Bidirectional interlinking converter (BIC) is the core equipment in a hybrid AC/DC microgrid connected between AC and DC sub-grids. However, the variety of control modes and flexible bidirectional power flow complicate the influence of AC faults on BIC itself and on DC sub-grid, which potentially threaten both converter safety and system reliability. This study first investigates AC fault influence on the BIC and DC bus voltage under different BIC control modes and different pre-fault operation states, by developing a mathematical model and equivalent sequence network. Second, based on the analysis results, a general accommodative current limiting strategy is proposed for BIC without limitations to specific mode or operation condition. Current amplitude is predicted and constrained according to the critical requirements to protect the BIC and relieving the AC fault influence on the DC bus voltage. Compared with conventional methods, potential current limit failure and distortions under asymmetric faults can also be avoided. Finally, experiments verify feasibility of the proposed method.

Since the fault dynamic of droop-controlled inverter is different from synchronous generators (SGs), protection devices may become invalid, and the fault overcurrent may damage power electronic devices and threaten the safety of the microgrid. Therefore, it is imperative to conduct a comprehensive fault analysis of the inverter to guide the design of protection schemes. However, due to the complexity of droop control strategy, existing literatures have simplified asymmetric fault analysis of droop-controlled inverters to varying degrees. Therefore, accurate fault analysis of a droop-controlled inverter is needed. In this paper, by analyzing the control system, an accurate fault model is established. Based on this, a calculation method for instantaneous asymmetrical fault current is proposed. In addition, the current components and current characteristics are analyzed. It was determined that fault currents are affected by control loops, fault types, fault distance and nonlinear limiters. In particular, the influences of limiters on the fault model, fault current calculation and fault current characteristics were analyzed. Through detailed analysis, it was found that dynamics of the control loop cannot be ignored, the fault type and fault distance determine fault current level, and part of the limiters will totally change the fault current trend. Finally, calculation and experimental results verify the correctness of the proposed method.