer Figure: silica gel breather

2.9 ON LOAD Tap Changer (OLTC):

This arrangement is employed for changing the turn ratio of the transformer to regulate the

system voltage while the transformer is delivering load. It is invariable practice to provide the

tapping at the neutral end of the HV windings of a transformer.

Construction:

The On load Tap changer consists of a high speed resistor transition diverter switch, Tap

selector switch, driving mechanism and external driving shaft.

Diverter Switch:

The driver switch consists of contact compartment, transition resistors, and spring acting

mechanism. A perfect oil tight structure is employed between diverter switch and the transformer

tank, to prevent mixing of oil outside the driver switch chamber with the oil inside, which will be

contaminated due to switching operations.

The tips of arcing contacts are made of copper-tungsten alloy which has got excellent are

resisting characteristics. Current limiting resistors consist of nickel chromium wire wound on heat

resisting bobbins held at both ends by the insulating plates.

Oil in the diverter switch is maintained under a seperate conservator head. The pipe leading

to the conservator from the diverter switch chamber is filled with a gas and oil operated relay, the

normally open velocity actuated contact of which is connected to trip circuit of the transformer.

Selector switch:

The tap selector operates under no load condition and therefore, there need not be any fear

of arc generation, damage of contacts or deterioration of main transformer oil.

The tap lead Wires rom the winding of the transformer are brought and terminated at the

fixed contacts of the tap selector. Tap change operation requires two rotary switches per phase.

The odd numbered tappings are connected to one switch and even numbered tappings to the other

in such a way that the two switches come into use alternately.

The selected odd and even numbered contacts are fitted on either insulating bars mounted

vertically to diverter switch chamber.

Principle of Operation:

The odd moving contact is on tap 7 and even side contact on tap 8 of the tap selector. The

odd and even collector contacts are connected to the diverter switch contacts O&E respectively.

Since Diverter switch is making contacts at O (Odd) flow of current will be through tap 7.When a

signal is given to the driving mechanism for changing the tap from 07 to 06, it will first operate

the even contact of tap selector from tap O8 to 06 without changing the position of odd contact.

Simultaneously with the operation of the tap selector, the spring mechanism, which is the energy

accumulator mounted on the top of the diverter switch will be charged.

On completion of tap selector operation ,energy accumulated in the spring will be released

for instantaneous transfer of the diverter switch moving contacts from tap O to E through the

resistor contacts Or and ER thus changing the flow current from tap 07 to 06.

For reverse operation i.e. for changing tap 06 to 07, the tap selector shaft will not operate

as its contacts is already on tap 07. But the transfer of the diverter switch moving contacts from E

to O via ER-OR-O will take place completing the tap changing operation.

CHAPTER III

PROTECTION OF POWER TRANSFORMERS

A power transformer is subjected to various faults through-out its life time. The protection

of power transformer should be effective enough to isolate the power transformer from internal

and external faults of the transformer minimizing the fault clearing time as well as increasing the

life time of transformer. The protection incorporated for an oil immersed type power transformer

is as follows

Buchholz relay:

The gas actuated Buchholz relay is a protective device designed to give indication faults

occurring in oil field conservator type transformers, on load rap changers. All types of faults

occurring within an oil field transformers are accompanied by gas generation. This phenomenon

has been effectively utilized by buchholz relays to provide the best known protection arrangement

for transformers.

The high sensitivity and capability of the relay has been successfully proved to detect faults

stated below:

Defective core laminations.

Breakdown of core insulations.

Local overheating of windings.

Phase to phase, phase to earth or internal short circuits.

Insulation breakdown of major nature.

Construction:

The relay comprises of flanged housing detachable front cover, terminal, box, toughened

glass windows, alarm and tripping device with mercury switches and valves for venting and relay

setting. The alarm and the tripping device consist of 2 counter balanced aluminum buckets which

are hinged and the tilting of these buckets operates the mercury switches.

Air release valve is provided at the top of the housing for releasing the trapped air and for

taking out the gas samples. One more valve is provided for introducing air inside the relay to carry

out the gas volume and surge tests.

The front inspection glass is provided with scale to allow reading the accumulated volume

of the gas and observe color of the gas for fault analysis.IN Service/Test lock can be selected in

the form of movement of red indicator on the reading scale.

Operation:

When any fault occurs the bas formation takes place in the transformer and accumulates in

the Bucholz relay on its way to conservator in consequence, oil level in the relay drops and the

upper counter balanced and hinged bucket moves down tilting the mercury switch to activate alarm

circuit. If the oil level drops further lower bucket also operates and closes the contact for tip circuit.

