Space charge and AC electric breakdown strength in polyethylene

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 accumulated. This paper intends to expound the characteristics of space charge in polymeric under AC stresses and its importance to the breakdown strength of polyethylene. A numerical model based on space charge dynamics under high voltage electric fields has been used to explain the differences of the breakdown strength in 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.