Pwer Transformer

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2.Power transformers

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Posted by arjunarjun September 21, 2008 September 21, 2008 advertisments advertisments 2.1 Introduction 2.1 Introduction

Transformer is the most important unit in

Transformer is the most important unit in an electrical distribution network. All transformers arean electrical distribution network. All transformers are subjected to thorough tests at

subjected to thorough tests at the manufacturer’s works before despatch to the destination of the manufacturer’s works before despatch to the destination of erection.erection. Due to limitations in transport, large

Due to limitations in transport, large capacity transformers are dis-assembled into variouscapacity transformers are dis-assembled into various components before despatch. At site, the

components before despatch. At site, the transformers are re-assembled with the various componentstransformers are re-assembled with the various components like bushings, coolers, conservator etc. and then the internal body is dried out to remove the surface like bushings, coolers, conservator etc. and then the internal body is dried out to remove the surface moisture sticking to the paper

moisture sticking to the paper insulation during exposure at site. As insulation during exposure at site. As erection of transformers involveerection of transformers involve assembly of various components, pre-test inspection of transformers have greater importance than assembly of various components, pre-test inspection of transformers have greater importance than other parts of an

other parts of an electrical system. The following paragraphs explain the pre-test inspections/pre-electrical system. The following paragraphs explain the pre-test inspections/pre-commission checks and the pre-inspections/pre-commission tests to

commission checks and the pre-commission tests to be conducted on power transformers prior tobe conducted on power transformers prior to energisation of the unit.

energisation of the unit.

2.2 Recording the salient parameters

As the service life of a transformer is expected to cover very many years, it

As the service life of a transformer is expected to cover very many years, it is necessary to record theis necessary to record the salient parameters of the transformer for future reference. Rated capacity, rated

salient parameters of the transformer for future reference. Rated capacity, rated voltage ratio,voltage ratio, connection, make, maker’s serial number, year of manufacture, date of

connection, make, maker’s serial number, year of manufacture, date of completion of erection,completion of erection, insulation dry out details at site etc. may be documented in a

insulation dry out details at site etc. may be documented in a register as permanent record. It is alsoregister as permanent record. It is also necessary to record the serial

necessary to record the serial number, rating and make of various components like number, rating and make of various components like bushings, tapbushings, tap changer, tap changer control cubicle, cooler control cubicle, cooling fans,

changer, tap changer control cubicle, cooler control cubicle, cooling fans, oil pumps, Buchholtzoil pumps, Buchholtz relay,temperatur

relay,temperature indicators, heat exchangers, oil flow e indicators, heat exchangers, oil flow meter, water flow meter, pressure gauges, oilmeter, water flow meter, pressure gauges, oil level gauge etc. For easy reference, the details of the main body and various components may be level gauge etc. For easy reference, the details of the main body and various components may be recorded in separate pages of a register. This register will serve as a

recorded in separate pages of a register. This register will serve as a record of the service of therecord of the service of the transformer.Det

transformer.Details of replacement of components may ails of replacement of components may also be recorded in also be recorded in the same register.the same register. 2.3 Pre-commission checks

2.3 Pre-commission checks

Before commencing the pre-commission tests, it is

Before commencing the pre-commission tests, it is necessary to visually inspect various parts,necessary to visually inspect various parts, components and accessories of the

components and accessories of the transformer and also to conduct operational check for transformer and also to conduct operational check for variousvarious protective devices. Check lists may

protective devices. Check lists may be followed for the visual be followed for the visual inspection and the operational checksinspection and the operational checks so that the pre-commission checks are conducted in a systematic manner and also that no check/test so that the pre-commission checks are conducted in a systematic manner and also that no check/test is omitted. Model check lists for General checks and Functional checks are given at the end of is omitted. Model check lists for General checks and Functional checks are given at the end of thischapter in Appendix 2.1 and 2.2

thischapter in Appendix 2.1 and 2.2 respectively.respectively. 2.3.1 General checks

2.3.1 General checks

(i) General arrangement

The General arrangement of the electrical installation shall

The General arrangement of the electrical installation shall be checked for concurrence with thebe checked for concurrence with the scheme approved by the Department of

scheme approved by the Department of Electrical Inspectorate. Special emphasis may be given to Electrical Inspectorate. Special emphasis may be given to thethe following:

following: · size of cables · size of cables

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· size of bus bars · size of bus bars · size of bus trunking · size of bus trunking · size of

· size of earthing conductorsearthing conductors · adequacy of various clearances · adequacy of various clearances · spacing between supports

· spacing between supports · ventilation

· ventilation

· oil drain facilities · oil drain facilities · fire protection walls · fire protection walls · fire

· fire fighting arrangementsfighting arrangements (ii) Terminations

(ii) Terminations

The transformer terminal connections may be checked for the

The transformer terminal connections may be checked for the following:following: · flexibility and area of cross section of flexible connections at bushings · flexibility and area of cross section of flexible connections at bushings · clearances of live

· clearances of live jumper connections from transformer tank and accessoriesjumper connections from transformer tank and accessories · socket size

· socket size

· perfection of crimpings · perfection of crimpings · tinning of contact s

· tinning of contact surfaces to prevent bimetalic actionurfaces to prevent bimetalic action · clearances inside cable end box

· clearances inside cable end box · clearances of bus bar trunking · clearances of bus bar trunking · conformity of cable end

· conformity of cable end box with the relevant IP( box with the relevant IP( Ingress Protection) classificationIngress Protection) classification · correctness of cable glanding and adequacy of cable gland earthing or pig tail

· correctness of cable glanding and adequacy of cable gland earthing or pig tail · support of cables

· support of cables at terminations and unsupported lengthsat terminations and unsupported lengths (iii) Perfection of connections

(iii) Perfection of connections

Connections to the following shall be

Connections to the following shall be checked for proper surface contact, seating and checked for proper surface contact, seating and tightness.tightness. · to bushings