Commissioning:

While mounting the relay care should be taken to see that the arrow on the relay is pointing

towards the conservator and the air vent valve at top. Ensure that pipe ascends to conservator at

angle between 1° -9° as per specification of manufacturer & the relay is kept in service position.

After installation of the relay and when the transformer has been filled with oil up to the

oil conservator the air trapped in the gas chamber must be allowed to escape through the air vent

valve at top.

Gas Analysis:

Depending on the nature of fault i.e. with winding, paper insulation, oil flashover in the

transformer the nature of gases formed will vary. The type of fault formed in the transformer can

be detected by diagnosing the gases collected in the Bucholz relay.

Gases collected in the Bucholz relay is allowed to pass through a test tube filled with Silver

Nitrate (AgNO3) solution by releasing the Air release plug .The precipitate formed on the walls of

the test tube is observed for diagnosing the fault in the transformer as stated below.

Nature of gas Probable fault

Colorless and odorless Air trapped in oil or insulation.

Grayish white with pungent smell, non over heating of insulation, press board

Flammable

Yellowish inflammable Decomposing of wood insulation

Dark grey inflammable Flash over in oil or due to excessive

Overheating of oil caused by a fault in the

Winding or core.

Oil surge relay:

This relay is sensitive to both low oil level and oil surge conditions. It is usually installed

for Protection against faults in the one Load Tap Changer.

The housing is provided with two flanges for connecting pipes leading to the tap changer

head and to the oil conservator. The flap Valve position can be checked through the inspection

glass on front of the housing.

For checking the tripping function of the relays well as subsequently resting the flap valve,

two push buttons are installed in the terminal box. One pushbutton for actually the trip manually

and the other pushbutton for resetting the trip contact.

OPERATION:

The protective relay is energized by an oil surges from the tap changer to the oil conservator

only. The oil flow operates the flap valve being trapped in to the OFF position. At that moment

the contact is actuated, the circuit breakers are operated and the transformer is switched OFF the

line. It is not energized by the tap changer being subject to nominal load or permissible overload.

Commissioning:

The protective relay has to be mounted in the pipe leading from the tap changer head to the

oil conservator. The relay must be located as near as possible to the tap changer head. Pipe work

rising to the conservator should be arranged at an angle of 5° above the horizontal to ensure the

effective operation of the protective relay with the test push buttons in the top of the housing. The

arrow on the terminal box cover must point towards the oil conservator.

Pressure relief valve:

The pressure relief valve is protective device for oil filled transformer. It is designed to

relieve the excessive pressure which may build up by fault or an arcing inside the transformer tank.

S. No Description

1. Base

2. Gasket

3. O Ring

4. Diaphragm

5. Cover

6. Springs

7. Rod retaining spring

8. Lock nut

9. Switch operating rod

10. Visual indicator

11. Stopper

12. Visual indicator

When the pressure inside the tank exceeds a preset limit, acts on the diaphragm inside

which in turn causes the diaphragm to lift up 4m from its seat. A movable rod attached to this

diaphragm operates a micro switch and makes the transformer to switch OFF. This lifting is

instantaneous and allows venting off the excess pressures inside the tank

It also gives a visual indication of the valve operation by raising a flag.

The diaphragm resets to its original position as soon as the pressure inside the tank drops

below the set limit.

Oil and Winding Temperature Indicators:

The temperature indicator is used as an Oil temperature Indicator (OTI) or as a Winding

Temperature Indicator (WTI) for the protection of liquid immersed power transformers.

Construction:

A Sensing bulb (20) a measuring bellow (2) and a small bore capillary tube (19) connecting

the two form the measuring system. A second bellow called as compensating bellows connected

with a second capillary (19) running parallel to the first capillary and terminated at the head of the

bulb form the temperature compensation system. The bellows are linked to a compensating lever

(17) in such a manner that the effect of ambient temperature changes on the capillary line and

measuring bellows is compensated. The movement of measuring bellows is related only to

temperature being measured by sensing bulb. This movement is amplified by the link and lever

mechanism (4) which directly drives the rotating disc (l3) carrying the control switches and pointer

indicating the temperature.

A heater coil (3-Bellows heater) is fitted around the measuring bellows (2) and supplied

from a current transformer mounted in the busing of the transformer. The heater coil indicates the

hot spot temperature of the windings over top oil temperature for a given load. The measuring

bellows reacts to this simulated temperature rise in addition to the top oil temperature measured

by the sensing bulb and the instrument functions as WTI indicating the temperature of the winding

An adjustable shunt resistor is provided for shunting a portion of current through the heater

coil to obtain precise thermal image. All internal electrical contact of mercury switches are wired

to terminal blocks.

