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Home / Technical Articles / Transformer Routine Dielectric (Insulation) Test
Transformer Routine Dielectric (Insulation) Test
Transformer Routine Dielectric (Insulation) Test (on photo: OSB laboratory of BEST Transformers)

Insulation tests to be performed

The following insulation tests are performed in order to meet the transformer insulation strength expectations. Unless otherwise requested by the customer, the following test are performed in the following order (IEC 60076-3) :

1. Switching impulse test:

To confirm the insulation of the transformer terminals and windings to the earthed parts and other windings, and to confirm the insulation strength in the windings and through the windings.

2. Lightning impulse test

to confirm the transformer insulation strength in case of a lightning hitting the connection terminals.

3. Separate source AC withstand voltage test

To confirm the insulation strength of the transformer line and neutral connection terminals and the connected windings to the earthed parts and other windings.

4. Induced AC voltage test (short duration ACSD and long duration ACLD)

To confirm the insulation strength of the transformer connection terminals and the connected windings to the earthed parts and other windings, both between the phases and through the winding.

5. Partial discharge measurement

To confirm the “partial dicharge below a determined level” property of the transformer insulationstructure under operating conditions.

According to standards, the transformer windings are made to meet the maximum operating voltage Um and the related insulation levels.

The transformer insulation levels and the insulation test to be applied according to IEC 60076-3 is shown in the below table.

Maximum operating
Um kV
duration AC
duration AC
voltage test
Um ≤ 72,5type
(note 1)
(note 1)
72,5 < Um ≤ 170routinenaspecialroutineroutine
170 < Um ≤ 300routineroutine
(note 2)
(note 2)
≥ 300routineroutineroutinespecialroutine
Note 1: In some countries, in transformers with Um ≤ 72,5 kV applied as routine test and the ACLD test is applied as routine or type test.

Note 2 : If the ACSD test is defined, the SI test is not applied.

In case of a transformer with one or more thanone gradual insulation, if foreseen by the induced voltage test, the switching impulse test isdetermined according to the maximum Um voltage winding.

The foreseen test voltage can not be reached in lower Um voltage windings. In this case, the ratio between the tap changer’s optimum tap position and the windings shall be such arranged that, the lowest Um voltage winding reaches the most appropriate value. This is acceptable (IEC 60076-3).

If chopped wave is requested during ligthning impulse (LI) test, the peak value of the chopped wave is 1.1 times the full wave value (10% higher). For transformers with the high voltage winding Um> 72.5 kV, the lightning impulse (LI) test is a routine test for all windings of the transformer.

Repeating the dielectric tests

If no modification is made in the internal insulation of a transformer, only maintenance is made, or if insulation tests are required for a transformer which is in operation, and if no agreement is made with the customer, test is performed with test voltages at 80% of the original test values. However, the long duration induced voltage test (ACLD) is always repeated with 100% of the original value.

For new transformers with factory tests completed, tests are repeated always with 100% of the original values.


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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

One Comment

  1. Hanafy Kamel
    Sep 10, 2019

    “Why do we do both tests,(Separate source AC withstand voltage test & Induced AC voltage test ) isn’t the INDUCED TEST enough to get the job done?”

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