Providing dc fault ride-through capability to H-bridge MMC-based HVDC networks

This paper proposes a framework to achieve dc fault ride-through capability in multi-terminal dc networks (MTdc), when H-bridge multilevel modular converters (MMC) are used. The studied network consists of four voltage-source converters (VSC) for high voltage direct current (HVdc) transmission. Two of these VSC converters connect two offshore wind farms (OWF) to the main HVdc link between two asynchronous onshore grids, in a radial configuration. In case of a dc fault, H-bridge MMCs are able to block the fast developing currents and drive them to zero, allowing for fast mechanical disconnectors to isolate the faulty cable segment and reconfigure the grid layout. In this paper, the effect of the dc fault location to the grid behavior is analysed both at the fault isolation phase, as well as at the grid restoration phase. Moreover, the worst-case dc fault scenario for the studied network is identified. Finally, the total fault recovery time of the MTdc network is estimated. The study showed that H-bridge MMCs are unable to isolate the faulty part of the network without de-energizing the MTdc grid. However, the proposed framework allows for fast grid restoration within 3.6 s without the need for expensive dc breakers.