Switches are identified by markings S1, S2, S3, S4 on them. Terminal no's wired to each circuit

are detailed below:

Terminal No's Switch No's Wired to

1, 2 Sl Alarm

3, 4 S2 Tip

5, 6 S3 Cooler Control-l

7, 8 S4 Cooler Control-2

9, 10, 11 Bellows heater & adjustable

Shunt

Magnetic oil Gauge:

A float is used as sensor of liquid level inside the conservator tank. Swing of hinged float

due to change in liquid level is utilized to indicate level in a calibrated dial and to operate the

switches for external alarm units.

Use of magnetic coupling in the indicator achieves complete sealing off the liquid inside

the conservator from surroundings atmosphere. This result in avoiding any leakage of costly oil

and avoiding contamination of insulating oil due to the seepage of surrounding air in the

conservator.

The details of the subassembly parts are as below:

(l)Gear Assembly

(2) Magnetic Couple

(3) Float with arm

(4) Cam assembly with Mercury Switch

(5) Dial with pointer

The float is hinged and swings up or down when oil level rises or falls. This rise or fall

rotates the bevel gear and thus the pinion of the gear assembly. The pinion in turn rotates the

driving magnet inside the conservator. The follower magnet positioned outside carries a pointer

and a cam. The pointer reads oil level and the cam set 0 operate the mercury switch at a

predetermined low level.

Differential Protection:

Differential protection also called as unit protection, is the Main Protection of Power

Transformer as it faults on the unit it is protecting, which is situated between the CT’S. The relay

therefore can be instantaneous in operation, as it does not have to coordinate with any other relay

on the network. This differential protection, as its name implies, compares currents entering and

leaving the protected zone and operates when the differential current between these currents

exceed a predetermined level.

The type of differential scheme normally applied to a transformer is called the current

balance or circulating current scheme as shown in Figure below:

Fig: Circulating current scheme in the case of through faults

The CTs and relay were connected in such a way that the relay is operated by circulating

current between secondary of CT. Differential current does not arise and hence the relay does not

operate for external faults as shown in the fig

Under internal fault conditions (i.e. faults between the CTs) the relay operates

Since both the CT secondary currents add up and pass through the relay as seen in Fig

Fig: Circulating current scheme in the case of internal faults

Unfortunately the circulating current protection mentioned above may operate even for

through faults due to the following factors which need careful consideration:

(a) Transformer vector group (i.e. phase shift between HV and LV)

(b) The possibility of zero sequence current entering the relay may destabilize the

Differential for an External earth fault.

(c) Magnetizing in-rush currents (from one side only)

(d) Mismatch of HV and LV CTs

(e) Varying currents due to on-load tap changer (OLTC).

Factor (a) can be overcome by connecting the HV and LV CTs in star/delta respectively

(Or vice versa) opposite to the vector group connections of the primary windings, so counteracting

the effect of the phase shift through the transformer.

The delta connection of CTs provides a path for circulating zero sequence current, thereby

stabilizing the protection for an external earth fault as required by factor (b).

NOTE: To counter factors (a) & (b), the Thumb rule for differential protection is that the CTs are

to be connected in star for delta connected winding, and vice versa.

As the magnetizing current in-rush is predominantly 2nd, harmonic filters are utilized to

stabilize the protection for this condition (c).

And Finally, It is necessary to bias the differential relay to overcome the current unbalances

caused by factor (d) & (e) i.e. mismatch of CTs and taps of OLTC respectively. And Hence a

Percentage Biased Differential relay is preferred instead of a circulating current principle.

Under-load or through-fault conditions, the CT secondary currents circulate, passing

through the bias windings to stabilize the relay, whilst only small. Out of balance spill currents

will flow through the operate coil, not enough to cause operation. In fact the higher the circulating

current the higher will be the spill current required to trip the relay as shown in the relay

characteristics (fig).

Most transformer differential relays have a bias slope setting of 20%, 30% and 40% as

shown in fig . The desired setting is dictated by the operating range of the OLTC, which is

responsible for the biggest current unbalance under healthy conditions;

Over Current and Earth Fault Protection:

Over current and Earth Fault protection is the Back Up protection for power

transformer. Unlike Differential Protection which operates for internal faults (between CTs), Over

current protection will operate even for through faults, above the pickup current setting, thus

isolating the transformer from feeding the fault.

Over current and earth fault protection is provided on both HV and LV Side of the

winding and hence proper time gradation is to be provided for relay operation starting from the

feeders connected to LV side of transformer to the Over Current protection on HV Side, to avoid

complete black out for an fault on feeder connected to LV Side of transformer.