· to bushings

· to the tap changer · to the tap changer · to earth leads · to earth leads

· to control and protective cables · to control and protective cables · to thermometers

· to thermometers (iv) Earthing

(iv) Earthing

Check the size of earthing conductors, tinning of contact surfaces, area of contact and seating, Check the size of earthing conductors, tinning of contact surfaces, area of contact and seating, effectiveness of bolting, socketing, riveting, welding etc.for the earthing of the

effectiveness of bolting, socketing, riveting, welding etc.for the earthing of the following:following: · Duplicate earthing for neutral and

· Duplicate earthing for neutral and bodybody · Main tank and top cover

· Main tank and top cover · Fan motors

· Fan motors · Pumps · Pumps

· On Load Tap Changer (OLTC) chamber · On Load Tap Changer (OLTC) chamber · Tap changer driving gear

· Tap changer driving gear · Divertor switch

· Divertor switch · Cable

· Cable glands/terminationglands/termination · Marshalling box

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(v) Control cable connections

Check the control cable connections between the

Check the control cable connections between the followingfollowing · transformer accessories and marshalling box

· transformer accessories and marshalling box · marshalling box and sub-station panel

· marshalling box and sub-station panel · tap changer control cubicle

· tap changer control cubicle and sub-station paneland sub-station panel (vi) Radiator

(vi) Radiator

Check the radiator for release of air and position of valves. The valves shall be

Check the radiator for release of air and position of valves. The valves shall be in open position.in open position. (vii) Main conservator and OLTC conservator

(vii) Main conservator and OLTC conservator

Check the oil level in the main conservator and OLTC conservator . The

Check the oil level in the main conservator and OLTC conservator . The conservator shut off valveconservator shut off valve in the Buchholtz relay pipe line shall be in open position.

in the Buchholtz relay pipe line shall be in open position. (viii) Bushings

(viii) Bushings

Check the oil level in the bushings if sealed

Check the oil level in the bushings if sealed bushings are used. Release air from bushings if airbushings are used. Release air from bushings if air release plugs are provided.

release plugs are provided. (ix) Breather

(ix) Breather

Check the oil level in the oil seal

Check the oil level in the oil seal of the breather. Check the colour of the silica gel in the of the breather. Check the colour of the silica gel in the breather .breather . (x) Cooler units, fans and pumps

(x) Cooler units, fans and pumps

Check fans and pumps for proper mounting. The number of fans and their position on the radiators Check fans and pumps for proper mounting. The number of fans and their position on the radiators shall be in

shall be in conformity with the general arrangement drawings.conformity with the general arrangement drawings. · Check the direction of rotation of cooling fans and pumps · Check the direction of rotation of cooling fans and pumps · Check the direction of oil flow

· Check the direction of oil flow

· Check flow of water in heat -exchangers · Check flow of water in heat -exchangers

· Measure the Insulation Resistance (IR) of fans and pumps · Measure the Insulation Resistance (IR) of fans and pumps · Check the settings for operation of fan motors and oil pumps · Check the settings for operation of fan motors and oil pumps

· Check the cooler unit for correct indication of oil flow and setting of the thermometer · Check the cooler unit for correct indication of oil flow and setting of the thermometer (xi) Winding Temperature Indicator (WTI) and

(xi) Winding Temperature Indicator (WTI) and Oil Temperature Indicator (OTI)Oil Temperature Indicator (OTI)

· Check whether thermometer pocket is

· Check whether thermometer pocket is filled with oilfilled with oil · Check whether the

· Check whether the connections of the CT connections of the CT for winding temperature indicator to the thermometerfor winding temperature indicator to the thermometer pocket is properly made as per the instructions given on the WTI terminal box.

pocket is properly made as per the instructions given on the WTI terminal box. · Check whether the contacts of WTI and OTI for alarm and trip are

· Check whether the contacts of WTI and OTI for alarm and trip are set at required temperaturesset at required temperatures depending upon ambient temperature and loading conditions. For oil filled transformers, the depending upon ambient temperature and loading conditions. For oil filled transformers, the maximum permissible temperature rise above the ambient temperature is usually taken

maximum permissible temperature rise above the ambient temperature is usually taken as 450C foras 450C for oil and 550C for winding. In the case of cast

oil and 550C for winding. In the case of cast resin transformers, the alarm contact of the windingresin transformers, the alarm contact of the winding temperature relay is usually designed to operate at 1400 C and trip contact at 1600 C

temperature relay is usually designed to operate at 1400 C and trip contact at 1600 C forfor transformers upto 1000 kVA. For higher ratings, the

transformers upto 1000 kVA. For higher ratings, the temperatures are 1600 C and temperatures are 1600 C and 1800C1800C respectively.

respectively.

· Calibration of the WTI/OTI may be checked with hot oil. Working of the WTI/RTD repeaters shall · Calibration of the WTI/OTI may be checked with hot oil. Working of the WTI/RTD repeaters shall be checked at the control room.

be checked at the control room. (xii) Buchholtz relays

(xii) Buchholtz relays

· Check the angle of mounting of the Buchholtz relay using a spirit level · Check the angle of mounting of the Buchholtz relay using a spirit level · Check the floats for free movement

· Check the floats for free movement · Release air in the Buchholtz relay · Release air in the Buchholtz relay · In the case of

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when the pumps are switched on when the pumps are switched on xiii) Magnetic Oil Level Gauge

xiii) Magnetic Oil Level Gauge

Move the float level of the oil level indicator up and down between the end

Move the float level of the oil level indicator up and down between the end positions to ensure thatpositions to ensure that the mechanism does not get stuck at any point. The

the mechanism does not get stuck at any point. The low oil level alarm of the gauge shall be checked.low oil level alarm of the gauge shall be checked. (xiv) Arcing horn gap

