Space charge and AC electrical breakdown strength in polyethylene

Breakdown strength is an essential feature of dielectrics. Theoretically, the intrinsic strength is independent of the configuration of the dielectrics or electrodes and the category of applied electric fields. However, the measured breakdown strengths of a same material under direct high voltage (HVDC) and alternative high voltage (HVAC) are significantly different. This paper aims to give an explanation of this observed phenomenon based on constant intrinsic electrical breakdown strength and reveal the significant influence of charge transport on the breakdown process. The effect of space charge on dielectric performances is generally neglected under high voltage alternate current (HVAC) electric fields, due to the limited charge amount observed. The characteristics of space charge in polymeric under AC stresses and its importance to the breakdown strength of polyethylene are also presented in this paper. A numerical model based on space charge dynamics under high voltage electric fields has been used to explain the differences in the breakdown strength of polyethylene under AC and DC stresses. Bipolar charge transport theory is adopted to analyse the charge transportation processes. Space charge profiles at the time of breakdown occurrence under different electric fields (including DC voltage, 0.5, 5 and 50 Hz AC voltages) and in the samples with different thicknesses (from 50 to 200 μm) are simulated. The relationship between space charge and the breakdown strength is discussed. The simulated results agree well with the trend of experimental breakdown results: generally, breakdown occurs easier under AC stress than DC stress; the breakdown strength is increased with the decrease of the sample thickness. The results suggest the presence of space charge under AC stresses has a great impact on electric breakdown strength of the material.