To achieve selectivity and coordination by time grading two philosophies are

available, namely:

1. Definite time lag (DTL), or

2. Inverse definite minimum time (1DMT).

(1).Definite Time Characteristic:

The relays are graded using a definite time interval of approximately 0.5 s. The relay R3 at

the extremity of the network is set to operate in the fastest possible time, whilst its upstream relay

R2 is set 0.5 s higher. Relay operating times increase sequentially at 0.5 s intervals on each section

moving back towards the source as shown in Fig.

Draw Back:

The problem with this philosophy is, the closer the fault to the source the higher the

Fault current, the slower the clearing time — exactly the opposite to what we should be

trying to achieve.

(2)Inverse Time Characteristic:

A relay having IDMT Characteristic will incorporate lower operating time for higher fault

currents. Various inverse characteristic curves can be selected among Normal Inverse, Very

Normal Inverse, and Extreme inverse curves. For a radial feeder, time grading by means of relays

having IDMT characteristics is shown in fig: below.

IEC Standard Normal Inverse 3.0 sec curve is used for the Over current and Earth Fault

protection of power transformer and the characteristic curve can be expressed as:

Relay Operating time (in sees) = (0.14*TL) / (PSM0.02 -1)

Where TL=Time Setting Multiplier adopted.

PSM=Plug Setting Multiplier = Fault current on CT Primary

_______________________________________

(CT Ratio Adopted) *(Current Setting Adopted on relay)

Example: For a feeder having CT Ratio adopted=400/I, and Relay settings adopted for a relay

having nominal current of 1A as Pick up=100% and Time Lever=0.I; the relay operating time for

a fault current of 800Amps on feeder can be obtained as below:

Plug Setting Multiplier=800/(400*100% of 1 A)=2 & TL=0.1

Then, relay operating time (in secs) = (0.14*0.1)/ (20.02-1) =1sec.

Thus, for a fault current of 800Amps, the relay will operate in 1 sec.

For protection of power transformer proper time gradation is to be provided for I-1V and

LV side IDMT relays.

For an HV winding fault the UV breaker is tripped but the fault can continue to be fed via the

Low voltage side, the back-feed coming from the, adjacent transformer(s) as shown in Fig where

the LV

Protection is set high to 'coordinate with downstream requirements. Thus, the Transformer

Protection should always trip both HV and LV circuit breakers for operation of HV Side IDMT

Relay as shown in Fig:

CHAPTER-4

PRE COMISSIONING TESTS OF A POWER TRANSFORMER

Introduction:

Various tests are performed on power transformer at various stages starting from the first

stage of i.e. transformer construction to the commissioning stage at the site. The tests conducted

can be classified as Type Tests, Routine Tests, etc. The test results obtained for various tests at the

time of commissioning were used as reference values for diagnosing the transformer during any

faults in the life time of transformer. Among various tests, Routine tests performed on power

transformer were mentioned in detail.

Routine Test performed on Power Transformer at site at the time of commissioning

includes:

• Insulation Resistance Test (Megger)

• Ratio Test

• Magnetizing Current test.

• Magnetic Balance test.

• Short Circuit test.

• Winding Resistance.

• Tan delta test of windings & bushings.

(1).Insulation Resistance Test (Megger):

Purpose of IR test is to check over all insulation of transformer .Insulation resistance is a

relative measure of the insulation structure. The test equipment used is a DC Insulation tester

(Megger).The IR test results are correlated with transformer insulation quality by parameters like

Absorption Ratio, Polarization Index.

Test Arrangement

Ensure that the neutral of the power transformer is disconnected from earth and the Oil

temperature of the transformer under test is to be noted as the temperature is also one of the

deciding factors of IR values obtained by this test.

Insulation resistance Value

Voltage Applied: -- Temperature (in °C):

Sl.NO Configuration IR value in mega ohms

1 HV-LV 15 sec 60 sec P.1=60 sec/15 sec

2 HV-body

3 LV-body

(3).Magnetizing current test:

The exciting current test at low voltage is very useful in location of problem such as defects

in magnetic core balance in magnetic core structure, shining of winding, failures in turn to turn

insulation, or i problem in tap changers. The acceptance criteria for the results of exciting current

measurement should be based on the comparison with the previous site test results or factory

results .The general pattern of Magnetizing current test would have two similar higher reading of

magnetizing current on outer phases and lower reading of magnetizing current on the center phase

in case of YNyno three phase transformers.

Test Arrangement:

Apply 3 Phase, 4 wire 415 volts AC from UV side & keep LV, neutral isolated. Measure

magnetizing current in U, V&W phases of HV winding.

Voltage applied Current measured (mA)

IU-IV Phase 415Volts IU phase --------

IV-IW phase 415 Volts IV phase --------

IW-IU phase 415 Volts IW phase --------

Similarly apply 3 Phase, 4 wire 415 volts AC from LV side & keep HV, neutral isolated.

