Based on IEC-76, there are few testing need to be carried out to ensure the power transformer are technically sound to be used.
In general, testing of power transformer are divided into three namely
a) Routine Test
b) Type Test and
c) Special Test
Other than the above testing, Transformer Oil Test needs to be done prior to the energizing of power transformer.
Routine Test consists of a few tests as below:
a) Ratio Test
b) Winding Resistance Test
c) Load Loss Test
d) No Load Loss Test
e) Induced Over-voltage Withstand Test
f) Separate Source Withstand Test
g) Test On-Load Tap Changer
h) Lightning Impulse for Line Terminal
i) Temperature Rise Test
And for Special Test will be:
a) Insulation Resistance Test
b) Sound Level Test
c) Lightning Impulse Chopped On The Tail
d) Tan Delta Test
Okay, we narrow down into individual test.
Routine Test
a) Ratio Test
This testing is to check that the deviation of the voltage ratio does not exceed the limit of the transformer standard used (generally 0.5%). It is also to ensure that the turn ratio of the winding is correct and that any tapping on the windings have been made at the correct position.
At the same time, the vector relationships are also being checked.
A ratiometer will be used during the testing and the result shall be compared with those calculated.
b) Winding Resistance Test
The resistance between all pairs of the phase terminal of each transformer winding are measured using Direct Current (DC). The measurement is performed foe each connection of windings and for each tapping connection. Furthermore, the corresponding winding temperature is measured.
The measured resistance are needed in connection with the load loss measurement when the load loss are corrected to correspond to the reference temperature. The resistance measurement will also show whether the winding joints are in order and the winding are correctly connected.
c) Load Loss Test
This test is to determine the load losses of the transformer and the impedance voltage at the rated frequency and the rated current. The measurements are made separately for each winding pair. (e.g. pair 1-2, 1-3 and 2-3).
In the testing, current is generally supplied to the HV winding and the LV winding is short-circuited.
d) No Load Loss Test
This test is to fine out the electrical losses due to the core of the transformer. In the no-load measurement, the no-load losses and the no-load current of the transformer are determined at rated voltage and rated frequency.
e) Induced Over-voltage Withstand Test
The objective of the test is to ensure that the insulation between the phase windings, turns, coils, tapping leads and terminal can withstand the temporary over-voltage.
Double rated voltage (e.g. 11kV becoming 22kV) and double rated frequency (e.g. 50Hz becoming 100Hz) is applied to the terminal of the LV windings. The HV windings are left open-circuited.
The test considered successful if no collapse of the test voltage occurs.
f) Separate Source Withstand Test
This test is to ensure that the insulation between the windings and the insulation between windings and earth parts can withstand the temporary overvoltage and switching overvoltage which may occur in service.
The test is made with single phase voltage of rated frequency. The test voltage is applied for 60 seconds between the winding under test and all terminals of the remaining windings, core and tank of the transformer connected together to earth.
g) Test On-Load Tap Changer (OLTC)
i. Once the OLTC is fully assembled on the transformer, the following tests are to be performed at 100% of the rated auxiliary supply voltage with the exception of Test No.2.
ii. Test No. 1
8 complete operating cycle with the transformer not energized.
iii. Test No. 2
1 complete operating cycle with the transformer not energized with 85% of the rated auxiliary
supply voltage.
iv. Test No. 3
1 complete operating cycle with the transformer energized at rated voltage and frequency at no
load.
v. Test No. 4
10 tap change operation with ± 2 steps on the other side of the principle tapping with as far as
possible the rated current of the transformer with one winding short circuited.
Type Test
a) Lightning Impulse For Line Terminal
This test is to ensure the transformer insulation can withstand the lightning over voltage which may occur in service.
This test basically performed with the standard lightning impulse of negative polarity. Conforming to the IEC 76-3 Clause 12.1 Standard, the Front Time (T1) and the Time To Half Value (T2) are defines as below:
Front Time (T1) = 1.2µs ± 30%
Time To Half Value (T2) = 50µs ± 20%
The test will be conducted based on the below sequence:
i. One (1) impulse of a voltage between 50% and 75% of the full test voltage.
ii. Three (3) subsequent impulse at full voltage.
Oscilloscope is to be used during the testing and it shall clearly shows the applied voltage impulse shape for Front Time and the Time To Half Value.
b) Temperature Rise Test
The objective of this test is to check the temperature rise of the oil and the windings do not exceed the limit agreed on or specified by the standard.
The test is performed by using the short circuit method. The temperature rise of the windings is determined by the resistance method.
Special Test
a) Insulation Resistance Test
The measurement is to determine the leakage current resistance of the insulation. This measurement given the informations about the condition of the insulation and the leakage current of the transformer.
As a rule of thumb, the insulation resistance is inversely proportional with the impurity contents and the moisture of the insulation itself.
b) Sound Level Test
The objective of this testing is to ensure the sound produced by the transformer comply with specification such as IEC 551and meets the transformer guarantee value given by the transformer manufacturer.
The customer are advised to insist the sound spectrum analysis
c) Lightning Impulse Chopped On The Tail
The purpose of the chopped lightning impulse test is to ensure that transformer insulation can withstand the voltage stresses caused lightning impulse which may occur during service.
The test is the special test line terminal of a winding. When it has been agreed to make this test, it shall be combine with the lightning impulse test. The peak value of the impulse shall be same as for the full impulse.
The impulse is chopped by means of a triggered-type chopping gap connected to the terminal where the impulse is applied. The delay of the chopping gap ignition impulse in relation to the ignition impulse generated is adjustable, thus the time Tc from the start of the impulse to the chopping can be adjusted.
The test is performed with the impulse of negative polarity. The duration Tc from the beginning of the impulse to be chopping can vary within the range of 2 to 6µs.
Front Time (T1) = 1.2µs ± 30%
Time To Half Value (T2) = 50µs ± 20%
Time To Chopped (Tc) = 2 to 6µs
The test is combine with the full impulse test in a single sequence. The recommended order of the difference impulse application is:
i. One (1) reduced full impulse.
ii. One (1) 100% full impulse.
iii. One (1) or more reduced chopped impulse.
iv. Two (2) 100% chopped impulse.
v. Two (2) 100% full impulse.
