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An Important OverView about Power Transformers for Electrical Tech!


Cover this document an important overview about Power Transformers provides a main ideas of Power Transformers for Distribution or Substations areas for instance. Lest go to play information about it, we recommend to read this ;)

2.1 Introduction
Transformer is the most important unit in an electrical distribution network. All transformers are subjected to thorough tests at the manufacturer’s works before despatch to the destination of erection.Due to limitations in transport, large capacity transformers are dis-assembled into various components before despatch. At site, the transformers are re-assembled with the various components like bushings, coolers, conservator etc. and then the internal body is dried out to remove the surface moisture sticking to the paper insulation during exposure at site. As erection of transformers involve assembly of various components, pre-test inspection of transformers have greater importance than other parts of an electrical system. The following paragraphs explain the pre-test inspections/pre-commission checks and the pre-commission tests to be conducted on power transformers prior to 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 is necessary to record the salient parameters of the transformer for future reference. Rated capacity, rated voltage ratio,connection, make, maker’s serial number, year of manufacture, date of completion of erection,insulation dry out details at site etc. may be documented in a register as permanent record. It is also necessary to record the serial number, rating and make of various components like bushings, tap changer, tap changer control cubicle, cooler control cubicle, cooling fans, oil pumps, Buchholtz relay,temperature indicators, heat exchangers, oil flow meter, water flow meter, pressure gauges, oil 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 record of the service of the transformer.Details of replacement of components may also be recorded in the same register.
2.3 Pre-commission checks
Before commencing the pre-commission tests, it is necessary to visually inspect various parts,components and accessories of the transformer and also to conduct operational check for various protective devices. Check lists may be followed for the visual inspection and the operational checks 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 thischapter in Appendix 2.1 and 2.2 respectively.
2.3.1 General checks
(i) General arrangement

The General arrangement of the electrical installation shall be checked for concurrence with the
scheme approved by the Department of Electrical Inspectorate. Special emphasis may be given to the
· size of cables
· size of bus bars
· size of bus trunking
· size of earthing conductors
· adequacy of various clearances
· spacing between supports
· ventilation
· oil drain facilities
· fire protection walls
· fire fighting arrangements
(ii) Terminations
The transformer terminal connections may be checked for the following:
· flexibility and area of cross section of flexible connections at bushings
· clearances of live jumper connections from transformer tank and accessories
· socket size
· perfection of crimpings
· tinning of contact surfaces to prevent bimetalic action
· clearances inside cable end box
· clearances of bus bar trunking
· conformity of cable end box with the relevant IP( Ingress Protection) classification
· correctness of cable glanding and adequacy of cable gland earthing or pig tail
· support of cables at terminations and unsupported lengths
(iii) Perfection of connections
Connections to the following shall be checked for proper surface contact, seating and tightness.
· to bushings
· to the tap changer
· to earth leads
· to control and protective cables
· to thermometers
(iv) Earthing

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 following:

· Duplicate earthing for neutral and body

· Main tank and top cover
· Fan motors
· Pumps
· On Load Tap Changer (OLTC) chamber
· Tap changer driving gear
· Divertor switch
· Cable glands/termination
· Marshalling box
(v) Control cable connections
Check the control cable connections between the following
· transformer accessories and marshalling box
· marshalling box and sub-station panel
· tap changer control cubicle and sub-station panel
(vi) Radiator
Check the radiator for release of air and position of valves. The valves shall be in open position.
(vii) Main conservator and OLTC conservator
Check the oil level in the main conservator and OLTC conservator . The conservator shut off valve in the Buchholtz relay pipe line shall be in open position.
(viii) Bushings
Check the oil level in the bushings if sealed bushings are used. Release air from bushings if air release plugs are provided.
(ix) Breather
Check the oil level in the oil seal of the breather. Check the colour of the silica gel in the breather .
(x) Cooler units, fans and pumps
Check fans and pumps for proper mounting. The number of fans and their position on the radiators shall be in conformity with the general arrangement drawings.
· Check the direction of rotation of cooling fans and pumps
· Check the direction of oil flow
· Check flow of water in heat -exchangers
· Measure the Insulation Resistance (IR) of fans and 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
(xi) Winding Temperature Indicator (WTI) and Oil Temperature Indicator (OTI)
· Check whether thermometer pocket is filled with oil

