Page 2 of 65 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.
Page 3 of 65 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)
Type Tests
11. Lightning impulse voltage withstand test, transformer winding Um < 300 kV
Page 4 of 65 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
Mechanical Tests
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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.
Page 6 of 65 3.1 Measurement of winding resistance
General
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.
3.1.1.1 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.
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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.
3.1.2.1 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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3.1.2.2 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 steady-state 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.
Page 9 of 65 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 phase-angle correction.
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(a)
1.1
1.2
2.2
2.1
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.
3.2.2.1 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
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.
3.2.2.2 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
Fig : 1 - Polarity by Alternating Voltage Test
source V 1.1 2.1 1.2 2.2
Page 11 of 65 2V 2W 1W Dy-I 1V 2U 1U 1W 1V N 1U+2U 2V 2W
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<1V-2W 1V-2V <1U-1W
Fig 2 : For HV-Delta / LV-Star Transformer
Example 2 Connect 1U to 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W VOLTAGE RELATION 1W-2W = 1V-2W 1W-2V > 1V-2V 1U-N= (1U-2W)+(2W-N) Yd11
Fig 3 : For HV-Star / LV-Delta Transformer
3.3 Measurement of no-load loss and excitation current
General
No-load (excitation) losses are those losses that are incident to the excitation of the transformer. No-load (excitation) losses include core loss, dielectric loss, conductor loss in the winding due to excitation current, and conductor loss due to circulating current in parallel windings. These losses change with the excitation voltage.
Excitation current (no-load current) is the current that flows in any winding used to excite the transformer when all other windings are open-circuited. It is generally expressed in percent of the rated current of the winding in which it is measured.
3.3.1 No-load loss test
The purpose of the no-load loss test is to measure no-load losses at a specified excitation voltage and a specified frequency. The no-load loss determination shall be based on a sine-wave voltage. The average-voltage voltmeter method is the most accurate method for correcting the measured no-load losses to a sine-wave basis and is recommended. This
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method employs two-parallel-connected voltmeters; one is an average-responding (possibly rms calibrated) voltmeter; the other is a true rms-responding voltmeter. The readings of both voltmeters are employed to correct the no-load losses to a sine-wave basis, using equation given in paragraph for waveform correction of no-load losses.
Test voltage will be 90%, 100%, 110% ,guarnteed at 100% and for reference purpose at 90 and 110%
Connection diagrams
Tests for the no-load loss determination of a single-phase transformer are carried out using the schemes depicted in figure 4 and figure 5. Figure 4 shows the necessary equipment and connections for the case where instrument transformers are not required. When instrument transformers are required, which is the general case, the equipment and connections shown in figure 5 apply. If necessary, correction for losses in connected measuring instruments may be made by disconnecting the transformer under test and noting the wattmeter reading at the specified test circuit voltage. These losses represent the losses of the connected instruments (and voltage transformer, if used). They may be subtracted from the earlier wattmeter reading to obtain the no-load loss of the transformer under test.
Tests for the no-load loss determination of a three-phase transformer shall be carried out by using the three wattmeter method. Figure 6 is schematic representation of the equipment and connections necessary for conducting no-load loss measurements of a three-phase transformer when instrument transformers are necessary.
Nowadays, digital power analysers or power meters are available for determination of losses (both no-load and load). Selection of these power analysers shall be based on the desired accuracy at low power factors
without instrument transformers
Fig : 4 - Connections for no-load loss test of a single-phase transformer
S o u rc e A V AV W
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with instrument transformers
Fig : 5 - Connections for no-load loss test of a single-phase transformer
S o u rc e A Current Transformer TransformerVoltage W V AV B
Fig : 6 - Three phase transformer connections for no-load loss and excitation for Y-Y connected CT
transformer neutral present to provide return path from transformers when no delta winding is
A Note : Source neutral
should be available
A
C sourcePower CT
current tests using three-wattmeter method
delta-connected winding or Line-to-Line for for Y-connected winding be connected Line-to-Neutral Note : Voltmeters should
A N V W3 a Vr VT r a C N V W2 V VT A A CT r V Va 1 W N B VT N 1U 1V 1W 2U 2V 2W
Voltage and frequency for no-load loss test
The operating and performance characteristics of a transformer are based upon rated voltage and rated frequency, unless otherwise specified. Therefore, the no-load loss test is conducted with rated voltage impressed across the transformer terminals, using a voltage source at a frequency equal to the rated frequency of the transformer under test, unless otherwise specified.
