The invention belongs to the technical field of power
system relay protection and particularly relates to a DC line
relay protection method for a multi-terminal flexible medium-
voltage DC power
distribution system. The method comprises the steps of main protection and the
backup protection. The main protection reflects an inter-
electrode short-circuit fault of a DC line and a unipolar grounding fault of a small transition
resistor, wherein a fault type and a fault interval are identified based on a current state matrix. The
backup protection adopts the duration of the
voltage imbalance as a starting criterion, so as to protect a judging fault interval of positive and negative current imbalance state matrixes at an installation part and isolate faults. The method determines the action
signal of a DC breaker according to the switching-on / off signals of the main protection and the
backup protection. The main protection only depends on the status
signal values -1, 0 and 1 of two adjacent protection and transmission current elements, so that the requirement on a communication
system is reduced. The backup protection judges the occurrence of a fault only based on the
voltage, wherein the unbalanced current calculation is aiming at the
positive current and the negative current of the same protection part. Therefore, the generation of errors due to the communication
delay or the non-synchronized communication can be avoided.