Separate Source Voltage Withstand Test of Transformer

Separate source voltage withstand test of Transformer
(Power frequency)
Q. Explain with neat sketch power frequency high voltage test on 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.

Separate source voltage withstand test of Transformer

• This test is made with the help of single phase a.c. voltage of sinusoidal waveforms as far as possible and at a frequency not less than 80% of rated frequency.
• The test is started at a voltage not greater than 1/3rd of the test voltage and is rapidly increased to the test voltage value.
• The full test voltage is applied for 60 seconds between winding under test and all terminals of the remaining winding, core, frame and tank of the transformer connected together to earth.
•  At the end of the test, the voltage is rapidly reduced to less. than 1/3rd of the test voltage and then the supply is switched off.
• The peak value of voltage is measured with the help of digital peak voltmeter associated with capacitive voltage divider. The test voltage is then (peak value/√2). The test voltage that should be applied is obtained from the previous table.