For the determination of the no-load losses of a single-phase transformer or a three-phase transformer, the frequency of the test source should be within ± 0.5% of the rated frequency of the transformer under test. If the excitation frequency is beyond the specified tolerance, then the test voltage shall be adjusted to maintain the V/f ratio corresponding to the ratio of rated voltage and rated frequency. The voltage shall be adjusted to the specified value as indicated by the average-voltage voltmeter. Simultaneous values of rms voltage, rms current, electrical power and the average voltmeter readings shall be recorded. For a three-phase
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transformer the average of the three voltmeter readings shall be the desired nominal value of the voltage.
The most difficult cases, both with regard to voltage wave shape distortion and power measurements usually arise when testing large single-phase transformers.
Instrument error at low power factor
At low power factors, such as those encountered while measuring the load losses and impedance voltage of power transformers, judicious selection of measurement method and test system components is essential for accurate and repeatable test results. The phase-angle errors in the instrument transformers, measuring instruments, bridge networks and accessories affect the load loss test results. Procedures for correcting the load losses for meeting phase-angle errors are described in IEC Publication 60076-8
Correction of no-load losses
The eddy current component of the no-load loss varies with the square of the rms value of excitation voltage and is substantially independent of the voltage waveform. When the test voltage is held at the specified value as read on the average-voltage voltmeter, the actual rms value of the test voltage may not be equal to the specified value. The no-load losses of the transformer corrected to a sine-wave basis shall be determined from the measured value by means of the following equation:
2 1 kP P P Pc m + = Where
Pc is the no-load losses, corrected for waveform
Pm is measured no-load losses
P1 is per unit hysteresis loss
P2 is per unit eddy-current loss
2
=
a rE
E
k
Er is the test voltage measured by rms voltmeter.
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The actual per unit values of hysteresis and eddy-current losses should be used if available. If actual values are not available, it is suggested that the two loss components be assumed equal in value, assigning each a value of 0.5 per unit for CRGO.
The above equation is valid only for voltage with moderate waveform distortion. If waveform distortion in the test voltage causes the magnitude of the correction to be greater than 5%, then the test voltage waveform must be improved for an adequate determination of the no-load losses and currents. For large single phase transformers, it is expected that the difference between rms voltages and average voltage will be greater than 5%, which should be accepted in view of test voltage source limitation.
The new generation of power analysers are equipped with software for automatic calculation of corrected losses based on the input data of voltages and power.
3.3.2 Measurement of excitation (no-load) current
The excitation (no-load) current of a transformer is the current that maintains the rated magnetic flux excitation in the core of the transformer. The excitation current is usually expressed in per unit or in percent of the rated line current of the winding in which it is measured. Measurement of excitation current is usually carried out in conjunction with the tests for no-load losses. RMS current is recorded simultaneously during the test for no-load losses using the average-voltage voltmeter method. This value is used in calculating the per unit or percent excitation current. For a three-phase transformer, the excitation current is calculated by taking the average of the magnitude of the three line currents. The tolerance for no-load current should be as per IS 2026 Part -1
3.4 Measurement of short-circuit impedance and load loss 3.4.1 General
The load losses of a transformer are those losses incident to a specified load carried by the transformer. Load losses include I2R loss in the windings due to load current and stray losses due to eddy currents induced by leakage flux in the windings, core clamps, magnetic shield, tank walls and other conducting parts. Stray losses may also be caused by circulating currents in parallel windings or strands. Load losses are measured by applying a short circuit across either the high voltage winding or the low voltage winding and applying sufficient voltage across the other winding to cause a specified current to flow in the windings. The power loss within the transformer under these conditions equals the load losses of the transformer at the temperature of test for the specified load current.
