Current based Distance Protection in Closed-Ring Grids with Distributed Generation

Abstract

Conventional methods of distance protection relays measure an impedance. Based on voltages and currents, the relay determines the distance between relay and fault location and enables selective fault clearing. Closed-ring structures increase the grid capacity for connecting distributed energy resources as inter infeeds. However, closed-ring structures and inter infeeds can cause malfunctions of conventional distance protection and disturb the proper operation of non-directional short-circuit current indicators particularly. This paper proposes a voltage-less distance protection method which complies with closed loop structures and inter infeeds properly in case of unbalanced faults. The proposed approach is based on current comparison and negative-sequence system quantities. Thus, it remains unaffected from inter infeeds and fault resistances. The approach serves as a supplement to conventional distance protection methods which are applied for symmetrical faults and if the closed-ring structure is dissolved. Finally, the paper presents a protection grading strategy that significantly reduces the average fault clearing time.