Effect of Surface Smoothness on Electrical Tree Characteristics at XLPE-SIR Interface

Cable accessories are the weakest parts in the whole cable system. The performance matching of the insulating materials on the interface is very important, and the same important is the smoothing of the insulation surfaces. The surface smoothness of the cable insulation depends on the peeling and smoothing techniques applied on site in a less well-controlled environment. The effect of surface smoothness of XLPE cable insulation on the electrical treeing characteristics at the XLPE-SIR interface was investigated in this paper. XLPE and SIR samples were pressed together under different interfacial pressures by a specific pressing device, with the surface of the XLPE sample roughed by using abrasive papers. AC voltages were applied on a set of pin-plane electrodes placed at the interface. The pin-plane distance was 2 mm. The needle diameter was 0.25 mm and the tip curvature radius was (10±1) μ m. Under different interfacial pressures, the voltages for electrical trees to initiate were recorded and statistically analyzed for both smooth and rough XLPE surfaces. Meanwhile, the tree morphology and breakdown channel were also captured by using a digital microimaging system. It was shown that when the XLPE surface was smooth, electrical trees would initiate and grow along the interface at a low pressure, while they tended to grow into SIR side at a higher pressure. When the XLPE surface was rough, however, breakdown occurred directly at a low pressure; and at a higher pressure, electrical trees initiated and grew along the scratches on interface mostly. Besides, the electrical tree initiation voltages of the smooth surfaces were all higher than those of rough surfaces. It was indicated that a smooth insulating surface and an appropriate high interfacial pressure can effectively suppress the initiation of electrical trees.