Investigation of the Thickness Effect on DC Breakdown Strength for HVDC Flexible Cable Insulation Associated with Space Charge

Abstract

Crosslinked polyethylene (XLPE) is the main insulation material in HVDC (high-voltage direct current) flexible cable. DC breakdown characteristics of XLPE are important indicators to determine operational safety. The accumulation of space charge could change the electric field distribution within the cable insulation layer, and could lead to insulation breakdown in severe cases. In response to these problems, this paper investigated on the breakdown strength and space charge characters of XLPE with different thicknesses. DC breakdown experiments were conducted for three different sorts of XLPE at positive and negative electric fields, where the thicknesses of the samples were 100 $\mu\mathbf{m},\mathbf{200}\ \mu \mathbf{m},\mathbf{300}\ \mu \mathbf{m}$ and $\mathbf{400} \mu\mathbf{m}$. Besides, the space charge measurements were carried out on $\mathbf{200}\ \mu \mathbf{m}$ and $\mathbf{400}\ \mu \mathbf{m}$ XLPE samples at 70kV/mm and 80kV/mm. The electric field distortion within the sample was calculated. The results show that the breakdown strength of XLPE decreases as the thickness of the XLPE increases. Power function was applied to describe the relationship between the breakdown strength and thickness. The homo charges injected from anode form packet-like charges which could move towards the cathode at high electric field. For thicker samples, more space charges are accumulated and there is a larger maximum electric field distortion. Therefore, the breakdown strength of thicker samples could be relatively lower than thinner samples.