Influence of Multi-Order Harmonic Voltage on Degradation in Epoxy Resin Layer of High Voltage Valve Side Bushing

The internal insulation characteristics of dry-type valve-side bushings in converter transformers under electrical and thermal complex stresses in ultra-high voltage direct current (UHVDC) transmission system are important for ensuring safe and stable operation. In this paper, an UHVDC transmission model is established to simulate the composite voltage waveform containing multiple high-frequency harmonic components. The insulation degradation experiments are conducted by using the harmonic voltage superimposed on the negative polarity DC voltage at 100 °C environment. The experimental results show that the initiation voltage of the electrical tree under multi-harmonic voltage reduces by 44%, and the insulation failure time shortens from 10⁴ s level to 10² s level, compared to the results under direct current (DC) superimposed on a fundamental frequency alternating current (AC) voltage. The morphology of the electrical tree gradually develops from branching to clumping. The phase of the partial discharge signal during the growth of the electrical tree shows "multi-peak" characteristics with the composite waveform, and the partial discharge exhibits periodic saturation behavior. The actual operating waveform of dry-type valve-side bushing causes more serious damage to the insulating properties of epoxy resin material, and the composite effect of high harmonics and high temperature should be taken into account when considering the design and operation of converter transformers.