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The impedance voltage of a transformer between a pair of windings is the voltage required to circulate rated current through one of two specified windings when the other winding is short circuited, with the windings connected as for rated voltage operation. Impedance voltage is usually expressed in per unit or percent of the rated voltage of the winding across which the voltage is applied and measured.
The impedance voltage is measured during the load loss test by measuring the voltage required to circulate test current in the windings. The measured voltage is the impedance voltage at the test frequency and the power loss dissipated within the transformer is equal to the load losses at the temperature of test and at rated load. The impedance voltage is corrected to the rated frequency and the load losses are corrected to a reference temperature using the formulas specified in this standard.
3.4.2 Factors affecting the values of load losses and impedance voltage
The magnitude of the load losses and the impedance voltage will vary depending on the positions of tap changers, if any in various windings. These changes are due to the changes in the magnitudes of load currents and associated leakage-flux linkages as well as being due to changes in stray flux and accompanying stray losses.
3.4.2.1 Temperature
Load losses are also a function of temperature. The I2R component of the load losses increases with temperature, while the stray loss component decreases with temperature. Procedures for correcting the load losses to the standard reference temperature are described in 3.5.5.
3.4.2.2 Instrument error at low power factor
At low power factors, such as those encountered while measuring the load losses and impedance voltage of power transformers, judicious selection of measurement method and test system components is essential for accurate and repeatable test results. The phase-angle errors in the instrument transformers, measuring instruments, bridge networks and accessories affect the load loss test results. Procedures for correcting the load losses for meeting phase-angle errors are described in IEC Publication 60076-8
3.4.3 Methods for measuring load losses and impedance voltage
Test Conditions
To determine the load losses and impedance voltage with sufficient accuracy, the following conditions shall be met.
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1. The temperature of the insulating liquid has stabilized and the difference between top and bottom oil temperatures does not exceed 5 0C.
2. The temperature of the windings shall be taken immediately either before or after the load losses and impedance voltage test in a manner similar to that described in 3.1.1. The average shall be taken as the winding temperature for computation of losses. 3. The conductors used to short-circuit the low voltage, high current winding of a
transformer shall have a cross-sectional area equal to or greater than the corresponding transformer winding leads.
4. The test current shall be at least 50 % of the rated current of the winding across which the voltage is applied.
5. The measurement of losses shall be done at the earliest after excitation of the transformer to the test current to avoid heating of the winding resulting in increase in resistance.
3.4.3.1. Wattmeter-voltmeter-ammeter method for load loss and impedance voltage test
The connection and apparatus needed for the determination of the load losses and impedance voltage of a single-phase transformer are shown in figure 7 and 8. Figure 8 applies when the instrument transformers are required, which is the general case.
For three phase transformers, three-phase power measurement utilizing two wattmeter is possible but can result in very large errors at low power factors encountered in load loss tests of transformers. It is recommended that the two-wattmeter method should not be used for loss tests on three-phase transformers of ratings preferably above 20 MVA, 66 kV class.
For three phase transformers, figure 9 shows the apparatus and connections using the three-wattmeter method. So u rc e A W V
Fig : 7 - Basic circuit for load loss and impedance measurement impedance tests with instrument transformers
SourceAC V
A
HV LV
CT VT
W
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N
impedance voltage tests using three-wattmeter method Fig : 9 - Three phase transformer connections for load loss and
CT AB, BC, CA A AN, BN, CN Volts can be read C A Power source CT B
(For any delta or Y Transformer under test
A N V W3 a Vr VT combination) VT V 2 W a Vr C N B 3U 3V 2U 2V A A CT V W1 a Vr VT 1U 1V Short
3.4.3.2 Measurement with Power analyser
Nowadays, digital power analysers or power meters are available for determination of load losses. Selection of these power analysers shall be based on the desired accuracy at low power factors.