(xiv) Arcing horn gap

Check arcing horn gaps of

Check arcing horn gaps of bushings for conformity with standard values. The standard values bushings for conformity with standard values. The standard values areare given below:

given below:

(xv) Tap changer

Check the sequence of operation of the tap changer for the following: Check the sequence of operation of the tap changer for the following: · manual operation

· manual operation

· local electrical operation · local electrical operation · remote electrical operation · remote electrical operation · parallel operation

· parallel operation

(xvi) General inspection

i. Heaters in cubicles, conservator, etc. shall be checked i. Heaters in cubicles, conservator, etc. shall be checked ii. Any other alarm/trip, contacts of

ii. Any other alarm/trip, contacts of flow meters, differential pressure gauges, etc. shall be flow meters, differential pressure gauges, etc. shall be checkedchecked iii. In the case of water cooled transformers, the pressure gauge readings on water and oil sides shall iii. In the case of water cooled transformers, the pressure gauge readings on water and oil sides shall be checked to ensure that the water pressure is less

be checked to ensure that the water pressure is less than the oil pressure. The quantity of oil andthan the oil pressure. The quantity of oil and water flow shall not be less than what is

water flow shall not be less than what is specifiedspecified iv. The angle

iv. The angle of protection of the lightning shield provided for outdoor transformers shall beof protection of the lightning shield provided for outdoor transformers shall be checked. The angle shall be less than 30

checked. The angle shall be less than 30 degreesdegrees v. Check whether roller blocks are

v. Check whether roller blocks are provided for the rollers of the provided for the rollers of the transformertransformer 2.3.2 Functional checks

2.3.2 Functional checks

After the visual inspection is

After the visual inspection is complete, it is necessary complete, it is necessary to test proper functioning of various protectiveto test proper functioning of various protective relays and instruments. The following functional checks may

relays and instruments. The following functional checks may be carried out.be carried out.

1. Check the operation of the Buchholtz alarm and trip by injecting air through the test pet cock. 1. Check the operation of the Buchholtz alarm and trip by injecting air through the test pet cock. 2. Test the OTI for alarm and trip.

2. Test the OTI for alarm and trip. 3. Test the WTI for alarm and trip. 3. Test the WTI for alarm and trip.

4. Check the working of the WTI / RTD

4. Check the working of the WTI / RTD (Resistance Temperature Device) repeater(Resistance Temperature Device) repeaters at thes at the control room.

control room.

5. Test the OLTC –

5. Test the OLTC – Oil surge relay for trip.Oil surge relay for trip. 6. Check alarm for low oil level .

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7. Check the REF relay for current setting 7. Check the REF relay for current setting

8. Check the differential relay for main and bias settings 8. Check the differential relay for main and bias settings

9. Check the back up over current and earth fault relays for current and time. 9. Check the back up over current and earth fault relays for current and time. 10. Check the over voltage relay for voltage and time.

10. Check the over voltage relay for voltage and time. 11. Check the instantaneous over

11. Check the instantaneous over voltage relay for voltage.voltage relay for voltage. 12. Check the over

12. Check the over fluxing relay for voltage, frequency and time.fluxing relay for voltage, frequency and time. 13. Check the cooler unit for

13. Check the cooler unit for · over current setting of fans · over current setting of fans

· over current setting of oil pumps · over current setting of oil pumps · cooler supply failure alarm

· cooler supply failure alarm · fan/pump trip alarm

· fan/pump trip alarm

.any mal- operation of the transformer Buchholtz relay when all the oil pumps are switched on .any mal- operation of the transformer Buchholtz relay when all the oil pumps are switched on simultaneously in forced oil

simultaneously in forced oil cooled transformerscooled transformers 2.4 Pre-commissioning tests

2.4 Pre-commissioning tests

2.4.1 Insulation Resistance test

Insulation Resistance test is the simplest and most widely used test to

Insulation Resistance test is the simplest and most widely used test to find out the soundness of find out the soundness of insulation between two windings or between windings and

insulation between two windings or between windings and ground. Insulation resistance is measuredground. Insulation resistance is measured by means of insulation testers popularly known as ‘Megger’. The ‘Megger’ consists of a D.C power by means of insulation testers popularly known as ‘Megger’. The ‘Megger’ consists of a D.C power source (hand operated or electrically driven D.C generator or

source (hand operated or electrically driven D.C generator or a battery source with a battery source with electronicelectronic circuit ) and a

circuit ) and a measuring system. Microprocessor based insulation testers are also measuring system. Microprocessor based insulation testers are also now available.now available. The insulation test reveals the

The insulation test reveals the condition of the insulation inside the condition of the insulation inside the transformer. The insulationtransformer. The insulation resistance values are affected by

resistance values are affected by temperature, humidity and presence of dirt on insulators andtemperature, humidity and presence of dirt on insulators and bushings.

bushings.

Selection of Insulation Testers

Insulation testers with test voltage of 500, 1000, 2500 and 5000 V are available. The recommended Insulation testers with test voltage of 500, 1000, 2500 and 5000 V are available. The recommended ratings of the insulation testers are given below:

ratings of the insulation testers are given below:

Factors influencing IR value

The IR value

The IR value of transformers are influenced byof transformers are influenced by 1. surface condition of the

1. surface condition of the terminal bushingterminal bushing 2. quality of oil

2. quality of oil

3. quality of winding insulation 3. quality of winding insulation 4. temperature of oil

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5. duration of application and value of test voltage 5. duration of application and value of test voltage Different IR values

Different IR values monitored in transformersmonitored in transformers The following IR values are

The following IR values are monitored in transformersmonitored in transformers

1. winding to ground. eg. HV to LV and earth connected together LV to

1. winding to ground. eg. HV to LV and earth connected together LV to HV and earth.HV and earth. 2. winding to winding. eg. HV to LV

2. winding to winding. eg. HV to LV

3. all windings to ground .eg. HV and LV to

3. all windings to ground .eg. HV and LV to earth.earth. Steps for measuring the IR

Steps for measuring the IR

1. Shut down the

1. Shut down the transformer and disconnect the jumpers and lightning arrestors.transformer and disconnect the jumpers and lightning arrestors. 2. Discharge the winding capacitance.

