الفهرس 
ACKNOWLEDGMENTSOnly 14 pages are availabe for public view
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Abstract
As the yearly electric energy demand grows, there is a significant increase in the penetration of distributed generation (DG) to fulfill this increase in demand. Penetration of a DG into an existing distribution system has many impacts on the system, with the system protection being one of the major issues. The presence of DG units in distribution systems increases the fault current level, which disrupts the existing coordination time interval of the protective overcurrent relays. One of the ways for eliminating the DG effects on the protection coordination is to install a unidirectional fault current limiter (UFCL) between the distribution system (upstream network) and the microgrid (downstream network). The UFCL does not limit fault current contribution of the upstream network when fault occurs in down stream but limits fault current contribution of the downstream network when fault occurs in the upstream. Moreover, it preserves the coordination between all of the relays. Load flow analysis is carried out using electrical transient analysis program (ETAP) in order to determine the value of pick-up current of the relays. A MATLAB program is prepared after being checked by comparison against the ETAP program for calculating symmetrical fault currents. This is a prerequisite for determining the time dial setting (TDS) value of overcurrent relays. In order to maintain the relay coordination, an initial value of the UFCL-resistance is assumed; 100 Ω and 50 Ω for the investigated 20-kV and 11-kV distribution systems. Then, incrementing the resistance value until the protection coordination is achieved. Several case studies are reported to investigate the effect of increasing the rating of main grid as well as changing the unit type and rating of the microgrid on the UFCL-resistance.