The new generation of power analysers are equipped with software for automatic calculation of corrected losses based on the input data of voltage, current, power, frequency and temperature.
3.4.4 Test procedure
3.4.4.1 Two-winding transformers and auto transformers
Load loss and impedance voltage tests are carried out using the connections and apparatus shown in figure 8 for single-phase transformers and figure 9 for three-phase transformers. With one winding short-circuited, a voltage of sufficient magnitude is applied to the other winding and adjusted to circulate test current in the excited winding. Simultaneous readings of wattmeter, voltmeter and ammeter are taken. If necessary, the corrections for the losses in external connections and connected measuring instruments should be made.
The procedure for testing three-phase transformers is very similar, except that all connections and measurements are three-phase instead of single-phase and a balanced three-phase source of power is used for the test. If the three line currents cannot be balanced, their average rms value should correspond to the desired value, at which time simultaneous reading of wattmeters, voltmeters and ammeters should be recorded.
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Single phase and three-phase auto transformers may be tested with internal connections unchanged. The test is made using the auto transformer connection. The input (or output) terminals are shorted and voltage is applied to the other terminals. The voltage is adjusted to cause test current to flow in the test circuit as shown in figure10. Simultaneous readings of wattmeters, voltmeters and ammeters are recorded for determination of load losses and impedance voltage.
Fig : 10 - Connections for impedance loss and impedance-voltage
S o u rc e V W A tests of an auto-transformer
For the purpose of measuring load losses and impedance voltage, the series and common windings of auto transformers may be treated as separate windings, one short circuited, the other excited. In this situation, where the transformer is connected in the two-winding connection for the test, the current held must be the test current of the excited winding, which may or may not be the same as rated line current. The load loss watts and applied volt-amperes will be same, whether series and common windings are treated as separate windings in the two-winding connection or are connected in the auto-transformer connection, so long as rated winding current atleast 50 percent is held in the first case and rated line current atleast 50 percent in the second case.
3.4.4.2 Three winding transformers
For a three winding transformer, which may be either single phase or three phase, three sets of impedance measurements are made between pairs of windings, following the same procedure as for two winding transformers. Measurement of the impedances Z12, Z23 and Z31
are obtained between windings 1, 2 & 3.
If the kVA capacities of the different windings are not alike, the current held for the impedance test should correspond to the capacity of the lower rated winding of the pair of the windings under test. However, all of these data when converted into percentage form should be based on the same output kVA, preferably that of the primary winding. An equivalent three-winding impedance network as shown in figure 11 can be derived from the following equations:
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Fig : 11 - Equivalent three-winding impedance network
1 3 Z 3 Z 1 Z 2 2 1 23 23 12 31 3 1 12 12 31 23 2 31 23 12 1 2 2 2 Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z − = + − = − = + − = + − = Where
Z12, Z23 and Z31 are the measured impedance values between pairs of windings, as indicated
all expressed on the same kVA base.
These equations involve complex numbers, but they may be used for the resistance (in-phase) component or the reactance (quadrature) component of the impedance voltage or of the impedance volt-amperes.
The treatment of the individual load losses for temperature corrections, etc., is the same as for two-winding, single phase transformers.
The total load losses of the three winding transformer is the sum of the losses in the branches of the equivalent circuit of figure 11 for any specific terminal load conditions.
3.4.5 Calculation of load losses and impedance voltage for test data
Load loss measurements vary with temperature and in general must be corrected to a reference temperature. In addition, load loss measurement values must be corrected for metering phase angle error. Impedance voltage measurement to vary with frequency and the values must be corrected for rated frequency.
