TESTING OF TRANSFORMER
The following Tests are specified in IS:2026 (Part I), specification for power transformers, Part-I, General:
TYPE TESTS:
· Measurement of winding resistance.
· Measurement of Voltage ratio and check of Voltage Vector relationship.
· Measurement of impedance voltage/short circuit impedance (principal tapping) and load loss.
· Measurement of no load loss and current.
· Measurement of insulation resistance.
· Dielectric tests
· Temperature rise test
· Tests on ON-LOAD tap-changers, where appropriate
ROUTINE TESTS
· Measurement of Winding resistance.
· Measurement of Voltage ratio and check of voltage vector relationship.
· Measurement of impedance voltage/short circuit impedance (Principal tapping and load loss)
· Measurement of no-load loss and current.
· Measurement of insulation resistance.
· Dielectric tests
· Tests on ON-LOAD tap-changers, where appropriate.
SPECIAL TESTS:
· Dielectric tests
· Measurement of Zero-sequence impedance of three phase transformers.
· Short circuit test.
· Measurement of acoustic noise level.
· Measurement of harmonics of the no-load current.
· Measurement of the power taken by the fans and oil pumps.
TESTS CARRIED OUT AT FACTORY:
Type/Routine/Special tests if any as specified by the MPPTCL in the P.O. specification will be done at the factory by the transformer manufacturer in the presence of the nominated Engineer of the MPPTCL. The test results will be authenticated by the Manufacturer’s Engineer and the MPPTCL Engineer who witnessed the test and sent to the MPPTCL for acceptance.
TESTS CARRIED OUT AT SITE:
A transformer may be tested at site under the following circumstances.
· A new or second hand/ repaired transformer to be commissioned :-
Pre- commissioning tests to be done.
· Tests to be conducted to determine the healthiness or other-wise of the transformer that has been taken out of service on the actuation of Buchholz/Differential relay or on observing some abnormal occurrence that has placed a question on the healthiness of the transformer :-
Routine test to be done
Pre-Commissioning Testing of Transformer:
After arrival of Transformer at site , thorough inspection shall be done immediately on arrival for any damages to X-mers in transport.
a. Transformer received filled with oil, oil level in main tank is to be verified by comparing the oil level shown by manufacturer before dispatch in their document. A sample of oil shall be taken from bottom of the tank and send the oil sample for oil parameter testing, test result found, should be comparable as per IS 1866 and factory test result.
b. If X-mer received is filled with nitrogen, the nitrogen gas pressure reading should be taken and ensure that gas pressure is positive. Dew point of N2 gas has also to be recorded with ambient temp and check wether the test results are comparable or not .
c. CORE INSULATION TEST : After above Take IR value of Core to Tank , Core to Frame and Frame to Tank (if external provision provided) with insulation tester at 2.5 KV or at Voltage level over which test is performed at factory.
d. Core-Tank-Frame are shorted to each other as shown in Figure (1) below, for IR Value measurement remove the shorting and take the reading and note down the result in Table(1) as shown below. The tank shall be grounded during the test and compare the result with factory test results. In no case, these IR value found, should be zero between Core-Tank-Frame.
Figure (1): Core-Tank-Frame Arrangement for Testing
|
Terminal |
Test voltage |
IR value measurement for 1 min |
|
Core-Tank |
|
|
|
Core-Frame |
|
|
|
Frame-Tank |
|
|
Table (1): Core-Tank-Frame IR value measurement format
If any abnormality noticed from above point a to c the X-mer should not be un- loaded from the trailer and proper intimations should be given to the manufacturers and higher authorities for necessary action.
After unloading of transformer on plinth and complete erection of main Tank, bushing, conservator , radiators and other accessories and complete oil filling of transformer, Pre-commissioning test are to be perform over transformer. The Procedure of various Pre- Commissioning test are elaborated below:
1. Ratio Test
Ratio measurements must be made on all taps to confirm the proper alignment and operation of the tap changers. The test should preferably be performed by automatic Transformer turns ratio (TTR) meter.
A. Using automatic Transformer turns ratio (TTR) meter.
Here Testing is describing using DV Power Three Phase transformer turn ratio tester TRT3X as shown in Figure (2). In addition to determining the voltage transformation ratio which is the primary objective of this test, the polarity between the windings and phase relationship between phases can also be easily established by the ratio meter in a single operation. Before using the ratio meter, the instruction manual for operating the ratio meter must be carefully studied and all precautions noted therein must be followed.
