TEST MANUAL OF TRANSFORMER...
Test Manual for Transformers 1. General
Test requirements, procedures and criteria for transformers are defined in national and international standards, i.e. IS 2026 and IEC Publication 60076 in general. This manual describes specific requirements for performing tests specified in IEC Publication 60076, IS 2026 and other standards applicable to distribution, power and regulating transformers. It is intended for use as a guide and reference for testing of transformers. The manual covers purpose, interpretation and explanation of specific conditions pertaining to the testing of transformer. The main objectives of this manual are following: To ensure system needs are met To obtain technical uniformity To provide inputs for proper interpretation of test results To eliminate unsuccessful practices
2. Necessity of tests on transformer
When all manufacturing processes have been completed, tests are performed on transformer at the manufacturer’s works to ensure the following purposes: 1) To prove that the design meets the specified job requirements and to obtain transformer characteristics. 2) To check that the quality requirements have been met and that performance is within the tolerance guaranteed. Tests performed for the former purpose are referred to as Type Tests and that for the latter purpose are referred to as Routine Tests (carried out on every unit manufactured). In addition to the aforesaid two category of tests, Special Tests may also be performed to obtain information useful to the user during operation or maintenance of the transformer. Transformer is important and vital equipment, it is therefore necessary to ensure its proper performance throughout its service life. Also during transportation, installation and service operation, the transformer may be exposed to conditions, which adversely affect its reliability and useful life. It is therefore necessary to do the field testing to ensure good operating health of transformers.
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Test Manual for Transformers 3. Tests The general requirements and details of the various category of tests (Routine Tests, Type Tests and Special Tests) are in accordance with IEC Publication 60076-2003. The Indian standard IS:2026 is being revised in accordance with IEC. The customer specific requirements are referred here as Additional tests and Mechanical Tests. The following tests are generally performed on the transformer which may also forms part of the customer acceptance:
A) Routine Tests
1. Measurement of winding resistance 2. Measurement of voltage ratio, polarity and check of voltage vector relationship 3. Measurement of no-load loss and excitation current 4. Measurement of short-circuit impedance and load loss 5. Measurement of Insulation resistance 6. Tests on on-load tap-changers, where appropriate 7. Switching impulse withstand voltage test, transformer winding Um > 170 kV 8. Lightning impulse withstand voltage test, transformer winding Um > 72.5 kV 9. Separate-source withstand voltage test 10. Induced AC over voltage withstand test with partial discharge measurement
(The tests at sl. no.7, 8, 9 and 10 above are referred as Dielectric Tests)
11. Lightning impulse voltage withstand test, transformer winding Um < 300 kV 12. Temperature rise test
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Test Manual for Transformers Special Tests 13. Lightning impulse test on neutral terminal 14. Long-duration induced AC voltage test (ACLD), transformer winding Um < 170 kV 15. Short-circuit withstand test 16. Measurement of zero-sequence impedances on three phase transformers 17. Measurement of acoustic sound level 18. Measurement of the harmonics of the no-load current 19. Measurement of the power taken by the fan and oil pump motors Additional Special Tests 20. Test with lightning impulse chopped on the tail 21. Magnetic circuit (Isolation) test 22. Determination of capacitances and dissipation factor between winding-to-earth and between windings 23. Magnetic balance test on three-phase transformers 24. Determination of transient voltage transfer characteristics 25. Dissolved gas analysis ( DGA ) of oil filled in the transformer before and after temperature rise test 26. Radio interference voltage ( RIV ) test, if applicable 27. Recurrent surge oscillographic ( RSO ) test 28. Determination of core hot spot temperature 29. Frequency response analysis ( FRA ) test 30. Measurement of magnetization current at low voltage 31. Functional tests on auxiliary equipments 32. Tests on oil filled in transformer
33. Oil pressure test on completely assembled transformer Page 4 of 65
Test Manual for Transformers 34. Jacking test and Dye-penetration test 35. Pressure relief device test
B)Recommended Field tests
1. Dew point measurement for large transformer filled with dry air or nitrogen filled 2. Winding resistance measurement 3. Vector group and polarity 4. Voltage ratio test 5. Measurement of magnetizing current 6. Magnetic balance test on three phase transformer 7. Magnetic circuit (Isolation) test 8. Measurement of short circuit impedance at low voltage 9. Insulation resistance measurement 10. Measurement of capacitance and dissipation factor 11. Dissolved gas analysis ( DGA ) on transformers above 50 MVA 12. Tests on oil filled in transformer as per IS 1866
The dielectric tests (Test Nos. A.8 to A.12) may be routine, type or special tests depending upon the voltage rating, specific customer requirements and referred standards. The purpose, interpretation and explanation for specific test conditions of the tests are briefly described as below. The tests and their sequence shall be mutually agreed between manufacturer and user.
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Test Manual for Transformers 3.1 Measurement of winding resistance
Resistance measurement helps to determine the following a) Calculation of the I2R losses. b) Calculation of winding temperature at the end of a temperature rise test. c) As a base for assessing possible damage in the field.
3.1.1 Determination of cold temperature
The resistance is measured at ambient (cold) temperature and then converted to resistance at 75 0C, for all practical purpose of comparison with specified design values, previous results and diagnostics. The cold temperature of the winding shall be determined as accurately as possible when measuring the cold resistance. The following should be observed.
184.108.40.206 Transformer windings immersed in insulating liquid
The temperature of the winding shall be assumed to be the same as the temperature of the insulating liquid, provided: a) The windings have been under insulating liquid with no excitation and with no current in the winding from three hours to eight hours (depending upon the size of the transformer) before the cold resistance is measured. b) The temperature of the insulating liquid has stabilized, and the difference between top and bottom temperature does not exceed 5 0C.
220.127.116.11 Transformer windings without insulating liquid
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Test Manual for Transformers The temperature of the winding shall be recorded as the average of several thermometers or thermocouples inserted between the coils, with care taken to see that their measuring points are as nearly as possible in actual contact with the winding conductors. It should not be assumed that the windings are at the same temperature as the surrounding air.
3.1.2 Resistance measurement methods The resistance of each winding shall be measured by any one of the following methods. If winding has tapping, then resistance shall be measured on at least principal, maximum and minimum taps.
18.104.22.168 Voltmeter-Ammeter method This method should be employed if the rated current of the transformer winding is one ampere or more. The following steps are performed to conduct this test. a) Measurement is made with direct current, and simultaneous readings of current and voltage are taken. b) To minimize errors of observation: 1) The measuring instruments shall have such ranges as will give reasonably large deflection. 2) The polarity of the core magnetization shall be kept constant during all resistance readings. c) The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured. This is to be avoid including in the reading the resistance of current-carrying leads, their contacts and extra length of leads. d) Readings shall not be taken until after the current and voltage have reached steady-state values. e) Readings shall be taken with not less than four values of current when deflecting instruments are used. f) The current used shall not exceed 15% of the rated current of the winding whose resistance is to be measured. Larger values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance.
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Test Manual for Transformers 22.214.171.124 Bridge method Bridge methods or high-accuracy digital instrumentation are generally preferred because of their accuracy and convenience. The current rating of the measuring instrument should not be very low for large inductive objects. In case of delta connected windings of a large rating transformer, the resistance meter should have adequate current rating. For star connected windings with neutral brought out, the resistance shall be measured by two methods 1) Between line and neutral 2) For small transformer with star connected windings, the resistance shall be measured between phases (line to line), and then resistance of the individual windings shall be determined by dividing the value by 2. This will rule out the effect of the resistance of the neutral lead and bus bars which is significant in comparison to phase resistance of small transformers. However, for the delta connected windings, measurements shall be made between pairs of line terminals. In this case the resistance per winding will be 1.5 X measured resistance between the pair of line terminals. In case of open delta connected winding, the resistance can be measured across all the three windings are in series and also individual winding resistance can be measured. Few precautions are to be carried out to minimize errors while performing the test as follows: a) Charged battery of sufficient capacity or at least 10 A shall be used with the bridge to avoid errors due to drop in battery voltage during measurements. b) To reduce the high inductive effect, it is advisable to use a sufficiently high current to saturate the core. Therefore the measuring instruments shall have high ranges as well as large deflection. c) The polarity of the core magnetization shall be kept same during all resistance readings. A reversal in magnetization of the core can change the time constant and result in erroneous readings. d) The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured. This is to avoid including in the reading the resistances of current-carrying leads and their contacts and of extra lengths of leads. e) To protect the voltmeter from injury by off-scale deflections, the voltmeter should be disconnected from the circuit before switching the current on or off. To protect the personnel from inductive kick, the current should be switched off by a suitably insulated switch. f) Readings shall not be taken until after the current and voltage have reached steadystate values. g) The current used shall not exceed 15% of the rated current of the winding whose resistance is to be measured. Larger values may cause inaccuracy due to heating of the winding and thereby changing its temperature and resistance.