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Temperature correction of load losses
Both I2R losses and stray losses of transformer vary with temperature. The I2R losses, Pr(Tm),
of a transformer are calculated from the ohmic resistance measurements (connected to the temperature, Tm, at which the measurement of the load losses and impedance voltage was
done) and the current that were used in the impedance measurement. These I2r losses subtracted from the measured load loss watts P(Tm), give the stray losses, Ps(Tm), of the
transformer at the temperature at which the load loss test was made. Ps (Tm) = P(Tm) – Pr(Tm)
Where
Ps(Tm) is the calculated stray losses (watts) at temperature Tm.
P(Tm) is the transformer load losses (watts), corrected in accordance with phase angle errors
in wattmeter at temperature Tm.
Pr (Tm) is the calculated I2R loss (watts) at temperature Tm
The I2R component of load losses increases with temperature. The stray loss component diminishes with temperature. Therefore, when it is desirable to convert the load losses from the temperature at which it is measured, Tm, to another temperature, T, the two components
of the load losses are corrected separately. Thus,
( )
( )
+ + = + + = T T T T T P P T T T T T P P k m k m s s m k k m r r Then P (T) = Pr (T) + Ps (T) WherePr (T) = I2R loss (watts) at temperature T, 0C
Ps (T) = stray losses (watts) at temperature T, 0C
P (T) = Transformer load losses (watts) corrected to temperature T, 0C Tk = 234.5 0C (copper) ≈ 235 0C
Tk = 225 0C (aluminium)
.
Calculation for impedance
The impedance shall be measured at rated frequency by applying an approximately sinusoidal supply to one winding, with the terminal of other winding short circuited, and with possible other winding open circuited. The supplied current should be equal to the relevant rated current. However, in case of limitation in the rating of supply source the current should
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not be less than the 50% of the rated current. Due to fluctuation in load the supply frequency may not be always be the rated frequency. Then frequency correction should be applied to calculate the actual impedance at rated frequency as following.
The formula for calculating the percentage impedance with current and frequency correction is
( )
% = × × × 100 t r test rated rated test f f I I V V Z WhereVtest =Test voltage
Vrated =Rated voltage
Itest = Test current
Irated = Rated current
ft = Test frequency
fr = Rated frequency
3.5 Measurement of insulation resistance
Insulation resistance tests are made to determine the insulation resistance from individual winding to ground or between individual windings. The insulation resistance in such tests is commonly measured in mega-ohms, or may be calculated from measurements of applied voltage and leakage current.
Note
1) The insulation resistance of electrical apparatus is subjected to wide variation in design, temperature, dryness, and cleanliness of the parts. When the insulation resistance falls below prescribed values, it can, in most cases of good design and where no defect exists, be brought up to that required standard by cleaning and drying the apparatus. The insulation resistance, therefore, may offer a useful indication as to whether the apparatus is in suitable condition for application of dielectric tests.
2) Under no conditions, test should be made while the transformer is under vacuum.
Instrumentation
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a) A variable-voltage dc power supply with means to measure voltage and current (generally in micro-amperes or milli-amperes)
b) A mega-ohmmeter
Mega-ohmmeters are commonly available with nominal voltages of 500V, 1000V, 2500V, and 5000V; dc or in multiples of 1000 V upto 10,000 V.
Voltage to be applied
The dc voltage applied for measuring insulation resistance to ground shall not exceed a value equal to the half of the rated voltage of the winding or 5 kV whichever is lower.
Procedure
Insulation resistance tests shall be made with all circuits of equal voltage above ground connected together. Circuits or groups of circuits of different voltages above ground shall be tested separately. All external insulating parts of the transformer shall be cleaned thoroughly to remove dust, moisture etc. before the test.
Examples:
a) High voltage to low voltage and ground, low voltage to high voltage and ground. b) Voltage should be increased in increments of usually one kilovolt and held for one
minute while the current is read.
c) The test should be disconnected immediately in the event the current begin to increase without stabilizing.
d) After the test has been completed, all terminals should be grounded for a period of time sufficient to allow any trapped charges to decay to a negligible value.