Figure(2) : Front Panel of DV Power Three Phase transformer turn ratio tester TRT3X
a. Colors and marking of test cable of DV Power Three Phase transformer turn ratio tester TRT3X shown in Table-(2)
|
IEC Test |
ANSI |
Australian |
Transformer |
Test Lead |
Australian |
Malaysian |
|
Lead |
Test Lead |
Test Lead |
Terminal |
Color IEC |
Test Lead |
Test Lead |
|
Marking |
Marking |
Marking |
Voltage |
Colors |
Colors |
|
|
1N |
H0 |
N |
Neutral |
Blue |
Black |
Black |
|
1U |
H1 |
A |
High |
Red |
Red |
Red |
|
1V |
H2 |
B |
High |
Black |
White |
Yellow |
|
1W |
H3 |
C |
High |
Yellow |
Blue |
Blue |
|
2N |
X0 |
n |
Neutral |
Blue |
Black |
Black |
|
2U |
X1 |
a |
Low |
Red |
Red |
Red |
|
2V |
X2 |
b |
Low |
Black |
White |
Yellow |
|
2W |
X3 |
c |
Low |
Yellow |
Blue |
Blue |
Table-(2) : Test lead marking and color
c. Figure (3) Shows the connection of TRT3X turn ratio tester to 3-Ph Auto- transformer, HV Test cable lead H1(Red), H2(Yellow),H3(Blue) and H0(Black) Connected to Transformer HV-1U,1V,1W and N respectively and LV Test cable lead X1(Red), X2(Yellow),X3(Blue) and X0(Black) Connected to Transformer LV- 2U,2V,2W and N.
Figure(3): Example of connecting TRT3x to a three-phase autotransformer
c. If the three-phase transformer autotransformer has a tertiary winding, it should be tested the same way as the secondary. The transformer is energized using the high voltage windings. Test leads 2U, 2V, 2W and 2N (X1, X2, X3 and X0) are in this case connected to the tertiary winding of the transformer.
d. Figure (4) Shows the connection of TRT3X turn ratio tester to 3-Ph conventional two winding transformer. HV Test cable lead H1(Red), H2(Black),H3(Yellow) and H0(Blue) Connected to Transformer HV-1U,1V,1W and 1N respectively and LV Test cable lead X1(Red), X2(Black),X3(Yellow) and X0(Blue) Connected to Transformer LV-2U,2V,2W and 2N
Figure(4): Example of connecting TRT3x to a three-phase conventional Two winding transformer
c. Figure (5) Shows the connection of TRT3X turn ratio tester to 1-Ph Auto- transformer,. HV Test cable lead H1(Red) and H0(Blue) Connected to Transformer HV-1U and 1N respectively and LV Test cable lead X1(Red) and X0(Blue) Connected to Transformer LV-2U and 2N.
Figure(5): Example of connecting TRT3x to a single-phase autotransformer
c. Figure (6) Shows the connection of TRT3X turn ratio tester to 1-Ph conventional two winding transformer,. HV Test cable lead H1(Red) and H0(Blue) Connected to
Transformer HV-1U and 1N respectively and LV Test cable lead X1(Red) and X0(Blue) Connected to Transformer LV-2U and 2N.
Figure(6) Example of connecting TRT3x to a single-phase conventional Two winding transformer
c. Single Phase Test Mode of TRT3X :
· If the test object is a three-phase transformer or three-phase autotransformer, the single-phase test is performed by applying a single- phase test voltage to each of the three H terminals in turns. The voltages across each of the corresponding X terminals are then measured. The ratio of these voltages is calculated and presented on the display. It is repeated for all three phases automatically, based on the selected/detected transformer vector group
· If the test object is a single-phase transformer or single-phase autotransformer, the TRT3x will apply one single-phase test voltage between selected H terminals, and will measure voltage across selected X terminals
d. Three Phase Test mode of TRT3X :
· The TRT3x will apply true three-phase test voltage between the terminals 1U-1V-1W (H1-H2-H3, A-B-C), and will measure a three-phase voltage between the terminals 2U-2V-2W (X1-X2-X3, a-b-c). Three turns ratios are calculated, as per the Table 3
|
Phase |
Applied voltage |
Measured voltage |
|
between terminals |
between terminals |
|
|
A |
1U-1V |
2U-2V |
|
H1-H2 |
X1-X2 |
|
|
A-B |
a-b |
|
B |
1V-1W |
2V-2W |
|
H2-H3 |
X2-X3 |
|
|
B-C |
b-c |
|
|
C |
1W-1U |
2W-2U |
|
H3-H1 |
X3-X1 |
|
|
C-A |
c-a |
Table-3 : Three Phase Turn Ratio
e. Viewing Result : When the test is completed, the TRT3x automatically displays the test results as shown in Figure(7) To scroll between all the results, ▲ and ▼ should be used.
