Maintenance of Power transformers

1. Commissioning of Power Transformer After, installation of power transformer, several pre-commissioning tests to be done before putting the transformer in service. Along with different pre- commissioning tests, different instruments associated to the transformer also to be checked. Common Pre-commissioning Tests of Transformer Insulation resistance values of various winding should be checked. During measurement of insulation resistance, the transformer must not be connected with any external lines or lightning arrestor etc. The IR values obtained should be comparable with those indicated in the manufacturer’s test reports. Oil sample from the top and the bottom of the tank, selector switch and diverter switch tank of the tap changer and radiator tank etc should be tested. The dielectric strength of the transformer oil should be more than 50 KV. Resistance measurement of winding should be done at all tap positions. The ratio test between the HV and LV at all tap positions should also be performed. For three winding transformers ratio between other pair of windings should also be taken. The phase relationship test should also be carried out. The tap changer should be undergone the following tests Manual operation Local electrical operation Remote electrical operation Parallel operation The operation should be checked from one end of the range to the other to check for the limit switch operation also. IR values of the motor and control wiring should be checked by 500 V Megger. Buchholz Relay Test Buchholz relay operation for alarm and trip should also be checked by injecting air to the test pocket provided in the relay. Low Oil Level Alarm Test The low oil level alarm of the magnetic oil gauge should be checked.

2. Temperature Indicator Test The contacts of the Oil Temperature Indicator and Winding Temperature Indicator for alarm trip and control should be checked and set at required temperature. Cooling Gear Test The IR values and setting for operation of oil pumps and fans motor should be checked. Alarm trip contact settings of differential pressure gauge, oil and water flow indicators, where provided should be checked. Marshalling Box The wiring from various accessories to the Marshalling kiosk should be checked Protective Relay Test Trapping of associated circuit breakers is to be provided by actual operation of differential relay, over current relay, earth fault relay and other protective relays as applicable. Magnetizing Current Test The magnetizing current can be measured by feeding 400 V, three phase 50 Hz supply from HV side keeping the LV side open circuited and the value in different phases could be compared. Additional checks during commissioning of power transformer 1.All oil valves are in correct position closed or opened as required. 2.All air pockets are cleared. 3.Thermometer pockets are filled with oil. 4.Oil is at correct level in the bushing, conservator tank, diverter switch tank etc. 5.Arcing horn of the bushing is set correctly 6.CT polarity is correct when bushing mounted CTs are provided. If all the above tests/checks are found satisfactory, a settling time of at least 24 hours, should be allowed for the oil and air released from all points at six hourly intervals, before commissioning the transformer. Whenever possible, while commissioning, the voltage should be build up slowly and brought to the full level in about 4 to 6 hours. If the circuit breaker is tripped on

3. differential, Buchholz or any other devices, the cause must be investigated before re-energizing the transformer. Sometimes tripping occurs maybe because of magnetic inrush current which depends upon the switching at a particular moment in the cycle. After commissioning magnetising current should be measured by connecting clip on meter in current transformer circuits and the reading should be compared with the test report value.