For a normally loaded distribution transformer in a clean indoor room, plan an annual visual and thermographic inspection, an annual oil sample with dissolved gas analysis on oil-filled units, and a fuller de-energised service every five years. That is the default. Everything that follows is about knowing when your unit is not the default case and the intervals have to come in.
What belongs in the annual round
The annual inspection is deliberately light, because it has to actually happen every year. Most of it is done energised, in an hour or two per unit, and its purpose is to catch the slow changes: a weeping gasket, a bushing that runs warmer than its neighbours, a breather that has gone pink, a fan that no longer starts.
- Visual check of tank, gaskets, valves and the containment bund for leaks and corrosion.
- Thermography under load, covering HV and LV terminals, cable connections and the cooling surfaces; compare the three phases against each other, not just against a limit.
- Silica gel breather colour and oil level in the conservator and the oil cup.
- Oil sample for DGA and basic oil properties, on oil-filled units.
- Function check of temperature indicators, alarms and, where fitted, fans and pumps.
- Cleaning: dust off cooling surfaces, clear ventilation grilles, remove anything stored against the unit.
What belongs on a three to five year cycle
The deeper work needs the transformer out of service, so it is planned into a shutdown you already have. It covers the electrical measurements you also took at commissioning: insulation resistance and polarisation index, turns ratio at every tap, and DC winding resistance per phase and per tap, each compared against the commissioning values rather than a generic threshold.
Add the mechanical items in the same window: retorque terminal and busbar connections to the manufacturer values, renew gaskets that have started to weep, treat corrosion before it reaches the weld seams, measure the earthing resistance, and carry out secondary injection testing on the protection relays. An on-load tap changer follows its own schedule based on the operation counter, typically service at the manufacturer figure or after five to seven years, whichever comes first.
When to tighten the intervals
Halve the interval, or move oil sampling to six-monthly, if the unit runs above roughly 80 % continuous load, carries a strongly cyclic or harmonic-rich load such as EV charging or arc furnaces, stands outdoors without shelter, or works in a coastal, dusty or chemically loaded environment. The same applies to units over 25 years old and to any transformer for which no spare and no rental replacement is arranged.
Tighten temporarily after events as well. A through-fault, a serious overload, a protection operation or a lightning event all justify an extra oil sample and an extra thermographic pass, even if the unit came back on load without complaint.
What continuous monitoring changes
Live monitoring of oil and winding temperature, load and power quality does not remove the annual physical inspection, because a sensor cannot see a leak or a rusting bund. What it changes is the deeper work: instead of opening the unit on the calendar, you open it when the data says something has shifted. That is the difference between a five-year service performed on schedule and one performed for a reason.
The practical rule is to write the interval and the trigger down in the same place. A programme that only lives in people's heads degrades within a few years into a vague sense that somebody looked at it recently. Protrafo can run the programme as a service contract, and pairing it with Protrafo Insights extends the standard two-year warranty to five.
