Electric breakdown under quasi-uniform field conditions and effect of emission shields in polyethylene

Electric breakdown strengths were measured using a quasi-uniform field spherical electrode assembly for polyethylene with and without an emission shield between the electrode and the polyethylene surface. The results show that the emission shield suppresses carrier injection from the electrodes and enhances the breakdown strength. The short-circuit breakdown strength is always lower than the normal breakdown strength under DC voltage conditions. With the carrier injection from one electrode suppressed by an emission shield, polarity effects during breakdown were observed. These effects, for measurements using a quasi-uniform field electrode assembly, are similar to those measured using an extremely nonuniform field needle-plate electrode assembly. The AC breakdown strength measured under quasi-uniform field conditions is about the same as that measured under extremely nonuniform field conditions, and it is also not much affected by the DC bias voltage. All of the breakdown phenomena can be interpreted on the basis of K.C. Kao´s et al. (1982) electrical discharge and breakdown model. One significant finding is that a suitable emission shield and a suitable method of reducing the energy released from nonradiative transition due to trapping and recombination may greatly improve the breakdown behaviour for high voltage polymeric-insulated cables