Before any transformer is put into service, a thorough commissioning process must be carried out to ensure safe and reliable operation. This begins with a series of pre‑start checks. The transformer should be clean and free of debris on the top cover, with no visible damage or oil leaks from the main body or its fittings. Oil levels in oil‑filled units must be at the correct mark, and all valves on the oil system need to be in the open position. The tap changer should be set to its normal operating position, and the cooling fans or pumps, if fitted, must be tested to confirm they run properly. Fire‑fighting equipment and oil drainage arrangements are also verified, and the protective relay settings are checked against the approved schedule. Grounding connections are inspected for tightness, and the phase and colour coding of the primary and secondary leads are confirmed to match the design drawings.
Once the physical checks are complete, the actual electrical testing begins. A full set of routine tests is performed, starting with measurement of DC resistance of the windings. This is done using a bridge instrument of adequate accuracy, and the results are corrected to a standard reference temperature. The test helps detect loose connections, broken strands, or poor contacts in the tap changer. Next, the turns ratio and vector group are verified at every tap position to ensure correct voltage transformation and phase displacement. Insulation condition is assessed by measuring insulation resistance, absorption ratio, and polarisation index with a 2500 V megger. These values must not fall below 70 % of the factory readings. In addition, dielectric loss factor and capacitance are measured to spot any moisture or ageing in the insulation system. An AC withstand voltage test is applied to check the overall dielectric strength, and for oil‑filled transformers, a sample of insulating oil is analysed for dissolved gases and water content. Other supplementary tests include core and clamp insulation checks, bushing tests, no‑load loss and short‑circuit impedance measurements, and winding deformation analysis using frequency response. Only after all these results meet the required standards can the transformer move to the next stage.
The protection and control systems are then commissioned. This involves testing each protective relay individually with a secondary injection test set to verify its pick‑up and timing characteristics. The current transformer wiring is checked by drawing a vector diagram to confirm correct polarity and phase alignment. Simulated internal and external faults are applied to observe how the protection scheme responds, ensuring that differential, overcurrent, and earth‑fault relays operate as intended. Control circuits are also exercised, and all auxiliary contacts and interlocking devices are proved through dry‑run operations.
With the electrical and protection tests cleared, the transformer is ready for its first energisation. This is done by closing the circuit breaker from the high‑voltage side, applying full rated voltage for a short duration. The first shot is held for at least ten minutes while the transformer is closely monitored for any abnormal sounds or indications. Usually, a series of five such impulse energisations is performed, with the neutral point solidly earthed each time. During these no‑load runs, the operator listens carefully to the core sound: a steady hum is normal, but a loud or uneven noise may suggest overvoltage or loose core laminations, while crackling or popping indicates possible partial discharge or arcing.
After the impulse tests prove satisfactory, the transformer is left running with no load for about 24 hours, during which oil temperature, level, and any leaks are regularly recorded. If all remains stable, a gradual load is applied, starting with a partial load for several hours and then stepping up to full load. The full‑load trial typically lasts 48 hours, with continuous observation of winding hot‑spot temperature, cooling system performance, and oil expansion. Only when this final load test is completed without any anomaly is the transformer considered ready for handover and official commercial operation. Throughout the entire process, detailed records of every test result, including instrument readings, relay settings, and oil analysis reports, are compiled and filed for future reference and maintenance planning.