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Test Manual for Transformers
3.2 Measurement of voltage ratio, polarity and check of voltage vector relationship
3.2.1 Ratio test
General The turn ratio of a transformer is the ratio of the number of turns in the high-voltage winding to that in the low-voltage winding. When the transformer has taps, the turn ratio shall be determined for all taps and for the full winding. The ratio tests shall be made at rated or lower voltage and the voltage shall be applied to the winding with higher voltage rating. In the case of three-phase transformers, when each phase is independent and accessible, single-phase supply should be used; although, when convenient, three-phase supply may be used. Tolerances for ratio The tolerances for ratio shall be as specified in IS 2026 Part 1 and IEC 60076-1.
Ratio test methods
Various types of ratio test methods are given in IS: 2026 Part 1 and IEC 60076 -1. Out of those, Ratio Bridge method is most commonly adopted. In this method, the turn ratio on each tapping between pairs of winding shall be measured by a direct reading ratio meter. This method gives more accurate results as compared to other methods described in aforesaid standards. The modern ratio bridge can also be used to test polarity, phase relation and phase sequence. More accurate results can be obtained using a ratio bridge that provides phaseangle correction.
3.2.2 Polarity and Vector group verification
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Test Manual for Transformers Polarity and phase-relation tests are of interest primarily because of their bearing on paralleling or banking two or more transformers. Phase-relation tests are made to determine angular displacement and relative phase sequence. Phase-relation or vector group verification test is performed on a three phase transformer or on a bank of three single-phase transformers. The details of Additive and Subtractive polarity are given in IS: 2026-Part 1 and IEC 60076-1. 126.96.36.199 Polarity by alternating-voltage test For a single-phase transformer having a ratio of transformation of 30 to 1 or less, the polarity test shall be done as follows. The line terminal of high voltage winding (1.1 ) shall be connected to the adjacent line terminal low-voltage lwinding (2.1) as shown in figure 1 source
2.2 Fig : 1 - Polarity by Alternating Voltage Test
Any convenient value of alternating voltage shall be applied to the full high-voltage winding and readings shall be taken of the applied voltage and the voltage between the right-hand adjacent high-voltage and low-voltage leads. When the later reading is greater than the former, the polarity is additive. When the later reading is less than the former (indicating the approximate difference in voltage between that of the high-voltage and low-voltage windings), the polarity is subtractive.
188.8.131.52 Verification of vector group
The phasor diagram of any three-phase transformer that defines the angular displacement and phase sequence can be verified by connecting the HV and LV leads together to excite the unit at a suitably low three-phase voltage, taking voltage measurements between the various pairs of leads and then either plotting these values or comparing them for their relative order of magnitude with the help of the corresponding phasor diagrams, e.g. as Page 10 of 65
Test Manual for Transformers shown in figure 2 and 3. Typical check measurements are to be taken and their relative magnitudes are then compared. Example 1
CONNECT 1U TO 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W VOLTAGE RELATION 1W-2V= 1W-2W 1W-2V< 1W-1U 1V-2V 220 kV
This test is intended to verify the switching impulse withstand strength of the line terminals and its connected windings to earth and other windings, the withstand strength between phases and along the winding under test. The impulses are applied either directly from the impulse voltage source to a line terminal of the winding under test, or to a lower voltage winding so that the test voltage is inductively transferred to the winding under test. The detailed test procedures and specific test requirements are addressed in IEC Publication 60076-3. Switching impulse waves Polarity The polarity of test voltage shall be negative because this reduces the risk of erratic external flashovers in the test circuit. Wave shape
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Test Manual for Transformers The voltage impulse shall have a virtual front time of at least 100 µs, a time above 90% of the specified amplitude of at least 200 µs, and a total duration from the virtual origin to the first zero passage of at least 500 µs but preferably 1000 µs.
Test sequence and records
The test sequence shall consists of one impulse of a voltage between 50% and 75 % of the full test voltage and three subsequent impulses of full voltage. If the oscillographic or digital recording should fail, that application shall be disregarded and a further application made. Oscillographic or digital records shall be obtained of at least the impulse wave-shape on the line terminal under test and preferably the neutral current.
During the test the transformer shall be in a no-load condition. Windings not used for the test shall be solidly earthed at one point but not short-circuited. For a single phase transformer, the neutral terminal of the tested winding shall be solidly earthed. A three-phase winding shall be tested phase by phase with the neutral terminal earthed and with the transformer so connected that a voltage of opposite polarity and about half amplitude appears on the two remaining line terminals which may be connected together. To limit the voltage of opposite polarity to approximately 50% of the applied level, it is recommended to connect high ohmic damping resistors (10 kΩ to 20 kΩ) to earth at the non tested phase terminals.
Failure detection The test is successful if there is no sudden collapse of voltage or discontinuity of the neutral current if recorded on the oscillographic or digital records. Additional observation during the test (abnormal sound effect etc.) may be used to confirm the oscillographic records, but they do not constitute evidence in themselves.
3.8 Lightning Impulse withstand voltage test
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Test Manual for Transformers This test is intended to verify the impulse withstand strength of the transformer under test. This test shall only be made on windings that have terminals brought out through the transformer tank or cover. When non-linear elements or surge diverters are installed for the limitation of transferred over voltage transients, the evaluation of test records may be different compared to the normal impulse test. These non-linear protective devices connected across the windings may cause difference between the reduced full wave and the full-wave impulse oscillograms. To prove that these differences are indeed caused by operation of these devices, this should be demonstrated by making two or more reduced full-wave tests at different voltage levels to show the trend in their operation. The detailed test procedure and specific test requirements are addressed in IEC 60076-3. Impulse wave The test impulse shall be a full standard lightning impulse: 1.2 µs ± 30% / 50 µs ± 20 %. But in some cases this standard impulse shape cannot reasonably be obtained, because of low winding inductance or high capacitance to earth. In such cases wider tolerance may be accepted by the agreement between purchaser and customer. It is recommended to use IEC Publication 60722 as a guide for non-standard wave shapes. Test sequence The test sequence shall consists of one impulse of a voltage between 50% to 75% of full test voltage, and three subsequent impulses at full voltage. If, during any of these applications, an external flashover in the circuit or across a bushing spark gap should occur, or if the oscillographic recording should fail on any of the specified measuring channels, that application shall be disregarded and a further application made.
Test Connections During test on line terminals The impulse test sequence is applied to each of the line terminals of the tested winding in succession. In the case of a three phase transformer, the other line terminals of the winding shall be earthed directly or through a low impedance, not exceeding the surge impedance of the connected line. If the winding has neutral terminal, it shall be earthed directly or through a low impedance such as a current measuring shunt. In the case of separate-winding transformer, terminals of windings not under test are earthed directly or through impedances, so that in all circumstances, the voltage appearing at the terminals is limited to not more than 75% of their rated lightning impulse withstand voltage for star connected windings, and 50% for delta- connected windings.
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Test Manual for Transformers In case of auto transformer, when testing the line terminal of the high voltage winding the non-tested line terminal shall be earthed through resistors not exceeding 400 Ω to get the impulse waveform as needed.
Impulse test on a neutral terminal Impulse withstand capability of neutral may be verified by : a) Indirect application: Test impulses are applied to any one of line terminals or to all three line terminals connected together. The neutral is connected to earth through an impedance or is left open. Then standard lightning impulse is applied to the line terminal which shall not exceed 75% of the rated LI withstand voltage of the line terminal. b) Direct application: Test impulse corresponding to the rated withstand voltage of the neutral is applied directly to the neutral with all line terminals earthed. In this case, however a longer duration of front time is allowed, upto 13 µs.
Records of test The oscillographic or digital records obtained during calibrations and tests shall clearly show the applied voltage impulse shape (front time, time to half value and amplitude). The oscillograms of the current flowing to earth from the tested winding shall also be recorded. Test sequence The test sequence shall consist of one impulse of a voltage between 50% to 75% of full test voltage, and three subsequent impulses at full voltage. If, during any of these applications, an external flashover in the circuit or across a bushing spark gap should occur, or if the oscillographic recording should fail on any of the specified measuring channels, that application shall be disregarded and a further application made.
