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Flexible direct current (DC) distribution systems have emerged as the development trend for future distribution grids. However, these systems are vulnerable to DC faults, rapid fault identification and faulted line selection method are required to enhance the security of the entire system. A novel transient highfrequency impedance comparison based DC protection for flexible DC distribution systems is proposed in this paper. The controlindependent high frequency impedance model of power converter is also investigated. Based on this model, the proposed method identified the faulted lines by comparing high frequency impedance measurement differences. For DC bus with multiple branches, this technique minimizes the threshold calculation job, which is usually difficult to process for the transient value based protections. Strict synchronization of data is also not required for this method. The simulation model of four-terminal flexible DC distribution networks is built in PSCAD/EMTDC to verify the effectiveness of the proposed protection model. Simulation results prove that the protection is robust to fault transition resistances and the measurement noise.

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Due to different fault ride through controls of both the wind farm and the voltage-source converter high-voltage alternating current (VSC-HVDC) converters, the offshore wind farms that are connected to the power system through VSC-HVDCs have unique fault transients. During a fault on AC side transmission lines, the wind farm and the VSC-HVDC together provide faulted current, whose amplitude and phase will be decided by their different control strategies collectively. The fault transients are different from those in the traditional AC grid. Thus, the performance of traditional protection will be affected. In this study, the AC fault current expression is deduced, while the influencing factors on the current's amplitude and phase are analysed in detail. Based on these characteristics, the performance of distance relays under the grid side fault of the VSC-HVDC-connected offshore wind farm integration system is evaluated. Simulations based on PSCAD have verified the theoretical analysis.

Distributed Generations (DGs) in the distribution systems are connected into the buses using power electronic converters. During fault, it is challenging to provide a constant impedance model for DGs in the system frequency due to the variable converter control strategies. System frequency impedance measurement based fault locations can be influenced by the converters' fault behavior. This paper addresses this problem by proposing a wide-area high frequency impedance comparison based fault location technique. The high frequency impedance model of DG is provided. Based on the constant DG impedance model in high frequency range, the faulted line sections can be distinguished by comparing the measured impedance differences without requiring the exact distribution system parameters. Simulation results show that the proposed wide-area transient measurements based fault location method can provide accurate faulted sections in the distribution systems with DGs regardless of the load and DG output variations, measurement noise, unbalanced loads and islanding operations.