Abstract— The expansion of offshore wind generation is increasing the need for interoperable and resilient HVDC transmission systems. While vendor‑agnostic control architectures proposed by the Aquila project have enabled unbalanced operation in multi‑terminal HVDC networks, significant challenges remain for offshore applications, particularly in hybrid bipolar configurations combining links with and without Dedicated Metallic Returns (DMRs). These challenges are amplified under N‑1 contingencies and when bipolar offshore converters operate in islanded mode forming asymmetric AC networks and power flow with respect of each pole. This paper investigates the control challenges associated with connecting offshore wind farms to rigid bipolar HVDC systems under simultaneous AC asymmetry of network impedance and power infeed between poles and DC‑side unbalance of the transfer network. Emphasis is placed on maintaining neutral‑to‑earth voltage and current within acceptable limits in the absence of a DMR, while preserving power transfer capability and peer‑to‑peer redundancy between parallel converters. A dedicated control strategy for bipolar offshore HVDC stations operating under islanded conditions is proposed, enabling stable unbalanced DC operation while accommodating asymmetric AC power generation and offshore network topology variations without violating AC or DC operating constraints. The proposed control strategy is validated in EMT simulations within RSCAD/RTDS environment. The tested cases include point-to-point rigid bipolar HVDC connected offshore windfarm, where comparison against library Virtual Synchronous Machine (VSM) – Voltage Source Converter (VSC) control of RSCAD, and a 5-terminal multi-purpose HVDC grid with a hybrid of full- and rigid- bipolar schemes subject to loss of pole end. All results showcase the feasibility of managing neutral voltage/current in various steady scenarios under both balanced and unbalanced HVDC operation.
Updated on 18th May 2026. All copyrights reserved by the National HVDC Centre