TL;DR
High oil temperature means the transformer is overloaded or the cooling system has failed. Check the load current against nameplate kVA, inspect the radiator and fans, and reduce load if oil temperature is above 95 degrees Celsius.
What you might see
- oil temperature gauge above 95 degrees Celsius
- high-temperature alarm from monitoring relay
- cooling fans running at full speed
- oil expansion tank at maximum level
Likely causes
Transformer overloaded above nameplate kVA rating
Cooling radiator fins clogged with dirt or debris reducing convection
Forced-air cooling fan seized or not running
High ambient temperature reducing the transformer's thermal headroom
Required tools
- Clamp ammeter (for load measurement and fan circuit check)
- IR thermometer or thermal camera
- Soft brush for radiator cleaning
Safety first
- Pad-mount transformer enclosures contain live medium-voltage (typically 4 to 35 kV) terminals. Enclosure doors must only be opened by qualified electricians with appropriate PPE, switching permits, and utility coordination.
- Never operate a transformer with oil temperature above 105 degrees Celsius for more than a short period. Insulation damage is cumulative and permanent.
Procedure
- 1
Read the oil temperature gauge and the load current on all three phases. Calculate the load as a percentage of nameplate kVA.[1]
- 2
Check whether the cooling fans are running. On ONAN/ONAF units, fans should start automatically when oil temperature exceeds the setpoint in the relay.[1]
- 3
Inspect the radiator banks for dirt, leaves, or blocked fin passages. Use a soft brush or low-pressure air to clean from the top downward.
Warning: The transformer enclosure contains high-voltage terminals. Keep clear of all bushing and connection compartments during external inspection. - 4
Check the cooling fan motor nameplate current. Clamp an ammeter on the fan circuit and compare to the nameplate. A fan drawing no current means a seized motor or open circuit.
- 5
If load is above 110% of nameplate kVA for more than 2 hours, shed load from the transformer. Sustained overload accelerates insulation aging.[1]
- 6
If fans are running and radiators are clean but temperature is still high, check for partial clogging of the oil circulation path: inspect the top and bottom oil valves to the radiator, both must be fully open.
- 7
Log the event: date, time, oil temperature peak, and corrective action taken. This data is required for transformer condition assessment.
Sources
ABB Distribution Transformer Operation and Maintenance Manual, ABB
ABB Pad-Mount Distribution Transformer installation and maintenance instructions, thermal loading and cooling system procedures (general)
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