· Check whether the connections of the CT for winding temperature indicator to the thermometer 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 set at required temperatures depending upon ambient temperature and loading conditions. For oil filled transformers, the maximum permissible temperature rise above the ambient temperature is usually taken as 450C for oil and 550C for winding. In the case of cast resin transformers, the alarm contact of the winding temperature relay is usually designed to operate at 1400 C and trip contact at 1600 C for transformers upto 1000 kVA. For higher ratings, the temperatures are 1600 C and 1800C respectively.
· Calibration of the WTI/OTI may be checked with hot oil. Working of the WTI/RTD repeaters shall be checked at the control room.
(xii) Buchholtz relays
· Check the angle of mounting of the Buchholtz relay using a spirit level
· Check the floats for free movement
· Release air in the Buchholtz relay
· In the case of forced oil cooled transformers, make sure that the Buchholtz relay does not operate when the pumps are switched on
xiii) Magnetic Oil Level Gauge
Move the float level of the oil level indicator up and down between the end positions to ensure that the mechanism does not get stuck at any point. The low oil level alarm of the gauge shall be checked.

(xiv) Arcing horn gap

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

(xv) Tap changer
Check the sequence of operation of the tap changer for the following:
· manual operation
· local electrical operation
· remote electrical operation
· parallel operation

(xvi) General inspection
i. Heaters in cubicles, conservator, etc. shall be checked
ii. Any other alarm/trip, contacts of flow meters, differential pressure gauges, etc. shall be checked
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 than the oil pressure. The quantity of oil and water flow shall not be less than what is specified
iv. The angle of protection of the lightning shield provided for outdoor transformers shall be checked. The angle shall be less than 30 degrees
v. Check whether roller blocks are provided for the rollers of the transformer
2.3.2 Functional checks
After the visual inspection is complete, it is necessary to test proper functioning of various protective
relays and instruments. The following functional checks may be carried out.
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.
3. Test the WTI for alarm and trip.
4. Check the working of the WTI / RTD (Resistance Temperature Device) repeaters at the
control room.
5. Test the OLTC – Oil surge relay for trip.
6. Check alarm for low oil level .
7. Check the REF relay for current setting
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.
10. Check the over voltage relay for voltage and time.
11. Check the instantaneous over voltage relay for voltage.
12. Check the over fluxing relay for voltage, frequency and time.
13. Check the cooler unit for
· over current setting of fans
· over current setting of oil pumps
· cooler supply failure alarm

· fan/pump trip alarm

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

2.4 Pre-commissioning tests
2.4.1 Insulation Resistance test

Insulation Resistance test is the simplest and most widely used test to find out the soundness of insulation between two windings or between windings and ground. Insulation resistance is measured 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 a battery source with electronic circuit ) and a measuring system. Microprocessor based insulation testers are also now available. The insulation test reveals the condition of the insulation inside the transformer. The insulation resistance values are affected by temperature, humidity and presence of dirt on insulators and bushings.
Selection of Insulation Testers
Insulation testers with test voltage of 500, 1000, 2500 and 5000 V are available. The recommended
ratings of the insulation testers are given below:

Factors influencing IR value
The IR value of transformers are influenced by
1. surface condition of the terminal bushing
2. quality of oil
3. quality of winding insulation
4. temperature of oil
5. duration of application and value of test voltage
Different IR values monitored in transformers
The following IR values are monitored in transformers
1. winding to ground. eg. HV to LV and earth connected together LV to HV and earth.
2. winding to winding. eg. HV to LV
3. all windings to ground .eg. HV and LV to earth.

Steps for measuring the IR
1. Shut down the transformer and disconnect the jumpers and lightning arrestors.
2. Discharge the winding capacitance.
3. Thoroughly clean all bushings
4. Short circuit the windings.
5. Guard the terminals to eliminate surface leakage over terminal bushings.
6. Record the temperature.
7. Connect the test leads (avoid joints).
8. Apply the test voltage and note the reading. The IR. value at 60 seconds after application of the test voltage is referred to as the Insulation Resistance of the transformer at the test temperature.
Minimum value of IR
The following values of IR at 30deg. C can be considered to be the minimum requirement for new oil
filled transformers.

The transformer IR values in oil drained condition will be 15 to 20 times more than in oil filled condition.
Influence of temperature on IR
IR. values decrease sharply with the rise in temperature of the oil. The following correction factors may be used for arriving at the IR value with difference in temperature.

