Investigating Transient Fault Characteristics of XLPE Cable in a Bipolar VSC-HVDC System - Cable Modeling and Field Verification

The high-voltage direct current (HVDC) system based on the voltage source converters (VSC) has been increasingly commissioned for its advantages of providing more controllability and flexibility for network operations. In a VSC-HVDC system, the cross-linked polyethylene (XLPE) cable is one of the main assets. The cable is exposed to transient overvoltage and overcurrent, which can cause its insulation degradation leading to faults and even failures of the cable. It is thus essential to investigate the transient propagation process along the XLPE cable in the VSC-HVDC system and subsequently determine the proper insulation coordination. This systematic study should be achieved by adopting an appropriate cable transient model. In this paper, with consideration of cable's multilayer structure, frequency-dependent and temperature-dependent material parameters, and cable joints, an improved transient model of the XLPE cable operated in a practical bipolar VSC-HVDC system is proposed. The sensitivity analysis of the cable transient model is performed. The accuracy of the model is verified through a field test with a 1.2/50 μs surge impulse on a 10.9 km long ±320 kV XLPE cable operating in the bipolar VSC-HVDC system. It is found that high temperature can increase the damping of wave propagation, whereas frequency-dependent permittivity of the semiconducting layer can affect the wave propagation in the inter-sheath modes. Cable joints have an insignificant effect on wave attenuation but can contribute to the waveform distortion for the surge impulse. The effect of the semiconducting layers and corrugated sheath on the attenuation and wave velocity may cause severe damping along the cable, leading to varying waveforms with lower oscillated amplitudes and shorter durations in the system. The findings in the paper can pave a way for implementing an accurate transient analysis for the XLPE cable.