Failure detection Grounded current oscillograms In this method of failure detection, the impulse current in the grounded end of the winding tested is measured by means of an oscilloscope or by a suitable digital transient recorder connected across a suitable shunt inserted between the normally grounded end of the winding and ground. Any differences in the wave shape between the reduced full-wave and final full-wave detected by comparison of the two current oscillograms, may be indication of failure or deviations due to no injurious causes. They should be fully investigated and Page 30 of 65
Test Manual for Transformers explained by a new reduced wave and full-wave test. Examples of probable causes of different wave shapes are operation of protective devices, core saturation, conditions in the test circuit external to the transformer. The ground current method of detection is not suitable for use with chopped-wave tests. Other methods of failure detection Voltage Oscillograms: Any unexplained difference between the reduced full-wave and final full-wave detected by comparison of the two voltage oscillograms, or any such differences observed by comparing the chopped-waves to each other and to the full-wave up to the time of flashover, are indications of failure. Noise: Unusual noise within the transformer at the instant of applying impulse is an indication of trouble. Such noise should be investigated. Measurement: Measurement of voltage and current induced in another winding may also be used for failure detection. 3.9 Separate source voltage withstand test Duration, frequency, and connections A normal power frequency, such as 50 Hz, shall be used and the duration of the test shall be one minute. The winding being tested shall have all its parts joined together and connected to the terminal of the testing transformer. All other terminals and parts (including core and tank) shall be connected to ground and to the other terminal of the testing transformer. Application of voltage for Separate Source Withstand test The test shall be commenced at a voltage not greater than one-third of the full value and be brought up gradually to full value in not more than 15 s. After being held for the specified time of 60 seconds, it should be reduced (in not more than 5s) to one thirdor less of the maximum value and the circuit opened. Failure detection Careful attention should be started given for evidence of possible failure that could include items, such as an indication of smoke and bubbles rising in the oil, an audible sound such as a thump, or a sudden increase in test circuit current. Any such indication should be carefully investigated by observation, by repeating the test, or by other test to determine if a failure has occurred. 3.10 Induced AC voltage withstand tests with partial discharge measurement
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Test Manual for Transformers This test is intended to verify the AC withstand strength of each line terminal and its connected winding(s) to earth and other windings, the withstand strength between phases and along the winding(s) under test. As per IS 2026 Part 3-1981 and IEC Pub. 60076-3 of 1981, the test is normally performed with partial discharge measurement (Method 2) for transformers with highest voltage winding of ≥ 300 kV. For transformer with highest voltage winding of < 300 kV, the test is performed without partial discharge measurement (Method 1). However, with the latest reversion of IEC 60076-3 in 2000, the methods for induced over-voltage withstand test are reformed as AC short duration test (ACSD) and AC long duration test (ACLD). For Um >72.5 kV, the test is normally performed with partial discharge measurements to verify partial discharge free operation of the transformer under operating condition,?? the requirements for partial discharge measurement during the ACSD test may be omitted. This shall be clearly stated at the enquiry and order stages. However, ACLD test is always performed with the measurement of partial discharge during the whole application of test. An alternating voltage shall be applied to the terminals of one winding of the transformer. The voltage shall be as nearly as possible sinusoidal and its frequency is sufficiently above the rated frequency to avoid excessive magnetizing current during the test. The test voltage is the peak value of voltage divided by √2 .The test time at full test voltage shall be 60 sec for test frequency up to and including twice the rated frequency. For frequency above twice the rated frequency the time duration of test shall be:
Rated frequency , but not less than 15 sec Test frequency
Table below shows the different conditions of induced AC voltage test as defined in IEC publication 60076-3. The time duration for the application of test voltage with respect to earth is shown in figure 12
Induced AC voltage test Type of test
Type of winding
Highest voltage of equipment Um ≤ 72.5 kV
> 72.5 kV
Test voltage level
As per Table2 of IEC 60076-3 U1=from Table D.1 of IEC 600763 U2= 1.3 Um/√3
Test Duration (Refer Fig 12) 60 sec
C= 120x Rated Frq. Test freq.
No PD measurement PD level should be ≤ 300 pC at level U2
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Test Manual for Transformers Non-uniformly insulated
Phase to earth test
U1=from Table D.2 of IEC 60076-3 U2= 1.5Um/√3 Phase U1=from to table phase D.2 of IEC test 60076-3 U2= 1.3 Um/√3 U1= 1.7 Um/√3 U2= 1.5 Um/√3
AC Short duration
Delta ≤ 245 kV connected HV
U1= 1.7 Um/√3 U2= 1.5 Um/√3 U1= 1.7 Um/√3 U2= 1.5 Um/√3
Star < 300 kV Uniformly connected AC and nonHV Long uniformly ≥ 300 kV Duration insulated
C= 120x Rated Frq. Test freq.
PD level should be ≤ 500 pC at level U2
With PD measurement It should be ≤ 300 pC at level U2 D=30 min C= 120x Rated Frq. Test freq. D=30 min C= 120x Rated Frq. Test freq. D=60 min C= 120x Rated Frq. Test freq.
PD level should be ≤ 500 pC at level U2 PD level should be ≤ 500 pC at level U2
1.1 Um / 3
A = 5 min B = 5 min C = test time D = 5 min for ACSD E = 5 min ACLD
1.1 Um / 3
and 30/60 min for
Fig. 12 Time sequence for the application of test voltage with respect to earth
Here Um=Highest voltage for equipment U1= Test voltage Page 33 of 65
Test Manual for Transformers U2= Partial discharge evaluation level
3.10.1 Short-duration induced AC withstand voltage test (ACSD) 184.108.40.206 ACSD for transformers with uniformly insulated high-voltage windings A three-phase winding shall preferably be tested with symmetrical three-phase voltages induced in the three winding phases. If the winding has a neutral terminal, this may be earthed during the test. Transformers with Um ≤ 72.5 kV The phase-to-phase test voltage shall not exceed the rated induced AC withstand voltages as specified in IEC 60073-3. The test voltage across an untapped winding of the transformer shall be as close as possible to twice the rated voltage. Normally, no partial discharge measurements are performed during this test. The test shall commence at a voltage not greater than one-third of the test value and the voltage shall be increased to the test value as rapidly as is consistent with the measurement. At the end of the test, the voltage shall be reduced rapidly to less than one-third of the test value before switching off. The test is successful if no collapse of the test voltage occurs. Transformers with Um > 72.5 kV The test is performed at two voltage levels U1 and U2 (U1 as in table D.1 of IEC-60076-3 and U2= 1.3 Um/√3) levels associated with partial discharge measurements. The phase-to-phase voltage is same as described earlier. The partial discharge performance shall be according to the time sequence for the application of the test voltage as shown in fig 12 : The voltage with respect to earth shall be: -
Switched on at a level not higher than one-third of U2.
Raised to 1.1 Um/√3 and held there for a duration of 5 min.
Raised to U2 and held there for a duration of 5 min
Raised to U1, held there for the test time calculated earlier
Immediately after the test time, reduced without interruption to U2 and held there for a duration of at least 5 min to measure partial discharges Page 34 of 65
Test Manual for Transformers -
Reduced to 1.1 Um /√3 and held there for a duration of 5 min
Reduced to a value below one-third of U2 before switching off
The test is successful if - no collapse of test voltage occurs - the continuous level of ‘ apparent charge’ at U2 during 5 min does not exceed 300 pC on all measuring channels - the partial discharge behaviour does not show a continuing rising tendency - the continuous level of apparent charges does not exceed 100 pC at 1.1 Um/√3 A failure to meet the partial discharge criteria shall lead to consultation between purchaser and supplier about further investigations. 220.127.116.11 ACSD for transformers with non-uniformly insulated high-voltage windings For three phase transformers, two sets of tests are required as per IEC60076-3. a) A phase-to-earth test with rated withstand voltage between phase and earth b) A phase-to-phase test with earthed neutral and with rated withstand voltage between phases The test sequence for a three-phase transformer consists of three single phase applications of test voltage with different points of the winding connected to earth at each time. Other separate windings shall generally be earthed at the neutral if they are star-connected and at one of the terminals if they are delta-connected. The test time and test sequence for the application of voltage shall be as given in figure12. For the partial discharge performance evaluation, during the phase-to-phase test U1= From Table D.2 of IEC-60076-3
U2 =1.3 Um
For the three single-phase tests for the phase-to-earth insulation, U1= From Table D.2 of IEC-60076-3
U2 =1.5 Um/√3
The test is successful if no collapse of the test voltage occurs and if the partial discharge measurements fulfil the requirement as in 600076-3 with the following alteration: The continuous level of ‘apparent charge’ at U2 during the 5 min does not exceed 500 pC on all measuring terminals for single-phase tests at U2 = 1.5 Um/√3 line to earth, or 300 pC for phase to phase tests at U2 = 1.3 Um/√3 or as may be required at extremely low a.c. coordination values at 1.2 Um. Page 35 of 65
Test Manual for Transformers The detailed test procedure and specific test requirements are addressed in IEC-60076-3.
