Electrical Properties Evaluation of Return Insulation for 500  kV EHVDC XLPE Cables Factory Joints Using TOPSIS Method

This study evaluates the insulation performance of different regions of the return insulation of factory‐joint in 500 kV extra‐high voltage direct current (EHVDC) cross‐linked polyethylene (XLPE) cable. For this purpose, the test samples were sliced from its inner, middle, and outer layers of return insulation. These samples underwent differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), volume resistivity, breakdown field strength, and space charge measurements. The insulation performance of each layer was quantitatively assessed using the technique for order preference by similarity to ideal solution (TOPSIS) method. Results indicate the following (in order)—cross‐linking byproducts: inner > middle > outer layer, degree of crystallinity: inner < middle < outer layer, the lamellar thickness: inner > outer > middle layer, and the porous defects: inner > outer > middle layer. According to TOPSIS, the outer layer exhibits the best overall insulation performance. The study finds that optimally reducing the cooling rate after vulcanization during factory joints manufacturing process can enhance the crystallinity and lamellar thickness of the outer and middle layers of the return insulation in EHVDC XLPE cable joints. This optimization can effectively improve the electrical performance of the return insulation of factory joints.