Insulation Characteristics and Failure Mechanisms of Ultra-High Voltage DC Bushings: A Review

Ultra-high-voltage direct current (UHVDC) bushings (i.e., valve-side bushings of converter transformers and DC wall bushings), serve as key components connecting converter transformers to valve halls and DC switchyards. These bushings primarily adopt multi-layer composite insulation systems composed of oil-impregnated paper (OIP) and epoxy resin-impregnated paper (ERIP) as the main insulation. Due to the extremely harsh service conditions, DC bushings are subjected to long-term electrical, thermal, and mechanical overstress. The complex coupling of electrical, thermal, and mechanical fields within the insulation structure significantly accelerates the degradation of solid insulation materials, increasing the risk of premature failure, thereby posing serious challenges to the safe and reliable operation of converter transformers and valve halls. Moreover, under long-term electro-thermal synergistic stresses, interfacial insulation failure has emerged as a critical weakness in composite insulation systems. With regards to the above issues, this article reviews the state-of-the-art progress on the insulation characteristics and failure mechanisms of UHVDC bushings, with particular emphasis on analyzing partial discharge behavior and charge transport properties in OIP/ERIP composite materials. The key challenges and future research priorities are identified, which aims at providing a valuable insight for failure mechanism analysis and optimization of insulation design in DC bushings.