Effect of Thickness on the Space Charge Behavior and DC Breakdown Strength of Cross-Linked Polyethylene Insulation

In high voltage direct current (HVDC) cable systems, study of electric field distortion created by space charge accumulation is very important for examining insulation performance. Therefore, in this paper the influence of space charge on short-time breakdown characteristics of cross-linked polyethylene (XLPE) are studied. Trap energy distribution is an important factor affecting space charge behavior in dielectrics. Dielectrics with various thicknesses exhibit different space charge phenomena due to various trap distribution characteristics, which regulate the injection, migration and accumulation processes of charges. Consequently, trap characteristics can affect many parameters, such as charge accumulation depth and injection barrier. XLPE with thickness less than 100 μm show lower density of deep traps which can accumulate homogeneous bulk charges as compared to thick XLPE with thickness greater than 100 μm. This phenomenon determines the internal field strength distortion rate which is 20% larger for thin XLPE and it is much higher than that for thick XLPE. The external factor which is considered for analysis is applied field strength and it shows a linear proportional affect on the electric field distortion. A quantitative relationship model between applied field strength and distortion field strength of XLPE with different thicknesses was established. Linear extrapolation was used to obtain the trend of applied, distortion, and actual field strengths at different XLPE thicknesses when breakdown occurred. When the thickness of XLPE is less than 100 μm, distortion field strengths is higher than 50kV/mm and increases with decreasing thickness, resulting in a significant effect on the breakdown phenomenon and breakdown strength increases with increasing thickness. And when the thickness of XLPE is greater than 100 μm, the distortion field strengths is less than 50 kV/mm which also confirms the volume effect on breakdown phenomenon and breakdown strength decreases with increasing thickness.