High-Frequency Partial Discharge Characteristics of Solid-State Transformer Interturn Multilayer Insulation Under Repetitive Electrical Stress

The solid-state transformer (SST) was regarded as the ideal power router in the energy Internet. However, the partial discharge (PD) in tightly packed SST is one of the main causes of interturn multilayer insulation premature failure. The effect of high-frequency (HF) electrical-thermal coupling stress on SST multilayer insulation's PD characteristics and failure mechanism is not clarified, which is investigated in this article. An HF PD measurement system is established, and a comparative study of the PD characteristics of multilayer polyimide (PI) insulation at different test frequencies and temperatures is explored. The experimental results show that when the frequency exceeds 10 kHz, the PD exhibits a rabbit-ear-like discharge pattern. However, the higher temperature will lead to lower PD inception voltage with large PD magnitudes, which may accelerate the degradation of interturn insulation. According to the phase-resolved PD, PD magnitude, PD number, and discharge phenomenon of the PI layer, the failure process of the multilayer PI system is divided into three stages: electric tree development, electric tree breakdown, and point prebreakdown. In addition, we propose the PD development model of multilayer PI insulation. The research results can provide a theoretical reference for elucidating the PD mechanism under HF stresses.