This test is made to prove that the transformer insulation will withstand voltage surges which may be caused by lightning or switching; this includes insulation to ground, insulation between turns and windings, and the flashover value of the associated bushings. A high-voltage wave of standard values, and approximating a lightning surge, is imposed on the unit to be tested. The surge generator usually consists of a number of capacitors connected so that they be charged in parallel from a relatively low-voltage source and discharged in series to give a high voltage across the test piece. A �standard� impulse wave is illustrated below;
The �standard� wave reaches its peak voltage value in 1.5 microseconds and reduces to half the voltage value in 40 microseconds. The value of the voltage applied depends on the rating of the insulation, and may vary from 5 to 30 times the voltage rating of the insulation. Since rather elaborate and costly equipment is needed, impulse tests are usually performed only on large station-type power transformers, and rarely made after they leave the factory.
Electrical Transformers and Power Equipment by Anthony J. Pansini