TL;DR

Oil leaks are most commonly from degraded flange gaskets or radiator welds. Identify the leak source, de-energize the transformer, and replace the gasket or have the weld repaired by a qualified transformer service shop.

What you might see

oil staining on the transformer tank or base pad
oil level in the expansion tank falling over weeks
visible oil seeping from a flange or valve
environmental spill response triggered

Likely causes

Tap to mark as ruled out

Degraded gasket at a radiator, inspection plate, or drain valve flange
Weld crack in the radiator section from thermal stress or age
Loose or damaged sampling valve allowing slow seepage
Over-pressurized tank from a blocked pressure relief valve

Required tools

Replacement flange gasket (correct dimensions)
Torque wrench for flange bolts
Clean dielectric mineral oil (correct specification)
Oil containment materials for environmental compliance
PPE for medium-voltage work (for the isolation step)

Safety first

Transformer oil is combustible. Never use open flame or spark-producing tools near the oil or near leaking oil. Control spills immediately to prevent fire hazards.
All isolation and grounding of the transformer must be done by authorized personnel under a switching permit before any hands-on repair.

Procedure

8 steps · ~90 min

1
Walk around the transformer and identify the leak location. Use chalk or tape to mark the seeping point before cleaning the area.
Warning: Stay clear of the high-voltage compartments during the external inspection.
2
Check the oil level in the expansion tank. If the level is below the minimum mark, the transformer may not be operated until oil is restored.
3
If the leak is from a drain valve or sampling cock, try tightening the valve packing nut slightly to stop the seep. Do not overtighten.
4
If the leak is from a flange gasket, arrange a de-energization with the utility or electrical authority. Gasket replacement requires the transformer to be isolated.[1]
5
After isolation and grounding, drain oil below the leak point level. Remove the flange bolts, replace the gasket with a new nitrile or cork-nitrile gasket of the correct dimensions, and re-torque the flange bolts evenly in a cross pattern.[1]
6
Refill oil to the correct level with new dielectric mineral oil of the correct specification. Do not mix oil types or grades.
7
If the leak is from a radiator weld, this requires a qualified transformer repair shop. The radiator must be removed and re-welded. Do not attempt field welding on an oil-filled transformer.
8
Document the leak volume for environmental compliance and the repair action taken.

Sources

ABB Distribution Transformer Operation and Maintenance Manual, ABB
ABB Pad-Mount Distribution Transformer installation and maintenance instructions, oil sampling, oil fill, and gasket replacement procedures (general)
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