Directional relays without voltage sensors for distribution grids: Optimal boundaries for fault direction estimation

Nowadays, with the presence of Distributed Generators (DGs) in the medium voltage (MV) networks, the false tripping problem can occur to traditional overcurrent protections. Directional relay is a solution. To reduce the cost, a principle for the directional relay using only current measurements, which is based on symmetrical components method, was proposed and tested with simulations in case of Inverter-Interfaced DGs [1]. Calculated from the sequence currents, both the I₂/I₁ and ΔI₂/ΔI₁ ratios created two separate areas on the complex plane from which upstream or downstream faults could be easily determined. However, in case of Synchronous Generators, these two areas become so close that they can overlap each other. Hence the overlap causes some errors in the directional algorithm. In this paper, optimal boundaries for separating the two areas are studied. The algorithm shows good performances in most situations with these boundaries defined by Support Vector Machine technique.