Communication-less fault detection in radial multi-terminal offshore HVDC grids

The realization of multi-terminal HVDC grids requires availability of adequate fault handling methods. This includes techniques for fault detection and location, and dc-breakers for faults clearing. The characteristics and consequences of dc-side faults are very different compared to ac-system faults, and conventional ac-protection methods are not necessarily suited in HVDC grids. This paper presents a study on fault detection in radial offshore HVDC grids, with emphasis on the identification of suitable fault detection criteria based on local measurements, referred to as markers. For selection of the best suited marker, four different requirements have been applied; dependability, speed, security and selectivity. This implies that all relevant dc-faults must be detected as fast as possible, and no other transient events should be detected as faults. In addition it must be possible to identify the faulted section/cable for all relevant faults based on local measurements, to allow for selective disconnection. The work is based on simulations of a radial three-terminal VSC HVDC system in PSCAD.