Simplified Approach for Investigating Overvoltages in DC Cables in a ±320 kV Symmetrical Monopolar HVDC System

Abstract

Transient analysis of DC cable systems has gained momentum, especially due to pressing demands on the availability and reliability of offshore interconnectors. Such investigations form a vital part of the overall insulation coordination studies, which are typically performed by utilities and transmission system operators. Detailed converter models are typically limited to the respective manufacturers, and from a cable manufactory perspective, the converters are usually considered as a black box component. This makes it challenging for cable manufacturers to obtain such models for performing studies on their DC cables, mainly aimed at the converter/cable interface. In this paper, reduced converter models are utilized for performing DC-side fault studies. Detailed cable models have been implemented, considering the properties of every cable layer. The focus is on pole-to-ground faults for a symmetric monopolar half bridge VSC based modular multilevel system. The simulation results are analyzed and recreated in the form of a special overvoltage test performed on a full-scale DC cable system, i.e. a cable, along with joints and terminations. Based on the presented study results, reduced converter models can be utilized as an alternative to detailed models within the cable industry. In this manner, it is possible to achieve a first-hand impression of the profile of transient stresses that the DC cables may experience. This can provide an advantage regarding developmental activities, especially in designing special and non-standard high voltage tests.