2. Discharge the winding capacitance. 3. Thoroughly clean all bushings

3. Thoroughly clean all bushings 4. Short circuit the windings. 4. Short circuit the windings. 5. Guard the terminals to

5. Guard the terminals to eliminate surface leakage over terminal bushings.eliminate surface leakage over terminal bushings. 6. Record the temperature.

6. Record the temperature.

7. Connect the test leads (avoid joints). 7. Connect the test leads (avoid joints).

8. Apply the test voltage and note the reading. The IR.

8. Apply the test voltage and note the reading. The IR. value at 60 seconds after application of thevalue at 60 seconds after application of the test voltage is referred to as the Insulation Resistance of the transformer at the test temperature. test voltage is referred to as the Insulation Resistance of the transformer at the test temperature. Minimum value of IR

Minimum value of IR

The following values of IR at 30deg. C can be

The following values of IR at 30deg. C can be considered to be the minimum requirement for new oilconsidered to be the minimum requirement for new oil filled transformers.

filled transformers.

The transformer IR values in oil drained condition will be 15 to 20 times more than in

The transformer IR values in oil drained condition will be 15 to 20 times more than in oil filledoil filled condition.

condition.

Influence of temperature on IR

IR. values decrease sharply with

IR. values decrease sharply with the rise in the rise in temperature of the oil. The temperature of the oil. The following correction factorsfollowing correction factors may be used for arriving at the IR value with difference in temperature.

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Interpretation of Insulation Resistance value.

While interpreting IR values, importance shall be given to the variation of the values over a period While interpreting IR values, importance shall be given to the variation of the values over a period of time rather than absolute values. For conclusive analysis, use only results from tests performed at of time rather than absolute values. For conclusive analysis, use only results from tests performed at identical conditions as IR values are affected by value of test voltage, temperature of oil, duration of identical conditions as IR values are affected by value of test voltage, temperature of oil, duration of application of voltage, humidity, extent of stress applied etc. IR values recorded over a period of application of voltage, humidity, extent of stress applied etc. IR values recorded over a period of time may be plotted as a curve to

time may be plotted as a curve to study the history of the insulation resistance. A curve showing astudy the history of the insulation resistance. A curve showing a downward trend indicates a loss

downward trend indicates a loss of IR due to of IR due to unfavourable conditions such as oildeterioration,unfavourable conditions such as oildeterioration, excessive moisture in paper, deterioration/damage to terminal bushings etc. A

excessive moisture in paper, deterioration/damage to terminal bushings etc. A very sharp drop is very sharp drop is aa cause for concern and action shall be taken to ascertain the exact cause

cause for concern and action shall be taken to ascertain the exact cause of insulation failure and forof insulation failure and for corrective steps.

corrective steps. Points to note

Points to note

1. Transformers with OLTC have

1. Transformers with OLTC have lower IR values when compared with lower IR values when compared with transformers with off circuittransformers with off circuit tap changer.

tap changer.

2. Auto transformers have lower IR

2. Auto transformers have lower IR when compared to two winding transformers.when compared to two winding transformers. 3. Transformer windings with graded insulation have lower IR

3. Transformer windings with graded insulation have lower IR when compared to fullywhen compared to fully insulated windings.

insulated windings. 4. If the

4. If the non-measured winding terminals are not guarded, the megger will give non-measured winding terminals are not guarded, the megger will give a low reading.a low reading. 5. Avoid meggering when the transformer is

5. Avoid meggering when the transformer is under vacuum.under vacuum. 2.4.2 Dielectric absorption and polarisation index tests

2.4.2 Dielectric absorption and polarisation index tests

Dielectric absorption and polarisation index tests give a

Dielectric absorption and polarisation index tests give a good indication of the condition of good indication of the condition of thethe insulation. This test is

insulation. This test is based on the comparison of based on the comparison of absorption characteristics of good insulationabsorption characteristics of good insulation versus absorption characteristics of humid or contaminated insulation.

versus absorption characteristics of humid or contaminated insulation. Instruments/mater

Instruments/materials ials requiredrequired

Motorised or electronic insulation testers of appropriate voltage Motorised or electronic insulation testers of appropriate voltage Stop watch

Stop watch

Logarithmic paper Logarithmic paper Procedure for test

Procedure for test

In this test, a test voltage is

In this test, a test voltage is applied for an extended period of time, usually thirty minutes, using aapplied for an extended period of time, usually thirty minutes, using a megger of appropriate voltage. The megger readings are

megger of appropriate voltage. The megger readings are taken every 10 seconds for taken every 10 seconds for the first minutethe first minute and thereafter every minute – upto

and thereafter every minute – upto 30 minutes. The procedure for measurement of IR 30 minutes. The procedure for measurement of IR under paraunder para 2.4.1. is followed here. Hand

2.4.1. is followed here. Hand cranked instruments are not suitable as cranked instruments are not suitable as continuous application of continuous application of voltage is not possible

voltage is not possible with such instruments. Motorised or battery operated insulation testers arewith such instruments. Motorised or battery operated insulation testers are used for the test. A curve is

used for the test. A curve is drawn showing the variation in the value of IR. against time on adrawn showing the variation in the value of IR. against time on a logarithmic paper. The resultant curve is known

logarithmic paper. The resultant curve is known as dielectric absorption curve. A typical dielectricas dielectric absorption curve. A typical dielectric absorption curve is shown in fig. 2.1.

