Commissioning of Power Transformer

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Key learnings:
  • Transformer Commissioning Definition: Transformer commissioning is defined as the process of preparing a power transformer for service by conducting various tests and adjusting settings.
  • Pre-Commissioning Tests: These include insulation resistance tests and checks on winding resistance and ratios to ensure the transformer meets manufacturing and safety standards.
  • Critical Checks: Key operational tests during commissioning involve tap changer functionality, relay tests, and cooling system verification.
  • Troubleshooting Guidance: After initial commissioning, any issues like tripping must be investigated to address potential problems like magnetic inrush current.
  • Final Commissioning Steps: Before full operational use, ensure all settings are correct, oil levels are appropriate, and the system has been allowed to settle, with final checks performed progressively.

After installation of power transformer, several pre-commissioning tests must be performed before putting the transformer into service. Additionally, various instruments associated with the transformer should 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 tested from one end of the range to the other to ensure the limit switch functions properly.
  • The insulation resistance (IR) values of the motor and control wiring should be checked using a 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.

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

In the Magnetizing Current Test, measure the magnetizing current by supplying 400 V, three-phase 50 Hz from the HV side while keeping the LV side open-circuited, and then compare the values across different phases.

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 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.

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