3.11 Lightning impulse voltage withstand test, transformer winding Um < 300 kV Refer to the clause no 3.8 of this manual
3.12 Temperature Rise Test
Temperature rise test To prove that temperature rise comply to limits specified in standards and to derive thermal characteristics for the transformer, A heat run is carried out supplying full load losses for sufficient time to ensure that the temperature rise of the winding and oil reach steady state values. The transformer shall be assembled completely with its cooling equipment. It is desirable to put the specified conservator with the transformers, if available. Alternatively, temporary conservator of approximately same capacity can be used for the purpose of the test. The top oil temperature is measured by a thermometer in a pocket at the top of the transformer tank, and this is used to verify that steady conditions have been reached. Final winding temperatures cannot be measured directly. The transformers shall be tested in the combination of connections and taps that give the highest winding temperature rises as determined by the manufacturer and reviewed by the purchaser’s representative when available. This will generally involve those connections and taps resulting in the highest losses. All temperature rise test shall be made under normal (or equivalent to normal) conditions of the means of cooling. The temperature–rise test shall be made in a room that is free from drafts as practicable and equipped with its protective device. Cooling air temperature Precautions should be taken to minimise variations of cooling air temperature specially when the steady state is approached. Rapid variation of reading should be prevented by providing at least three sensors, and average of their readings shall be used for evaluation. The sensors shall be distributed around the tank 1m to 2m away from the tank or cooling surface and protected from direct radiation. Cooling water temperature The temperature is measured at the intake of the cooler. Readings of temperature and rate of water flow should be taken at regular interval. Page 36 of 65
Test Manual for Transformers Test method Short circuit method During this test the transformer is subjected to the calculated total losses, previously obtained by two separate determination of losses, namely load loss at reference temperature and no load loss. The purpose of this test is
to establish the top oil temperature rise in steady-state condition with dissipation of total losses
to establish the average winding temperature rise at rated current and with the top oil temperature rise as determined above.
This is achieved in two steps: a) Total loss injection First the top oil and average oil temperature rises are established when the transformer is subjected to a test voltage such that the measured power is equal to the total losses of the transformer. The test current will be above rated current to the extent necessary for producing an additional amount of loss equal to the no-load losses, and winding temperature rise will be correspondingly elevated. The oil temperature and cooling medium temperature are monitored, and the test is continued until a steady- state oil temperature rise is established. The test may be terminated when the rate of change of top oil temperature rise has fallen below 1°C per hour and has remained there for a period of 3 hour. b) Rated current injection When the top oil temperature rise has been established, the test shall immediately continued with the test current reduced to the rated current for the winding combination connected. This condition is maintained for 1 h, with continuous observation of oil and cooling medium temperatures. At the end of one hour, the resistance of windings are measured with suitable method. During the hour with rated current the oil temperature falls. The measured values of winding temperature shall therefore be raised by the same amount as the average oil temperature rise has fallen from the correct value. The corrected winding temperature value minus the cooling medium temperature at the end of the total losses injection period is the average temperature rise. By the agreement, the two steps of the test may be combined in one single application of the power at a level between load loss and the total loss. The temperature- rise figures for the top oil and for the windings shall then be determined using the correction rules. The power injected during the test shall however be at least 80% of the total loss figure.
Determination of average winding temperature Page 37 of 65
Test Manual for Transformers The average winding temperature is determined via measurement of winding resistance. A reference measurement (R1,θ1) of all winding resistances is made with the transformer at ambient temperature, in a steady condition. When the resistance R2 at different temperature (θ2) is measured this yields the temperature value
Copper : θ 2 = Alu min ium
R2 (235 + θ 1 ) − 235 R1 : θ2 =
R2 (225 + θ 1 ) − 235 R1
The external cooling medium temperature at the time of shutdown is θa The winding temp. rise is then, finally :
∆ θw =θ2-θa Determination of winding temperature before shutdown Immediately after disconnection of test power supply and removal of short circuit connection the resistance of winding is measured with a suitable measuring circuit. The winding has large electrical time constant therefore accurate reading obtained only after a certain time delay. The resistance of the winding varies with time as the winding cools down. It shall be measured for a sufficient time to permit the extrapolation back to instant of shutdown. The detailed procedure to determine the resistance at the instance of shutdown is accordance with IEC-60076-2. Corrections : If the specified values of power or current have not been obtained during the test, the result shall be corrected according to the following relation. They are valid within a range of ±20% from target value of power and ±10% from target value of current. The oil temperature rise above ambient during the test is multiplied by :
Total losses Test losses
X= 0.8 for distribution transformers X= 0.9 for larger transformers with ON cooling X= 1.0 for transformers with OF or OD cooling The average winding temperature rise above average oil temperature during the test is multiplied by:
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Test Manual for Transformers Rated Current Test Current
Y=1.6 for ON and OF cooled transformers Y= 2.0 for OD cooled transformers 3.13 Lightning impulse test on neutral terminal Details of this test covered in the clause3.8 of this manual 3.14 Long-duration induced AC voltage test (ACLD), transformer winding Um < 170 kV ACLD with non-uniformly and/or uniformly insulated high-voltage windings This test is not a design proving test, but a quality control test and is intended to cover temporary over voltages and continuous service stress. It verifies partial discharge-free operation of the transformer under operating conditions. This test is not recommended for three phase transformers supplied from low voltage side with a delta connected high voltage winding The time sequence and application of voltage is as follows: -
Switched on at a level not higher than one-third of U2.
Raised to 1.1 Um/√3 and held there for a duration of 5 min.
Raised to U2 and held there for a duration of 5 min
Raised to U1, held there for the test time calculated earlier
Immediately after the test time, reduced without interruption to U2 and held there for a duration of at least 60 min when Um ≥ 300 kV or 30 min for Um < 300 kV to measure partial discharges
Reduced to 1.1 Um /√3 and held there for a duration of 5 min
Reduced to a value below one-third of U2 before switching off
During the whole application of the test voltage, partial discharges shall be monitored. The voltage to earth shall be: U1 =1.7 Um /√3 U2 =1.5 Um /√3 The partial discharges shall be observed and evaluated as follows. - Measurements shall be carried out at the line terminals of all non-uniformly insulated winding. Page 39 of 65
Test Manual for Transformers - The measuring channel from each terminal used shall be calibrated with repetitive impulses between the terminal and earth, and this calibration is used for the evaluation of readings during the test. - Before and after the application of test voltage, the background noise level shall be recorded on all measuring channel. - During the raising of voltage upto level U2 and reduction from U2 down again, possible inception and extinction voltages should be noted. Measurement of apparent charge shall be taken at 1.1 Um/√3. - No values of apparent charge are assigned to the application of U1. - During the whole of the second period at voltage of U2, the partial discharge level shall be continuously observed and readings shall be recorded every 5 min. The test is successful if - no collapse of the test voltage occurs - the continuous level of partial discharges does not exceed 500 pC during the long duration test at U2. - the partial discharge behaviour shows no continuously rising tendency at U2. Occasional high bursts of non-sustained nature should be disregarded - the continuous level of apparent charges does not exceed 100 pC at 1.1 Um/√3. A failure to meet the partial discharge acceptance criteria shall therefore not immediate rejection, but lead to consultation between purchaser and supplier about further investigations.
3.15 Short circuit withstand test General This test identifies the requirement for power transformer to sustain without damage the effects of over current originated by external short-circuit. The test demonstrates the thermal ability and dynamic effects of power transformer to withstand the rated short-circuit forces. The detailed procedures describing the magnitude of current, test duration, no. of tests and evaluation criteria shall be as per IEC 60076-5.
