柔性高压直流双极带金属回线系统电缆电磁暂态建模与极线故障特性

As a key transmission device in voltage source converter (VSC)-based high-voltage direct current (HVDC) system, cross-linked polyethylene (XLPE) cables are affected by various transient events. The long-term exposure to transient electrical stress can lead to gradual insulation degradation. To improve the accuracy of the transient calculation, an analytical method for determining the equivalent distributed circuit parameters of cable was proposed after the multilayer structure and the material properties were considered. Based on the proposed model, a VSC-HVDC bipolar system with metallic return was modeled in PSCAD/EMTDC to study the mechanisms of overvoltage and overcurrent under the pole-to-ground fault. Transient characteristics of both pole and metallic return cables were obtained, with an in-depth investigation into the variations of fault-induced transients under long-distance transmission scenarios. The results show that, under a pole-to-ground fault, a zero-crossing damped oscillation will appear at the non-fault section of the faulty cable. The pseudo-period of the oscillation is related to the fault distance and wave velocity, while the oscillation amplitude is influenced by the fault distance, fault impedance, valve-side impedance, cable characteristic impedance, and attenuation characteristics. The reverse voltage peak caused by the oscillation increases with distance from the fault point, reaching a maximum value close to −1 pu (base value 320 kV). Pole-to-ground faults can lead to similar and significant overcurrent in both the faulty pole and the metallic return cable. The peak overcurrent is determined by the uncontrolled rectification steady-state stage after MMC blocking and the trip time of the AC circuit breaker. A grounding fault at the sending end of the DC cable results in the highest current peak. Short cable routes (kilometer scale) experience larger transient currents, while long cable routes (hundred-kilometer scale) are more prone to severe overvoltage in the metallic return cable and the healthy pole during single-pole faults. The study aims to provide an accurate cable model and simulation reference for evaluating transient electrical stresses and insulation coordination for cables in VSC-HVDC system.