Active Injection for Single-Ended Protection in DC Grid Using Hybrid MMC

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.