- 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, commissioning is the set of site tests and instrument checks that prove the unit matches the factory report and can be energised. Those checks must finish before the transformer is put into service.
Common Pre-commissioning Tests of Transformer
Measure insulation resistance of each winding with the transformer isolated from lines and any lightning arrestor. Compare the IR values with the manufacturer’s test report. Take oil samples from the top and bottom of the tank, from the tap-changer selector and diverter tanks and from the radiator.
The dielectric strength of the transformer oil must meet the manufacturer’s or IEC 60422 limit for that voltage class. Many site specifications use 50 kV as a working acceptance. Measure winding resistance on every tap. Measure the HV to LV ratio on every tap. On a three winding transformers unit, measure the other winding pairs as well.
Carry out the phase-relationship or vector-group test. Then run these checks on the tap changer
- 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.
- Check insulation resistance of the motor and control wiring with a 500 V Megger.
Buchholz Relay Test
Check Buchholz relay alarm and trip by injecting air through the test pocket on the relay, using the method in the relay maker’s sheet.
Low Oil Level Alarm Test
Check the low-oil-level alarm on the magnetic oil gauge.
Temperature Indicator Test
Check and set the alarm, trip and control contacts of the Oil Temperature Indicator and Winding Temperature Indicator at the required temperatures.
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
Check the wiring from each accessory into the marshalling kiosk.
Protective Relay Test
Trip the associated circuit breakers by actual operation of the differential relay, over current relay, earth fault relay and other protective relays that apply.
Magnetizing Current Test
A common site magnetizing-current check applies 400 V, three-phase 50 Hz to the HV winding with the LV open and compares the three phases. That is a reduced-voltage check. The factory no-load current in IEC 60076 is taken at rated voltage. Compare either result with the test report for the method used.
Additional checks during commissioning of power transformer
- All oil valves are in correct position closed or opened as required.
- All air pockets are cleared.
- Thermometer pockets are filled with oil.
- Oil is at correct level in the bushing, conservator tank, diverter switch tank etc.
- Arcing horn of the bushing is set correctly
- CT polarity is correct when bushing mounted CTs are provided.
If those tests pass, many makers ask for a stand of at least 24 hours, with trapped air released from every vent about every six hours, before first energisation. Where the grid allows it, raise voltage slowly over about 4 to 6 hours. If a breaker trips on differential, Buchholz or another protection, find the cause before re-energising. Switching at some points on the voltage wave can produce magnetic inrush current that looks like a fault. After energisation, measure magnetising current with a clip-on meter in the current transformer circuits and compare the reading with the test report.
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No worries Bakhshi! Happy to hear you found it useful!