Figure(7) : Results after test
c. Results of the transformation turns or voltage ratio are absolute, and may be compared with the specified values measured during factory testing. The turns ratio tolerance should be within 0.5 % of the nameplate specifications
A. When automatic Transformer turns ratio (TTR) meter not available.
When a ratio meter is not available, the ratio may be determined by using testing board as shown in Figure(8) .connect all three phases of HV,IV,LV and Neutral Bushing top through insulated wire at respected terminal marked on testing board and testing is done in steps as elaborated below:
a. 3-Phase Supply given to the HV winding of the transformer terminal 1U-1V-1W and measuring the voltage applied to the H.V. side i.e. 1U-1V,1V-1W & 1W-1U and measure the voltage induced in the LV Windings by taking measurement at the LV
terminals i.e 2U-2V,2V-2W & 2W-2U and then calculate the ratio and note down the result as shown in Table(4) .
|
TAP NO |
STD. RATIO |
MEASURED RATIO & % DEVIATION |
|||||
|
1U-1V/2U- 2V |
% DEV |
1V- 1W/2V- 2W |
% DEV |
1W-1U/2W-2U |
% DEV |
||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Table-(4) : Format for Three Phase Turn Ratio
b. % Deviation is calculated from formula = (Cal. Ratio – Std. Ratio)/Std. ratio * 100 Where Cal. Ratio is Calculated ratio and Std. Ratio is Standard Ratio of Transformer.
c. % Deviation should be less then <+- 0.5 %
d. This is not an accurate method, as the instruments used to measure the voltage may not have the required accuracy.
e. The ratio is to be determined at all the taps. The test can be started from one extreme tap and measurement taken for each tap up to the other extreme by operating the OLTC without switching off supply voltage while operating the OLTC. Increase or decrease in the LV side voltage for each tap should more or less correspond to the percentage mentioned for that tap in the name-plate.
1 Magnetic Balance Test.
This test is a low voltage test conducted at factory and site by applying single phase voltage between phase and neutral of a winding and measuring voltage induced in other two phases of the same winding and all the three phases of other windings at Normal tap. This test is done to find out whether the magnetic paths of the transformer core are balanced.
l. For 3-Ph Auto-transformer having tertiary winding, Figure(8) shown the Testing Board for magnetic balance test of Transformer.
Figure(8) Arrangement for measurement of magnetic Balance
Connect all three phases of HV,IV,LV and Neutral Bushing top through insulated wire at respected terminal marked on above testing board as 1U,1V,1W ,2U,2V,2W & 3U,3V,3W N respectively and testing is done in three steps as elaborated below :
Figure(9) Measurement of magnetic Balance when 1-ph supply given on HV side and
measure on HV,IV & LV side
Repeat the test for Remaining Two phase of HV winding i.e for 1V-N & 1W-N and note down the result as shown in Table-5.
|
230V Supply at terminal |
Measure Induce Voltage in Volts at Terminal |
||||||||
|
1U- N |
1V- N |
1W-N |
2U-N |
2V-N |
2W-N |
3U- 3W |
3U-3V |
3V- 3W |
|
|
1U-N |
230 |
|
|
|
|
|
|
|
|
|
1V-N |
|
230 |
|
|
|
|
|
|
|
|
1W-N |
|
|
230 |
|
|
|
|
|
|
Table-5 :1-ph 230V supply given to HV terminal and voltage measure at HV,IV &LV Terminal
ü Apply 1-Ph Voltage 230 V across one phase of Intermediate Voltage (IV) Winding Terminal and neutral i.e 2U-N, then measure Induced voltage in other two IV terminals across neutral i.e 2V-N & 2W-N and Lowest Voltage (LV) winding terminal 3U-3W,3U-3V & 3V-3W as shown in Figure(10).
