Evaluation of control schemes for island VSC-HVDC links operating in parallel with AC interconnectors

The main objective of this paper is to comparatively assess alternative control schemes for high voltage dc (HVDC) links based on voltage source converters (VSCs), adapted for island systems with mixed ac and dc interconnectors operating in parallel. A suitable study-case system is selected, which corresponds to the planning for the future interconnection of Crete island to the Greek mainland system, where a multitude of generating units on the island, i.e. ac and dc interconnectors along with conventional local thermal units, is expected to operate in parallel. Particular emphasis is placed on the ability of the island VSC to maintain synchronism under severe contingency events, such as following the sudden loss of the external ac interconnector or local thermal plants, which lead the VSCs to transition from grid-connected to grid-forming operation. A detailed average value model is developed for the VSC-HVDC link in Matlab/Simulink, where various disturbances are simulated.