Crosslinking Modification Design and High-Frequency Discharge Characteristics of PI–EP Composite Insulation Interface

The insulation interface between polyimide (PI) and epoxy resin (EP) composite is always the weak point in high-frequency power transformers. This paper proposes a novel modified way for PI-EP composite insulation interface. Firstly, the dopamine is grafted onto the PI surface. Then, the amino and phenolic hydroxyl groups introduced by dopamine crosslink with epoxy groups of EP. Thus, the chemical connection between PI and EP interfaces is achieved. The infrared spectroscopy, nuclear magnetic resonance hydrogen spectroscopy and other measurement methods are used to verify the effectiveness of interfacial crosslinking modification. Based on the built composite insulation interface discharge test platform, the interface discharge characteristics of modified PI-EP samples are further studied under high-frequency electrical-thermal stresses. Experimental results show that compared with unmodified samples, the modified ones show significantly improved performances in terms of partial discharge suppression, breakdown performance and resistance to electrical-thermal aging. Among them, the sample with the alkali treatment time of 60 minutes show the best interfacial insulation performance. Under severe conditions of 200 °C and 30 kHz, its partial discharge inception voltage is about 22% higher than that of the unmodified ones, and its breakdown voltage and time under aging conditions increase by 39% and 116%, respectively. Further analysis indicates that the compact interface structure of the modified samples can better maintain the overall stability of the molecular chain structure under electrical-thermal stresses, thereby improving the insulation performance of the interface. The above research results can provide a reference for optimizing the insulation design of high-frequency power transformers.