Why Capacity Sets the Maintenance Interval
An oil-immersed power transformer works on the principle of electromagnetic induction: the primary winding produces a magnetic flux in the core, the flux links the secondary winding, and the ratio of turns fixes the ratio of voltages. Everything that keeps that chain working, from the insulating oil to the bushings and the tap changer, ages at a rate that depends on load and on temperature. Capacity is used as the practical proxy for that load, because a larger transformer carries a larger current, runs hotter, and has more oil volume and more insulation to lose if a fault is missed. That is why maintenance frequency is written as a function of rated capacity rather than as a single interval for all units.
Inspection Intervals by Rated Capacity
| Rated capacity and installation | Minimum check of the maintenance plan |
|---|---|
| Above 320 kVA and below 3200 kVA | At least once a month |
| 3200 kVA and above, attended substations and indoor units | At least once every 10 days |
| 320 kVA and below, small transformer rooms and pole-mounted units | At least once every two months, plus before and after each period of use |
Units rated at exactly 3200 kVA and above move into the short interval because the consequences of a failure scale with the energy stored in the windings and with the number of customers affected. Small pole-mounted and room-installed transformers of 320 kVA and below are given the longest interval, but they carry an additional condition: the plan is checked before and after each period of use, which in practice means each time the unit is energised or taken out of service. A unit that stands idle for months is not thereby exempt from inspection.
Checks That Are Carried Out Every Time
Whatever the interval, the visual and functional checks are the same. The bushings should be clean, free of damage, and show no crack or discharge traces on the porcelain or the cement joints. The oil conservator and the oil level should be normal, with the level meeting the scale requirements of the oil gauge, and there should be no oil leakage anywhere on the tank, the gaskets or the radiator joints. The transformer room must be weather-tight: doors, windows and latches complete, the door kept locked, no rain ingress, and lighting and air temperature appropriate for the equipment installed. Each of these items can be recorded on a short standard form, and the value of the record comes from comparing one visit with the next rather than from any single entry.
Insulation Resistance and Oil Testing
Interval inspection of the oil level and the bushings is not the same as testing the insulation system. Winding insulation resistance is measured with a megohmmeter at a defined voltage and corrected to a reference temperature, and the reading is judged against previous results for the same unit rather than against a fixed number. Oil condition is assessed by dielectric strength and by moisture content, with the sample drawn from a low point of the tank so that free water and sediment are captured. A downward trend in oil dielectric strength or a rising moisture figure points to a sealing problem long before the transformer fails. Records for the current year and the two previous years should travel with the unit, because a trend line is worth far more than an isolated test result.
Acting on Alarms and Abnormal Conditions
During severe climate change, whether very cold or very hot, the transformer oil level should be inspected more often than the normal plan requires, since contraction and expansion can expose the windings or overfill the conservator. When a gas relay gives an alarm signal, the plan calls for an external inspection of the transformer as well, to check for oil leakage, a low oil level, a blocked breather or any sign of an internal fault. An alarm is treated as information, not as a fault on its own; it is the combination of the alarm with the oil level, the temperature reading and the condition of the oil that decides whether the unit is taken out of service for internal examination.
Frequently Asked Questions
Q: How often must a transformer above 320 kVA but below 3200 kVA be checked?
A: The maintenance plan for such a unit is checked at least once a month.
Q: What interval applies to a 3200 kVA unit in an attended substation?
A: For on-duty substations and indoor transformers of 3200 kVA and above, the maintenance plan is checked at least once every 10 days.
Q: Is there a special rule for small pole-mounted transformers?
A: Yes. Units of 320 kVA and below installed in small transformer rooms or on poles are checked at least once every two months, and also before and after each period of use.
Q: What should be inspected when the weather turns extreme?
A: An additional check of the oil level. Severe cold or heat changes the oil volume, and the level can fall below or rise above the range indicated on the oil gauge.
Q: What does a gas relay alarm require?
A: An external inspection of the transformer, looking for oil leakage, an abnormal oil level, a blocked breather or any other visible change. The relay signal alone does not identify the fault.
Q: Which condition items belong on the routine form?
A: Bushing condition, oil level against the gauge scale, absence of leakage, conservator condition, and the state of the transformer room including doors, windows, latches, lighting and air temperature.