Fault Handling Method in an HVDC System with Half-Bridge MMCs and Hybrid DC Circuit Breakers

This article proposes a fault handling method for an HVDC system with half-bridge modular multilevel converters (MMC) and hybrid dc circuit breakers (DCCB). In an HVDC system, the clearance of the dc fault is still challenging, and the hybrid DCCBs attract attention as one of the fault clearing methods. However, the hybrid DCCBs have an operation delay of several milliseconds when clearing a dc fault. It causes a dc voltage sag during the period, resulting in a large current to flow through the MMCs and shuts down the entire HVDC system. In order to avoid the shutdown of the entire HVDC system and resume the power transmission immediately, this article proposes the fault handling method in the HVDC system which connects the offshore wind farm to the onshore ac grid. In the proposed method, the onshore MMC is temporarily blocked while the offshore MMC continues the switching operation with the dc current control. The proposed method is verified by experiments. They demonstrated a fast restoration of power transmission 25 ms after the dc fault, even if the hybrid DCCBs have an operation delay of 5 ms.