Measure magnetizing current in u, v &w phases of LV winding.

Voltage applied Current measured (mA)

2u-2v Phase 415Volts 2u Phase ………

2v-2w phase 415 Volts 2v phase ………

2w-2u phase 415 Volts 2w Phase ……..

(4).Magnetic Balance Test:

Magnetic Balance Test is conducted to check healthiness of windings, core assembly

condition and flux distribution in the transformer. When the supply is fed to outer phase of a

transformer, the voltage induced in the center phase shall be 50 to 90% of the applied voltage.

However when the center phase is excited then the voltage induced in the either of the outer phases

will be nearly 50% of the applied voltage.

Test Arrangement:

Ensure that primary and secondary windings are kept open. Apply 230Volt AC, on

primary/secondary winding in one phase and measure the voltages in remaining phases. Sum of

the measured voltages in two phases should be equal to the applied voltage

UN VN WN

Supply Measured voltage Measured voltage

Measured voltage Supply Measured voltage

Measured voltage Measured voltage Supply

(5).Short Circuit Test:

Short Circuit test is performed to check the measured Percentage impedance of the

transformer in agreement with name plate details on the power transformer. During internal fault,

any damage to the winding of the transformer may result change in the per unit impedance of

transformer and hence can be reflected in the test results of short circuit test.

Test Arrangement:

Short circuit test is conducted by shorting the LV windings and applying the 3Ph supply to

HV winding. Ensure that the LV Winding phase terminals are shorted to its corresponding neutral

and apply 3ph, 4 wire 440V AC supplies to the HV winding and measure the HV & LV winding

line currents.

Short Circuit Test

HV Winding cut-rents (Amps) LV Winding Currents (Amps)

U V W N u v w n

____ ____ ____ ____ ____ ____ ____ ____

The measured currents in HV and LV windings should be in agreement with calculated values for

a particular applied voltage. )

HV Current at Applied voltage= (Rated HV Current/Impedance in volts)* Applied Voltage Where

Impedance volts= (% Impedance* HV Volts)/100.

NOTE: (1) Conducting the short circuit test by shorting the HV winding and applying the 3ph

supply to LV winding may result in heavy currents on LV side which cannot be supplied by the

LT 3ph Source.

(2)Measurement of currents during short circuit test should be done quickly as the magnitude of

currents produced would be of large value.

(6).Vector Group Test:

Vector group test is conducted to check the HV and LV windings arrangement was in agreement

with the vector group specified by the manufacturer. Various test patterns are to be followed for

various vector groups. The following test arrangement is to be followed for testing a power

transformer having YNynO vector group.

Test Arrangement:

Connect the primary and secondary of one phase together and then apply 3Ph,

4wire 440Volts AC supply to the HV winding of power transformer and measure thd voltages as

specified below. ,

Supply applied on HV Side:

1U1V:

IV1 W:

IWIU:

Sl.NO Verifying conditions measured values

1 1U2n-I-IN2n=lUIN

2 1 W2w= I V2v

3 1 W2w<1W2v

(7).Winding Resistance Test:

Transformer winding resistance is to be measured at site in order the check for

abnormalities due to loose connection broken strands of conductor, high contact resistance in tap

changers, high voltage leads and bushings.

Test Arrangement:

Ensure that I-IV and LV windings are kept open and neutrals disconnected. This test is

to be conducted at all taps of the transformer. DC current of approximately. I °Amps is injected

and the voltage developed across I the test terminals is used to measure the resistance of the

winding which will be of order of ml.

Winding Resistance

Winding Tap Phase U Phase V Phase W

g No

HV 1 to 25 mΩ mΩ mΩ

LV mΩ mΩ mΩ

The test results obtained from this test should be in agreement with the factory test results.

NOTE: Winding resistance test is to be conducted after conducting all the above mentioned tests

failing which the test results occurred would be erratic due to core magnetization in a particular

direction with the injected DC Current during this test.

Transformer Failures

To devise or strategy in order to control the transformer failures, it is very

essential to understand the complete product fundamentally by knowing its internal construction

details, the mechanism of manufacturing each internal component, material composition and

parameters of process required to be controlled while manufacturing alone with quality aspects

during material procurement, manufacturing assembly and testing. The above process is to be

followed by important aspect of transportation, site storage, care during erection and

commissioning and finally the critical requirements during operation and maintenance phase of

the equipment Since large number of transformer failures was taking place it became important to

study and analyze every individual failure through feedback mechanism and take the necessary

corrective action. In the above process the failure data was generated and described in the

subsequent paragraphs

Reasons for Failure

Insulation failure due to aging

Failure due to short circuit forces

Failure due to Design deficiencies

Failure due to manufacturing defects

Failure due to Maintenance problems

Failure due to Quality Problems

1. Insulation failure due to aging

Transformer insulation life is considered to be 30years and all the failures have place

much before the total expected life. So, the failure of transformers due to this cause is none.

