When the power converter connects to the high-frequency transformer breaks through the bottleneck and reaches a frequency of 100 kHz or even higher, the high-frequency transformer’s inter-turn insulation faces more serious high-frequency discharge and high-temperature problems. In order to improve the service performance of oil-immersed high-frequency transformer insulation paper, composite K-BNNS particles are prepared by ultrasonic stripping, heat treatment, and thermomagnetic stirring. Then, K-BNNS particles are mixed with PMIA (polymeric m-phenylenediamine solution) slurry to produce composite aramid paper. And the effects of K-BNNS particles with different contents on the thermal conductivity, dielectric properties, partial discharge properties, and mechanical properties of aramid paper are explored. It can be found that, when the addition of composite particles (K-BNNS) is 10%, the comprehensive performance of composite aramid paper is the best. Compared with Nomex paper, the in-plane and through-plane thermal conductivity of composite insulating paper F-10 increased by 668.33% and 760.66%, respectively. Moreover, the high-frequency breakdown voltage increased by 48.73% and the tensile strength increased by 2.49%. The main reason is that the composite particles form a complete thermal conductive network in the aramid paper matrix and a large number of hydrogen bonds with the matrix, which enhances the internal interface bonding force of the material and changes the charge transport mechanism.

Aramid paper serves as an insulating material in high-frequency power electronic transformers, and the incorporation of composite K-BNNS particles has been shown to enhance the insulating properties of aramid paper. Partial discharge (PD) is a predominant phenomenon that can lead to insulation failure in high-frequency transformers. Therefore, this paper primarily investigates the PD performances of different nanoparticle doping concentrations on aramid paper. Firstly, composite aramid papers containing K-BNNSs at different concentrations are prepared, namely 5%, 8%, 10% and 13%, respectively. Then, the experimental platforms of PD for composite aramid paper are established, and the PD performances, surface potentials, and hydrogen bonds under different high-frequency applied voltages are discussed. The experiment results show that the composite aramid paper with 10% K-BNNSs nanoparticle content has the optimal insulation performance. In the needle-plate and column-plate models, the PD amplitude decreases by 65.35% and 27.33%, respectively, when compared with non-doped aramid paper. Moreover, the breakdown voltage improves by 32.2% and 38.5%, respectively. After that, the influence mechanisms of residual charges and hydrogen bonds on the PD characteristics of composite aramid paper are analyzed. The results obtained in this paper can provide important reference for the design and selection of insulation materials for high-frequency transformers.