Utilization of symmetrical components in a communication-assisted protection scheme for radial MV feeders with variable or reduced short-circuit currents

Distribution networks are evolving toward the vision of microgrids, which are defined as power systems that contain both loads and Distributed Energy Resources and operate in a grid-connected or islanded mode. Correct operation of the OverCurrent protection is compromised in a typical microgrid because the fault current is significantly lower in the islanded mode compared to the grid-connected mode and consequently a single set of settings for the OC relays is not sufficient. This paper propose a communication-assisted protection scheme that is able to operate correctly in a radial Medium Voltage feeder with variable or reduced-short circuit currents. The proposed method is effective against the Single Phase to Ground fault and utilises symmetrical components of the fault current. More precisely, the Single Phase to Ground fault is indicated by evaluation of the ratio of zero-sequence current over positive-sequence current, which is a complex number. It is shown that the presence the fault in a radial feeder is associated with a non-zero magnitude of this ratio. Moreover, the closest relay placed upstream the fault will notice the highest magnitude of the ratio of zero-sequence current over positive-sequence current, while the relays placed downstream the fault detect a zero magnitude for their corresponding ratios. Protection relays calculate the magnitude of the proposed ratio and then exchange the obtained value with the adjacent upstream relays using communication. Therefore, each relay would have access to its own ratio and to the ratio provided by the adjacent relay. Finally, the relay with the highest calculated magnitude of the ratio of zero-sequence to positive-sequence current will trip, thus clearing the fault. The new protection scheme is implemented in a test Medium Voltage microgrid using PSCAD and simulation results prove its effectiveness against various Single Phase to Ground faults. The proposed method solves some of the protection issues associated with a high penetration level of Distributed Generation, as it is less sensitive to the variable short-circuit power and reduced short-circuit currents compared to the OverCurrent protection. In addition, coordination of the relays that use the proposed algorithm does not require a prior knowledge of the load or fault currents. The setting procedure of the relays that use the new protection algorithm is relatively simple in a radial feeder and a single set of settings is valid for a wide range of network conditions.