Unscheduled event handling capability and swift recovery from transient events are indispensable study areas to ensure reliability in offshore multiterminal high-voltage dc (MT-HVdc) grids. This article focuses on enhancing the reliability of half-bridge modular multilevel converters (HB-MMCs) in MT-HVdc grids by introducing a predictive dc fault ride-through (DC-FRT) recovery controller and fault separation devices. A novel dc protection-informed zonal DC-FRT scheme for HB-MMCs is proposed, incorporating a model predictive planner for optimized control inputs based on local and interstation measurements and converter constraints. A real-time digital simulator environment simulates the approach, which improves lower level control during fault interruption and suppression by utilizing fault detection and location information. In addition, the study examines two control schemes to assess the impact of communication delays in MT-HVdc grids, a critical factor for system stability and reliability during faults. These schemes include a centralized scheme with delays in input and output signals and a decentralized approach focusing on external signal delays. Both are compared against a baseline centralized control with no delays. These approaches explore alternatives for the placement of the proposed controller, considering potential delays in interstation high-speed communication. The findings underscore the significance of the proposed DC-FRT control in reinforcing MT-HVdc systems against faults, which contributes to efficient recovery and grid stability.

The objective of this paper is to investigate the coordinative performance of different types of high voltage DC (HVDC) circuit breakers (CBs) in multi-terminal DC (MTDC) grids. Several different HVDC CB technologies are emerging as a solution for the protection of offshore MTDC grids. There is a need for coordinative operation between different types of DC CBs in the same network. In this paper, two typical types of DC CBs are modelled in detail and implemented in a 4terminal MTDC grid in PSCAD environment, by considering operation time, interruption capability and interruption characteristics. Since the requirement of the DC CBs depends on the magnitude of the interrupted current where they are implemented, the fault scenarios in all terminals are studied and the worst scenarios are selected to demonstrate the coordinative performance of different DC CBs. Four cases are defined and demonstrated by two different types of CBs at each terminal of the cable. DC CBs perform differently with the change of the operating time and the locations where they are implemented. The performances and energy absorption are compared and analyzed. The obtained results can be used as DC CB’s selection optimization methodology for future MTDC grids.