2. Failure due to short circuit forces

A large number of transformers have failed in this category. The main reason of failure

due to this category is mechanical inadequacy in the transformer to withstand the force experienced

during the short a circuits

2. A) Mechanical forces in transformers

Any current coil placed in a magnetic field experiences forces governed by the laws of

electromagnetic Induction /interaction in the case of transformers, a common flux is linking all the

windings and this windings are subjected to radial and axial forces whenever there is variation in

current. These changes can be due to normal changes in load current or due to faults in the system

externally to the transformer or switching operation of the transformer itself.

In transformers the axial flux in limbs interacts with the current in the windings to produce radial

forces in the windings. Whether these forces create hoop stress or compressive stress in windings

depend upon the polarity of the respective windings chosen. This is, by convention, in such a way

to generate hoop stress in the outer winding and compressive stress in the inner windings

In addition to above forces, axial forces are generated in the windings due to two factors

• Leakage flux

• Difference in the heights of the windings

All efforts are made in the design itself to keep the leakage flux as low as possible. Symmetry in

the windings is due to the difference in no of turns, insulation and cross section of the conductors

this asymmetry is made up by using permali wood blocks, insulation packing etc. and symmetry

is achieved during assembly stage in the shop floor. However, since the insulation paper is used in

the transformer shrinks in course of the time while transformer is in service, it can give rise to

asymmetry also any deficiency ignored during manufacturing makes the possibility of axial forces

in the transformer windings. the disturbance caused due to axial forces cause further asymmetry

in the windings resulting into increase in the intensity of subsequent axial forces every time I thus

any slackness in the windings has a compounding effect on the axial forces To prevent the

movement of the windings explained as above and thus to ensure the trouble free operation of the

transformers during the entire expected life, coil clamping arrangement is provided the clamping

is such as not to allow even minor movements of the windings which otherwise may cause so

damage to the windings insulation at a vulnerable points the clamping arrangements in general

consist of circular wooden blocks placed on top of the windings force applied over this blocks

using screws supported from yoke structure the arrangement ensures that clamping of the windings

is uniform all above the periphery.

For checking mechanical strength of a transformer to withstand short circuit forces, presently

adequate facilities are not available in the country. A transformer designer takes care of this aspect

based on certain guidelines as per NEMA/IEEE standards. However the short circuit testing has

been incorporated in the transformer specifications as type test for which the large capacity

transformers are being sent abroad where such facilities are available.

Failure due to Design deficiencies

While designing a transformer mainly two aspects need to be tackled very carefully. These

are dielectric length of the transformer and mechanical strength of the support structure so that

various parts and windings tlo not get dislodge or deformed due to axial and radial forces produced

during service. For ensuring the dielectric strength the transformer is subjected to various voltage

stress tests in shop floor these test are

f) Induced Over Voltage withstand test

g) Lightning impulse test

h) Switching impulse test

Once these tests are done on a transformer in the shop floor and there is no failure, it is presumed

that the transformer has been designed electrically as per the specifications. However during

service the transformer internal conditions deteriorate and dielectric strength as tested during shop

floor may not remain same and failure may occur.

The present day transformer design is being done by computer aided software available with the

designers. With the help of this software the total geometry of the transformer can be plotted and

the flux mapping and voltage stress in each area can reassessed. The total assessment can be done

in 3-d plots and the 'necessary corrective actions can be taken in the design itself. Use of shunts

and provision of little extra insulation at the places of high leakage flux or voltage stress are few

such techniques used by the designers now a day. Due to competition in the market, the designers

keep very little safety margin to make the product more, competitive. It is however, essential that

the sufficient safety margins in the internal electrical clearances are kept so that no failure due to

weakness in the dielectric- strength takes place this aspect is taken care of by specifying the

increased BIL (Basic Insulation Level) of I425K V in place of 1300KV.This necessitates the

manufacturer to keep additional electrical clearance and automatically the safety margins are

increased.

4. Failure due to manufacturing defects

Among the broad categories of causes the transformer failures have been

attributed to the defects inherent in the equipment due to flaws occurring due to manufacturing

account for a large number of total failures. The transformer manufacturing is mainly done by

skilled technicians. The reliability of the transformer 0 is dependent on workers skill to a large

extent. A small mistake during the manufacturing cycle can result in the failure of the transformer.

