Research on Overvoltage for XLPE Cable in a Modular Multilevel Converter HVDC Transmission System

Due to the rapid development of the voltage-source converter (VSC), the demands for HVDC cables have increased significantly. Although more than ten VSC-HVDC projects are under construction and many more are in operations globally, the testing criteria for HVDC XLPE cables are mainly referenced to test guideline TB 496 recommended by CIGRE. Due to the large stray capacitance of the VSC-HVDC cables, the switching impulse overvoltage stress of the cables is the main electrical parameter concerned, a unified overvoltage factor of 2.1 p.u. being generally accepted. However, the switching impulse overvoltage stress of a VSC-HVDC cable system depends on not only the VSC topology but also the cable parameters and the volt-ampere characteristics of the dc cable surge arresters used. This paper studies the switching impulse overvoltage stresses for the dc cable system of a symmetrical monopole VSC-HVDC transmission system using the modular multilevel converter technology. The studies were performed according to the key technical parameters for a typical 1000-MW/320-kV VSC-HVDC transmission scheme, the XLPE dc cable links being assumed. The results show that a switching impulse overvoltage level of 2.3 p.u. should be applied during the type test of the overvoltage capability of the HVDC cables.