Material science of epoxy resin cast insulators

Epoxy resin systems used in high voltage epoxy insulators are thermoset polymers formulated with hardeners, fillers, and additives to achieve high dielectric strength, mechanical strength, and environmental resistance. Typical cast epoxy systems for medium voltage applications offer breakdown strength in the range of roughly 20–40 kV/mm, depending on the formulation and filler system, enabling compact insulation distances. ​ Key material-level advantages include: High dielectric strength: Epoxy cast resin insulation provides very low electrical conductivity and excellent insulation resistance, helping prevent surface flashover and internal breakdown under high voltage stress. ​ Tailorable thermal performance: Special formulations maintain insulating and mechanical properties up to around 120–150 °C class, suitable for cast resin transformers, CTs, PTs, and busbar systems. ​ Strong adhesion and low shrinkage: Epoxy bonds well to metals, fibers, and inserts, and exhibits lower shrinkage than many other polymers, reducing internal stress and void formation during curing. ​ Compared to porcelain, epoxy is lighter, impact-resistant, and allows complex geometries and integrated features (grading sheds, mounting bosses, embedded conductors) in a single monolithic part. Compared to oil-impregnated insulation, epoxy eliminates the need for tanks, seals, and oil maintenance while avoiding environmental and fire hazards associated with oil leaks. ​Key material-level advantages include: High dielectric strength: Epoxy cast resin insulation provides very low electrical conductivity and excellent insulation resistance, helping prevent surface flashover and internal breakdown under high voltage stress. ​ Tailorable thermal performance: Special formulations maintain insulating and mechanical properties up to around 120–150 °C class, suitable for cast resin transformers, CTs, PTs, and busbar systems. ​ Strong adhesion and low shrinkage: Epoxy bonds well to metals, fibers, and inserts, and exhibits lower shrinkage than many other polymers, reducing internal stress and void formation during curing. ​ Compared to porcelain, epoxy is lighter, impact-resistant, and allows complex geometries and integrated features (grading sheds, mounting bosses, embedded conductors) in a single monolithic part. Compared to oil-impregnated insulation, epoxy eliminates the need for tanks, seals, and oil maintenance while avoiding environmental and fire hazards associated with oil leaks. ​ Electrical, mechanical, and thermal performance Electrical behavior Cast resin insulation is designed to manage both bulk dielectric strength and surface performance under pollution and humidity. Epoxy resin cast insulators offer: High insulation resistance and low dielectric loss, ensuring low partial discharge at rated voltage when manufactured correctly. ​ Stable performance in humid and polluted atmospheres when combined with appropriate fillers and surface design (sheds, creepage path optimization). ​ Customizable internal geometry to control electric field distribution and reduce localized stress points that could trigger partial discharge. ​ Mechanical strength Epoxy insulators exhibit high compressive, flexural, and impact strength, with good resistance to vibration and mechanical shock, which is critical in switchgear cubicles, RMUs, and bus ducts. The lower weight compared to porcelain reduces handling risk and mechanical loading on enclosures and support structures. ​ Thermal performance Well-formulated cast resin insulation maintains dimensional stability, low thermal expansion, and good thermal endurance over wide temperature ranges. Low thermal expansion, combined with good adhesion to inserts, helps preserve interface integrity during load cycles and short-circuit events. ​ Design and manufacturing considerations Modern epoxy resin cast insulators and epoxy bushings are typically produced by vacuum casting or pressure gelation (APG) processes to achieve void-free parts and consistent quality. Critical considerations include: ​ Resin system and filler selection for target voltage class, thermal class, and environmental conditions. ​ Proper drying and preheating of molds and inserts to avoid moisture-related voids and partial discharge sites. ​ Optimized geometry for creepage, clearance, and field control to comply with IEC/IS standards for insulation coordination. ​ For transformer bushings and busbar spouts, designers often integrate conductor clamping, earthing points, and grading features directly into the epoxy component, reducing part count and assembly time. In GIS and compact switchgear, epoxy housings can be cast to match existing busbar layouts, enabling drop-in replacements or upgrades without major structural changes.

EPOXY RESIN CAST