DC grid re-energization analysis during DC fault ride-through of bipolar MTDC systems with energy-based MMC control in grid-following

DC Fault Ride-Through (FRT) capability is essential to minimise impact of DC faults in hybrid AC/DC power system. It ensures safe operation, and enable a smooth system restoration, which are key factors to enhance the overall system’s resilience and reliability (R&R). In this context, the DC grid re-energization after a DC fault introduces a relevant power disturbance at the converter AC point of connection (PoC). This can affect the converter operation, and consequently, both AC and DC grid performance. These challenges are further exacerbated under weak grid conditions, which can lead to transgression of the operational margins of the system, imposing stricter requirements on coordinated control and protection (C&P) strategies. This paper investigates and compares two control strategies for multi-terminal DC (MTDC) grid re-energization during DC FRT events when facing a weak grid condition. The first strategy, Vdc-droop reactivation, enables the converters in Vdc control mode to share DC voltage restoration after reclosing the DC Circuit Breaker (DCCB). Second, Vdc-master control assigns a single converter to restore DC voltage in a centralised manner. Simulation results demonstrate that the Vdc-master control approach achieves smoother voltage restoration and reduced power disturbances, assuring system recovery and improved post-fault stability compared to the Vdc-droop method when facing a weak grid condition. These findings provide valuable insights for the design of robust FRT and re-energization schemes in future hybrid AC/DC grids.