Development of a 500-kV DC XLPE Cable System

Abstract

thereby lowering insulation performance, so that these space charges must be suppressed at the design stage, in terms of the performance characteristics for XLPE insulation material under DC voltage. The following dielectric breakdown characteristics are inherent to DC voltage: • heat generated due to current in the insulation (Joule heat); and • local high electric field due to space charge. Consequently, the properties required for the insulating material for DC XLPE cables are: (1) high resistivity to reduce leakage current and suppress heat generation; and (2) absence of local high electric fields due to space charge formation. The authors identified a polyethylene with a polar group (hereinafter called DC-XLPE) as the insulation with the required properties. It is known that the polar groups work as trap sites for electric charge 5). The existence of trap sites slows down the mobility of electric charges, thereby increasing the resistivity of the polymer insulation. The trap sites also have the effects of preventing the uneven distribution of the space charges thereby stabilizing the behavior of space charge. Furthermore, existing facilities can be used to manufacture the cable, because requirements for the handling of PE material for DC cables are the same as those for ordinary PE material. Figures 1 and 2 plot the volume-resistivity curve and space charge formation for the subject DC-XLPE. Figure 1 shows that the XLPE satisfies requirement (1) above, i.e., at least 10 times the volume resistivity of the conventional XLPE material for AC cables (hereinafter called AC-XLPE). Figure 2 shows that the XLPE satisfies requirement (2), allowing scarcely any accumulation of space charges, but that the AC-XLPE allows significant