TESTING OF TRANSFORMER

TESTING OF TRANSFORMER

When a transformer is manufactured in an industry as per a particular design, testing is required to determine. whether the performance of the transformer is as per the designed data or not. If not how much variations are there in the actual values and the designed values and whether it is within the permissible limits as specified by ISI or not. If the variations in the results such as efficiency, regulation, iron losses copper losses, no-load current, etc are not within the permissible limits. the reason either for higher values or low values should be investigated and the manufacturing process should be modified accordingly. From testing, it can also determine the quality Of workmanship quality of material used, etc. Thorough testing in all respect is also required when a new design of the transformer is used for manufacturing it, to check all results and to see whether the assembled transformer will fulfill the designed conditions or not.

Hence, every transformer manufactured in an industry is required to undergo certain tests. The tests to be conducted on transformers can be classified as follows as per ISI 2026-1962.
(i) Routine Tests: Routine tests are those tests, which are conducted on each and every transformer manufactured in an industry. They are as enlisted below:

(a)      Measurement of winding resistance.

(b)     Measurement of impedance, voltage short circuit impedance and copper loss.

(c)      Measurement of insulation resistance.

(d)     Polarity test.

(e)      Phasing out the test.

(f)       Magnetizing current and core loss.

(g)     HV test.

(h)     Load losses.

(i)       No-load loss and no-load current.
(j)     Dielectric strength of the transformer of oil.
(k)   Voltage tests-windings

 (i) Separate source

                          (ii) Induced voltage

                       (l)     Core insulation voltage test.

(ii)     Type Tests: Type tests are those tests that are carried out on only a few pieces, manufactured in a lot of the same design. It means that 100 transformers are manufactured of the same design then type tests will be conducted only on 1 or 2 transformers only out of 100. They are as follows:

(a)   Temperature rise tests.

(b)   Impulse test.

(c)   Noise level test.

(iii)   Supplementary Test: Supplementary tests are those which are conducted on the transformers when additional information is required in respect of a particular transformer, either by the manufacturer or by the purchaser,. The supplementary tests can be as enlisted below:

(a)   Efficiency test. (b) Back to backtest.

(iv)) Special Test: Special tests are the tests required to be performed on the transformers used for specified purposes only. And these tests are conducted only when the results of such tests are demanded by the purchaser. They can be classified as follows:

(a)   Measurement of harmonics present in the transformer EMFs.

(b)   Measurement of zero phase sequence impedance of the three-phase transformer.
(c) Partial discharge.

(d)    Radio interference.

(e)    Vibration test.

(f) Star-delta test.

(g) Short-circuit withstand the test.

(v) Commissioning Test: The tests to be carried out on a transformer before connecting it to supply, depends upon the kVA rating, voltage rating, and also the facilities available at the site at the time of switching on. Also, it may depend upon the conditions laid down by the purchaser. The test is carried out as per ISS 1886-1967. Generally following tests can be carried out as site:

(a)   General observations.

(b)   Secondary injection test.

(c)   Primary injection test.

(d)   An overall inspection of the control and relay panels, cable junction boxes is carried out and observed if any connection is left open.

(e)   Tests on relay such as over current, earth fault, reverse power, etc is carried out for their operation.

(f)    The voltage ratio with equal tappings on all phases can be checked.

(g)   Measurement of earth resistance and necessary steps for bringing it within the suitable value.

(h)   Tests carried out on Buchholz-relay for its contact operation.

(i)     The test can be carried out, on cooling fans, cooling water pumps about their direction of rotation, speed, and troubles in starting, etc.

(J) If no loading tap changing arrangement is there its operation can be checked with the indication of tap the number on panel and buzzer etc at the time of change.

(k)    If required phasing out is carried out.

(l)   Insulation measurement with megger between high voltage and between low voltage and core, high voltage and core etc. at the same time on current transformer and potential transformer used in measurement and protection systems.

(m)              Oil in conservator should be upto the indication mark on glass-plate.

          (n)                  If a neutral earthing switch is provided it should be closed before switching on supply.

(o) The transformer can then be loaded and the meters both on high voltage and low voltage sides be checked for their readings on all the phases. It should be equal for same loading

(p)    Operation of on load, the tap-changing arrangement can then be checked.

(q)             If possible transformers above 1000 kVA rating should be connected to variable supply and the voltage then be increased to rated value gradually.