DC fault analysis of VSC based multi-terminal HVDC systems

The use of Voltage Source Converter (VSC) based HVDC transmission has been strongly suggested for the interconnection of various power networks. However, VSCs susceptibility to DC faults, particularly the potential damage caused to the converter IGBTs due to overcurrent, is an issue that must be addressed. This paper analyses the behaviour of a VSC-based HVDC system under varying DC fault conditions. A two terminal multi-terminal DC system, consisting of a single sending and receiving end is used as a case study. Recharging of the DC link Capacitor after a DC line-to-line fault was found to act as additional damping for the AC fault current, helping to reduce the AC current offset. Also, it was discovered that the development of potential earth loops with the system as a result of DC lineto-earth faults can lead to substantial overcurrent if an effective earthing scheme is not in place.