Partial power operation of Multi-level Modular Converters under subsystem faults

The large number of proposals for wind farm developments across northern Europe and the North Sea have lead to the suggestion that a European Supergrid using High Voltage DC (HVDC) would be beneficial. Grid formats of HVDC will use Voltage Source Converters (VSCs) with a fixed voltage polarity and power flow controlled through current magnitude and direction. The multi-level form of VSC looks set to displace 2-level for reasons of lower switching power loss and lower harmonics. Given the significant size of the power transfers involved compared to total capacity in GB and other smaller systems, there is a potential loss-of-infeed problem if a converter station or line fails. This paper examines the extent to which Multi-level Modular Converters (MMC) can continue to transfer some power following the loss of a subsystem such that a loss-of-infeed is limited to some fraction of the total capacity. The subsystem faults tested were a single DC line to ground fault and a transformer phase fault. In both cases the converter is able to transfer 50% of rated power continuously with the fault applied. This result means that the HVDC converter stations and routes could possibly be built at higher ratings than the system loss-of-infeed limit.