Polarisation Index (PI)
The purpose of polarisation index test is to determine if equipment is suitable for operation or even for an overvoltage test. The polarisation index is a ratio of insulation resistance value at the end of 10 min test to that at the end of 1 min test at a constant voltage.
The total current that is developed when applying a steady state dc voltage is composed of three components:
1) Charging current due to the capacitance of the insulation being measured. This current falls off from maximum to zero very rapidly.
2) Absorption current due to molecular charge shifting in the insulation. The transient current decays to zero more slowly.
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3) Leakage current which is the true conduction current of the insulation. It has a component due to the surface leakage because of the surface contamination.
The advantage of PI is that all of the variables that can affect a single IR reading, such as temperature and humidity, are essentially the same for both the 1 min and 10 min readings. Since leakage current increases at a faster rate with moisture present than does absorption current, the IR readings will not increase as fast with insulation in poor condition as with insulation in good condition. After 10 min the leakage current becomes constant and effects of charging current and absorption current die down.
Acceptable PI value for power transformer shall be better than 1.5. For distribution transformer it should be at least 1.3 .
Interpretation of results
Insulation resistance may vary with applied voltage and temperature any comparison must be made with measurements at the same voltage.
The significance of values of insulation resistance tests generally requires some interpretation, depending on the design and the dryness and cleanliness of the insulation involved. When a user decides to make insulation resistance test, it is recommended that insulation resistance values be measured periodically (during maintenance shutdown) and that these periodic values be plotted. Substantial variations in the plotted values of insulation resistance should be investigated for cause.
3.6 Tests on On-load Tap-changers
3.6.1 Operation test
With the tap-changer fully assembled on the transformer the following sequence of operations shall be performed without failure:
a. With the transformer un-energised, eight complete cycles of operations (a cycle of operation goes from one end of the tapping range to the other, and back again).
b. With the transformer un-energised, and with the auxiliary voltage reduced to 85% of its rated value, one complete cycle of operation.
c. With the transformer energized at rated voltage and frequency at no load, one complete cycle of operation
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d. With one winding short circuited and, as far as practicable, two rated current in the winding, 10 tap-change operations across the range of two steps on each side from where a coarse or reversing changeover selector operates, or otherwise from the middle tapping.
3.6.2 Auxiliary circuits insulation test
After the tap changer is assembled on the transformer, a power frequency tests shall be applied to the auxiliary circuits as specified in IEC 60076-3.
DIELECTRIC TESTS
The purpose of dielectric tests is to demonstrate that the transformer has been designed and constructed to withstand the specified insulation levels. The insulation requirements for the transformers and the corresponding dielectric tests are given in IS 2026 Part-3 and IEC Publication 60076-3 with reference to specific windings and their terminals. For oil immersed transformers, the requirements apply to the internal insulation only.
The dielectric tests shall generally be made at the manufacturer premises with the transformers approximately at ambient temperature.
Transformers, including bushings and terminal compartments when necessary to verify air clearances, shall be assembled prior to making dielectric tests, but assembly of items, such as radiators and cabinets, which do not affect dielectric tests is not necessary. Bushing shall, unless otherwise authorised by the purchaser, be those to be supplied with the transformer.
If a transformer fails to meet its test requirements and the fault is in a bushing, it is permissible to replace this bushing temporarily with another bushing and continue the tests on the transformer to completion without delay. A particular case arises for tests with partial discharge measurements, where certain types of commonly used high-voltage bushings create difficulty because of their relatively high level of partial discharge in the dielectric. When such bushings are specified for the transformer, it is permitted to exchange them for bushings of a partial discharge free type during the testing of transformer.
Test levels and other test parameters shall be as per IEC Publication 60076-3 and the corresponding IS 2026 Part-3.
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It is recommended to measure voltage at the high voltage terminal of its transformer. The measuring system shall be in accordance with IEC Publication 60071-2.
In conducting low frequency tests for transformers of 100 kVA and less to be tested at 50 kV or less, it is permissible to depend on the ratio of testing transformer to indicate the proper test voltage.
