Fast DC fault recovery technique for H-bridge MMC-based HVDC networks

This paper proposes a post-fault control technique for H-bridge multilevel modular converters (MMC), which in combination with a restoration framework, can achieve fast operation recovery in multi-terminal direct current (MTdc) networks under different dc fault types. The studied network consists of four voltage-source converters (VSC) for high voltage dc (HVdc) transmission. A meshed MTdc grid topology is used for the connection of two asynchronous grids with two offshore wind farms (OWFs). The effect of different dc fault types on the grid restoration time is evaluated. Following the proposed restoration steps, the grid is able to restore its operation within 7.4 s in case of a pole-to-ground fault. This time is reduced to only 34 ms, if a pole-to-pole fault occurs. To overcome the main time constraint of the restoration framework, a control technique is proposed that allows momentarily the connection of the two dc pole cables through the MMC-VSC switch valves. In this way, the dc grid is discharged faster and thus, the operation can be faster restored. The study showed that, with the proposed strategy, an H-bridge MMC-based network can restore its operation after a pole-to-ground dc fault within 158 ms without the need for expensive dc breakers.