A novel fault-dependent-time-settings algorithm for overcurrent relays

A typical structure of electric power distribution networks is radial or a normally open-loop structure with a single supply path between the high-voltage supply substation and the end-consumers. The selectivity of a protection scheme for the radial network is generally achieved through time-coordination of different protection devices (relays, reclosers and fuses). However, future distribution networks may become more meshed (normally closed-loop) in order to integrate distributed generators (DGs) and more active demand-side response. This will, in turn, require novel protection schemes since the current time-coordinated protection which assumes radial network structure may no longer be effective. Furthermore, it is also recognized that today´s practice of automatically disconnecting DGs when a fault occurs in their vicinity may cause un-necessary generation shortages. It is, instead, desired to adjust protection so that a DG unit remains connected to the system when nearby faults occur as long as there are no related safety problems. This paper introduces a novel fault-dependent time settings algorithm for overcurrent relays which is capable of overcoming these problems. The implementation of the proposed algorithm on the existing protection scheme is enabled by using communications among the relays. The proposed algorithm ensured a reduced relay tripping time in the primary protection zone by enhancing the selectivity of the existing protection scheme. Consequently, it becomes possible to operate distributed generators during times when nearby faults occur. While the use of the proposed algorithm is illustrated in the distribution network, the same algorithm is also applicable to the transmission networks.