Selective Protection Relaying in the Presence of Distributed Generation and Closed-Ring Structures

Adverse effects of high shares of distributed energy resources (DER) in distribution grids, like overvoltage or overloading, can be mitigated by closing open-ring structures. In order to obtain acceptable selectivity in case of faults on these closed-ring structures, an additional protection relay in the coupling point (“coupling relay is beneficial. If the coupling relay (CR) is a conventional overcurrent or distance relay, it suffers from shortcomings like a highly reduced reach towards the substation caused by the necessity to discriminate low fault currents against high currents in non-faulty operation states. Furthermore, DER fault current contributions may lead to sympathetic tripping of the CR in case of faults on adjacent feeders. The proposed CR approach overcomes those challenges by using a negative-sequence reactance criterion. The method’s reactance results are not disturbed by phenomena like fault resistance or variable DER infeed. There is no risk of sympathetic tripping and the CR can be properly coordinated with other protection relays on spur feeders of the ring. This is achieved without communication channels between the CR and other relays or DERs. The approach is validated by simulations of a typical German 20-kV-distribution grid using PSS® Sincal.