Most plant transformers get one of two treatments: nothing at all until
something goes wrong, or a calendar-driven routine that nobody reviews. Both
tend to produce the same outcome eventually, for opposite reasons.
What works better is a small number of checks done consistently, with the
results written down. The writing down is the part that gets skipped, and it is
the part that carries the value — a single reading tells you almost nothing,
while four years of the same reading tells you what is happening.
Five minutes, no tools, no shutdown.
- Oil level in the conservator gauge, against the temperature mark. A
falling level is a leak; a rising one usually means the breather is blocked.
- Leaks at gaskets, valves, radiator joints and the bottom of the tank.
Fresh oil on the plinth is worth investigating the same week.
- Silica gel breather. Blue or orange means dry, depending on the type;
pink or clear means it is saturated and is no longer protecting anything.
This is the single most neglected item on any transformer, and it is the main
route by which moisture gets in.
- Temperature gauges — oil and winding — with a note of the reading and the
load at the time. Neither figure means anything without the other.
- Noise. A change in the hum is worth reporting even if you cannot describe
it precisely.
- Ventilation. In a plant room, check nothing has been stacked against the
radiators since last month. This happens constantly.
Plan this into a scheduled shutdown rather than trying to work around
production.
- Oil sample for BDV. The core annual test. Record it against previous
years — the trend matters far more than the absolute number.
- Insulation resistance, HV to LV, HV to earth, LV to earth, with the oil
temperature noted. IR varies strongly with temperature, so a reading without
a temperature is not comparable to anything.
- Winding resistance on all three phases. Imbalance between phases points
at a developing joint problem before it becomes a fault.
- Thermal imaging under load. This finds loose connections, blocked
cooling paths and unbalanced loading — and it is the highest-yield annual
check on most industrial installations.
- Bushings — clean them, check for chips, cracks and tracking.
- Protection devices: Buchholz relay, pressure relief, oil and winding
temperature alarms. Verify they actually operate rather than assuming.
- Earthing. Continuity and connection condition on the tank and neutral
earths.
- Tap changer. If it is off-circuit and has not been moved in years,
operating it through its full range and back re-wipes the contacts. Contacts
that never move develop resistance where they sit.
- Full oil analysis: moisture in ppm, acidity, and dissolved gas analysis
(DGA) if the unit is important enough to justify it. DGA identifies the
type of fault developing inside a sealed tank — thermal, partial discharge
or arcing — from the gas ratios.
- Oil filtration where the BDV trend calls for it.
- Gasket replacement on any joint with a history of weeping.
- Full internal inspection for units in severe duty, or after any
through-fault heavy enough to have been felt by the switchgear.
If you do nothing else, track these three:
- Oil BDV trend. Falling BDV is the earliest broad warning available.
- Winding temperature at a known load. The same load producing a higher
temperature than last year means the cooling path has degraded — blocked
radiators, oil loss, or fouled surfaces.
- Inter-phase winding resistance imbalance. A phase drifting away from the
other two is a joint problem, and joint problems do not improve on their own.
Insulation life is about temperature and moisture, not years. A unit kept
dry, correctly loaded and properly cooled will run for decades. One that is
chronically overloaded or has wet oil will not, regardless of how young it is.
Age is a poor predictor; operating history is a good one.
Keep the commissioning test certificate. It is the baseline every later
reading gets compared against. Without it you are comparing today's IR value
against a generic threshold, which is very much weaker than comparing it
against what this specific transformer did when it was new.
If a set of records does not exist, start one now. In three years it will be
the most useful document you have about that machine.
Related: Onsite maintenance ·
Transformer repair and rewinding ·
Transformer oil filtration: when and why
