Transformer Testing

For the purpose of quality assurance and ensuring that the finished transformer conforms to customer requirements and is ready for service a barrage of tests are performed on the transformer. These can be broadly classified into the following 3 types: Routine Test, Type Test and Special Test.

1) Routine Tests:

These are standard tests performed necessarily on all transformers. They are further divided into two subcategories:

Dielectric Tests (for insulation testing)

a)    Separate Source Power Frequency Test on HV and LV
b)    Induced over voltage test
c)    Insulation Resistance of HV and LV winding
d)    Dielectric value of oil

Parametric Tests (for winding, loss and efficiency testing)

a)    Winding Resistance
b)    No load loss and No load Current
c)    Load Loss and Impedance
d)    Turns ratio on all taps and all phases

2)   Type Test:

This test is done on one unit of a particular design. It verifies the response of the design to the expected boundary conditions of the design. Hence it is not necessary to perform it on all units manufactured. Generally the testing and certification for the validation of a design to the conformity of these tests is considered for a period of five years. There are two tests that come under this paradigm:

a)    Temperature Rise Test
b)    Impulse Test

3)   Special Tests:

These are performed at the request of the customer by third party testing organizations. These give an idea about the design integrity, manufacturing quality, resistance to fault currents, operation quality etc. These are the following:

a)    Short-Circuit Test
b)    Unbalanced Current Test
c)    Magnetic Balance Test
d)    Measurement of Zero Sequence Impedance
e)    Measurement of noise level

As seen from the above list there are a total of 15 tests to be performed on a transformer. These tests are to be performed in the same sequence as it has been written above.

The routine tests need to be performed on all transformers that are manufactured, whereas type and special tests have to be performed under the conditions that have been mentioned.

It is necessary to perform the above tests in the same order because; each test can cause slight changes in the mechanical and electrical characteristics of the transformer. For example it can cause some of the insulated material to come under excessive levels of dielectric stress which may not be a regular occurrence under normal working conditions. They can cause modifications to the geometry of the core coil assembly thereby affecting the building factor. Also, if the tests are performed in a haphazard fashion without taking into account the parametric variations induced due to the previous tests, then probably the test results are not indicating the correct or operational values.

It is recommended to perform tests on Transformer as per the relevant BIS and IEEE standards.

Thank you!

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