Induced Over Voltage Withstand Test 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.

Induced over-voltage withstand test of Transformer

• The test is started by increasing applied voltage not greater than 1/3rd of test voltage, the voltage is then increased rapidly to full test voltage value for 60 seconds. 
• The frequency of applied voltage should be twice the rated frequency of transformer.
•  Then the voltage is rapidly reduced to 1/3rd of full test voltage before switching off.
•  The purpose of using double frequency is to avoid excess excitation current during test. The double frequency supply is obtained from a separate alternator. 
• If the test frequency exceeds twice the rated frequency of transformer the duration of the test shall be (120 x Rated frequency /Test frequency) sec. but not less than 15 seconds.This test is also known as flash test.