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Polarisation Index is the ratio of Insulation Resistance at 10 minutes to Insulation Resistance at 1 Polarisation Index is the ratio of Insulation Resistance at 10 minutes to Insulation Resistance at 1 minute of application of test voltage.

minute of application of test voltage.

Polarisation Index =Insulation Resistance at 10 minutes/Insulation Resistance at 1 minute. Polarisation Index =Insulation Resistance at 10 minutes/Insulation Resistance at 1 minute. Interpretation of Polarisation Index and

Interpretation of Polarisation Index and Dielectric Absorption CurveDielectric Absorption Curve

A steady increase in

A steady increase in insulation resistance with continuous application of test voltage indicates thatinsulation resistance with continuous application of test voltage indicates that the insulation is clean

the insulation is clean and dry. Flat or and dry. Flat or ambulated curves demand reconditioning of the insulation.ambulated curves demand reconditioning of the insulation. Polarisation index is a good

Polarisation index is a good appraisal of the condition of appraisal of the condition of the insulation.the insulation. The following are the guidelines for

The following are the guidelines for evaluating the condition of transformer insulation with respectevaluating the condition of transformer insulation with respect to Polarisation Index values.

to Polarisation Index values.

Polarisation indices with respect to insulation resistance between HV and LV + earth , LV and

Polarisation indices with respect to insulation resistance between HV and LV + earth , LV and HV+HV+ earth , earth and

earth , earth and HV+LV are evaluated to ascertain HV+LV are evaluated to ascertain the real condition of the the real condition of the transformer insulation.transformer insulation. 2.4.3 Two Voltage Test (Step Voltage Test)

2.4.3 Two Voltage Test (Step Voltage Test)

This test is an extension of the

This test is an extension of the dielectric absorption test. This has been recommended as a moredielectric absorption test. This has been recommended as a more conclusive indication of presence of moisture. Two

conclusive indication of presence of moisture. Two separate dielectric absorption tests made atseparate dielectric absorption tests made at different voltages help to detect moisture in the winding. The higher test voltage should be about 4 different voltages help to detect moisture in the winding. The higher test voltage should be about 4 to 5 times the lower one, (eg.2500 V and 500 V)

to 5 times the lower one, (eg.2500 V and 500 V) but should not be so high as but should not be so high as to damage theto damage the insulation. A wide spread between the

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moisture. If the insulation resistance value decreases

moisture. If the insulation resistance value decreases substantially at a higher voltage, say substantially at a higher voltage, say more thanmore than 25 percent, it is a reasonable indication of presence of moisture in the insulation system.

25 percent, it is a reasonable indication of presence of moisture in the insulation system. 2.4.4 Measurement of Tan delta

2.4.4 Measurement of Tan delta

Various insulation resistance tests explained above indicate mainly the

Various insulation resistance tests explained above indicate mainly the surface conditions andsurface conditions and presence of moisture in the

presence of moisture in the insulation. Measurement of loss factor, commonly referred to as insulation. Measurement of loss factor, commonly referred to as tan deltatan delta reveals the internal condition of the

reveals the internal condition of the insulation. With alternating currents, the absorption of theinsulation. With alternating currents, the absorption of the dielectric is intimately connected with the loss of power in the dielectric. This loss within an dielectric is intimately connected with the loss of power in the dielectric. This loss within an insulation structure is associated with the

insulation structure is associated with the oscillation of polar molecules trying to orient oscillation of polar molecules trying to orient themselvesthemselves with the alternating electric field. Hence current flowing through the

with the alternating electric field. Hence current flowing through the insulation leads the voltage byinsulation leads the voltage by some angle which is slightly less than 90 degrees. This

some angle which is slightly less than 90 degrees. This small angle between pure capacitive currentsmall angle between pure capacitive current and actual current represented by d (delta) is known as loss angle. The dielectric loss in

and actual current represented by d (delta) is known as loss angle. The dielectric loss in an insulationan insulation is given by V2 w C

is given by V2 w C tan d and hence proportional to tan d. If the insulation is perfect, thetan d and hence proportional to tan d. If the insulation is perfect, the

characteristic of tan delta versus the applied voltage is almost a horizontal line. If voids have crept in characteristic of tan delta versus the applied voltage is almost a horizontal line. If voids have crept in the insulation during manufacture or service, there will be

the insulation during manufacture or service, there will be substantial increase in tan delta substantial increase in tan delta with thewith the applied voltage. Hence the absolute values

applied voltage. Hence the absolute values of tan delta in of tan delta in a commercially manufactured equipmenta commercially manufactured equipment have comparatively little practical signific

have comparatively little practical significance. But the variation in tan delta – ie. ance. But the variation in tan delta – ie. D tan delta – withD tan delta – with respect to time is

respect to time is very important. The values found during maintenance testing should be very important. The values found during maintenance testing should be comparedcompared with the initial values recorded before commissioning the

with the initial values recorded before commissioning the equipment. A stable value of tan equipment. A stable value of tan delta isdelta is indicative of insulation stability and small increase is indicative of normal ageing. Tan delta is indicative of insulation stability and small increase is indicative of normal ageing. Tan delta is measured using tan delta measuring equipment.

measured using tan delta measuring equipment. 2.4.5 Transformer Ratio Test

2.4.5 Transformer Ratio Test

Transformer ratio test is conducted to ensure that the turns ratio tally with the name plate details and Transformer ratio test is conducted to ensure that the turns ratio tally with the name plate details and also that tap changer connections are done correctly. Ratio test is done using a transformer turns also that tap changer connections are done correctly. Ratio test is done using a transformer turns ratio tester or with voltmeters. With the turns ratio tester, the turns ratio is directly read on the tester ratio tester or with voltmeters. With the turns ratio tester, the turns ratio is directly read on the tester for each tap and for each phase of the

for each tap and for each phase of the winding.The turns ratio can also be tested by applying a singlewinding.The turns ratio can also be tested by applying a single phase ac voltage (approximately 230V) on the HV side and measuring the voltage on the low

phase ac voltage (approximately 230V) on the HV side and measuring the voltage on the low voltage side at all tap positions.

voltage side at all tap positions.

