Criteria Setting of Earthfault Relay Coordination for Bhutan Substation Considering the Interconnected Indian Grid Influence

Earthfault relays (51N) operate based on zero sequence fault current. Since most of the faults that occur on the power system are the earthfault, therefore, coordination of earthfault relays plays a vital role in discriminating the fault. More importantly, the criteria settings must be developed for earthfault relay coordination. Single Line to Ground (SLG) fault current of the system must be known for earthfault relay coordination, however for Bhutan grid, the fault current depends on gigantic Indian grid because of its interconnection. In this paper, short circuit studies pertaining to the SLG fault have been carried out in DIgSILENT to ascertain the SLG fault current. Further, the accuracy of the earthfault relay coordination based on the desired Coordination Time Interval (CTI) for a 132/33/11 kV substation in Bhutan has been validated through DIgSILENT. This paper recommends the earthfault relay coordination methodology and relay setting criteria for those system which is connected to a large grid, like the case of tiny Bhutan and gigantic Indian grid.