Commissioning is the controlled sequence between delivery and live service: arrival inspection, electrical measurements, verification of the protection, a first energisation and a documented handover. For a distribution transformer it is a day of work; for a power transformer it runs over several days. It is also the last realistic moment to catch a transport or assembly fault before load is applied, which is why none of the steps are optional.
What has to be checked before anything is energised
Start with the mechanical side. Confirm the nameplate matches what was ordered, read the shock indicators if the unit travelled with them, check oil level and the colour of the silica gel breather, and look for leaks around gaskets and valves. Verify that tank earthing and the neutral connection are made with the correct cross-section, that clearances and the oil containment bund meet the site drawing, and that ventilation openings are clear.
Then measure. Field measurements are compared against the factory test report, not against a general rule of thumb, and every value goes into the report you hand over at the end.
- Insulation resistance winding-to-earth and winding-to-winding, plus the polarisation index; a healthy dry winding gives a PI above 1.5.
- Turns ratio at every tap position, within 0.5 % of the nameplate value.
- DC winding resistance per phase and per tap, compared with the factory records; investigate a spread between phases above roughly 2 %.
- Oil breakdown voltage and water content on oil-filled units, as a baseline for later sampling.
- Protection and instrument wiring: Buchholz relay, oil and winding temperature, pressure relief, tap-changer position and any monitoring.
Oil-filled units need settling time
If the transformer travelled full of oil, let it stand upright for at least 12 hours before the first energisation so entrained air can rise out of the windings. Units shipped drained under nitrogen are vacuum-filled on site and then need considerably longer to rest, typically 24 to 72 hours depending on voltage class and the manufacturer instruction.
Take an oil sample before energising. It costs almost nothing at that moment and gives you the dissolved gas baseline that every later analysis will be compared against.
The first energisation and the load steps
Energise from the high-voltage side with the low-voltage side open, after agreeing the switching request, the protection settings and the earthing removal with the grid operator. Expect an inrush current of several times rated current on closing; the protection must ride through it without tripping. Leave the unit unloaded for about half an hour and listen, look and measure: an even hum, no smell, stable temperatures.
Then apply load in steps rather than all at once, and record oil and winding temperature, load current and voltage at each step. A rising temperature that does not stabilise, or a hum that changes character under load, is a reason to stop rather than to continue.
What the handover file should contain
The documentation is what proves the unit was put into service correctly, and it is what a later fault investigation starts from. It should hold all measured values with instrument type and ambient conditions, the agreed protection settings, the tap position left in service, the oil analysis baseline, photographs of the installation and the signed handover date on which the warranty starts. Protrafo supplies two years as standard, extendable to five with Protrafo Insights.
The practical trade-off is simple. A commissioning day costs a fixed and modest amount; an undetected transport fault costs an outage plus a repair. The working rule is that you never energise on the strength of a factory report alone. Turns ratio and insulation resistance are measured again on site, on the day, by the people who will sign for it.
