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Testing of Transformer | Type Tests, Routine Tests and Special Tests of Transformer
An electrical transformer is tested at the transformer manufacturer premises and at the consumer’s site and also periodically in regular and emergency basis during its service life to confirm its specifications and performance.
Types of Transformer Tests
Tests of transformer done at the manufacturer’s premises −
- Type tests
- Routine tests
- Special tests
Tests of transformer done at the consumer’s site −
- Pre-commissioning tests
- Periodic tests
- Emergency tests
Type Tests of Transformer
The type tests of a transformer are performed at the manufacturer’s premises to prove the design expectations and consumer’s specifications. The type tests are performed in a prototype unit, not in all manufactured units and these tests confirm the basic and main design criteria of the transformer.
Following transformer tests are included in the type tests −
- Transformation ratio test
- Winding resistance test of transformer
Measurement of core losses and no-load current through open-circuit test
Measurement of short circuit impedance and copper losses through short-circuit test
- Transformer vector group test
- Measurement of insulation resistance
- Dielectric test of transformer
- On-load tap-changer test
- Temperature rise test
- Vacuum test of tank and radiators
Routine Tests of Transformer
The routine tests of a transformer are performed to confirm the operational performance of the transformer and being performed on every units manufactured.
Following tests are included in the routine tests −
- Winding resistance test of transformer
- Transformation ratio test
Measurement of core losses and no-load current through open-circuit test
Measurement of short circuit impedance and copper losses through short-circuit test
- Transformer vector group test
- Dielectric test of transformer
- On-load tap-changer test
Oil pressure to check against the leakages of joints and gaskets.
Special Tests of Transformer
The special tests of transformer are performed depending upon customer’s requirements to gathering the information which is useful during the operation and maintenance of the transformer.
The special tests of transformer include the following tests −
- Dielectric test of transformer
- Short-circuit test
Measurement of acoustic noise level
Measurement of no-load current harmonics
Measurement of zero-sequence impedance of the 3-phase transformer
Measurement of power drawn by cooling fans and oil pumps
Tests on accessories e.g. buchholz relay, oil preservator, temperature indicators, pressure relief devices etc.
The some important tests of transformer are discussed below.
Winding Resistance Tests of Transformer
The winding resistance tests are performed to determine the I2R loss in the windings of the transformer. These tests are carried out as a type test as well as routine test. There are various methods available for the measurement of resistance of the transformer winding as follows −
- Ammeter-voltmeter method
- Bridge method
- Kelvin bridge method
- Automatic winding resistance measurement kit.
Transformation Ratio Test
The voltage transformation ratio is the principle factor which greatly affects the performance of a transformer. Therefore, the transformation ratio test is an essential test of transformer. The transformation ratio test is performed by applying a 3-phase 415 V supply to the high voltage winding, with keeping the low-voltage winding open circuited, the induced voltage is then measured at HV-winding terminals and LV-winding terminals of the transformer to determine the actual transformation ratio of the transformer.
Insulation Resistance Test of Transformer
The insulation resistance test of transformer is carried out by the megger, so it is also known as megger test of transformer. This test is performed to ensure the healthiness of the overall insulation of the transformer.
The procedure of the insulation tests of transformer is as follows −
Step 1 – Disconnect the all line and neutral terminals of the transformer.
Step 2 – The megger leads are to be connected between the LV and HV bushing studs to measure the insulation resistance between the LV and HV windings.
Step 3 – The megger leads are to be connected between the HV bushing stud and earth terminal of the transformer (body of the transformer) to measure the insulation resistance between the HV winding and the earth.
Step 4 – The megger leads are to be connected between the LV bushing stud and the body of the transformer (earth terminal) to measure the insulation resistance between the LV winding and the earth.
All the readings taken above should be tending to infinity for a healthy transformer.
Dielectric Test of Transformer
The dielectric test of a transformer is also a kind of insulation test. This test is carried out to ensure the expected insulation strength of the transformer. The dielectric test is mainly performed in following steps −
Step 1 – This step is called as separate source voltage withstand test of transformer, in which a single phase supply of prescribed voltage level is applied to the winding of the transformer for 60 seconds. Another winding and the body of the transformer is connected to the earth and it is observed that any failure of insulation occurs or not during the test.
Step 2 – This step is called as the induced voltage test of the transformer, in which a 3- phase voltage supply that is twice of the rated voltage of secondary winding in magnitude and frequency is applied to the secondary winding of the transformer for 60 seconds and the primary winding is kept open-circuited. The test is started with a voltage lower than 1/3 of the full rated voltage and it has to be quickly increased upto the desired value. The test would be successful if no failure of insulation occurs at full test voltage during the test.
In addition to the dielectric tests of the transformer, there are other types of test for checking the insulation strength of the transformer such as BDV test of transformer oil, lightening impulse test, switching impulse test and partial discharge test etc.
Temperature Rise Test of Transformer
The temperature rise test of a transformer is performed to ensure that the temperature of the transformer windings and oil is in the specified limits or not. This test is done with the help of winding temperature indicator (WTI) and oil temperature indicator (OTI) meters.
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