A Comparative Analysis of Partial Discharge in 13 Combined Insulation Structures of 11 Materials Used in Cast-Resin Dry-Type Transformers

Partial discharge (PD) in cast-resin dry-type transformers (CRDTs) is an important issue that needs to be overcome. In this study, PD in insulation materials used in CRDT windings is investigated. Four types of samples are prepared using 11 different types of materials, including epoxy resin, three-ply glued polyester insulation, prepreg insulation, glass fiber mesh, polyester adhesive tape, polyester film, polyimide film, rectangular conductors with polyester insulation, round conductors with polyester insulation, round conductors with enamel insulation, and foil conductors without insulation, and subjected to PD measurements. The samples are placed into a mold for resin casting that is similar to those used to fabricate large cast-resin windings, and the material behavior is tested. PD measurements are carried out in the high-voltage laboratory of the BEST Transformer Company. The measured data are used as inputs to models of the samples that are simulated using finite element analysis software to determine the PD inception electric field. The PD patterns of the samples are compared with well-known PD patterns in the literature. Finally, a comparative analysis of the materials is performed, and strong and weak materials against PD are identified. The results of this study show that laminated insulation materials or fiberglass meshes, both with epoxy resin, which have PD inception at a mean value of 3.4 kV/mm of electric field intensity, are more susceptible to PD than the other insulation materials investigated. In contrast, insulated conductors or nonlaminated polyester or polyimide films, all with epoxy resin, which have PD inception above a mean value of 5.3 kV/mm of electric field intensity, are robust to PD.