Flexible DC Grid Protection Scheme Based on Fitting Slope Magnitude

In view of the current problems of insufficient ability to withstand high resistance in the single-terminal protection of multi-terminal flexible DC grids, a flexible DC grid protection scheme based on the fitting slope is proposed. First, the expression of the current change rate during faults inside and outside the zone is theoretically derived. And it is found that there are obvious differences in the magnitude and steepness of the current during faults inside and outside the zone. Then a protection scheme is proposed based on this difference. The slope of the fitted current is used to identify faults inside and outside the zone. In view of the limitation that the fitted current may malfunction under high resistance conditions, it is proposed to use the fitted power to construct a fitted power assist criterion. Finally, a four-terminal flexible DC grid model is built to verify the protection scheme. The simulation results show that the proposed protection method can accurately identify faults and fault poles inside and outside the zone, and still has good sensitivity under high-resistance faults.