Distributed ω ‐ Q Control of Offshore Wind Farms Integrated via the Diode‐Rectifier‐Unit‐Based HVDC

ABSTRACT

Diode‐rectifier‐unit‐based high‐voltage direct current (DRU‐HVDC) may significantly reduce the cost of large‐scale offshore wind farms (OWFs) integration. However, the diode rectifier cannot stabilise the frequency of the offshore inner grid. This paper proposes a distributed ω ‐ Q control, enabling the wind turbine generators (WTGs) to form the frequency of the offshore grid. The frequency control mechanism and the formation of stable system frequency equilibrium points are analysed to provide a design basis for the proposed distributed ω‐Q control and the self‐synchronisation principle among WTGs under this control scheme is also revealed. A black‐start control strategy for WTGs and OWFs is given, where small‐capacity energy storage devices are coupled to a part of the WTGs as the first drive power. The offshore AC fault ride‐through control strategies are realised by reshaping the current limit curve, ensuring the regular operation of the frequency control during the fault. Additionally, with the impedance‐based method, the capability of the proposed distributed ω‐Q control to maintain system small‐signal stability across various OWF generation conditions is analytically verified. Finally, simulation results in PSCAD/EMTDC verify the proposed control strategies, theoretical analysis and design method.