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Increased fault current level may exceed the rating of the existing network's equipment and disrupt coordination of overcurrent relays (OCRs). Introduction of new dispersed generation (DG) units in a grid connected microgrid is the main reason for increasing the fault current level. To overcome the above-mentioned problems, fault current limiters (FCLs) can be utilized between the microgrid (downstream) and main grid (upstream). Most FCLs have a bidirectional current limiting function. Whereas in the case of a fault in downstream, it is advised to disable the FCL in order to prevent some problems occurring in the microgrid. Power quality (PQ) and reliability of the microgrid as well as coordination between the upstream and downstream OCRs are affected by the bidirectional FCL in case of fault in downstream. In this paper a unidirectional fault current limiter (UFCL) is proposed to achieve a proper interaction between the upstream and downstream. During the fault condition in the main network, the UFCL acts normally; however, in case of fault occurrence or heavy load startup in downstream, the UFCL is disabled to avoid the aforementioned problems in the microgrid. Several simulations and experiments are carried out to demonstrate the effectiveness of the UFCL. Index Terms-Fault current limiters, microgrid, power quality, protection, relays, short-circuit current, unidirectional fault current limiter (UFCL). NOMENCLATURE Downstream Microgrid.Fault current seen by relay.Relay pick-up current. Impedance correction factor for transformer. Impedance correction factor for DG unit.Load losses at rated current. Power of transformer.Power of DG unit.Turns ratio.Tripping time of primary relay.Tripping time of back-up relay. UpstreamMain network and primary distribution network.

Islanding in power systems is a challenge that results in various uncertainties in the system parameters, degrades power quality, and may endanger the maintenance workers. This study presents a new passive islanding detection approach for grid-connected distributed generation (DG) units. The proposed method employs a Kalman filter (KF) to extract and filter the harmonic contents of the voltage signal measured at DG terminals. A three-phase voltage signal is measured at the point of common coupling and taken as a test signal for islanding detection. First, a residual signal is generated using the KF to detect different changes in the power system. In the second step, multiple harmonic contents are estimated by the KF to calculate a criterion named selected harmonic distortion (SHD). The variation of SHD classifies between islanding and normal conditions. The IEEE 13-bus test system simulated in Matlab/Simulink is used as a testbed to assess the performance of the proposed approach. The proposed method is extensively analysed under various islanding and non-islanding scenarios. The results demonstrate that the proposed method can successfully differentiate between islanding and non-islanding events. Moreover, it provides high reliability by eliminating the non-detection zone and stands robust against the false operation.