Frequency Control of Island VSC-HVDC Links Operating in Parallel With AC Interconnectors and Onsite Generation

The main scope of this paper is to propose a suitable frequency control scheme for high-voltage dc-links (HVDC) based on voltage source converters (VSCs), operating in island systems with on-site conventional generation and external ac interconnectors. The proposed droop-type and inertia emulator is built upon the power synchronization control (PSC) concept, where grid synchronization is achieved without the need of a dedicated synchronization unit. The dynamics of the proposed PSC-based scheme are assessed both in frequency and time domain, utilizing a realistic study-case system, which corresponds to the current planning for the interconnection of Crete to the Greek mainland system, involving ac and dc interconnectors operating in parallel with conventional local thermal units. To demonstrate the benefits offered by the proposed controller in the context of frequency response, a detailed average value model is developed for the VSC-HVDC link in MATLAB/Simulink, where severe contingencies are simulated, such as the sudden loss of the external ac interconnector or local generation, leading from mainland grid-connected to islanded operation.