Charge transport and accumulation around HVDC insulators

Abstract

High voltage direct current (HVDC) based transmission of electric power, in comparison with high voltage alternating current (HVAC) transmission over a distance of over 800 km, has the advantages of higher transmission capability, lower line loss, more confined damage by fault, and that all AC lines connected to an HVDC system do not need to be synchronized. However, under DC conditions the electric field across the insulation space remains unidirectional and stable, which results in charge accumulation, leading to significant differences in the dielectric behavior and flashover characteristics of insulation materials in comparison with those under AC conditions. There exists limited amount of test data and research results on HVDC insulation behaviour. It is therefore necessary to carry out in-depth study of the discharge mechanisms and behavioral characteristics of the insulation materials required for HVDC transmission systems. The research in this thesis aims at the understanding of the charge transport and accumulation process inside and around insulators made of epoxy composite material. Firstly, the physical mechanisms are critically reviewed with appropriate experimental results selected for model verification. The model developed in the work encompasses all important mechanisms. Material properties obtained under different conditions are reviewed and values for the cases investigated in the present work determined. Charge transport is dominated by the drift of two types of oppositely charged particles in electric field with nonlinear generation source terms, giving rise to extreme difficulties for convergence of computation in strong electric field. As a result, special code is developed for a commercial software package (COMSOL based on finite element method) to implement the model and also the complicated boundary conditions. To gain confidence in the model and its implementation including the boundary conditions and material properties the model is verified in the work against experimental cases with different gas, geometry and applied voltage. An experimental study of the effect of surface charge accumulation on the potential distribution along the surface of an epoxy insulator under HVDC stress was carried out under non-ideal conditions.