3.16 Measurement of zero–phase–sequence impedances on 3-phase transformers Zero–phase–sequence impedance tests of three–phase transformers Page 40 of 65
Test Manual for Transformers The zero–phase–sequence impedance characteristics of three–phase transformers depend upon the winding connections, and in some cases, upon the core construction. Zero–phase– sequence impedance tests apply only to transformers having one or more windings with a physical neutral brought out for external connection. In all tests, one such winding shall be excited at rated frequency between the neutral and the three line terminals connected together. External connection of other windings shall be as described in succeeding paragraphs for various transformer connections. Transformers with connections other than as described in succeeding paragraphs shall be tested as determined by those responsible for design and application. The excitation voltage and current shall be established as follows: If no delta connection is present on the transformer, the applied voltage should not exceed 30% of the rated line–to–neutral voltage of the winding being energized, nor should the phase current exceed its rated value. If a delta connection is present, the applied voltage should be such that the rated phase current of any delta winding is not exceeded. The percent excitation voltage at which the tests are made shall be shown on the test report. The time duration of the test shall be such that the thermal limits of any of the transformer parts are not exceeded. Single–phase measurements of excitation voltage, total current, and power shall be similar to those described in for load loss measurements. The zero–sequence impedance in percent on kVA base of excited winding for the test connection is: E Ir . Z (% ) = 300 E r I
Where E = measured excitation voltage Er = rated phase–to–neutral voltage of excited winding I = measured total input current flowing in the three parallel–connected phases Ir = rated current per phase of the excited winding A zero-sequence test shall be made on the winding with the available neutral. A single–phase voltage shall be applied between the three shorted line terminal and neutral. The external terminals of all other windings may be open–circuited or shorted and grounded.
The zero-sequence impedance is dependent upon the physical disposition of the windings and the magnetic parts and measurement of different windings may not therefore agree.
3.17 Measurement of acoustic sound level Page 41 of 65
Test Manual for Transformers
This test shall be done in accordance with the clauses given in NEMA TR1 and IEC-6007610. The detailed test procedure is given in the ANSI/IEEE standard, which has been approved by NEMA. Audible sound from transformer originates principally in the transformer core and is transmitted, either through the dielectric fluid or the structural support, to other solid surfaces from which it is radiated as airborne sound. The audible sound also contains the noise emitted by any dielectric fluid mechanical cooling system. Measurement should be made in an environment having an ambient sound pressure level at least five decibels below the combined sound pressure level of the transformer and the ambient sound pressure level. The transformer shall be located so that no acoustically reflecting surface is within 3 m of the measuring microphone, other than the floor or ground. The transformer shall be connected and energised at rated voltage and rated frequency, and shall be at no load with the tap changer on principal tap. Pumps and fans shall be operated as appropriate for the rating being tested. Sound measurements shall begin after the transformer being tested is energised and steady- state sound level conditions are established. Measurements may be made immediately on the transformers that have been in continuous operation. The rated voltage shall be measured line-line for ∆ connected windings and line-neutral for Y connected windings. The voltage shall be measured with a voltmeter responsive to the average value of the voltage but scaled to read the rms value of a sinusoidal wave having the same average value. The voltmeter should be connected between the terminals of the energized windings. The reference sound-producing surface is a vertical surface that follows the contour of a taut string stretched around periphery of the transformer or integral enclosure (Fig 12). The contour shall include radiators, coolers, tubes, switch compartments, and terminal chambers, but exclude bushing and minor extensions. The measurement shall be done with the microphone, which shall be calibrated as recommended by the sound level meter manufacturer before and after measurement. The first microphone locations shall coincide with the main drain valve. The number of microphone position is not less than 4. The microphone shall be located on the measurement surface spaced 0.3 m from the reference sound- producing surface. When fans are in operation, the microphone shall be located 2 m from any portion of radiators and coolers. For transformers having an overall tank or enclosure height of les than 2.4 m, measurements shall be made at half height. For transformers having an overall tank height of 2.4 m or more, measurements shall be made at one-third and at two-thirds height. The sound power rating of the transformer is determined using the following five steps: a) Measure ambient sound pressure level Page 42 of 65
Test Manual for Transformers b) Measure combined ambient and transformer sound pressure levels c) Compute ambient corrected sound pressure levels d) Compute average sound pressure levels e) Calculate sound power levels The detailed calculation is done in accordance with the ANSI/IEEEC57.12.90-1993. The average sound level of transformers should not exceed the values given in table 0-2 through 0-4 of NEMA TR1 when measured at the factory in accordance with the conditions outlined in ANSI/IEEEC57.12.90-1993. Microphone Location Fan cooled surface 2m
Reference sound producing source
Measurement surface 1m
2/3 Height Height 1/3 Height
Microphone location for measuring audible sound from transformers
3.18 Measurement of the harmonics of the no-load current The harmonics of the no–load current in all the phases are measured by means of harmonic analyzer and the magnitude of the harmonics is expressed as a percentage of the fundamental component. 3.19 Measurement of power taken by the fans and oil pump motors The measurement shall be done by suitable instruments at rated voltage. 3.20 Test with lightning impulse chopped on tail
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Test Manual for Transformers This test is a special test and should be used for special applications on the line terminals of a winding. When this test is performed it shall be combined with the full lightning impulse test. The peak value of the chopped impulse shall be 1.1 times the amplitude of the full impulse. Usually the same settings of the impulse generator and measuring equipment are used, and only the chopping gap instrument is added. The standard chopped lightning impulse shall have a time to chopping between 2 µs and 6 µs. The triggered type chopping gap should be used with adjustable timing, although a plain rodrod gap is allowed. The chopping circuit should be so arranged that the amount of over swing to opposite polarity of the recorded impulse will be limited to not more than 30% of the amplitude of the chopped impulse. For this purpose, it is permitted to put a resistance in service with the chopping gap. The test is combined with the full impulse test in a single sequence. The order of application is : one reduced level impulse one full level impulse one or more reduced level chopped impulse(s) two full level chopped impulses two full level impulses The same type of measuring channels and oscillographic or digital records are specified as for the full-wave impulse test. The detection of faults during chopped impulse test depends essentially on a comparison of the oscillographic or digital records of full level and reduced level chopped impulses. The neutral current record presents a superposition of transient phenomena due to the front of the original impulse and from the chopping. Account should therefore be taken of the possible variations, of the chopping time delay. The recordings of successive full impulse tests at full level constitute a supplementary criterion of a fault, but they do not constitute in themselves a quality criterion for the chopped impulse test.
3.21 Magnetic circuit (Isolation ) test This test is done to detect the presence of inadvertent ground if exists. This test is done with help of megger or by AC supply. During this test other terminals should be in open circuit position. This test is done by applying the voltage alternate between the core clamp to end frame, core clamp to tank and between end frame to tank. The value of test voltage is varying according the customer requirement and electrical specification. The duration of test voltage application is 60 seconds. Alternatively the test is performed with the help of megger. In which the value of insulation resistance is measured between two terminals. This test shall be conducted in accordance with IS-2026 Part 1. Page 44 of 65
Test Manual for Transformers The tests will be successful if the terminals withstand the required AC voltage for test duration. The values of the insulation resistance in mega-ohm (MΩ ) should as follows; Test voltage : 2 kV New equipment
: > 10 MΩ
Service aged equipment
: > 1 MΩ
: < 1 MΩ
Destructive circulating current
: < 100 KΩ
3.22 Determination of capacitance and dissipation factor between winding to earth and between windings Capacitance and tan delta are usually determined for winding to earth and between windings by bridge measuring technique, such as Schering Bridge. The test specimen shall have the following requirements: All windings immersed in insulating liquid. All winding short-circuited. All bushings are in place. The applied voltage for measuring capacitance and tan delta shall not exceed half of the low frequency test voltage, for any part of the winding or 10 kV whichever is lower. This test may be performed with or without guard for the circuit combination as shown below.
