Study on Space Charge Accumulation Allowance for DC Cables Using Bipolar Transport Calculation

Charge injection and accumulation in DC cables have a possibility to enhance the local electric field especially when impulse voltage with reversed polarity is applied. Allowance level of charge accumulation was assessed based on the safety margin included in this polarity reversal testing. The bipolar transport model was utilised to obtain a realistic charge distribution under a temperature gradient condition. The field enhancement due to the space charge accumulation was evaluated by comparing the calculation results obtained for ohmic and Schottky injections. It was found that the allowance of additional charge should be evaluated at the inner conductor, where the charges injected by the Schottky effect were stacked to enhance the local field under the voltage reversal. This paper also assessed how far we can grasp the space accumulation allowance to clarify the remaining issues.