New protection schemes based on impedance for active distribution lines with high impedance fault detection capability

The extensive penetration of microgrids in the distribution network poses challenges to the control system, coordination with the network, and especially the traditional current-based protection systems despite many advantages such as reducing power outages, increasing reliability and resiliency, enhancing the flexibility of the power system in supplying loads, and improving the power quality. The main reason for the incorrect performance of current protection schemes is the change in the network fault level due to the connection and disconnection of the distributed generator or the microgrid operating mode change from the grid connected to the islanded and vice versa. To improve the performance of the protection system in the presence of microgrids, some schemes have been presented in two general categories: schemes based on modifying the network behavior and schemes based on modifying the protection system. In the first category, the network behavior during the faults is modified by external equipment to operate the conventional protection schemes properly. In the second category, the protection schemes are modified to correct performance according to the network behavior change during the short circuit faults. Due to the limitations of the schemes in the first category, the schemes of the second ones are more practical. Among the second category schemes, the impedance-based ones can operate in both grid-connected and islanded modes of the microgrid due to their directional nature and independence of the fault level of the network.This article rearranges and reforms the conventional sequence equivalent circuits of the lines during the short circuit faults and presents new equivalent circuits. Also, a protection scheme is proposed using these circuits for low-voltage and medium-voltage overhead and cable lines in smart AC microgrids. The basis of the proposed scheme is the large change in impedance in the proposed delta sequence equivalent circuit. This scheme employs the voltage and current data at the relay location and the magnitude of the positive sequence voltage at the other line end. Therefore, minimum data exchange between two ends of the line and low sampling rate are the features of the proposed scheme. This scheme is independent of the configuration of the microgrid, its operating mode, and uncertainties in the microgrid. Also, it can detect high resistance and high impedance short circuit faults in the grid-connected and islanded mode with a detection time of fewer than two cycles in low voltage lines and about three cycles in medium voltage lines. A case study microgrid is simulated in PSCAD software, and the proposed scheme is implemented on it by MATLAB software. The results show the accurate performance of the proposed protection scheme in detecting short circuit fault types in different conditions.