Figure(10) : Measurement of magnetic Balance when 1-ph supply given to IV terminal and voltage measure a ,IV & LV
Repeat the test for Remaining Two phase of IV winding i.e 2V-N & 2W-N and note down the result as shown in Table-6.
|
230V Supply at terminal |
Measure Induce Voltage in Volts at Terminal |
|||||
|
2U-N |
2V-N |
2W-N |
3U-3W |
3U-3V |
3V-3W |
|
|
2U-N |
230 |
|
|
|
|
|
|
2V-N |
|
230 |
|
|
|
|
|
2W-N |
|
|
230 |
|
|
|
Table-6 :1-ph 230V supply given to IV terminal and voltage measure a ,IV & LV
Terminal
Note :- When voltage is applied
on IV side don’t measure the voltage on HV
side as due to step up phenomena high
voltage is induce on HV side which will damage the Voltage
meter.
ü Apply 1-Ph Voltage 230 V across one phase of Lowest Voltage (LV) winding terminal i.e 3U-3W, then measure Induced voltage in other two LV terminals
i.e. 3U-3V & 3V-3W as shown in Figure(11)
Figure(11) : Measurement of magnetic Balance when 1-ph 230V supply given to LV terminal and voltage measure LV Terminal
Repeat the test for Remaining Two phase of LV winding i.e 3U-3V & 3V-3W and note down the result as shown in Table-7.
|
230V Supply at terminal |
Measure Induce Voltage in Volts at Terminal |
||
|
3U-3W |
3U-3V |
3V-3W |
|
|
3U-3W |
230 |
|
|
|
3U-3V |
|
230 |
|
|
3V-3W |
|
|
230 |
Table-7 :1-ph 230V supply given to LV terminal and voltage measure LV
Terminal
Note :- When voltage is applied on IV side don’t measure the voltage on HV & LV side as due to step up phenomena high voltage is induce on HV side which will damage the Voltage meter.
ü Compare the measured result with factory test results and the identical results confirm no damage due to transportation.
m. For 3-Ph Conventional transformer having Two Winding or Three Winding, Figure(8) shown the Testing Board for testing magnetic balance test of Transformer connect all three phases of HV,LV and Neutral Bushing top through insulated wire at
respected terminal marked on testing board as 1U,1V,1W ,2U,2V,2W & 1N,2N respectively and testing is done in three steps as elaborated below:
ü Apply 1-Ph Voltage 230 V across one phase of Highest Voltage (HV) winding terminal and neutral i.e 1U-N, then measure Induced voltage in other two HV terminals across neutral i.e 1V-N & 1W-N and Low Voltage (LV) Winding Terminal and neutral 2U-N,2V-N & 2W-N as shown in Figure(12).
Figure(12) : Measurement of Magnetic Balance when 1-ph 230V supply given to HV terminal and voltage measure at HV & LV
Repeat the test for Remaining Two phase of HV winding i.e. 1V-N & 1W-N and note down the result as shown in Table-8.
|
230V Supply at terminal |
Measure Induce Voltage at Terminal |
|||||
|
1U-N |
1V-N |
1W-N |
2U-N |
2V-N |
2W-N |
|
|
1U-N |
230 |
|
|
|
|
|
|
1V-N |
|
230 |
|
|
|
|
|
1W-N |
|
|
230 |
|
|
|
Table-8 :1-ph 230V supply given to HV terminal and voltage measure at HV & LV
Terminal
ü Apply 1-Ph Voltage 230 V across one phase of Low Voltage (LV) Winding Terminal and neutral i.e 2U-N, then measure Induced voltage in other two LV terminals across neutral i.e 2V-N & 2W-N as shown in Figure(13)
Figure(13) : Measurement of Magnetic Balance when 1-ph 230V supply given to LV terminal and voltage measure at LV
Repeat the test for Remaining Two phase of LV winding i.e for 2V-N & 2W-N and note down the result as shown in Table-9.
|
230V Supply at terminal |
Measure Induce Voltage at Terminal |
||
|
2U-N |
2V-N |
2W-N |
|
|
2U-N |
230 |
|
|
|
2V-N |
|
230 |
|
|
2W-N |
|
|
230 |
Table-9 :1-ph 230V supply given to LV terminal and voltage measure on LV
Terminal
Note :- When voltage is applied on LV side don’t measure the voltage on HV side as due to step up phenomena high voltage is induce on HV side which will damage the Voltage meter.
