Search

Premium Membership ♕

Experience matters. Learn from experienced electrical engineers. Study specialized LV/HV technical articles, papers and courses.

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.

Winding
structure
Maximum operating
voltage
Um kV
Tests
Lightning
impulse
(LI)
Switching
impulse
(SI)
Long
duration AC
(ACLD)
Short
duration AC
(ACSD)
Applied
voltage test
uniform
insulated
Um ≤ 72,5type
(note 1)
nana
(note 1)
routineroutine
uniform
and
gradually
insulated
72,5 < Um ≤ 170routinenaspecialroutineroutine
170 < Um ≤ 300routineroutine
(note 2)
routinespecial
(note 2)
routine
≥ 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.

Resource: BEST Transformer – Tests (BALIKESİR ELEKTROMEKANİK SANAYİ TESİSLERİ A.Ş.)

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information
Edvard Csanyi - Author at EEP-Electrical Engineering Portal

Edvard Csanyi

Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.

I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.

Profile: Edvard Csanyi

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?”

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

  −  2  =  three

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

EEP Hand-Crafted Video Courses

Check more than a hundred hand-crafted video courses and learn from experienced engineers. Lifetime access included.
Experience matters. Premium membership gives you an opportunity to study specialized technical articles, online video courses, electrical engineering guides, and papers written by experienced electrical engineers.