Protecting a HVDC link against accidental isolation from its receiving AC system

When a high-voltage DC (HVDC) scheme is isolated from its receiving AC system, the inverter may continue to operate, generating its own AC bus voltages; this is defined as islanding. If islanding is allowed to continue unrestricted, main circuit components may in some conditions be damaged. It is therefore necessary to provide a suitable protection system. The author describes a protection scheme developed for an HVDC link that prevents damage due to islanding while still permitting the link to automatically restart on reclosure of the isolating breaker. Oscillograms showing the protection in operation on both the GEC ALSTHOM HVDC simulator and during tests carried out as part of the commissioning of the McNeill HVDC link are included.<>