Interpretation of Insulation Resistance value.
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 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 time may be plotted as a curve to study the history of the insulation resistance. A curve showing a downward trend indicates a loss of IR due to unfavourable conditions such as oildeterioration, excessive moisture in paper, deterioration/damage to terminal bushings etc. A very sharp drop is a cause for concern and action shall be taken to ascertain the exact cause of insulation failure and for corrective steps.
Points to note
1. Transformers with OLTC have lower IR values when compared with transformers with off circuit tap changer.
2. Auto transformers have lower IR when compared to two winding transformers.
3. Transformer windings with graded insulation have lower IR when compared to fully
insulated windings.
4. If the non-measured winding terminals are not guarded, the megger will give a low reading.
5. Avoid meggering when the transformer is under vacuum.
2.4.2 Dielectric absorption and polarisation index tests
Dielectric absorption and polarisation index tests give a good indication of the condition of the insulation. This test is based on the comparison of absorption characteristics of good insulation versus absorption characteristics of humid or contaminated insulation.
Instruments/materials required
Motorised or electronic insulation testers of appropriate voltage
Stop watch
Logarithmic paper
Procedure for test
In this test, a test voltage is applied for an extended period of time, usually thirty minutes, using a megger of appropriate voltage. The megger readings are taken every 10 seconds for the first minute and thereafter every minute – upto 30 minutes. The procedure for measurement of IR under para 2.4.1. is followed here. Hand cranked instruments are not suitable as continuous application of voltage is not possible with such instruments. Motorised or battery operated insulation testers are used for the test. A curve is drawn showing the variation in the value of IR. against time on a logarithmic paper. The resultant curve is known as dielectric absorption curve. A typical dielectric absorption curve is shown in fig. 2.1.

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

Interpretation of Polarisation Index and Dielectric Absorption Curve
A steady increase in insulation resistance with continuous application of test voltage indicates that the insulation is clean and dry. Flat or ambulated curves demand reconditioning of the insulation. Polarisation index is a good appraisal of the condition of the insulation.

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

Polarisation indices with respect to insulation resistance between HV and LV + earth , LV and HV+ earth , earth and HV+LV are evaluated to ascertain the real condition of the transformer insulation.
2.4.3 Two Voltage Test (Step Voltage Test)
This test is an extension of the dielectric absorption test. This has been recommended as a more conclusive indication of presence of moisture. Two separate dielectric absorption tests made at 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) but should not be so high as to damage the insulation. A wide spread between the two dielectric absorption curves indicates presence of moisture. If the insulation resistance value decreases substantially at a higher voltage, say more than 25 percent, it is a reasonable indication of presence of moisture in the insulation system.
2.4.4 Measurement of Tan delta
Various insulation resistance tests explained above indicate mainly the surface conditions and presence of moisture in the insulation. Measurement of loss factor, commonly referred to as tan delta reveals the internal condition of the insulation. With alternating currents, the absorption of the dielectric is intimately connected with the loss of power in the dielectric. This loss within an insulation structure is associated with the oscillation of polar molecules trying to orient themselves with the alternating electric field. Hence current flowing through the insulation leads the voltage by some angle which is slightly less than 90 degrees. This small angle between pure capacitive current and actual current represented by d (delta) is known as loss angle. The dielectric loss in an insulation is given by V2 w C tan 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 the insulation during manufacture or service, there will be substantial increase in tan delta with the applied voltage. Hence the absolute values of tan delta in a commercially manufactured equipment have comparatively little practical significance. But the variation in tan delta – ie. D tan delta – with respect to time is very important. The values found during maintenance testing should be compared with the initial values recorded before commissioning the equipment. A stable value of tan delta is indicative of insulation stability and small increase is indicative of normal ageing. Tan delta is
measured using tan delta measuring equipment.
2.4.5 Transformer Ratio Test
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 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 winding.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 voltage side at all tap positions.
The results of the voltage ratio test may be recorded in tabular form as given below:

Voltage ratio test ( by voltmeter )

2.4.6 Short circuit current measurement
This test is carried out as a check for any loose contact in the tap changer, lead connections etc. In this test, all the 3 windings in the LV side of the transformer are short circuited. All contacts in the tap changer, lead connections and terminals are checked for proper contact. From a variable voltage source, a 3 phase balanced low voltage a.c supply is applied 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 at the applied voltage.
The value of HV current at the applied voltage is calculated as follows:

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

Wide difference between the measured and calculated values of HV short circuit current is an indication of loose contact in tap changer or lead connections.
2.4.7 Measurement of Magnetising current
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 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 core type transformer, the middle phase magnetising current will be approximately half that in other windings. In YyO, Dy1 and Dy11 connections, the currents in ‘u’ and ‘w’ phases will be nearly double the current in ‘v’ phase. In a Yd1 connected transformer, currents in ‘v’ and ‘w’ phases will be nearly equal and the current in ‘u’ phase more than that in ‘v’ and ‘w’ phases. In a Yd11 connection, currents in ‘u’ and ‘v’ phases will be nearly equal and the current in ‘w’ phase more than that in ‘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 some defect in the transformer 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 transformer. Sample format for recording the magnetising current is given below:

2.4.8 Test for magnetic balance
This test is done to find 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 connections. A single phase supply of nearly 230V is applied to one phase of the star connected winding and the induced voltage in othertwo phases are measured. The voltage may preferably be 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 conducted on the LV side after taking necessary precautions against accidental contact with the HV bushings.

