A cast resin transformer is a dry type transformer whose windings are cast in epoxy resin under vacuum. The resin is not a coating on top of an insulated winding. It is the insulation, and it is also what holds the winding in shape when a fault tries to move it.
What the casting process does
The winding is dried and placed in a mould, the mould is evacuated, and resin is drawn in to fill the space under vacuum. Curing then produces a solid body with no voids and no trapped air. Air pockets are the weak point of any high-voltage insulation system, so removing them at the manufacturing stage is what makes the design work at medium voltage.
Both LV and HV windings are normally cast, or the LV winding is cast and the HV winding is produced by the same process in a separate mould. The result is a coil that presents a clean, smooth surface to the surrounding air, which is what allows the electric field to distribute evenly and keeps partial discharge low.
Partial discharge, and why it is the number that matters
Partial discharge is a small, local breakdown in a void or at an edge where the field is concentrated. It does not fail the transformer immediately. It erodes the insulation slowly, and the rate accelerates, so a unit with a high partial discharge level early in life is likely to end its life early.
Cast resin construction controls this because a void-free casting has nothing to discharge inside. Verified partial discharge levels are measured at the factory and reported on the test certificate, and that figure is worth asking for when comparing units.
What the encapsulation changes in service
- Moisture. The winding is sealed. Humidity changes on the outside of the coil do not reach the insulation, so the unit can sit in a damp or condensing environment without a drying-out routine.
- Mechanical. The casting is rigid. Electromagnetic forces during a short circuit are resisted by the solid insulation rather than by clamping alone, so the winding moves less under fault conditions.
- Fire. The resin system is formulated to be self-extinguishing and it contains no liquid to burn or to pool. There is nothing to leak, so no bunding and no oil containment.
- Contamination. Dust, salt, and corrosive vapours sit on the outside of a smooth surface instead of migrating into an insulation channel.
Where cast resin gets specified
- Indoor substations in commercial buildings, hospitals, and data centres.
- Underground installations, tunnels, and basements where an oil spill would be a problem.
- Coastal and highly humid sites.
- Chemical, food, and pharmaceutical plants where a leak would contaminate the process.
- Renewable energy and marine installations with limited fire compartment volume.
The trade-offs
Two things go the other way. First, the casting adds weight and cost relative to an open-wound unit of the same rating. Second, resin is not a coolant, so heat has to leave through the casting to the air; a cast resin unit of a given rating is physically larger than an equivalent oil-immersed unit and dissipates less heat per unit of surface area.
Casting also means the winding cannot be opened for repair. A faulted cast resin unit is normally replaced rather than rewound, and that should be part of the cost comparison over the life of the installation.
Specifying one
Rating, primary and secondary voltage, vector group, impedance, temperature rise class, ambient and altitude, enclosure or IP rating, and the partial discharge and impulse test levels are required. Given those, the design follows. See the cast resin transformer range, or send us the requirements.
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