Improving the Electric Field Distribution in Stress Cone of HTS DC Cable Terminals by Nonlinear Conductive Epoxy/ZnO Composites

Nonuniform electric field distribution is one of the key issues for the superconducting cable terminals. This paper tries to modify the electric field distribution in HTS dc cable terminals by the nonlinear conductive epoxy/ZnO insulation, instead of the insulation with a fixed conductivity. The nonlinear conductivity of epoxy/ZnO composites is measured, and a finite element simulation model of the +100 kV HTS dc cable terminal is established to analyze the electric field distribution at the steady and transient state. The results show that the field-dependent conductivity has a great effect on the dc electric field distribution. If the nonlinear coefficient is excessively large, the electric field in the polypropylene laminated paper (PPLP) will be extremely strong; otherwise, the electric field in the terminal insulation will be extremely strong. The epoxy/ZnO composites with an appropriate nonlinear conductivity can effectively uniform the electric field distribution at the triple-junction and the stress cone surface. When the switching overvoltage impulse superimposed dc voltage is applied, the maximum field is always located in the cable insulation of PPLP, which is because that the electric field distribution in the stress cone is mainly dependent on the capacitance characteristics of the insulation instead of the conductivity. The nonlinear conductive epoxy/ZnO composites could play an important role in promoting the development of stress cone of HTS dc cable terminals.

I. Introduction

Superconducting cable terminals are the important part of high temperature superconducting (HTS) cable system, in which the stress cone connects the superconducting conductor and current lead. Nonuniform electric field is one of the key issues for the stress cone of superconducting cable terminals, which leads to the partial discharge, accelerates the insulation degradation and even failure [1], [2]. More importantly, it is unavoidable to experience transient overvoltage caused by the switch of thyristors and the polarity reversal in HVDC converters, which aggravates the concentration of local electric field in terminal insulation [3], [4]. It is infeasible to uniform electric field in stress cone simply by designing its structure in cable terminals, due to the resistance-dependent field distribution in polymeric insulation, which is completely different from the capacity-dependence under ac field [5].