ü Compare the measured result with factory test results and the identical results confirm no damage due to transportation.
m. Also the applied voltage may be expressed as 100% voltage and the induced voltage may be expressed as percentage of the applied voltage. This will help in comparison of the two results when the applied voltages are different. The voltage induced in the centre phase shall be 50 to 90% of the applied voltage. However, when the centre phase is excited then the voltage induced in the outer phases shall be 30 to 70% of the applied voltage
1 Magnetizing current Test :
This test is performed to locate defects in the magnetic core structure, shifting of windings, failure in turn to turn insulation or problem in tap changers. These conditions change the affective reluctance of the magnetic circuit, thus affecting the current required to
establish flux in the core. Abnormal value of magnetizing current can indicate inter turn short in the windings or problem in the core like core shifting etc., So magnetizing current is a very important parameter to be taken before commissioning a new Transformer
Note :- This test should be done before DC measurements of winding resistance to reduce the effect of residual magnetism. Magnetizing current readings may be effected by residual magnetism in the core. Therefore, transformer under test may be demagnetized before commencement of magnetizing current test.
A. Using Omicron CPC-80 Capacitance and Tan Delta measurement kit.
For 3-Ph Auto-transformer having tertiary winding: Here Testing is describing using Omicron CPC-80 Capacitance and Tan Delta measurement kit shown in Figure(14) for measurement of magnetizing current.
Before using the Omicron CPC-80, the instruction manual for operating the Kit must be carefully studied and all precautions noted therein must be followed
Figure(14) : Omicron CPC-80 Capacitance and Tan Delta measurement kit used for magnetizing current measurement
Setup of Omicron CPC-80 kit for measurement of magnetizing current:
i. Power Supply for Kit should be phase controlled with ground
ii. Ground terminal of kit and supply board should be same.
iii. Properly connect both kits via serial interphase connector and Ext. Booster cable as per the slots shown in Figure(14) .
iv. Connect the HV and LV leads as per slot an color code as shown in Figure(14).
v. Power on the Kit and create a template of power transformer as describe in instruction manual.
vi. Select the Supply voltage between 0 to 10 KV at which magnetizing current will be measure depending upon the current measuring limit of Kit.
vii. Keep Step of variac as 5 and disable or kept OFF other ICON as shown in Figure(16)
Figure(15) : Omicron CPC-80 Front panel
Figure(16) : Omicron CPC-80 Magenetizing current measurement tempalate
i. Now Push Start Button two times for starting the Test.
ii. When Testing Done Result Shown on Screen as shown in Figure(16)
ü For HV Side- Connect HV Lead of Testing Kit on R phase of Highest Voltage (HV) winding terminal i.e 1U and LV Lead of Testing Kit on neutral i.e N as
shown in Figure(17), then Start the Kit as explain above and measure magnetizing current of HV R-phase i.e for 1U-N.
Figure(17) : Measurement of HV Magnetizing current
Repeat the test for Remaining Two phase of HV winding i.e. 1V-N & 1W-N and note down the result as shown in Table-10.
|
Tap No. |
Supply Voltage |
Measured Magnetizing Current |
||
|
1U-N |
1V-N |
1W-N |
||
|
|
230 V |
|
|
|
|
5 KV |
|
|
|
|
|
10 KV |
|
|
|
|
Table-(10) :Measurement of Magnetizing Current of HV Terminal.
ü For IV Side- Connect HV Lead of Testing Kit on R phase of Intermediate Voltage (IV) winding terminal i.e 2U and LV Lead of Testing Kit on neutral i.e N as shown in Figure(18), then Start the Kit as explain above and measure magnetizing current of IV R-phase i.e for 2U-N.