Method I Test without guard
Method II Test with guard
Two winding HV to LV and ground LV to HV and ground HV to LV to ground
Two winding HV to LV and ground LV to HV and ground HV to LV to ground LV to ground, Guard to HV
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Test Manual for Transformers Three winding HV to IV ,LV and ground IV to HV ,LV and ground LV to IV ,HV and ground HV and IV to LV and ground HV and LV to IV and ground IV and LV to HV and ground HV, IV and LV to ground
Three winding HV to LV and ground, guard on IV HV to ground, guard on LV & IV LV to IV & ground, guard on HV LV to ground, guard on HV & IV IV to HV & ground, guard on LV IV to ground, guard on HV & LV HV & LV to IV & ground HV & IV to LV & ground
Temperature correction factors The temperature correction factors for the insulation power factor depend upon the insulating material, their structure, moisture, etc. Values of correction factor ‘K’ listed in the below table are typical and satisfactory for practical purpose for use as given in equation
Test temperature T, 0C 10 15 20 25 30 35 40 45 50 55 60 65 70
Correction Factor ‘K’ 0.80 0.90 1.00 1.12 1.25 1.40 1.55 1.75 1.95 2.18 2.42 2.70 3.00
FP20 = Fpt /K Where, FP20 is the power factor corrected to 20 0C Fpt is the power factor measured at T T is the test temperature 0C K is the correction factor
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Test Manual for Transformers Insulation temperature may be considered to be that of the average liquid temperature. When insulation power factor is measured at a relatively high temperature and the corrected values are unusually high, the transformer should be allowed to cool and the measurements should be repeated at or near 20 0C 3.23 Magnetic balance test on 3-phase transformers This test is conducted only in three phase transformer to check the imbalance in the magnetic circuit. In this test, no winding terminal should be grounded; otherwise results would be erratic and confusing. Evaluation criteria The voltage induced in the center phase shall be 50 to 90% of the applied voltage on the outer phases. However, when the center phase is excited then the voltage induced in the outer phases shall be 30 to 70% of the applied voltage. Zero voltage or very negligible voltage induced in the other two windings should be investigated. 3.24 Determination of transient voltage transfer characteristics When the low - voltage winding cannot be subjected to lightning over voltage from the low voltage system, this winding may, by agreement between supplier and purchaser, be impulse tested with surges transferred from high voltage winding. This method is also used when the design is such that an impulse directly applied to the low voltage winding could result in unrealistic stressing of higher voltage windings, particularly when there is a large tapping winding physically adjacent to the low voltage winding. With the transferred surge method, the tests on the low voltage winding are carried out by applying the impulse to the adjacent high voltage winding. The line terminals of the low voltage winding is connected to earth through resistance of such value that the amplitude of the transferred impulse voltage between line terminals and earth, or between different line terminal or across a phase winding, will be as high as possible but not exceeding the rated impulse withstand voltage. The magnitude of the applied impulses shall not exceed the impulse level of the winding to which the impulses are applied.
The details of the procedure shall be same as the test on line terminal of HV winding.
3.25 Dissolved Gas Analysis (DGA) of oil filled in the transformer Page 47 of 65
Test Manual for Transformers Introduction For many years the method of analyzing gasses dissolved in the oil has been used as a tool in transformer diagnostics in order to detect incipient faults, to supervise suspect transformers, to test a hypothesis or explanation for the probable reasons of failures or disturbances which have already occurred and to ensure that new transformers are healthy. Finally, DGA could be used to give scored in a strategic ranking of a transformer population. In this respect the dissolve gas analysis (DGA) is regarded as a fairly mature technique and it is employed by several transformer companies around the world either in own plant or in cooperation with an affiliated or independent laboratory. The idea with dissolve gas analysis is based on the fact that during its lifetime the transformer generates decomposition gasses–essential from the organic insulation – under the influence of various stresses– both normal and abnormal. The gasses that are of interest for the DGA analysis are the following; − H2 Hydrogen − CH4 Methane − C2 H4 ethylene − C2H6 Ethane − C2H2 acetylene − (C3H6 propene )– not always measure − (C3H8 propane ) – not always measure − CO carbon monoxide − CO2 carbon dioxide − O2 oxygen − N2 Nitrogen − TCG total combustible gas content (= H2 + CH4 + C2H4 + C2H6 + C2H2 + CO) All these gases except oxygen and nitrogen may be formed during the degradation of the insulation. The amount and the relative distribution of these depend on the type and severity of the degradation and stress. Around the world and during the years several different schemes have been proposed as evaluation scheme for the DGA. The most commonly known schemes are the one proposed by Rogers and the scheme laid down in IEC publication 60599 Procedure The DGA procedure consists of essential four steps: − Sampling of oil from the transformer − Analysis of these gases from oil − Analysis of the extracted gas mixture in a gas chromatography, GC. − Interpretation of the analysis according to an evaluation scheme. Sampling, extraction and analysis. Page 48 of 65
Test Manual for Transformers The oil samples should preferable be taken in the moving oil so that the gas generated somewhere easily and rapidly is transported from the point of generation to the sampling point. Suitable locations are valves in the cooler/radiator circuit. To take samples from these locations is not always possible because of design limitations. Other places from which to draw samples are the cover, bottom valve, the conservator and from the buchholz relay. In addition, it is very important that the sampling is made in such a way that the contamination of the sampling vessel is held at a minimum and that gas are not lost during sampling or transpiration to the laboratory. The removal of the gasses from the oil can be accomplished by various methods: − Partial degassing (single–cycle vacuum extraction) − Total degassing (multi cycle vacuum extraction) − Stripping by flushing the oil with another gas − By the head–space technique in which gases are “equalized” between a free gas volume and the oil volume. After extraction in the extracted gas mixture is fed into adsorption columns in a GC where the different gases are absorbed to various degrees and reach the detector after different periods of time. In this way the gas mixture is separated into individual chemical compounds, identified and their concentration in volume gas STP/volume oils is calculated and expresses in ppm (STP = standard temperature and pressure). It should be emphasized that this extraction and analysis may involve analytical errors which mean that it may be difficult to directly compare the results from two different laboratories. One should not jump from one lab to another but try to stick to one well-reputed lab. Interpretation There are several different approaches how to explain and interpret the analyzed gas composition and to diagnose the condition of the transformer. Essentially the following methods are at had; − Identification of the key gas , The key gas identify a particular problem, e.g., H2 indicates a PD − Determination of rations between gasses, normally between gas levels. − Determination of rates of increase (“production rates”), in ppm / day or ml gas/day Around the world and during the years several different schemes have been proposed as evaluation schemes for the DGA. The most common known schemes are the one proposed by Rogers forming the basis for the ANSI method and the scheme laid down in IEC Publication 60599. Both these methods are using ratios gas concentrations. In order to get a feeling for DGA the “key-gas method” is appropriate.
