Explain various aspects of transformer testing. What measures can be taken to enhance the life and efficiency of the transformer. Explain various reasons leading to transformer failure.
For confirming the specifications and performances of an electrical power transformer it has to go through numbers of testing procedures. Some tests are done at manufacturer premises before delivering the transformer. Mainly two types of transformer testing are done at manufacturer premises- type test of transformer and routine test of transformer. In addition to that some transformer tests are also carried out at the consumer site before commissioning and also periodically in regular & emergency basis through out its service life.
Type of Transformer Testing
Tests done at factory
- Type tests
- Routine tests
- Special tests
Tests done at site
- Pre-commissioning tests
- Periodic/condition monitoring tests
- Emergency tests
Type Test of Transformer
To prove that the transformer meets customer’s specifications and design expectations, the transformer has to go through different testing procedures in manufacturer premises.
Some transformer tests are carried out for confirming the basic design expectation of that transformer. These tests are done mainly in a prototype unit not in all manufactured units in a lot. Type test of transformer confirms main and basic design criteria of a production lot.
Routine Tests of Transformer
Routine tests of transformer is mainly for confirming operational performance of individual unit in a production lot. Routine tests are carried out on every unit manufactured.
Special Tests of Transformer
Special tests of transformer is done as per customer requirement to obtain information useful to the user during operation or maintenance of the transformer.
Pre Commissioning Test of Transformer
In addition to these, the transformer also goes through some other tests, performed on it, before actual commissioning of the transformer at site. The transformer testing performed before commissioning the transformer at site is called pre-commissioning test of transformer. These tests are done to assess the condition of transformer after installation and compare the test results of all the low voltage tests with the factory test reports. Type tests of transformer includes
- Transformer winding resistance measurement
- Transformer ratio test.
- Transformer vector group test.
- Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test).
- Measurement of no load loss and current (Open circuit test).
- Measurement of insulation resistance.
- Dielectric tests of transformer.
- Temperature rise test of transformer.
- Tests on on-load tap-changer.
- Vacuum tests on tank and radiators.
Routine tests of transformer include
- Transformer winding resistance measurement.
- Transformer ratio test.
- Transformer vector group test.
- Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test).
- Measurement of no load loss and current (Open circuit test)
- Measurement of insulation resistance.
- Dielectric tests of transformer.
- Tests on on-load tap-changer.
- Oil pressure test on transformer to check against leakages past joints and gaskets.
That means Routine tests of transformer include all the type tests except temperature rise and vacuum tests. The oil pressure test on transformer to check against leakages past joints and gaskets is included. Special Tests of transformer include
- Dielectric tests.
- Measurement of zero-sequence impedance of three-phase transformers
- Short-circuit test.
- Measurement of acoustic noise level.
- Measurement of the harmonics of the no-load current.
- Measurement of the power taken by the fans and oil pumps.
- Tests on bought out components / accessories such as buchhloz relay, temperature indicators, pressure relief devices, oil preservation system etc.
Transformer Winding Resistance Measurement
Transformer winding resistance measurement is carried out to calculate the I2R losses and to calculate winding temperature at the end of a temperature rise test. It is carried out as a type test as well as routine test. It is also done at site to ensure healthiness of a transformer that is to check loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings etc.
There are different methods for measuring of transformer winding, likewise
Current voltage method of measurement of winding resistance. ♣ Bridge method of measurement of winding resistance. ♠ Kelvin bridge method of Measuring Winding Resistance. ♠ Measuring winding resistance by Automatic Winding Resistance Measurement Kit.
NB:- Transformer winding resistance measurement shall be carried out at each tap.
Transformer Ratio Test
The performance of a transformer largely depends upon perfection of specific turns or voltage ratio of transformer. So transformer ratio test is an essential type test of transformer. This test also performed as routine test of transformer. So for ensuring proper performance of electrical power transformer, voltage and turn ratio test of transformer one of the vital tests.
The procedure of transformer ratio test is simple. We just apply three phase 415 V supply to HV winding, with keeping LV winding open. The we measure the induced voltages at HV and LV terminals of transformer to find out actual voltage ratio of transformer. We repeat the test for all tap position separately.
Magnetic Balance Test of Transformer
Magnetic balance test of transformer is conducted only on three phase transformers to check the imbalance in the magnetic circuit.
Procedure of Magnetic Balance Test of Transformer
- First keep the tap changer of transformer in normal position.
- Now disconnect the transformer neutral from ground.
- Then apply single phase 230 V AC
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