Advances in HVdc Bushing Using Nonlinear Materials: Theory, Materials, Structure, and Realization

Bushings are the key equipment in a high-voltage dc (HVdc) project. HVdc bushings not only provide the insulating medium between the HV conductor and the ground electrode but also physically fix the HV conductor in the transformer or in the feedthrough assembly. In operation, HVdc bushings always experience high electric field and load current. When a fault occurs, the bushing will inevitably face instantaneous overvoltage and overcurrent conditions. This harsh operating condition poses a serious threat to the long-term performance and reliability of HVdc bushings. However, the construction of extra-HVdc projects, ultra-HVdc projects, and flexible dc transmission projects is in full swing. The market demand for HVdc bushings is growing rapidly. Take China, for example: from 2013 to 2020, about 2,600 transformer bushings for ultra-HVdc converter transformers and 400 wall bushings for valve halls were needed. The annual demand for HVdc bushings is about 375, and the cost is more than 1.7 billion CNY per year. Considering the construction of extra-HVdc and flexible dc projects, the market scale of an HVdc bushing will increase in the future.