Figure(18) : Measurement of IV Magnetizing current
Repeat the test for Remaining Two phase of IV winding i.e. 2V-N & 2W-N and note down the result as shown in Table-11.
|
Tap No. |
Supply Voltage |
Measured Magnetizing Current |
||
|
2U-N |
2V-N |
2W-N |
||
|
|
230 V |
|
|
|
|
5 KV |
|
|
|
|
|
10 KV |
|
|
|
|
Table-11 : Measurement of Magnetizing Current of IV Terminal
Note :- For IV Side measure the Magnetizing current at all taps in order check correctness of tap Changer.
ü For LV Side- Connect HV Lead of Testing Kit on R phase of Low Voltage (LV) winding terminal i.e 3U and LV Lead of Testing Kit on B phase of Low Voltage (LV) winding terminal i.e. 3W as shown in Figure(19), then Start the Kit as explain above and measure magnetizing current of LV R-phase i.e for 3U.
Figure(19) : Measurement of LV Magnetizing current
Repeat the test for Remaining Two phase of LV winding i.e. 3U-3V & 3V-3W and note down the result as shown in Table-12.
|
Tap No. |
Supply Voltage |
Measured Magnetizing Current |
||
|
3U-3W |
3U-3V |
3V-3W |
||
|
|
230 V |
|
|
|
|
5 KV |
|
|
|
|
|
10 KV |
|
|
|
|
Table-12 :Measurement of Magnetizing Current of LV Terminal.
ü Compare the measured result with factory test results and the identical results confirm no damage due to transportation.
l. For 3-Ph Conventional transformer having Two Winding or Three Winding,
ü For HV Side- Connect HV Lead of Testing Kit on R phase of Highest Voltage (HV) winding terminal i.e 1U and LV Lead of Testing Kit on neutral i.e 1N as shown in Figure(20), then Start the Kit as explain above and measure magnetizing current of HV R-phase i.e for 1U-N.
Figure(20) : Measurement of HV Magnetizing current
Repeat the test for Remaining Two phase of HV winding i.e. 1V-N & 1W-N and note down the result as shown in Table-13
|
Tap No. |
Supply Voltage |
Measured Magnetizing Current |
||
|
1U-N |
1V-N |
1W-N |
||
|
|
230 V |
|
|
|
|
5 KV |
|
|
|
|
|
10 KV |
|
|
|
|
Table-13 :Measurement of Magnetizing Current of HV Terminal.
ü For LV Side- Connect HV Lead of Testing Kit on R phase of Low Voltage (LV) winding terminal i.e 2U and LV Lead of Testing Kit on neutral i.e 2N as shown in Figure(21), then Start the Kit as explain above and measure magnetizing current of LV R-phase i.e for 2U-N.
Figure(21) : Measurement of LV Magnetizing current
Repeat the test for Remaining Two phase of LV winding i.e. 2V-N & 2W-N and note down the result as shown in Table-14.
|
Tap No. |
Supply Voltage |
Measured Magnetizing Current |
||
|
2U-N |
2V-N |
2W-N |
||
|
|
230 V |
|
|
|
|
5 KV |
|
|
|
|
|
10 KV |
|
|
|
|
Table-14 : Measurement of Magnetizing Current of LV Terminal
Note :- For LV Side measure the Magnetizing current at all taps in order check correctness of tap Changer.
ü Compare the measured result with factory test results and the identical results confirm no damage due to transportation
m. The test values on the outside legs should be within 15 % of each other, and values for the centre leg should not be more than either outside for a three- phase transformers. Results compared to factory tests made under the same conditions should not vary more than 25%. If the measured exciting current value is 50 times higher than the value measured during Factory test, then there is likelihood of a fault in the winding which needs further analysis.
A. When Omicron CPC-80 kit is not available.
a. For 3-Ph Auto-transformer having tertiary winding, Figure(8) shown the testing board for testing magnetizing current of Transformer connect all three phases of
HV,IV,LV and Neutral Bushing top through insulated wire at respected terminal marked on testing board and testing is done in three steps as elaborated below :
For HV Side- Apply 1-Ph Voltage 230 V across one phase of Highest Voltage (HV) winding terminal and neutral i.e 1U-N, and measure the current with Ammeter or Current Clamp meter in 1U phase with proper current range selection as shown in Figure(21).
