To identify the fault area more accurately and improve the reliability and sensitivity of the traditional protection principle on DC lines, an active injection for single-ended protection in the DC grid is proposed. Considering the influence of fault impedance, the effect of frequency-dependent parameters, numerical differential error, and the strength of DC boundary, high controllability of hybrid modular multilevel converter is utilized to inject sinusoidal signals with different frequencies into the line at DC terminal of each converter. Then the single-ended fault identification equation is constructed to preliminarily identify the fault area. According to the amplitude difference of the injection signals in different fault areas, the fault areas of the DC grid can be further distinguished. Simulation results verify the effectiveness of the proposed protection principle, improved reliability and sensitivity of protection, and full length protection of line without DC boundary.

Traditional single-ended fault location method is influenced by the fault resistance, and the remote in feed, which reflects as the two equations built by fault phase have four unknowns. And in the grid-connected converter system, the weak feedback characteristic further reduces the performance of the method. To solve it, a single-ended fault location method based on control and protection coordination is proposed in this paper. First, using the high controlled-ability of the converter, the control strategy for fault ride-through (FRT) operation is divided into two stages by switching control modes. Then, according to the electrical quantities under two stages, four uncorrelated equations can be established, and the equations still have four unknown parameters. The fault distance can be calculated by solving them. The simulation results show that the proposed method has high accuracy and is immune to fault resistance and fault location, it solves the problem of insufficient equations in principle.

Considering the advantages and limitations of traditional identification method, combined with the strategy of active detection, the principle of DC grid pilot protection based on active detection is proposed to improve the sensitivity and reliability of hybrid MMC DC grid protection, and to ensure the accurate identification of fault areas in DC grid. By using the DC fault ride-through control strategy of the hybrid sub-module MMC, the fault current at the converter station DC terminal is limited. Based on the high controllability of hybrid MMC, sinusoidal fault detection signals with the same frequency are injected into the line at each converter station. Based on model recognition, the capacitance model condition is satisfied by the detected signals at both ends during external faults whereas not satisfied during internal faults. The Spearman correlation coefficients is then introduced, and the correlation discriminant of capacitance model is constructed to realize fault area discrimination of DC grid. The simulation results show that the active detection protection scheme proposed in this paper can accurately identify the fault area of DC grid, and is not affected by fault impedance and has low sampling rate requirement.

Accurate fault location plays an important role in the fault clearance of the transmission line. However, because of the influence caused by fault resistance and the remote end system, there are four unknowns in the two equations built by the fault phase. Thus, the equations cannot be solved. In this paper, a single-terminal fault location method for transmission lines integrated by the inverter-type source is proposed. For a single-phase to ground fault or phase-to-phase fault, due to the specific control strategy, the equivalent positive and negative-sequence impedances of the converter are unequal in most cases. However, the positive and negative-sequence impedances of other elements are approximately equal, and the boundary condition at the fault branch is definite. Combining the boundary condition and the sequence impedance difference, two uncorrelated equations can be built. Substituting them to the equations built by the fault phase, the fault location can be calculated accurately. And the influence caused by fault resistance and the remote end system is eliminated. The simulation results show that the proposed method can achieve fault location accurately. Moreover, it has higher reliability and is independent of control strategy, fault distance and fault resistance.

A fault identification scheme for protection and adaptive reclosing is proposed for a hybrid multi-terminal HVDC system to increase the reliability of fault isolation and reclosing. By analyzing the "zero passing" characteristic of current at the local end during the converter capacitor discharge stage, the fault identification scheme is proposed. The distributed parameter-based fault location equation, which incorporates fault distance and fault impedance, is developed with the injection signal and the distributed parameter model during the adaptive reclosing stage. The fault distance is determined using a trust region reflection algorithm to identify the permanent fault, and a fault identification scheme for adaptive reclosing is developed. Simulation results show that the proposed scheme is suitable for long-distance transmission lines with strong anti-fault impedance and anti-interference performance. Also, it is less affected by communication delay and DC boundary strength than existing methods.