Although a lot of mechanization of the manufacturing process has been done in 111) advanced

countries to overcome this problem in India we still rely on the work force and manual

manufacturing processes,

Other factors responsible for transformer failures due to manufacturing

deficiencies are lack of required skill for the critical jobs. Lack of proper facilities at works like

controlled and dust proof atmosphere for EHV transformer shortage of time required to meet the

yearly committed targets, unhealthy working environment like 4D strikes and labor trouble, on

availability of required material in tisme because of which substitute material is r used etc. Every

care is required to be taken to avoid such failures due to manufacturing deficiencies Fortunately .0

due to various diagnostics techniques the transformer deficiencies could be detected and advance

action could op be taken to avoid major failures. However the outage of the equipment during the

course of inspection and • rectification of the problem has resulted into loss of generation

5. Maintenance Problem

If a transformer is to give a long trouble free service, it should receive a reasonable

amount of attention L and maintenance during service period. Maintenance consists of regular

inspection, testing and reconditioning IPIP of the transformer wherever necessary. The

manufacturers specify the complete inspection of the transformer core-coil assembly at a

frequency of 8 to 10 years in case of large transformers. During this inspection the 6 windings are

to be cleaned with hot oil jet to remove sludge deposited on these in order to improve the cooling

efficiency. Apart from this the core-coil assembly is required to be tightened up to the original

torque values because during services these become loose. Although the inspection it is very

cumbersome, it will certainly improve the reliability of the equipment in the long run.

The transformer failures can occur in two ways

a) Slow developing faults if adequate care is not taken in time

b) Sudden faults

Transformer is subjected to various electrical and thermal stresses while in operation resulting into

liberation of various fault gasses from the decomposition of the hydrocarbon mineral oil used as

an insulating media and coolant. This solid insulating materials like paper, press boards, wooden-

-supports etc. also decomposed and liberate various gases which eventually get dissolved into the

oil. The most significant gases which are liberated under fault conditions are hydrogen (h2),

methane (ch4), ethane (c2h6), ethylene (c2h4), acetylene (c2h2), propane (c3h8), propylene

(c3h6), carbon monoxide (co), carbon dioxide (cot) .These gases are regularly monitored in the oil

samples by carrying out dissolve gas analyses (DGA) and the trend of gas analysis is further

analysed to interpret the nature of fault inside the transformer. Based on the interpretation and

severity of expected internal fault, the action is required to be taken and either internal inspection

is to be carried out to locate the fault or the oil needs to be filtered to improve the parameters like

break down voltage (BDV) and moisture etc. For slow developing or incipient faults dissolve gas

analysis is being regularly done as it a diagnostic technique. Reliability of this technique has been

proven on most of the occasions and most of the occasions and most of the failures have been

averted by site inspection and rectification of the defect

To avoid sudden failures in transformers, some of the maintenance aspects need greater attention.

One such aspect is the air ingress in case of transformers with oil forced air forced (OFAF) cooling

arrangement. The oil pumps develop high discharge pressure which is sufficient to circulate oil to

keep the transformer I temperature low so that hot spot temperature limit does not exceed under

any operating regime. All the valves pipes, pumps and expansion joints connecting coolers to the

transformers tank have gasket joints at connecting flanges. Any leakage in gasket joints on the

suction side of the pump will lead to ingress of air into the cooling circuit. This ingress air can take

a path along with the directed oil flow path and also can get locked in some critical winding area.

This localized air can cause localized hot spot in the absence of oil in that area. The entrapped air

might further get ionized due large electrical field resulting into insulation failure.

Apart from such catastrophic failures of the transformers, the ingress air will cause oxidation

of insulating oil and cellulosic insulating material. Oxidation of oil will result into sludge formation

thereby causing the oil properties to deteriorate very fast. The dissolved oxygen also plays major

part in accelerating the insulation aging. Thus, any air ingress is to be avoided and should be

attended at the earliest.

Another crucial aspect to avoid sudden transformer failures is to take proper care of bushings.

The high • voltage condenser bushings are built up around a center pipe which is called the core.

The insulating paper is wound around the core under heat and pressure. During winding of paper

insulation on the core, the grading layers of aluminum are integrated to achieve uniform voltage

distribution. The core is then dried under heat, vacuum and oil impregnation is done. The

impregnated core is assembled in outer shell consisting of lower and upper hollow porcelain,

mounting flange and top conservator. The entire bushing assembly is then sealed and. filled with

oil.

All condenser bushings are provided with a test tap, which is used for measurement of

capacitance and tan delta. When the cap of this test tap is screwed on, the tapping from the outer

layer of condenser is earthed. This earth connection is very critical and in case it is broken, the

entire voltage distribution across the condenser bushing, will get disturbed and lead to bushing

failure.

The tan delta and capacitance values of the bushings are to be monitored annually and the limit is

0.7% maximum for tan delta value for oil impregnated paper (01P) bushings and maximum

allowable capacitance is 110% of shop test results.