Figure(21) : Measurement of HV Magnetizing current
Repeat the test for Remaining Two phase of HV winding i.e for 1V-N & 1W-N and note down the result as shown in Table-15.
|
Tap No. |
230 V Supply Voltage At Terminal |
Measured Magnetizing Current in (mA) |
|
|
|
1U-N |
1U |
|
|
1V-N |
1V |
|
|
|
1W-N |
1W |
|
|
Table-15 : Measurement of Magnetizing Current of HV Terminal
ü For IV Side-Apply 1-Ph Voltage 230 V across one phase of Intermediate Voltage (IV) Winding Terminal and neutral i.e 2U-N, and measure the current with Ammeter or Current Clamp meter in 2U phase with proper current range selection as shown in Figure(22).
Figure(22) : Measurement of IV Magnetizing current
Repeat the test for Remaining Two phase of IV winding i.e for 2V-N & 2W-N and note down the result as shown in Table-16.
|
Tap No. |
230 V Supply Voltage At Terminal |
Measured Magnetizing Current in (mA) |
|
|
|
2U-N |
2U |
|
|
2V-N |
2V |
|
|
|
2W-N |
2W |
|
|
Table-16 : Measurement of Magnetizing Current of IV Terminal
Note :- For IV Side measure the Magnetizing current at all taps in order check correctness of tap Changer.
ü For LV Side-Apply 1-Ph Voltage 230 V across one phase of Lowest Voltage (LV) winding terminal i.e 3U-3W and measure the current with Ammeter or Current Clamp meter in 3U phase with proper current range selection as shown in Figure(23).
Figure(23) : Measurement of LV Magnetizing current
Repeat the test for Remaining Two phase of LV winding and note down the result as shown in Table-17.
|
Tap No. |
230 V Supply Voltage At Terminal |
Measured Magnetizing Current in (mA) |
|
|
|
3U-3W |
3U |
|
|
3U-3V |
3V |
|
|
|
3V-3W |
3W |
|
|
Table-17 : Measurement of Magnetizing Current of L V Terminal
ü Compare the measured result with factory test results and the identical results confirm no damage due to transportation
b. For 3-Ph Conventional transformer having Two Winding or Three Winding, Figure(8) shown the testing Board for testing magnetizing current of Transformer connect all three phases of HV , LV and Neutral Bushing top through insulated wire at respected terminal marked on testing board and testing is done in three steps as elaborated below:
ü For HV Side- Apply 1-Ph Voltage 230 V across one phase of Highest Voltage (HV) winding terminal and neutral i.e 1U-N, and measure the current with Ammeter or Current Clamp meter in 1U phase with proper current range selection as shown in Figure(24)
Figure(24) : Measurement of HV Magnetizing current
Repeat the test for Remaining Two phase of HV winding i.e for 1V-N & 1W-N and note down the result as shown in Table-18.
|
Tap No. |
230 V Supply Voltage At Terminal |
Measured Magnetizing Current in (mA) |
|
|
|
1U-N |
1U |
|
|
1V-N |
1V |
|
|
|
1W-N |
1W |
|
|
Table-18 : Measurement of Magnetizing Current of HV Terminal
ü For LV Side-Apply 1-Ph Voltage 230 V across one phase of Intermediate Voltage (IV) Winding Terminal and neutral i.e 2U-N, and measure the current with Ammeter or Current Clamp meter in 2U phase with proper current range selection as shown in Figure(25).
Figure(25) : Measurement of LV Magnetizing current
Repeat the test for Remaining Two phase of LV winding i.e for 2V-N & 2W-N and note down the result as shown in Table-19
|
Tap No. |
230 V Supply Voltage At Terminal |
Measured Magnetizing Current in (mA) |
|
|
|
2U-N |
2U |
|
|
2V-N |
2V |
|
|
|
2W-N |
2W |
|
|
Table-19 : Measurement of Magnetizing Current of LV Terminal
Note :- For LV Side measure the Magnetizing current at all taps in order check correctness of tap Changer..
ü Compare the measured result with factory test results and the identical results confirm no damage due to transportation
b. The test values on the outside legs should be within 15 % of each other, and values for the centre leg should not be more than either outside for a three- phase transformers. Results compared to factory tests made under the same conditions should not vary more than 25%. If the measured exciting current value is 50 times higher than the value measured during Factory test, then there is likelihood of a fault in the winding which needs further analysis