MMC control optimization approach to facilitate DC-side interoperability in MTDC networks

Abstract HVDC systems are expected to evolve from today's dominating point-to-point towards multi-terminal DC (MTDC) systems. Considering the European market structure, these MTDC networks are envisaged to be multi-vendor based. Due to proprietary reasons, the controls and tuning of the converters are not expected to be shared. Recent studies have shown interoperability based instabilities in multi-vendor based MTDC networks, which might lead to a shutdown of the network. To facilitate the envisioned development towards MTDC networks, the control interoperability between different converters in case of network extensions has to be ensured without having access to existing converters controls. This paper therefore presents an enhanced control tuning approach for Modular Multilevel Converters which are being connected to an existing DC network to ensure control interoperability. The existing network is seen as a black-box and only information accessible at the terminals of the new converter station is used. Using a frequency-sweep on the DC side, the equivalent MMC and network impedances at the terminals are derived. The proposed control optimization method incorporates the information gained from the impedance analysis and thereby ensures control stability. An example based on a three-terminal HVDC network demonstrates the validity of the concept.