Commissioning a distribution transformer: the 12 checks before the first switch-on

The transformer is in place, the cables are connected and the grid operator's crew is due tomorrow. The following twelve checks take about two hours and catch nearly every fault that would otherwise show up at the worst possible moment – with the network live.

Visual and mechanical

1. Nameplate matches the order – rating, voltages, vector group, serial number matches the test report.
2. No transport damage: bushings, fins, paint, oil level indicator in range, no oil traces (oil units); coils undamaged and clean, no cracks in the resin (dry units).
3. Clearances and ventilation as designed; nothing stored against the transformer.
4. Tap changer on the agreed position, locked; position recorded.

Electrical

5. Insulation resistance with a 5 kV megger (HV–earth, HV–LV) and 1 kV (LV–earth). Expected: > 1 000 MΩ HV–earth for a new oil unit at 20 °C; > 500 MΩ for cast resin. Values below 100 MΩ mean moisture – investigate before energising.
6. Voltage ratio on at least the nominal tap with a ratio meter or a low-voltage source; compare with the test report (±0.5 %).
7. Vector group / phase sequence: confirm L1-L2-L3 on both sides; for units to be paralleled, confirm the clock number by measurement, not by the nameplate alone.
8. Winding resistance (optional, but valuable as a fingerprint for future comparison).

Protection and earthing

9. Earthing: tank / frame earthed with the specified conductor; LV neutral earthed according to the network scheme (TN-C-S, TT…); HV cable screens earthed.
10. Protection devices tested: DGPT2 or DMCR relay (oil units) – alarm and trip contacts checked by simulating gas, pressure and temperature; PT100 monitoring relay (dry units) – sensor loop resistance, alarm and trip thresholds (typically 130 °C alarm / 150 °C trip for class F), fan start if fitted.
11. Upstream protection settings verified against the inrush current and the LV fault current (see our articles on inrush and on uk).
12. Cable connections torqued to the manufacturer's values; on cast-resin units check the flexible LV connections and the HV cable clearances to the coils.

Energisation

Energise from the HV side with the LV breaker open. Listen: a steady 100 Hz hum is normal; crackling, rattling or a rising tone is not – switch off. Measure the LV phase-to-phase and phase-to-neutral voltages on all three phases; they should be equal within 1 %. Leave the transformer at no load for 15–30 minutes, then close the LV breaker and load it in steps if possible.

After the first week

• Check the oil level indicator and the winding-temperature reading under load.
• Re-torque LV connections once after the first load cycles.
• Compare the measured load with the design – a transformer that runs above 80 % from day one deserves a conversation about the next rating.

Keynetgrid provides remote or on-site commissioning support with every PNACHO delivery – see Services. The operating manual delivered with the unit contains the manufacturer's specific torque values and relay settings.