Influence of Cross-linking Characteristics of DC Cable Insulation on DC Performance

The high-voltage DC cross-linked polyethylene (XLPE) insulated cable is a critical component of the flexible DC transmission system. Its insulating materials must meet stringent requirements for properties such as DC conductivity, space charge, and DC breakdown field strength. Unfortunately, high-performance cable materials needed for DC XLPE cables have been monopolized by foreign companies for a prolonged period, which has become a major bottleneck for the development of high-voltage DC cables in China. In this study, we first sliced the domestic and imported cable insulations, then tested the cross-linking degree (gel content) and microstructure of the cable slice samples for both types of insulation while in the undegassed state. This paper explains the variations in the micro cross-linking characteristics of the two types of cables and introduces a time-frequency domain hybrid analysis method based on polarization current. The method calculates the low-frequency dielectric constant and dielectric loss angle tangent of the cable, along with their change laws. By doing so, we can assess the state of cross-linking by-products in the cable insulation, laying a theoretical and experimental foundation for establishing a real-time monitoring approach for the cable degassing process.