Impulse Voltage Withstand Test on Transformers
Q. Describe the impulse voltage withstand test on transformers.
 |
| Figure (A) |
Lightening impulse test : (This is a type test)
In practice the transformer is subjected to lightening strokes and switching surges. In such case ordinary high voltage tests are not useful. The test voltage wave is generated by a special type of generator known as impulse or surge generator. Due to expense and complexity of this test it is applied only as a type test . It consists of application of a limited number of unidirectional surge voltages to one or two phases of L.V. and H.V. windings of transformer.
The arrangement of transformer for impulse testing is shown in Figure A.The capacitor C1 is previously is charged to a particular d.c. voltage is discharged by triggering the gap ‘S’. This discharge through wave shaping network (R8, Rdand R1 and other combination). The discharge voltage give rise to exponential waveshape. The waveshape can be controlled varying resistance R8.
The voltage impressed on the transformer suddenly rises to a very high value and then falls to zero value in a vary short period of time. This is called impulse voltage. The test schedule is as under :
- A full wave test followed by
- Two chopped wave test followed by
- One full wave test
These wave shapes one shown in Figure B and C.
 |
| Figure B & C |
The test impulse voltage shall be a full lighting impulse of (1.2 ± 30 %) X (50 ± 20 %) use as defined in (IS 2071, part II 1974). This standard wave means the value of the full test impulse voltage (peak) is adjusted to reach in max 1.2 μsec and fall of its half value (50 % of peak) in 50 μsec .
The magnitude of voltage that should be applied depends on the class of insulation used for transformer. The test results are recorded on a cathode ray oscilloscope. The record of waveform of sound insulation is available . The test results are compared with this record and if any difference is found, it means failure of insulation has occurred.