The results of the voltage ratio test may be recorded in tabular form as

The results of the voltage ratio test may be recorded in tabular form as given below:given below: Voltage ratio test ( by voltmeter )

Voltage ratio test ( by voltmeter )

2.4.6 Short circuit current measurement

This test is carried out as

This test is carried out as a check for any loose contact in the tap a check for any loose contact in the tap changer, lead connections etc. Inchanger, lead connections etc. In this test, all the 3 windings in the

this test, all the 3 windings in the LV side of the transformer are short circuited. All contacts in theLV side of the transformer are short circuited. All contacts in the tap changer, lead connections and terminals are

tap changer, lead connections and terminals are checked for proper contact. From a variable voltagechecked for proper contact. From a variable voltage source, a 3 phase balanced low voltage a.c supply is

source, a 3 phase balanced low voltage a.c supply is applied to the HV winding at rated tap and theapplied to the HV winding at rated tap and the current measured. The current measured at rated tap should tally with the calculated value of HV current measured. The current measured at rated tap should tally with the calculated value of HV current at the applied voltage.

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The value of HV current at the applied voltage is calculated as

The value of HV current at the applied voltage is calculated as follows:follows:

Repeat the test at different tap positions by lowering and raising taps. The current measured in the Repeat the test at different tap positions by lowering and raising taps. The current measured in the HV winding should tally with the

HV winding should tally with the calculated value of HV short calculated value of HV short circuit current.circuit current.

Wide difference between the measured and calculated values of HV short circuit current is an Wide difference between the measured and calculated values of HV short circuit current is an indication of loose contact in

indication of loose contact in tap changer or lead connections.tap changer or lead connections. 2.4.7 Measurement of Magnetising current

2.4.7 Measurement of Magnetising current

The magnetising current is measured to test any fault in the magnetic circuit and winding. The The magnetising current is measured to test any fault in the magnetic circuit and winding. The measured values are compared with the factory test values. A balanced three phase 415V ac supply measured values are compared with the factory test values. A balanced three phase 415V ac supply is applied to the LV winding and the simultaneous current readings of the three phases are taken is applied to the LV winding and the simultaneous current readings of the three phases are taken using low range a.c ammeters of the same accuracy class. For a

using low range a.c ammeters of the same accuracy class. For a core type transformer, the middlecore type transformer, the middle phase magnetising current will be approximately half that in

phase magnetising current will be approximately half that in other windings. In YyO, Dy1 and other windings. In YyO, Dy1 and Dy11Dy11 connections, the currents in ‘u’ and ‘w’ phases will be nearly double the current in ‘v’ phase. In a connections, the currents in ‘u’ and ‘w’ phases will be nearly double the current in ‘v’ phase. In a Yd1 connected transformer, curren

Yd1 connected transformer, currents in ‘v’ and ‘w’ phases will be ts in ‘v’ and ‘w’ phases will be nearly equal and the current in ‘u’nearly equal and the current in ‘u’ phase more than that in ‘v’ and ‘w’ phases. In a

phase more than that in ‘v’ and ‘w’ phases. In a Yd11 connection, currents in ‘u’ and ‘v’ phases willYd11 connection, currents in ‘u’ and ‘v’ phases will be nearly equal and the current in ‘w’ phase more than that in

be nearly equal and the current in ‘w’ phase more than that in ‘u’ and ‘v’ phases. If the measured‘u’ and ‘v’ phases. If the measured values widely differ from the above values or from the factory test values, there is reason to suspect values widely differ from the above values or from the factory test values, there is reason to suspect some defect in the

some defect in the transformer core and the manufacturer may be consulted. Thetransformer core and the manufacturer may be consulted. The

measured values of magnetising current may be used as bench marks for the service life of the measured values of magnetising current may be used as bench marks for the service life of the transformer. Sample format for recording the magnetising current is given below:

transformer. Sample format for recording the magnetising current is given below:

2.4.8 Test for magnetic balance

This test is done to

This test is done to find out the condition of stacking of core laminations, tightness of core bolts andfind out the condition of stacking of core laminations, tightness of core bolts and perfection of magnetic circuit. The HV and LV sides are isolated by removing the bushing

perfection of magnetic circuit. The HV and LV sides are isolated by removing the bushing connections. A single phase supply of nearly 230V is applied to one phase of

connections. A single phase supply of nearly 230V is applied to one phase of the star connectedthe star connected winding and the induced voltage in

winding and the induced voltage in othertwo phases are measured. The voltage othertwo phases are measured. The voltage may preferably bemay preferably be applied on the HV winding, as applying voltage to LV winding may induce very high voltage in the applied on the HV winding, as applying voltage to LV winding may induce very high voltage in the HV winding. If the HV winding is connected in delta, the test may be

HV winding. If the HV winding is connected in delta, the test may be conducted on the LV side afterconducted on the LV side after taking necessary precautions against accidental contact with the

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When the voltage is applied to the middle phase, the induced voltage measured on the two other When the voltage is applied to the middle phase, the induced voltage measured on the two other phases should be approximately equal. Where the voltage is applied to an extreme phase, the phases should be approximately equal. Where the voltage is applied to an extreme phase, the induced voltage on the middle phase should be substantially high when compared to the voltage induced voltage on the middle phase should be substantially high when compared to the voltage induced in the other extreme phase. In each test, the sum of

induced in the other extreme phase. In each test, the sum of the induced voltages in two phasesthe induced voltages in two phases should be nearly equal to the applied voltage.

should be nearly equal to the applied voltage.

