A high-voltage direct-current (HVDC) grid protection strategy to suppress dc fault currents and prevent overcurrent in the arms of modular multi-level converters (MMCs) is proposed in this paper. The strategy is based on the coordination of half-bridge (HB) MMCs and hybrid dc circuit breakers (DCCBs). This is achieved by allowing MMC submodules (SMs) to be temporarily bypassed prior to the opening of the DCCBs. Once the fault is isolated by the DCCBs, the MMCs will restore to normal operation. The performance of the proposed method is assessed and compared to when MMCs are blocked and when no corrective action is taken. To achieve this, an algorithm for fault detection and discrimination is used and its impact on MMC bypassing is discussed. To assess its effectiveness, the proposed algorithm is demonstrated in PSCAD/EMTDC using a four-terminal HVDC system. Simulation results show that the coordination of MMCs and DCCBs can significantly reduce dc fault current and the absorbed current energy by more than 70 and 90% respectively, while keeping MMC arm currents small.

To protect HVdc grids from dc faults, the concept of a hybrid dc circuit breaker is widely accepted due to its low conduction losses and fast interruption speed. For a well-built dc grid, a massive number of hybrid dc circuit breakers have to be installed. This will lead to high capital costs. An interlink dc circuit breaker based on an idea of sharing a main breaker branch between two circuit breakers is proposed to reduce the overall costs of circuit breakers in a dc grid. Compared to existing hybrid dc circuit breakers, the interlink hybrid dc circuit breaker can achieve the same dc fault interruption capability with fewer components. Novel structures of main breaker branches are designed and their parameters are determined to make the interlink hybrid dc circuit breakers be capable for both unidirectional and bidirectional interruption on demand. For a unidirectional interlink hybrid dc circuit breaker, the size of metal-oxide varistors (MOVs) is reduced by 50%. For a bidirectional interlink hybrid HVdc circuit breaker, the number of IGBTs and MOVs are reduced by 25%. The interlink hybrid dc breakers are verified and compared to the hybrid dc circuit breaker via a three-terminal HVdc grid in PSCAD/EMTDC.

For a grid-connected inverter requiring the ac volt- age magnitude and the active power control, both vector control and power synchronization control can be applied. The stability comparison based on the dq impedance stability analysis between both control are carried out via three factors including the grid impedance, the inner current loop and the virtual impedance. The dq impedances of the inverter based on both control are derived. The determinant of the impedance ratio is used for the stability analysis. The bode plot of the grid impedance and the inverter impedance are present to assist the stability analysis and explain their interactions. It is found that increasing the grid impedance and the cut-off frequency of the current loop stabilize the inverter with the power synchronization control, which is converse to the vector control. Furthermore, the inverter with the power synchronization control may suffer the instabilities when connecting to a strong grid. The virtual inductor and resistor are proposed to enhance the stability for the vector control and the power synchronization control respectively. The simulation validation using Matlab/Simulink is performed.

This letter proves that a grid-following converter can stably connect to a weak grid even short-circuit ratio (SCR) is 1. Root instability causes of this grid-following control are identified including fast control response, insufficient damping, and slow voltage support. A simple and effective tuning method is proposed to stabilize this connection. Three control modes are considered in this tuning method including current control, active power and voltage ( $ PV$ ) control, and active power and reactive power ( $ PQ$ ) control. The switching model of a two-level converter is used for the simulation validation.

Dynamic wireless charging (DWC), which enables charging, while electric vehicles (EVs) are in motion, is an attractive charging way. However, additional installed power tracks underneath the lane bring a significant concern to its economic viability. This article proposes a comprehensive framework to evaluate the economic viability of the DWC lane for private EVs. The investment of local power support with renewable energy integration and energy storage is also considered in the DWC system. Charging choices of private EVs among multiple charging providers are modeled to estimate the electricity demand of the DWC system. The grid impact of the DWC system is studied via a multibus ac network. Moreover, an optimization policy is proposed to maximize the DWC provider’s profit and minimize the grid impact by adjusting the charging price and electricity procurement at each horizon. It is found that the payback period of the DWC system with the proposed optimization policy is shortened by 25% compared with the fixed charging price strategy. Even with a tight grid impact limit, the payback period will not be significantly longer under the proposed optimization policy. When the efficiency of the DWC lane increases to 90% and the cost reduces to 50%, the payback period is shortened by 19% and 22%, respectively.