Rules for some particular transformers
In transformers where uniformly insulated windings having different Um values are connected
together within the transformer, the separate source AC withstand test voltages shall be determined by the insulation of the common neutral and its assigned Um.
In transformers which have one or more non uniformly insulated windings, the test voltages for the induced withstand voltage test, and for the switching impulse test, are determined by the winding with highest Um value, and the windings with lower Um values may not receive
their appropriate test voltages.
During switching impulse tests, the voltages developed across different windings are approximately proportional to the ratio of turns. Rated switching impulse withstand voltages shall only be assigned to the winding with the highest Um. Test stresses in other windings are
also proportional to the ratio of numbers of turns and are adjusted by selecting appropriate tappings to come as close as possible to the assigned value.
Insulation requirements and dielectric tests
The basic rules for insulation requirements and dielectric tests for different categories of windings are described in Table 1(Refer IEC Publication 60076-3)
TABLE 1 Tests Category of winding Highest voltage for equipment Um kV Lightning impulse (LI) Switching impulse (SI) Long duration AC(ACLD) Short duration AC(ACSD) Separate source AC
Uniform Um ≤ 72.5 Type Not
applicable Not applicable Routine Routine Uniform and non-72.5 < Um ≤ 170 Routine Not applicable
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170 < Um < 300 Routine Routine* Routine Special* Routine
uniform insulation
Um ≥ 300 Routine Routine Routine Special Routine
* If ACSD test is specified, the SI test is not required.
The standard dielectric requirements are verified by dielectric tests. They shall, where applicable and not otherwise agreed upon, be performed in the sequence as given below. 1) Switching impulse test (SI) for the line terminal
2) Lightning impulse test (LI) for the line terminals 3) Lightning impulse test (LI) for neutral terminal
4) Separate source AC withstand voltage test (applied potential test) 5) Short-duration induced AC withstand voltage test (ACSD)
6) Long-duration induced AC voltage test (ACLD)
3.7 Switching impulse withstand voltage test, transformer winding Um > 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.
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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.
Test connections
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.
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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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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
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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.
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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:
frequency Test
frequency Rated
×
120 , but not less than 15 sec
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
Test voltage level Test Duration (Refer Fig 12) Remarks ≤ 72.5 kV As per Table2 of IEC 60076-3 60 sec No PD measurement Uniformly insulated > 72.5 kV U1=from Table D.1 of IEC 60076-3 U2= 1.3 Um/√3 C= 120x Rated Frq. Test freq. PD level should be ≤ 300 pC at level U2
Page 33 of 65 Phase to earth test U1=from Table D.2 of IEC 60076-3 U2= 1.5Um/√3 PD level should be ≤ 500 pC at level U2 AC Short duration Non-uniformly insulated >72.5 kV Phase to phase test U1=from table D.2 of IEC 60076-3 U2= 1.3 Um/√3 C= 120x Rated Frq. Test freq. With PD measurement It should be ≤ 300 pC at level U2 Delta connected HV ≤ 245 kV U1= 1.7 Um/√3 U2= 1.5 Um/√3 D=30 min C= 120x Rated Frq. Test freq. PD level should be ≤ 500 pC at level U2 AC Long Duration Uniformly and non-uniformly insulated Star connected HV < 300 kV ≥ 300 kV U1= 1.7 Um/√3 U2= 1.5 Um/√3 U1= 1.7 Um/√3 U2= 1.5 Um/√3 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 Here
Um=Highest voltage for equipment
U1= Test voltage
Fig. 12 Time sequence for the application of test voltage with respect to earth
A = 5 min B = 5 min C = test time
D = 5 min for ACSD and 30/60 min for ACLD E = 5 min A B C D E Ustart U2 U1 U2 Um Ustart < / 3 1.1 / 3 Um 1.1
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U2= Partial discharge evaluation level
3.10.1 Short-duration induced AC withstand voltage test (ACSD)
3.10.1.1 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
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- 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.
3.10.1.2 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.
