Partial discharge analysis in Class F and H epoxy resin insulators: void size impact on high-voltage performance

Abstract Partial discharge (PD) describes localized electrical discharges that occur within insulating materials. Owing to its exceptional electrical and thermal properties, epoxy resin is widely utilized as an insulation medium in electrical systems. Insulation classes F and H are categorized based on their thermal endurance ratings, with insulation degradation remaining a primary cause of power equipment failures. PD-based diagnostic tools enable precise condition monitoring of insulation systems, critical for reliability. Voids inherent in solid dielectric materials often initiate insulation breakdown, particularly when PD activity originates within enclosed cavities. This paper investigates the relationship between void dimensions and insulation lifespan in dry-type transformers, focusing on PD-induced degradation within internal voids. Class-F and Class-H epoxy resin samples, incorporating artificial circular voids of 2 mm, 3.5 mm, and 4 mm diameters, were analyzed in collaboration with ESSENNAR R&D Center using IEC60270-compliant testing protocols. Key findings reveal that increased void size correlates with reduced partial discharge inception voltage (PDIV) and heightened discharge magnitudes and repetition rates. While observed discharges remained below the IEC60270 threshold of 140 pC, PDIV values approached the critical 10 pC limit in larger voids, underscoring the impact of void geometry on insulation performance. These insights emphasize the need for stringent quality control in epoxy resin applications to mitigate PD risks.