Tests may be carried out by connecting a series lamp (say

Tests may be carried out by connecting a series lamp (say 25 watts) at supply side to restrict higher25 watts) at supply side to restrict higher current, if any. If the

current, if any. If the series lamp glows brightly or the series lamp glows brightly or the induced voltage readings in different phasesinduced voltage readings in different phases indicate zero or very low

indicate zero or very low value or if the value or if the induced voltages show abnormal variation from theinduced voltages show abnormal variation from the expected values, fault in the winding can be suspected.

expected values, fault in the winding can be suspected.

For measuring the voltages, high impedance voltmeter like digital

For measuring the voltages, high impedance voltmeter like digital multimeter should be used. Themultimeter should be used. The test may be repeated by applying voltage to the second and third phases and

test may be repeated by applying voltage to the second and third phases and measuring the inducedmeasuring the induced voltages in other phases. When the magnetic circuit is balanced, there would be symmetry in the voltages in other phases. When the magnetic circuit is balanced, there would be symmetry in the value of measured induced voltages. The

value of measured induced voltages. The measured voltages may be recorded in measured voltages may be recorded in the sample formatthe sample format given below:

given below:

2.4.9 Phasor Group Test

Phasor relationship between HV and LV voltages is checked by this test. Without earthing the Phasor relationship between HV and LV voltages is checked by this test. Without earthing the winding neutral points, interconnect one phase of HV winding – say 1U – to the corresponding winding neutral points, interconnect one phase of HV winding – say 1U – to the corresponding phase of LV winding -2U and apply a balanced 3 phase

phase of LV winding -2U and apply a balanced 3 phase low voltage to the HV winding. The phaselow voltage to the HV winding. The phase sequence of the supply should be the same as

sequence of the supply should be the same as the specified phase sequence of the transformerthe specified phase sequence of the transformer winding. Connections for Dy 11 and

winding. Connections for Dy 11 and Dy1 transformers and the corrsponding vector groupings areDy1 transformers and the corrsponding vector groupings are given in figures 2.2 and 2.3. Measure the voltage between the primary and the secondary terminals. given in figures 2.2 and 2.3. Measure the voltage between the primary and the secondary terminals. The following requirements shall be fulfilled depending on the

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If two transformers are available for test, the phasor groups can be compared easily by applying If two transformers are available for test, the phasor groups can be compared easily by applying voltage from same source to identical bushings on the HV side and by measuring the voltage voltage from same source to identical bushings on the HV side and by measuring the voltage between identically marked terminals on the LV

between identically marked terminals on the LV sides with single interconnection between either thesides with single interconnection between either the neutrals or any one phase.

neutrals or any one phase.

2.4.10 Test for Transformer Oil

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Transformer oil is of petroleum origin and is used as a coolant and dielectric in transformers. Transformer oil is of petroleum origin and is used as a coolant and dielectric in transformers. Transformer oil in good condition and conforming to relevant standards

Transformer oil in good condition and conforming to relevant standards will prevent deterioration of will prevent deterioration of transformer insulation

transformer insulation. As the transformer oil is, to some extent, exposed to . As the transformer oil is, to some extent, exposed to air at site, it is alwaysair at site, it is always necessary to test the oil for various characteristics before the transformer is put to service. As the necessary to test the oil for various characteristics before the transformer is put to service. As the various tests for transformer oil are laboratory tests, details of these tests are beyond the scope of this various tests for transformer oil are laboratory tests, details of these tests are beyond the scope of this book. Test procedures for various tests are given in the relevant standards of BIS, list of which are book. Test procedures for various tests are given in the relevant standards of BIS, list of which are given at the end of this chapter in para

given at the end of this chapter in para 2.6.2.6.

From the point of view of field tests, what is important is the

From the point of view of field tests, what is important is the method of and precautions formethod of and precautions for collecting the transformer oil and the limit

collecting the transformer oil and the limit values of various characteristics. However a rough test values of various characteristics. However a rough test onon the moisture content of the oil can be made at

the moisture content of the oil can be made at site by conducting a simple test, popularly called thesite by conducting a simple test, popularly called the crackle test. In this test, a piece of

crackle test. In this test, a piece of steel tube of approximately 25 mm dia is closed at one end steel tube of approximately 25 mm dia is closed at one end andand the closed end is heated to just

the closed end is heated to just under red hot . Now the hot end is under red hot . Now the hot end is plunged into the oil sample withplunged into the oil sample with the ear close to the open end.

the ear close to the open end. If the oil contains large quantity of moisture, a sharp crackle will beIf the oil contains large quantity of moisture, a sharp crackle will be heard.Dry oil will only sizzle.

heard.Dry oil will only sizzle. Sampling of oil – General

Sampling of oil – General precautionsprecautions

Since the results of the tests prescribed for transformer oil largely depend on the impurities in the Since the results of the tests prescribed for transformer oil largely depend on the impurities in the sample sent for testing, it is essential to keep

sample sent for testing, it is essential to keep the oil free from any contamination. The followingthe oil free from any contamination. The following precautions shall be taken while

precautions shall be taken while collecting samples of transformer oil.collecting samples of transformer oil.

1. For collecting the sample, glass containers with glass stoppers are preferred over metal type. Wax 1. For collecting the sample, glass containers with glass stoppers are preferred over metal type. Wax shall not be used for sealing the containers. The stopper may be

shall not be used for sealing the containers. The stopper may be covered with a piece of cloth packedcovered with a piece of cloth packed with silica gel.

with silica gel.

2. The container may be warmed to above the ambient air temperature in order to avoid any 2. The container may be warmed to above the ambient air temperature in order to avoid any condensation of moisture.

condensation of moisture.

3. Before collecting the sample, all equipments used for handling the oil must be washed with clean 3. Before collecting the sample, all equipments used for handling the oil must be washed with clean transformer oil. The oil used

transformer oil. The oil used for washing must be discarded.for washing must be discarded.

