Impulse Test of Transformer

Lighting is a common phenomenon in transmission lines because of their tall height. This lightning stroke on the line conductor causes impulse voltage. The terminal equipment of transmission line such as power transformer then experiences this lightning impulse voltages. Again during all kind of online switching operation in the system, there will be switching impulses occur in the network. The magnitude of the switching impulses may be about 3.5 times the system voltage.

Insulation is one of the most important constituents of a transformer. Any weakness in the insulation may cause failure of transformer. To ensure the effectiveness of the insulation system of a transformer, it must confirms the dielectric test. But the power frequency withstand test alone can not be adequate to demonstrate the dielectric strength of a transformer. That is why impulse test of transformer performed on it. Both lightning impulse test and switching impulse test are included in this category of testing.

Lightning Impulse

The lightning impulse is a pure natural phenomenon. So it is very difficult to predict the actual wave shape of an lightning disturbance. From the data compiled about natural lightning, it may be concluded that the system disturbance due to natural lightning stroke, can be represented by three basic wave shapes.


1) Full wave
2) Chopped wave and
3) Front of wave


Although the actual lightning impulse disturbance may not have exactly these three shapes but by defining these waves one can establish a minimum impulse dielectric strength of a transformer.

If lighting disturbance travels some distance along the transmission line before it reaches the transformer, its wave shape may approach to full wave.


If during traveling, if flash-over occurs at any insulator of the transmission line, after the peak of the wave has been reached, the wave may become in form of chopped wave.

If the lightning stroke directly hits the transformer terminals, the impulse voltage rises rapidly until it is relieved by a flash over. At the instant of flash – over the voltage suddenly collapses and may form the front of wave shape.

The effect of these wave forms on the transformer insulation may be different from each other. We are not going here in detail discussion of what type of impulse voltage wave forms causes what type of failure in transformer. But whatever may be the shape of lightning disturbance voltage wave, all of them can cause insulation failure in transformer. So lighting impulse test of transformer is one of the most important type test of transformer.

Switching Impulse

Through studies and observations reveal that the switching over voltage or switching impulse may have front time of several hundred microseconds and this voltage may be periodically damped out. The IEC – 600060 has adopted for their switching impulse test, a long wave having front time 250 μs and time to half value 2500 μs with tolerances.