Overheating of cellulose CO, CO2 Page 49 of 65
Test Manual for Transformers Overheating of oil increasing temperature C2H4 C2H6 CH4 CH4
C2H4 CH4 C2H2
Partial Discharges (PD) increasing temperature H2 H2 C2H2 Discharges C2H2 H2 Chrematistics key gases – principal lay-out One looks for the most prominent gas – the one which differs most from a tacitly expected “normal” level (or change). For instance, at overheating of cellulose the main decomposition gases are CO and CO2. At a partial discharge of corona H2 is formed. (PD in cellulose involves the formation of carbon oxides). At a partial discharge of corona H2 is formed. (Pd in cellulose involves the formation of carbon oxides). At a more sever electric discharge such as arching C2H2 is formed. (Normally also H2 is formed with smaller amounts of methane and ethane. If CO is present cellulose is involved). Finally, at overheated oil it is the hydrocarbons that are formed – normally the saturated hydrocarbons such as C2H6 at lower temperatures and unsaturated such as C2H4 at higher temperature. Acetylene points to very high temperature. This scheme can also be used to understand the evaluation – scheme based on ratios. For instance, the IEC method uses 3 ratios, C2H2/ C2H4, CH4/ H2, C2H4/ C2H6 CH4/H2 is used to discriminate between a thermal fault and an electric fault. C2H2/C2H4 indicates the presence of a strong discharge of very severe electric problem and C2H4/ C2H6 is an indication of the oil temperature. 3.26 Radio interference voltage (RIV) test, if applicable The radio interference level to be permitted at the specified test voltage depends on many factors, such as the importance and vulnerability of the radio communications which are to be protected against interference and the distance of the receiver from the source of interference. For all reasons, agreement between the manufacturer and purchaser may be necessary to decide the followings: The value of test voltage, Details of the arrangement of the job The level of radio interference to be accepted This test is a type test and shall be made only in a special case when agreed by purchaser and manufacturer. The detailed test procedure should be in accordance with the IS 8263. Page 50 of 65
Test Manual for Transformers
3.27 Recurrent surge oscillographic (RSO) test The insulation of a transformer must be proportioned to the surge voltages, which will appear at the various points throughout the windings. The surge voltage distribution in the winding is independent of the magnitude of the applied voltage and that the same results may be obtained by applying a reduced surge voltage, of the order of a few hundred volts. In order to obtain the maximum possible amount of information it is desirable to have electrical contact with the maximum number of points on the winding. Hence this test is generally performed at pre-stage of transformer manufacturing after completion of terminal gear. High voltage impulse tests on a complete transformer are costly and take a great deal of time Furthermore, for high-voltage transformers the core and windings must be immersed in oil and mounted in the tank. This condition does not facilitate the collection of data. This test is conducted with a recurrent surge generator, which consists of a capacitor charged to a suitable voltage and discharged by means of a thyratron into a circuit which is designed to generate the required low-voltage surge of the standard wave shape. The charge and discharge sequence is repeated fifty times per second. The output voltage from the recurrent surge generator is applied to the terminal of the transformer being tested, in a similar manner to that in which a high voltage impulse test would be conducted. The surge voltage appearing at any point of the winding can be measured and displayed on the screen of the cathode-ray oscilloscope. The time base is arranged so that it is synchronized with the recurrent discharge of the capacitor. By this means it is possible to obtain a standing picture on the screen of the applied voltage and of the voltage appearing at the points along the winding, together with a time calibration wave. 3.28 Determination of core hot spot temperature This test is done to check uniform distribution of flux on every point on the core & to determine the core hot spot temperature. This test is done by exciting the core with suitable voltage, which is the voltage per turn is multiplied with the wound turn around the core. The required voltage is applied to transformer and note down the reading of temperature at the different point on the core using thermo vision camera or laser temperature scanner. The reading of the temperature should not vary too much from point to point. If the temperature is varying from one point to other then the flux is not distributed uniformly around the core. Scan the point around the core where the highest temperature occurs. This is the hot spot temperature of the core. 3.29 Frequency Response Analysis (FRA) Frequency response analysis (FRA) test is conducted on transformers & reactors to determine the frequency response of windings. The reference frequency responses obtained Page 51 of 65
Test Manual for Transformers during laboratory testing serve as ‘fingerprints’ to monitor the condition of the transformer or reactor during service. The frequency response of an electrical winding is obtained by application of sweep frequency (sinusoidal). The winding will have a characteristic frequency response for the applied signal at different frequencies. The response is uniquely determined by the winding arrangement involved and any winding movement or other fault will modify the frequency response due to changes in inductances and capacitances. The sweep frequency voltage is applied through network analyzers. The frequency response of the winding is determined between the frequency range of 10 Hz to 2 MHz. The FRA test is performed on one winding of the electrical equipment at a time. The transformer / reactor shall be electrically isolated from any other electrical connections or systems, including earth connections during FRA test. The two end terminals of each winding shall be made available for measuring the frequency response across the winding. # For star connected winding, the response shall be measured across the terminal & neutral. # For delta connected winding, the response shall be measured across two line terminals & in case of open-delta, across individual winding. # For auto connected winding, the response of series & common windings shall be measured separately. For a transformer, it is normal practice to earth one end of every winding that is not being tested, leave the other open end. Alternatively, all other windings may be left unconnected from each other and from earth. In every case, the termination of each winding for each test should be recorded. The frequency response of the winding is determined by plotting the ratio of the output from the winding to the input at atleast following frequency ranges. * * * *
10 Hz 100 Hz 1 KHz 10 KHz
to to to to
2 KHz 20 KHz 200 KHz 2 MHz
Alternatively frequency ranges specified by the customer can be selected. The test is normally conducted at maximum, mean and minimum taps, in case of windings having tappings. While making measurements at mean tap, care should taken to move the tap from higher voltage taps, for proper comparison of FRA results of different phases of same transformer or different transformers. The FRA results is analyzed for Changes in response of the winding Difference between the FRA records of different phases of the same transformer. Page 52 of 65
Test Manual for Transformers FRA test is primarily a condition assessment test and can be used in conjunction with other diagnostic tests for detailed analysis and interpretation of the transformer.
3.30 Measurement of magnetization current at low voltage
This test is performed at 415 V 3-phase(neutral un-grounded) for three phase transformer and 230 V 1-phase for single phase transformer. This test is performed to locate defect in magnetic core structure, shifting of windings, failures in turn insulation or 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 test results. The general pattern is two similar high readings on the outer phases and one lower reading on the center phase, in case of three phase transformers. An agreement to within 25% of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50% higher than the value measured during pre-commissioning checks, then the winding needs further analysis.
3.31 Functional tests on auxiliary equipments Acceptance test for Oil (OTI) and winding (WTI) temperature indicator A. Routine test 1. OTI (Range 20º-140º C) i) Each completely assembled instrument shall be tested for accuracy over the complete range i.e. at 40º, 60º, 80º, 100º & 120 º C by keeping the bulb in the hot oil bath continuously stirred. The accuracy of indication shall be ±1.5 % full scale deflection (FSD). 2. WTI ( Range 30º - 150º C) i) Each completely assembled instrument shall be tested by injecting the current to its heater coil. Oil bath shall be maintained at 60 0C, Total temperature and temperature rise shall be recorded for 0, 2, 3 and 4 amperes current. The accuracy of the indication i.e. oil bath temperature measured by standard. Thermometer plus rise in temperature due to injection of current in heating coil i.e. total temperature indicated by WTI shall be within ± 1.5 % FSD. Error allowed shall be 1.5 / 100 x 150 = ± 2.25 0 C Max. ii) In case of repeater, both WTI and repeater shall be tested together by injecting the current as mentioned in 2(I). Accuracy of the repeater readings shall be within ± 1.5 FSD i.e. ± 1.5 0C considering WTI readings as the reference temperature. Page 53 of 65
Test Manual for Transformers High Voltage Test on Insulation test of auxiliary wiring Unless otherwise specified the wiring for auxiliary power and control circuitry shall be subjected to a one minute power frequency withstand test of 2 kV r.m.s. to earth. Motors and other apparatus for auxiliary equipment shall fulfill insulation requirements according to the relevant IEC standard (which are generally lower than the value specified for the wiring alone, and which may sometimes make it necessary to disconnect them in order to test the circuits) B Type tests (one instrument of each lot / batch) Switch setting and operations: Switches shall be able to set between 50–140 0C and their operation shall be within ± 2.5% of pointer indication unless otherwise specified on purchase order. Switch setting will be done as below: OTI: Alarm (S1) – 95 0C Trip (S2) – 100 0C WTI: Alarm (S1) – 115 0C Trip (S2) – 125 0C – 85 0C Fan start (S3) Pump start (S4) – 95 0C Switch differential Each switch shall have adjustable differential (difference between make and break temperature) of 6 0C to 90 0C and will be set for 6 ± 1 0c differential. Switch Rating 5 Ampere, continuous 250 V, AC or DC for make or break. 3.32 Tests on oil filled in transformer 3.32.1 Dielectric strength The voltage at which the oil breaks down when subjected to an AC electric field with continuously increasing voltage contained in the specified apparatus is called dielectric strength. The voltage is expressed in kV. This test is performed to determine the dielectric breakdown voltage of service-aged oil. It is important to measure the oil’s ability to withstand electrical stress without failure. The dielectric strength of oil is determined by the two methods. First method utilizes spherical capped electrode in the test cell, which is recommended primarily for filtered, degassed and dehydrated oil prior to and during filling of electrical power equipment rated above 230 kV and above. The second method utilizes flat electrodes and recommended for all other apparatus. The detailed test procedure is in accordance with IS 6792. The acceptance value of oil for the different test voltage of transformer in general is recommended as per the table given below. System voltage of transformer Electric strength kV kV Page 54 of 65
Test Manual for Transformers Above 72.5 145 245
Upto and including 72.5 145 245 420
60 65 65 70
