Performance of directional relays without voltage sensors: Impact of distributed generation technologies

Radial distribution networks are mainly protected with overcurrent relays, which are used for both earth and phase fault protection. Nevertheless, higher capacitive current of underground cables can cause false tripping problem for overcurrent relay of a feeder during a line-to-ground fault on the adjacent feeder. The contribution of distributed generation (DG) to the fault current during a line-to-line fault also leads to a similar situation. To solve this problem, a novel algorithm of directional relay has been proposed in a previous work [1]. Based on the symmetrical components method, the algorithm only uses current measurements to determine fault direction and thereby suppresses the cost of voltage sensors. This paper presents effect of distributed generators on the directional algorithm by comparing the cases of Inverter-interfaced Distributed Generators (IIDGs) in PV systems and of synchronous generators (SGs) in CHP plants. Results show good performances in both cases during earth faults. However, during line-to-line faults, this algorithm gives better results in case of IIDGs.