Mitigation of sympathy trips in highly cabled non-effectively earthed radial distribution systems via MINLP

Abstract

Sympathy tripping problem simply refers to unnecessary operation of healthy feeder protection relays. Capacitive currents of underground cables are the main cause of sympathy tripping during the single line-to-earth faults (SLEFs) in non-effectively earthed distribution systems. Sympathy trips cause unnecessary loss of loads, increase the amount of energy-not-sold and deteriorate reliability indices. To prevent sympathy tripping in the presence of the circulating capacitive currents during the SLEF, pick-up current settings of earth fault relays should be optimized in accordance with the network topology. Accordingly, in this paper, a novel objective function and constraints have been formulated and solved using Mixed-Integer Non-linear Programming (MINLP) for optimizing the earth fault relay settings. The effectiveness of the proposed methodology has been tested by considering the effects of different cable cross-sections, fault locations, and fault resistances on a real distribution network. Therefore, with the developed MINLP approach, sympathy tripping problem is prevented. Selective protection can be provided by the optimized setting of earth fault relays in highly cabled non-effectively earthed radial distribution networks. More importantly, by using the proposed method it is possible to eliminate sympathy tripping in different radial typologies such as different numbers of parallel feeders, voltage levels, and neutral earthing structures.