A dry-type transformer works on the same principle as an oil-immersed unit but uses solid insulation and air instead of oil. That makes it the usual choice inside buildings, close to the load.
Construction
- Core. Grain-oriented silicon steel with mitred (45°) joints, so the flux follows the rolling direction and losses stay low. The core surface is coated against rust.
- Windings. Copper or aluminium. In cast-resin designs the high-voltage winding is vacuum-cast in epoxy resin, often with a glass-fibre reinforcement; the low-voltage winding is commonly foil-wound.
- Insulation. Epoxy resin gives good dielectric strength, mechanical strength and moisture resistance. Aramid paper is used in high-temperature (class H) designs.
- Temperature monitoring. Sensors embedded in the windings feed a temperature controller that starts cooling fans, raises an alarm or trips the supply as temperature rises.
- Enclosure. Optional steel or stainless steel housing, with a degree of protection chosen for the room, for example IP20 indoors or higher where dust or dripping water is a risk.
Strengths
- No oil, so no oil leaks, no oil fire and no oil pit
- Self-extinguishing resin, suited to buildings with strict fire rules
- Little routine maintenance
- Can be installed close to the load, which shortens low-voltage cables
- Lower noise than many oil units of the same rating
Limits
- Higher purchase price than an equivalent oil-immersed unit
- Air cooling gives less overload margin; fans can add capacity for short periods
- Needs a clean, ventilated room; heavy dust, salt or condensation must be designed for
See our cast-resin ranges: SCB10, SCB11, SCB12, SCB13, SCB14, SCB18 and the on-load tap changing SZCB10 (SCZB10).