Transformer Preventive Maintenance Checklist (Oil-Immersed Units)

Most transformer failures announce themselves weeks in advance — through a slow oil leak, a saturated breather, or a drifting oil level. A short, regular inspection routine catches these early. This checklist is written for substation operators and maintenance contractors who keep oil-immersed units running.

1. Daily / visual walk-by

Even a 60-second look on the way past the unit pays off. Check for obvious oil stains at flanges and bushings, listen for abnormal hum or vibration, and confirm the Buchholz relay and pressure relief device show no alarm. A sudden silence from the Buchholz relay test push-button is itself a sign to investigate.

2. Monthly inspection

Item	What to check	Action if abnormal
Silica gel breather	Colour of desiccant (blue = dry, pink = saturated)	Replace or regenerate the cartridge
Oil level indicator	Float position vs. temperature scale	Top up via conservator; inspect for leaks
Buchholz relay	Gas accumulation window, no alarm	Collect gas sample; vent if minor
Gaskets & seals	Weeping at joints	Torque bolts; replace seal set at next outage

3. Annual / scheduled outage

During a planned outage, do the work that needs the unit de-energised: dielectric testing of oil, tightness check of the pressure relief valve spring, inspection of the conservator rubber capsule for diaphragm fatigue, and verification that all protection devices trip correctly. Keep a calibrated standby relay and valve on the shelf so a failed unit is swapped, not waited for.

4. Keep the right spares on hand

The cheapest insurance is a small consolidated kit: gasket sets, silica gel refill cartridges, one conservator capsule, one standby Buchholz relay, and one pressure relief valve. See our spare-parts kit guide for quantities sized to your fleet.

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