4. Flexible steel hose may be used for handling the oil. Some kinds of

4. Flexible steel hose may be used for handling the oil. Some kinds of synthetic hoses are alsosynthetic hoses are also suitable. Ordinary rubber hose should not be used as oil dissolves the sulphur in the rubber and suitable. Ordinary rubber hose should not be used as oil dissolves the sulphur in the rubber and thereby gets contaminated. The hose used must be clean and free from dust, rust and scale. thereby gets contaminated. The hose used must be clean and free from dust, rust and scale. 5. The operator shall take special care to see

5. The operator shall take special care to see that his hands do not come in contact with the that his hands do not come in contact with the samplesample or the internal surface of

or the internal surface of the container.the container. 6. The transformer oil shall

6. The transformer oil shall be protected against all kinds be protected against all kinds of light radiation during transportation andof light radiation during transportation and storage.

storage.

Sampling procedure

1. Remove the valve shield if fitted. 1. Remove the valve shield if fitted.

2. Remove all visible dirt and dust from the valve with a

2. Remove all visible dirt and dust from the valve with a lintfree clean cloth.lintfree clean cloth. 3. Run off sufficient quantity of oil- say 1 litre – to

3. Run off sufficient quantity of oil- say 1 litre – to eliminate any contaminant that might haveeliminate any contaminant that might have accumulated in the drain cock.

accumulated in the drain cock.

4. Rinse the container with the oil being sampled. 4. Rinse the container with the oil being sampled.

5. Fill the container by allowing the oil to flow against the side

5. Fill the container by allowing the oil to flow against the side of the containers to avoid air traps.of the containers to avoid air traps. 6. Close and seal the container and store the

6. Close and seal the container and store the samples in a dark place.samples in a dark place. Evaluation of test results

Evaluation of test results

Three samples of oil from the top and bottom of the tank are

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transformers of capacity below 1 MVA and where very high reliability is not expected the five transformers of capacity below 1 MVA and where very high reliability is not expected the five characteristics given in table 2.1 shall be invariably tested and it shall be ensured that the

characteristics given in table 2.1 shall be invariably tested and it shall be ensured that the test resultstest results are within the

are within the minimum/maximminimum/maximum limits.um limits.

In the case of transformers where very high reliability is required and in all cases where the capacity In the case of transformers where very high reliability is required and in all cases where the capacity is 1 MVA or above, the additional characteristics given in table 2.2 shall also

is 1 MVA or above, the additional characteristics given in table 2.2 shall also be tested. The testbe tested. The test results shall be within the given limits.

results shall be within the given limits.

2.4.11 Relay Tests

All protective relays , CTs, PTs and control wiring shall be tested as

All protective relays , CTs, PTs and control wiring shall be tested as explained under the chapter forexplained under the chapter for protective relays. The relays shall be set to suit the

protective relays. The relays shall be set to suit the operating conditions and to coordinate with otheroperating conditions and to coordinate with other sections of the system. The relay test results shall be

sections of the system. The relay test results shall be documented for future referedocumented for future reference.nce. 2.5 Commissioning

2.5 Commissioning

After completing all the pre-commission tests given under section

After completing all the pre-commission tests given under section 2.4, the pre-test checks under2.4, the pre-test checks under section 2.3 are redone once again. All the protective relays and circuit breakers are tested for proper section 2.3 are redone once again. All the protective relays and circuit breakers are tested for proper working. The relay settings are kept at a low value so

working. The relay settings are kept at a low value so that the transformer will get isolated if there isthat the transformer will get isolated if there is any internal fault.Allow a settling time of at least 24 hours for oil and then release air

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points. Now the transformer may be

points. Now the transformer may be test charged from the incoming side test charged from the incoming side on no-load and operated foron no-load and operated for about two hours.

about two hours.

Observe the transformer hum for any abnormality. Any vibration or

Observe the transformer hum for any abnormality. Any vibration or abnormal magnetising currentabnormal magnetising current may also be

may also be observed. After continuous operation for about two hours, isolate the observed. After continuous operation for about two hours, isolate the transformer andtransformer and check the gas operated Buchholtz relay for any gas collection. Any dissolved air or air bubble that check the gas operated Buchholtz relay for any gas collection. Any dissolved air or air bubble that may be collected in the Buchholtz relay may be released and the transformer charged again on may be collected in the Buchholtz relay may be released and the transformer charged again on noload. All connected instruments may be checked

noload. All connected instruments may be checked for any abnormal indication. Now gradually loadfor any abnormal indication. Now gradually load the transformer to full capacity and keep it under constant observation for at least 24 hours of

the transformer to full capacity and keep it under constant observation for at least 24 hours of operation. Check the oil and

operation. Check the oil and winding temperature at full load and compare with winding temperature at full load and compare with factory test values.factory test values. After four or five days of service, test the oil for various characteristics, especially for BDV. Any After four or five days of service, test the oil for various characteristics, especially for BDV. Any gas collection in the Buchholtz relay may also be observed. If the test results and

gas collection in the Buchholtz relay may also be observed. If the test results and observations areobservations are found normal, the transformer may be cleared for

found normal, the transformer may be cleared for regular service. After the transformer is put inregular service. After the transformer is put in service for some weeks

service for some weeks with normal working temperature, all sealed joints shall be with normal working temperature, all sealed joints shall be re -tightened.re -tightened. The results of the various tests shall be

The results of the various tests shall be recorded and kept in the station as a permanent record forrecorded and kept in the station as a permanent record for future reference. Details such as place

future reference. Details such as place of erection, date of commissioning, protection given to theof erection, date of commissioning, protection given to the transformer etc. may be furnished to the

transformer etc. may be furnished to the manufacturer after commissioningmanufacturer after commissioning.. 2.6 List of Indian Standards relevant to testing of Power transformers

2.6 List of Indian Standards relevant to testing of Power transformers

Appendix

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