Impulse Test of Transformer
Q. State how insulation resistance of a transformer is measured.
Measurement of Insulation Resistance of Transformer
- Before commissioning the transformer it is necessary to know that the insulation resistance of a transformer is greater than a specified value, to avoid excessive leakage currents.
- Megger test for insulation are carried out between windings, between winding and core and between core and clamping bolts.
- The insulation resistance of each winding, in turn, to all the other windings, core and frame or tank connected together, and to earth shall be measured and recorded. Following table can be referred for megger test.
|
Voltage of winding
|
Voltage applied by megger
|
Min. insulation resistance(MΩ)
|
|
400 V
|
500 V
|
2
|
|
11 kV
|
1000 V
|
50
|
|
33 kV
|
2500 V
|
150
|
|
132 kV
|
2500 V
|
500
|
- The clear idea about insulation strength can not be obtained just by megger test because many times the insulation has to withstand high voltages developed in network due to switching surges etc. So following tests are conducted.
(a) Separate source voltage withstand test
(b) Induced over voltage withstand test
(c) Lightening impulse test
- These test are described briefly below. The magnitude of high voltage that should be applied is given in following table (ISS 2026 – 1962) and can be used as a guideline.
|
Voltage of transformer
(KV)
|
Test voltage
(KV)
|
|
3.3
|
16
|
|
6.6
|
22
|
|
11
|
28
|
|
22
|
50
|
|
33
|
70
|
|
66
|
140
|
|
110
|
230
|
|
132
|
275
|
|
220
|
460
|