Insulation Dielectric Test of Transformer

The dielectric test of transformer is generally performed in two different steps, likewise, separate source voltage withstand test and induced voltage withstand test of transformer, which we have discussed one by one below.

Separate Source Voltage Withstand Test of Transformer

This dielectric test is intended to check the the ability of main insulation to earth and between winding.


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

Separate source withstand test of transformer
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.

Nominal system
voltage rating
for equipment
Highest system
voltage rating
for equipment
Rated short duration
power frequency withstand
415V 1.1 KV 3 KV
11 KV 12 KV 28 KV
33 KV 36 KV 70 KV
132 KV 145 KV 230 / 275 KV
220 KV 245 KV 360 / 395 KV
400 KV 420 KV 570 / 630 KV
Winding with graded insulation, which has neutral intended for direct earthing,
is tested at 38KV

Induced Voltage Test of Transformer

induced withstand test of transformer
The induced voltage test of transformer is intended to check the inter turn and line end insulation as well as main insulation to earth and between windings-

  1. Keep the primary winding of transformer open circuited.
  2. Apply three phase voltage to the secondary winding. The applied voltage should be twice of rated voltage of secondary winding in magnitude and frequency.
  3. The duration of the test shall be 60 second.
  4. The test shall start with a voltage lower than 1/3 the full test voltage, and it shall be quickly increased up to desired value.

The test is successful if no break down occurs at full test voltage during test.

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