High dielectric strengths do not indicate the absence of oil contaminants. There should not be direct correlation between a certain breakdown voltage and failure, except in extreme cases. 3.32.2 Water content There is always some moisture present in any practical transformer. In addition, since the paper in the insulation system has a great affinity for water, most of the moisture present will be in the paper. The dielectric strength of the paper is very sensitive to the presence of moisture as is the oil, it is therefore important that the moisture content be known and its concentration controlled. An estimate of the moisture content of the paper is determined by measuring the moisture content of the oil. Dissolved water may or may not affect the electrical properties of the oil. The solubility of water in transformer oil increases with increasing temperature. Above a certain water content level which is called saturation water content, all the water cannot remain in solution and free water may be seen in the form of cloudiness or water droplets. Free water invariably results in decreased dielectric strength and resistivity and increased dielectric dissipation factor. The concentration of water-in-oil expressed in ppm. Concentration in ppm does not provide sufficient information to obtain an adequate evaluation of the insulation system dryness. Relative saturation provides a better evaluation under a wide range of operating condition and temperatures. The detailed test procedure, equipment and test condition for measurement of water content is in accordance with the IS 13567. In a transformer the total water content is distributed between the paper and the oil in a ratio that is predominantly in the favor of paper. Small changes in the temperature significantly modify the water content of the oil but only slightly that of paper. Using the graphs as given in the IS 13567, it is possible to obtain at a given temperature the water content of the paper from the measured water content of the oil assuming equilibrium conditions. The limiting value of water content are given in table below System voltage of transformer kV Above Upto and including 72.5 72.5 145 145 245 245 420
Water content ppm, max. 20 20 15 15
High water content accelerates the chemical deterioration of the insulating paper and is indicative of the undesirable operating conditions or maintenance requiring correction. Page 55 of 65
Test Manual for Transformers
3.32.3 Dielectric dissipation factor This test covers the determination of the power factor of new and service aged oil. This test is used to indicate the dielectric losses in the oil when used in an alternating electric field and of the energy dissipated as heat. A low power factor indicates low dielectric losses. It is useful as a means to ensure that sample integrity is maintained, and as an indication of changes in quality resulting from contamination and deterioration in service or as a result of handling. This test is satisfactorily performed in the field, as well as in a laboratory environment. The detailed test procedure and test equipment may be in accordance with IS 6262. Acceptable limit for the dielectric dissipation factor largely upon the type of apparatus and application. The power factor limits given for oil are based upon the understanding that this is an indicator test for contamination by excessive water or polar or ionic materials in the oil. High level of dissipation factor (.0.5 % at 25º C) is because of contaminants may collect in the areas of high electrical stress and concentrate in the winding. Very high dissipation factor ( > 1.0% ) in oil may be caused by the presence of free water which could be hazardous to the operation of a transformer. 3.32.4 Resistivity The resistivity ( specific resistance) in ohm-centimeters of a liquid is the ratio of the dc potential gradient in volts per centimeter paralleling the current flow within the specimen, to the current density in amperes per square centimeter at a given instant of time and under prescribed conditions. This is numerically equal to the resistance between opposite faces of a centimetre cube of liquid. Resistivity measurements are made at many different temperatures. But for acceptance test, it is generally done at a temperature of 90º C, while for routine testing, it is usually made at room temperature or 90º C. The average electrical stress to which specimen is subjected to shall not be less than 200 V/mm nor more than 1200 V/mm. the upper limit is set with the purpose of avoiding possible ionization if higher stresses are permitted. The detailed test procedure is as accordance with IS 6103. Useful information can be obtained by measuring resistivity at both ambient and at higher temperature such as 90º C. A satisfactory result at 90º C coupled with an unsatisfactory value at lower temperature is an indication of the presence of water or degradation products precipitated. 3.33 Oil pressure test on completely assembled transformer This test is done after completion of all electrical and temperature rise test. Transformer with cooling bank, bushing and other accessories shall be tested for any oil leakage at high pressure (normal pressure plus 35 kN per Sq.m measured at the base of tank) and at room temperature as specified by customer. The procedure for conducting this test is as follows: Page 56 of 65
Test Manual for Transformers 1. Conservator along with the protective relay shall be disconnected. 2. Calibrated pressure gauge shall be mounted at the bottom of the tank. 3. Bushings will remain mounted however conservator along with buchholz relay shall be isolated. 4. In welded cover type construction cooler bank, bushings shall be removed but all turrets and cover pipe work shall remain. 5. Fill the oil completely and release all trapped air. 6. The specified pressure shall be maintained for the specified test duration as specified in the test schedule or quality plan. 7. The test duration should be at least one hour unless otherwise specified. Criteria for oil pressure test During the pressure test, there shall not be any leakage. If there is pressure drop during the test either because of some trapped air inside the transformer or due to ambient temperature variation, the pressure shall be raised to the specified level. The unit will be considered to pass the test only if there is no visual oil leakage. Pressure drop shall be considered as failure of the unit in the test. 3.34 Jacking test and Dye-penetration test This test is done to check out the mechanical capability of jacking pads on bottom tank of transformer. The procedure for conducting the above test is as follows: 1. Bring the transformer on to rail track 2. The transformer should be filled with oil. 3. Place jack under jacking pads such that the C.L. of the jack ram coincides with the jack points on jacking pads at appropriate height. 4. connect the jacks to hydraulic pump unit 5. Raise pressure of oil slowly so that the transformer is lifted gradually. 6. Continue lifting till, the flanges of the rollers are above the R/L and its possible to turn the wheel. 7. The transformer must be held in the raised condition for 15 minutes. 8. Lower the transformer gradually after the expiry of time period by releasing oil pressure in stages till the wheel again rest on the rails. The transformer jacking pads should withstand this test without any deformation or any cracks. The dye-penetration test is done simultaneously with jacking test to check out any welding cracks in jacking pads. This is done by applying the dye paint to the welded joints of jacking pads. If there is no leakage of dye at welding joints, then the welded joint is perfect.
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Test Manual for Transformers 3.35 Pressure relief device test The pressure relief device shall be subjected to rising pressure (pneumatically). The pressure at which the device operates shall be noted. The operating pressure should be less than normal pressure plus 35 kN/m2.
B) FIELD TEST INTRODUCTION Transformer is important and vital equipment between generation station and the utility and therefore necessary to ensure its proper performance through out its service life. During transportation, installation and service operation, the transformer may be exposed to conditions, which adversely affect its reliability and useful life. Field-testing and condition monitoring are the techniques to ensure good operating health of power transformers. Interpretations are also included to provide additional information on the particular test and to provide guidance on acceptable criteria. There is not necessary any direct relationship between field tests and factory tests. Interpretation of measured results is usually based on a comparison with data obtained previously on the same unit under similar condition. It should be noted that some times the results of several types of tests should be interpreted together to diagnose a problem. Manufactures acceptance criteria shall also be consulted. B.1 Dew point measurement for large transformers filled with dry air or nitrogen filled Large rating transformers are transported to site from manufacturing works, without oil and filled with dry air or nitrogen due to weight limitations. Positive gas pressure is generally maintained at 0.175 kg/m2 during transportation and storage. As the insulation of transformer is hygroscopic, it absorbs moisture from atmosphere if positive pressure of gas is not maintained. After arrival of transformer at site it is necessary to check the gas pressure and if it is not positive there is every possibility that moisture must have gone inside the transformer during transportation. To ascertain this factor and to check the dryness of the insulation, dew point measurement is carried out at site. Dew point is the temperature at which the water vapours present in the gas filled in the transformer begin to condense. Page 58 of 65
Test Manual for Transformers It will not be possible to define a limit of dew point of nitrogen gas as dew point depends on the ambient temperature, pressure of the gas, moisture level of cellulose insulation etc. The procedure and acceptance limits are given in section K of this manual.
ELECTRICAL TESTS B.2 Winding resistance measurement Transformer winding resistances are measured at site in order to check for abnormalities due to loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings. The resistance is measured by two methods a) Voltmeter Ammeter method b) Bridge method The detailed test procedure of above methods is same as factory testing and is covered in section A.3.1.2 of this manual. Precautions shall be taken during field testing as given below. The test shall be conducted at all taps of the transformer winding and the measured value shall be converted to 75 0C. The acceptance criterion is usually agreement to within 5% of resistance measurements made separately on different phases, under field condition. But, for large transformers it is recommended to compare the resistance values with original data measured in the factory and in case of large variation, connection tightness to be checked. The current used for these measurements should not exceed 15% of the rated current in order to avoid heating the winding thereby changing its resistance. However, the current should not be too small, which may not be sufficient to avoid inductive effect, due to core magnetization The winding resistance shall be preferably done when the difference in the top and bottom temperature of the winding (temperature of oil in steady-state condition) is equal to or less than 5°C. Winding resistance measurement shall be done only after measurement of magnetization current (excitation current). The polarity of the core magnetization shall be kept constant during all resistance measurement. A reversal in magnetization of the core can change the time constant and result in erroneous readings. B.3 Vector group and polarity To determine the phase relationship and polarity of transformers Page 59 of 65
Test Manual for Transformers The procedure to find out vector group shall be general be same as defined in section A18.104.22.168. a) Connect neutral point of star connected winding with earth b) Join 1U of HV and 2U of LV c) Apply 415V, 3 phase supply to HV terminals d) Measure voltage across following terminals 1W-2V,1W-2W,1U-2W,1V-2V,1V-2W Example 1
For HV-Delta/LV-Star Transformer Connect 1U to 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W 1U