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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How this is packaged on real projects
On most sites the unit described here is not installed on its own. It goes into an assembly that includes the incoming switching, protection and outgoing feeders, which you can see on the commercial building distribution, compact substations and switchgear control and protection pages.
Frequently Asked Questions
What should be confirmed before ordering a cast resin transformer?
System voltage and frequency, fault level at the point of connection, the load with its duty, and the site conditions including ambient range and indoor or outdoor placement. With those settled the rest of the specification follows.
When should I choose dry type over oil immersed?
Indoors, and wherever fire risk, oil containment or maintenance access is a concern. Commercial buildings, hospitals, data centres and enclosed industrial spaces are the usual cases. Oil immersed remains common outdoors and at higher ratings, where cost and cooling favour it.
What cooling options are available?
Air natural is the baseline. Where the load profile or the enclosure temperature demands it, forced air cooling with fans raises the usable rating. The choice is made against your ambient range and load profile, not against a catalogue default.
Which standard applies, and is it certified?
The applicable standard and clause set are agreed against your project specification before production starts. Where a project names IEC 60076 for the transformer, we work to those clauses and the routine test report ships with the unit.
What do you need from us to quote?
The single-line diagram or a description of the application, the voltages, the load with its duty, the site conditions, and any standard the project specifies. Send what you have – if something is missing, it can be worked out together rather than delaying the enquiry.