Insulation Dielectric Test of Transformer

There are two variable steps to generally perform the dielectric test of a transformer, likewise, induced voltage withstand test and separate source voltage withstands the test of transformer.

Induced Voltage Test of Transformer

The induced voltage test of the transformer aims at checking the line end insulation and the inter-turn as well as main insulation to earth and between windings.

1. Keep the primary winding of the transformer open-circuited.
2. Three-phase voltages are applied to the secondary winding. The applied voltage should be twice the rated voltage of secondary winding in magnitude and frequency.
3. The duration of the test shall be 60 seconds.
4. The test shall begin with a voltage lower than 1/3 the full test voltage, and it shall quickly grow up to desired value.

The test will achieve success if no breakdown occurs at full test voltage during the test.

Separate Source Voltage Withstand Test of Transformer

This dielectric test aims at checking the ability of main insulation to earth and between windings.

Procedure

1. All tested three-line terminals of the winding are linked together.
2. Other winding terminals which are not under test and also a tank of the transformer should be connected to the earth.
3. Then apply a single-phase power frequency voltage of shape approximately sinusoidal for 60 seconds to the terminals of the winding under test.
4. Perform the test on all the windings one by one.
5. The test achieves success if no breakdown in the dielectric of the insulation occurs during the test.

In this transformer testing, the peak value of voltage is measured, that is why the capacitor voltage divider with digital peak voltmeter is employed as shown in the diagram above. The peal value multiplied by 0.707 (1/√2) is the test voltage.

The values of test voltage for different fully insulated winding are furnished below in the table.