A cast resin transformer needs no oil sampling, no breather and no containment bund, but it is not maintenance-free. Plan a visual inspection and cleaning at least once a year, thermography under load in the same visit, and a full de-energised clean with a torque check on the connections every three to five years. Dust and moisture are what kill these units, and both are entirely preventable.
Why dust is the main enemy
The epoxy resin around the windings is a good insulator until something conductive settles on it. A layer of dust holds moisture, and a damp deposit gives surface tracking a path to follow, phase to phase or phase to earth. The same layer also insulates the resin thermally, so the winding runs hotter at the same load, which ages it faster.
Clean with low-pressure dry compressed air or an industrial vacuum, working from the top down and reaching the cooling ducts between the coils. Do not use solvents that attack epoxy, and never hose down a warm unit: the thermal shock and the trapped moisture cause more damage than the dirt did.
The annual routine
Most of this is done in an hour, and most of it is looking rather than measuring. What you are hunting for is a change against last year, so photograph what you find.
- Visual check of the resin surfaces for cracks, crazing, discoloration and tracking marks around the ends of the coils.
- Thermography under load on HV and LV terminals, tap links and core clamps; compare the three phases and note the rise above ambient.
- Ventilation: fan operation and direction, clear inlet and outlet grilles, filters, and an enclosure that has not been stacked against.
- Temperature monitoring: verify the PT100 sensors in the low-voltage windings and confirm the alarm and trip settings match the insulation class.
- Housekeeping: nothing stored inside the enclosure, no cable entries left open, no rodent or bird ingress.
For class F insulation the usual settings are an alarm around 140 °C and a trip around 150 °C, but always take the values from the manufacturer plate rather than from a general table.
What to measure when the unit is de-energised
During the deeper service, measure insulation resistance winding-to-earth and winding-to-winding and expect readings in the gigaohm range. A disappointing value on a dry-type transformer almost always means moisture and dust on the surfaces rather than failed insulation, and it usually recovers after cleaning and drying. Add turns ratio and winding resistance so you have the same comparison set as at commissioning, and retorque all terminal connections to the manufacturer values once the unit has cooled.
If you suspect partial discharge, a PD measurement is the definitive test, but it needs specialist equipment and interpretation. IEC 60076-11 requires no more than 10 pC at the factory, and a field result well above that on a unit in service is a reason to plan an intervention rather than to keep watching.
After a long standstill
A dry-type transformer that has stood in a damp or unheated room absorbs moisture at the surface, and energising it wet is a genuine risk. Dry it out first with the space heaters or by circulating warm air, and re-measure insulation resistance until the value has stabilised for several hours before you energise.
The honest trade-off is that cast resin is cheaper to maintain, not free to maintain. You save the oil programme, the bund and the fire compartmentation, and you pay for it with a higher purchase price, less overload tolerance and a real dependence on clean, moving air. Budget roughly one inspection day a year per unit and the type performs exactly as advertised.