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 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 the induced voltages in two phases should be nearly equal to the applied voltage.
Tests may be carried out by connecting a series lamp (say 25 watts) at supply side to restrict higher current, if any. If the series lamp glows brightly or the induced voltage readings in different phases indicate zero or very low value or if the induced voltages show abnormal variation from the expected values, fault in the winding can be suspected.

For measuring the voltages, high impedance voltmeter like digital multimeter should be used. The test may be repeated by applying voltage to the second and third phases and measuring the induced voltages in other phases. When the magnetic circuit is balanced, there would be symmetry in the value of measured induced voltages. The measured voltages may be recorded in the sample format given below:

2.4.9 Phasor Group Test
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 phase of LV winding -2U and apply a balanced 3 phase low voltage to the HV winding. The phase sequence of the supply should be the same as the specified phase sequence of the transformer winding. Connections for Dy 11 and Dy1 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. The following requirements shall be fulfilled depending on the vector group of the transformers.

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 between identically marked terminals on the LV sides with single interconnection between either the neutrals or any one phase.
2.4.10 Test for Transformer Oil
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 will prevent deterioration of transformer insulation. As the transformer oil is, to some extent, exposed to air at site, it is always 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 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 2.6.
From the point of view of field tests, what is important is the method of and precautions for collecting the transformer oil and the limit values of various characteristics. However a rough test on the moisture content of the oil can be made at site by conducting a simple test, popularly called the crackle test. In this test, a piece of steel tube of approximately 25 mm dia is closed at one end and the closed end is heated to just under red hot . Now the hot end is plunged into the oil sample with the ear close to the open end. If the oil contains large quantity of moisture, a sharp crackle will be heard.Dry oil will only sizzle.
Sampling of oil – General precautions
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 the oil free from any contamination. The following precautions shall be taken while collecting samples of transformer oil.
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 covered with a piece of cloth packed with silica gel.
2. The container may be warmed to above the ambient air temperature in order to avoid any condensation of moisture.
3. Before collecting the sample, all equipments used for handling the oil must be washed with clean transformer oil. The oil used for washing must be discarded.
4. Flexible steel hose may be used for handling the oil. Some kinds of synthetic hoses are also 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.
5. The operator shall take special care to see that his hands do not come in contact with the sample or the internal surface of the container.
6. The transformer oil shall be protected against all kinds of light radiation during transportation and storage.
Sampling procedure
1. Remove the valve shield if fitted.
2. Remove all visible dirt and dust from the valve with a lintfree clean cloth.
3. Run off sufficient quantity of oil- say 1 litre – to eliminate any contaminant that might have accumulated in the drain cock.
4. Rinse the container with the oil being sampled.
5. Fill the container by allowing the oil to flow against the side of the containers to avoid air traps.

6. Close and seal the container and store the samples in a dark place.
Evaluation of test results
Three samples of oil from the top and bottom of the tank are tested for various characteristics. For 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 test results are within the minimum/maximum limits.

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 be tested. The test results shall be within the given limits.

2.4.11 Relay Tests
All protective relays , CTs, PTs and control wiring shall be tested as explained under the chapter for protective relays. The relays shall be set to suit the operating conditions and to coordinate with other sections of the system. The relay test results shall be documented for future reference.
2.5 Commissioning
After completing all the pre-commission tests given under section 2.4, the pre-test checks under 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 that 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 from all points. Now the transformer may be test charged from the incoming side on no-load and operated for about two hours.

Observe the transformer hum for any abnormality. Any vibration or abnormal magnetising current may also be observed. After continuous operation for about two hours, isolate the transformer and 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 noload. All connected instruments may be checked for any abnormal indication. Now gradually load the transformer to full capacity and keep it under constant observation for at least 24 hours of operation. Check the oil and winding temperature at full load and compare with factory test values. 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 observations are found normal, the transformer may be cleared for regular service. After the transformer is put in service for some weeks with normal working temperature, all sealed joints shall be re -tightened.

The results of the various tests shall be recorded and kept in the station as a permanent record for future reference. Details such as place of erection, date of commissioning, protection given to the transformer etc. may be furnished to the manufacturer after commissioning.

2.6 List of Indian Standards relevant to testing of Power transformers




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