Space charge and its role in electric breakdown of solid insulation

The presence of space charge in solid dielectric materials can lead to electric field distortion and affect the electrical performance. Local electric field enhancement in the solid dielectric materials may greatly contribute to the early aging and even electric breakdown. The above effect has been highlighted in many research papers under HVDC conditions and space charge has been considered as the key to understand many observed phenomena under high electric fields. On the other hand, the influence of space charge in the solid dielectric materials under HVAC conditions has not been fully investigated. In this paper, the effect of space charge on DC breakdown strength of polymeric material has been presented and the thickness-dependent breakdown strength explained using a bipolar charge injection/transport model. More importantly, the model has been extended to AC conditions. Simulations based the model shows the charge dynamics and different features when compared with the DC condition. It can be shown that the breakdown under AC condition is different from that under DC condition. Due to the different charge dynamics the highest electric field is likely to occur in the region close to the surface of the dielectric material under AC condition while in the bulk under DC condition. Consequently, the breakdown strength of a polymeric material under AC condition is lower than that under DC condition. The same model can also be used to explore the effect of frequency and voltage rise rate on the breakdown strength of the polymeric materials. The simulation results show qualitatively agreement with the observed breakdown results.