High-voltage direct current (HVDC) transmission systems have developed rapidly in China. Strong DC and weak AC power grids have some limitations, and cascading faults caused by HVDC commutation failures is a serious problem in high voltage AC/DC hybrid power grids. The lack of self-turn-off ability of semiconductor switch causes commutation failure. The first commutation failure is inevitable. However, the continuous commutation failure can be prevented by the proper control method. The key point is the duration time and values of advancing firing control and the extinction angle dynamic control. Hence, an improved method based on constant extinction area is proposed. The proposed method is based on the dynamic change of extinction angle in the inverter side and superimposed advancing firing control in the rectifier side. The proposed method is tested on a hybrid system with two HVDC transmission systems and one AC interconnection line. The model was developed using the electromagnetic transient software package PSCAD/EMTDC.

This paper proposes a novel protection method for single line-to-ground (SLG) faults in ungrounded low-inertia microgrids. The proposed method includes microgrid interface protection and unit protection. The microgrid interface protection is based on the difference between the zero-sequence voltage angle and the zero-sequence current angle at the microgrid interconnection transformer for fast selection of the faulty feeder. The microgrid unit protection is based on a comparison of the three zero-sequence current phase directions at each junction point of load or distributed energy resources. Methods are also included to locate the minimum fault section. The fault section location technology operates according to the coordination of microgrid unit protection. The proposed method responds to SLG faults that may occur in both the grid and the microgrid. Simulations of an ungrounded low-inertia microgrid with a relay model were carried out using Power System Computer Aided Design (PSCAD)/Electromagnetic Transients including DC (EMTDC).

A high-voltage hybrid AC/DC power system has already been developed in China. The interconnection of such a line-commutated converter-high-voltage direct current LCC- HVDC transmission system is extremely challenging. Traditional relay protection directional components may mis-operate during a commutation failure. This paper proposes a novel directional element for high-voltage hybrid AC/DC power systems. Different sequence components were analyzed, and a novel directional element based on pre-fault current and fault current components is proposed. PSCAD-based simulations were used to generate signals, and Fourier transforms were used to estimate the sequence components and phasors. Simulation results show that the proposed directional element works well in high-voltage hybrid AC/DC power systems.

This paper proposes a protection method for single line-to-ground (SLG) faults in an ungrounded offshore wind farm with fully-rated converter-based wind turbines. The proposed method uses the unsynchronized current phasors measured by unit protections installed at the connection point of the fully-rated converter (FRC)-based wind turbines (WTs). Each unit protection collects the unsynchronized current phasors from two adjacent nodes and synchronizes them by aligning the positive-sequence current to the same phase angle. The faulted section is identified by comparing the phase angles of the synchronized zero-sequence currents from adjacent nodes. Simulations of an ungrounded offshore wind farm with relay models were carried out using power system computer-aided design (PSCAD)/ electromagnetic transients including direct current (EMTDC).