Directional relays without voltage sensors for distribution networks with distributed generation: Use of symmetrical components

This paper presents two local algorithms using only currents measurements that could be used as a back-up protection (after the loss of the voltage sensors) in directional relays for distribution networks with distributed generation (DG), or as additional directional relay dispatched along the feeders. These algorithms are based on the symmetrical components (0-zero, 1-positive and 2-negative sequences) of the 3-phases currents. Due to the power flows generated by the DG the positive sequence current argument is unforeseeable, thus it is not possible to use only the positive sequence. Then, we first propose in this paper an algorithm using the I2/I0 ratio to locate a phase-to-ground fault upstream or downstream the detector. The second algorithm measures the zero and positive sequence components of the fifth harmonic of the current and calculates the I0_5/I1_5 ratio. The performances of these algorithms are analysed for several DG power, fault resistance, capacitive current and neutral grounding (resistive and compensative grounding). The fluctuations of the phasors measurements is also taken into account in the range ±5% for the modulus and ±5° for the argument. The present paper shows that these algorithms can be reliable in the major part of the studied cases.