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After successful erection, some development work is to be done before starting precommissioning test.
The development work includes the following:
-Topping up of oil: Final topping up is done with clean, dry and tested oil upto a level in the conservator commensurate with filling oil temperature.
-Air release: Since bushings used in distribution transformer are of oil communicating type, the trapped air inside the bushings should necessarily be removed before commissioning in order to allow the oil to come upto the top of the bushing terminals. This will reduce the possibility of corona discharge considerably.
-Trapped air inside the tank cover should also be removed through air release plug, if provided, on the tank cover.
-If radiators have air release plugs on top, the same should be operated to release trapped air, if any, from the radiators.
-Air from buchholz relay and explosion vent (if equalizer pipe not provided) should be released at this stage.
-Any other plug for releasing trapped air from the transformer, if provided, should also be operated.
-After completion of all the release operations, the oil level should be checked further to see that it is up to normal level.
-Test on buchholz relay: The slope of mounting angle of buchholz relay is checked with a spirit level to ensure that the inclination of pipe is approximately 5 to 10°. Further the status of alarm and trip contacts are checked by injecting dry air through the test petcock.
-Test on magnetic oil level gauge: The float of the oil level indicator is moved up and down between the end positions to check that the mechanism does not stick at any point. The low oil level alarm contact of the oil gauge should also be checked.
-Temperature indicator: The contacts of.OTI and WTI for alarm and trip are checked and set at required temperature depending upon ambient temperature and loading conditions.
If the foregoing instructions have been carefully followed, the transformer may be subjected to the following precommissioning test before final energization.
Checking of ratio, polarity and phase relationship:
The ratio should be checked at all taps and the results should match with the factory test results. For such measurements, turns ratio bridge is employed. In case of OLTC, continuity during tap change is to be ensured. Polarity and the inter phase connections should be checked.
Resistance measurement of the windings:
Wheatstone/Kelvin bridge should be used for the measurement of winding resistances. In the case of tapped winding, the resistance at all tap positions should be measured. The values obtained should match the factory test results.
Insulation resistance (IR):
IR values between the windings as well as windings to earth are checked. While checking IR, no external connection to the bushing terminals, including surge arrestor, should remain in the circuit.
Bushings are thoroughly cleaned before measuring IR value. It is preferred to use motor driven insulation tester (commonly known as megger) for such measurements. For 11/0.433 kV distribution transformer, 2500 V megger may be used for measuring IR values of both primary and secondary windings. Properly insulated leads with no crack or joint should be empolyed during measurement.
There is no proper guideline for minimum acceptable value of insulation resistance of the windings.
Some reference values for transformer are illustrated below.
A three phase, 415 volts supply is fed to the HV windings of a three phase transformer and the current drawn by the three phases are recorded. The currents should be uniform in nature. The currents on two side phases should be more or less equal while the middle phase current should be slightly less. In case, the currents in the phases are disproportionate i.e., one of the phase currents in quite high with respect to the others, it is a clear indication of failure either interturn or interlayer, in any of the windings.
Breakdown value of the oil sample:
Oil takes up moisture rapidly and that is why the condition of oil should always be checked before commissioning. Oil of muddy colour is certain to be wet. Water mixed or water saturated oils are heavier than dry oil and sink to the bottom of the tank. Sample should, therefore, be taken from the bottom sampling valve of the transformer. Sample should not be taken unless the oil is allowed to settle for 24 hours, if from a drum or two/three days, if from a transformer.
Dirt from the draw-off valve or plug should be removed. To ensure that the valve is cleaned, sufficient quantity of oil (at least 2 or 3 liters) should be allowed to flow into a separate container before collecting the sample for testing. The sample should be collected in a clear glass container with glass stopper. Cleanliness is essential as small amount of dirt or water affect the accuracy of the test results.
Wax should not be used for sealing the bottle containing the sample oil. However, the stopper can be covered with a pack of silica gel tied in a piece of dry and clean cloth.
The sample so drawn should be subjected to dielectric breakdown test as per the laid-down procedure discussed earlier or as per ISS. The acceptable value is above 50 kV.
Crack test of transformer oil:
A rough test may be made by closing the end of a steel tube, heating the closed end to just under dull red hot and plunging it into the oil sample with the ear close to the open end. If the oil contains large amount of moisture, a sharp crackle sound will be heard. Dry oil will only sizzle.
If all the above tests/ checks are found satisfactory, the transformer should be allowed at least 4/6 hours so that the oil settles. The protective devices should be set to the minimum possible.
Now the transformer may be switched on to the full voltage at no-load. Any abnormality during commissioning, such as vibration of radiator part or abnormal hum etc. should be noted. After few hours of energization at no-load, the transformer should be switched off. Abnormality noticed, if any, should be corrected. All protective devices should be reset to normal values. Transformer can now be re-energized safely and loaded gradually