Sammendrag
This paper presents a protection scheme for multi-terminal Voltage Source Converter (VSC)-based HVDC systems, designed to detect and clear cable-to-ground faults in a range from very low to relatively high impedance. The protection scheme consists of two complementary and integrated layers: a protection layer for low impedance faults without communication between the terminals and a slower layer for higher impedance faults based on differential protection. Moreover, the paper demonstrates that the two layers can partially overlap without conflicting and affecting selectivity. The operation of the scheme has been verified by numerical simulations in PSCAD and is demonstrated in a three-terminal system for a few selected cases.