TRANSFORMER TEST

Transformer Test

Transformer test are done to determine the electrical, thermal and mechanical suitability for the system where they will be applied or use. The major tests carried out in the transformers are:

Turn ratio test(open circuit test)

The performance of a transformer largely depends upon perfection of specific turns or voltage ratio of transformer. So transformer ratio test is an essential type test of transformer. This test also performed as routine test of transformer. So for ensuring proper performance of electrical power transformer, voltage and turn ratio test of transformer is one of the vital tests.

This is also used to make sure that the turn ratio between the windings of the transformers is correct; with this information, you can decide what the output voltage of the transformer will be. The ratio is calculated under no load condition.

Simultaneous readings of voltage are taken to high voltage and low voltage windings area taken after the voltage is applied on one winding and on each taps if the transformer has a tap changer. The ratio is the division between the high reading and low reading. If it is a three phase transformer each phase is tested individually. Expected variation should be between 0.5%.

Procedures: The instrument use to test is multimeter. Assume the transformer we are testing is a 60MVA/132/33kv transformer with 10 taps. First of all note the nominal tap of the transformer before you proceed (but normally written on the name plate of the transformer) then a supply voltage of 415volt is connected at the primary side of the transformer 132kv while the secondary side of the transformer is open circuited, then with your multimeter take the line voltage of both the primary and secondary side of the transformer i.e. R-Y, R-B, B-Y .This is done in all the taps 1-10

Insulation resistance test

Insulation resistance test of transformer is essential type test, it is commonly known as megger test. This test is carried out to ensure the healthiness of overall insulation system of an electrical power transformer. It measures the quality of insulation within the transformer. Some variation will be obtainable depending on the moisture, cleanliness and the temperature of the winding. It is recommended that core and tank should always be grounded when the test is performed. Each winding should be short circuited at the bushing terminals. Resistances are  measured between each winding and all other winding to ground.

Procedure of Insulation Resistance test of transformer

1) First disconnect all the line and neutral terminals of the transformer.
2) Megger leads to be connected to LV and HV bushing studs to measure Insulation Resistance IR value in between the LV and HV windings.
3) Megger leads to be connected to HV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the HV windings and earth.
4) Megger leads to be connected to LV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the LV windings and earth.

NB : It is unnecessary to perform insulation resistance test of transformer per phase wise in three phase transformer. IR values are taken between the windings collectively as because all the windings on HV side are internally connected together to form either star or delta and also all the windings on LV side are internally connected together to form either star or delta.

Measurements are to be taken as follows:
For Auto Transformer: HV-IV to LV, HV-IV to E, LV to E
For Two Winding Transformer: HV to LV, HV to E, LV to E
Three Winding Transformer: HV to IV, HV to LV, IV to LV, HV to E, IV to E, LV to E

Oil temperature should be noted at the time of insulation resistance test of transformer. Since the IR value of transformer insulating oil may vary with temperature.

Short circuit test

This test is done to determine the impedance voltage which gives an idea of how much load the transformer can carry.

Procedures: The instrument use to test is multimeter. Assume the transformer we are testing is a 60MVA/132/33kv transformer with 10 taps. First of all note the nominal tap of the transformer before you proceed (but normally written on the name plate of the transformer) then a supply voltage of 415volt is connected at the primary side of the transformer 132kv while the secondary side of the transformer is short circuited, then with your multimeter take the line voltage of both the primary and secondary side of the transformer i.e. R-Y, R-B, B-Y .This is done in all the taps 1-10

Magnetizing Current Test

Magnetizing current test of transformer 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 effective reluctance of the magnetic circuit, thus affecting the electric current required to establish flux in the core.

1) First of all keep the tap changer in the lowest position and open all IV & LV terminals.
2) Then apply three phase 415V supply on the line terminals for three phase transformers and single phase 230V supply on single phase transformers.
3) Measure the supply voltage and electric current in each phase.
4) Now repeat the magnetizing current test of transformer test with keeping tap changer in normal position.
5) And repeat the test with keeping the tap at highest position.

Generally there are two similar higher readings on two outer limb phases on transformer core and one lower reading on the centre limb phase, in case of three phase transformers. An agreement to within 30 % of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50 times higher than the value measured during factory test, there is likelihood of a fault in the winding which needs further analysis.