The composite insulation interface between polyimide (PI) and epoxy resin (EP) is becoming a current research hotspot as the insulation weak point in solid-state transformers. In this paper

the design of direct cross-linking reaction at the PI-EP composite insulation interface is taken as the modification idea

and the direct chemical bonding connection at the interface of PI and EP is realized by carboxylating the PI surface and introducing carboxyl groups capable of cross-linking reaction with epoxy groups. The validity of the interfacial cross-linking modification is verified by infrared spectroscopy

nuclear magnetic resonance and other characterization means. Based on the constructed interfacial discharge test platform for composite insulating materials

the interfacial discharge characteristics of the modified PI-EP samples under high-frequency electrothermal stress are further investigated. The results show that

compared with the unmodified samples

the interfacial cross-linking modification significantly improves the degree of interfacial bonding strength of the PI-EP samples

which can better maintain the overall stability of the molecular chain structure under the action of electrical and thermal stresses. Therefore

the modified PI-EP samples have more excellent inhibition of partial discharge performance

breakdown performance

and resistance to electrical and thermal aging performance. Among them

the PI-EP sample with an alkali treatment time of 45 min has the best interfacial insulation performance. Under the severe conditions of 180℃ and 30 kHz

its partial discharge inception voltage is improved by about 20%

breakdown voltage by about 37%

and endurance time under electrothermal aging by about 81% compared with that of the unmodified sample. The results can provide a reference for the optimization of solid-state transformer insulation design.