Quality Problems

Certain components used in the transformer, if not of the desired quality, can result

in serious damage to the transformer. Terminal bushings, copper conductor and OLTC are main

components which have resulted into failures during the past. Special care is mechanism for the

bought out items used in transformer manufacturing pan from this, process control for stabilization

of the coils during manufacturing to achieve uniform height of each coil is very important. As

explained previously, any discrepancy in the heights of different coils in a transformer will result

into large axial forces and eventual failure during service.

STEPS TO REDUCE TRANSFORMER FAILURES

To reduce transformer failures, various steps were taken at different stages

covering the aspects of deficiencies during manufacturing quality control, finalization of design,

operation and maintenance etc. The details of such steps taken in past by NTPC are as follows:

BASIC INSULATION LEVEL (BIL)

For 400 KV class transformers, the earlier specified B1L was 1300 KV which was

increased 1425 KV. long with this the switching impulse of 1050 KV has been increased to 1180

KV and induced over voltage with partial discharge measurement has been changed from 420/364

KV for 5 seconds/30 minutes to 460/510 KV for 5 seconds/1 hour. With this DEL, the

manufacturer shall be forced to keep larger internal clearances and eventually the safety margins

shall increase.

TEMPERATURE RISE LIMITS

Earlier oil temperature rise of 50 Degree C above an ambient of 50 Degree C was being

specified which has now been reduced to oil temperature rise of 35 degree C above an ambient of

50 Degree C.

In place of earlier specified winding temperature rise of 55 Degree C above 50 Degree C

ambient, the new limits for winding temperature rise are 40 Degree C over and above 50 Degree

C ambient temperature. *This new specification has forced the transformer manufacturers to

improvise the cooling efficiency and since p the overall operating temperatures are reduced, the

expected insulation life thereby gets enhanced.

TAP CHANGER ON GENERATOR TRANFORMER

To avoid the transformer failures due to on load tap changer problems, the generator

transformers are being specified with off circuit tap changer only. The provision of OLTC is being

kept in standby and unit -7 transformers.

IMPEDANCE OF GENERATOR TRANSFORMERS

In place of earlier specified impedance of 12 to 5% over the entire tap range for generator /3

transformers, the same is now been specified as 1.3.5% principal tap with variation of ± in order

to facilitate interchangeability of transformers.

TEMPERATURE RISE TEST

For 400 KV class transformers and generator transformers temperature rise test at I tO%

of rated current has been specified as routine test along with DGA before and after the temperature

rise test. This test certifies `at that the thermal design including the manufacturing are perfect and

the temperature rise during service period of the transformer shall remain within the specified

regime only.

CHECKING OF DESIGN CALCULATIONS

For ENV grade transformers, the design calculations are being asked from the

manufacturers and specific in checks are being carried out for transfer surge calculations and

withstand calculations against short circuits so that voltage withstand limits and forces are not

exceeded than the design values.

ADDITIONAL TESTS ON EHV TRANSFORMERS

Following additional test have been incorporated sc as to improve the reliability of transformers:

a) Short circuit tests to be included as type test

b) HV busing snap test

c) Chopped Wage test as routine test

d) RSO measurements as routine test on generator transformers.

DEW POINT MEASUREMENT

In order to ensure the complete dryness of the transformer, dew point measurement

technique has been introduced at works at the time of dispatch and at site during erection and

internal inspections. The permissible AO limit for dew points is — 30Degree C. If the required

dew point is not achieved, the transformer is required to be further dried up with the help of dry

nitrogen of dew point better than -50 Degree C and heating.

FREQUENCY RESPONSE ANALYSIS (FRA)

This test has been introduced as routine test for generator transformers and before

and after short circuit tests for all ENV transformers so as to ensure that no winding/core movement

has taken place during the testing, by monitoring the FRA signatures.

INCREASE IN CORE INSULATION

Earlier the magnetic circuit was being specified for insulation level of 2 KV for

one minute but multiple core earthing phenomenon was very common. To avoid this problem the

magnetic circuit is now being specified to withstand 10 KV for one minute.

9.0 CONCLUSIONS

Non-destructive testing used as diagnostic tool will be of a great importance in future in

order to predict any failure by trend analysis. Advance remedial actions can help in avoiding major

catastrophic failures and improve the equipment availability. Tests like dissolve gas analysis, tan

delta capacitance measurement. FRA, partial discharge measurement and vibration analysis along

with low voltage tests have helped in diagnosing the problems and are of immense future benefit.

The analysis, categorization and critical examination of the circumstances leading to failure

give very -useful information regarding the mechanism of failure. With proper analysis there is a

possibility of preventing majority of the failures.