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In a conventional distribution system, protection algorithms are designed to operate on a unidirectional high fault-current level. In a microgrid, a fault current from distributed generation (DG) may bring about a relay malfunction because of the bidirectional and relatively small fault current. Therefore, the conventional protection scheme is not applicable to microgrids and a new protection method must be developed. In this paper, two protection coordination algorithms which can be applied for facility and secondary microgrids are proposed, respectively. The proposed protection algorithms eliminate faults not by the EMS signal but by directional relays. Moreover, this makes the algorithms flexible regardless of the types and numbers of DG. The proposed protection algorithms were simulated at the KEPCO RI Microgrid Demonstration Site.

This paper considers a multiple-DG microgrid connected to the main grid at a point of common coupling (PCC) via a static switch. The DGs in the microgrid can be connected in series and parallel as well. Each DG can also be operated in unit power control (UPC) mode or feeder flow control (FFC) mode. In this paper the active power and frequency response in the microgrid with respect to different configurations (parallel/series) and different control modes (UPC/FFC) is investigated. Four possible cases are considered: case 1) Series-FFC configuration: the DGs are connected in series and the DGs' control modes are FFC; case 2) Series-UPC configuration: the DGs are connected in series and DGs' control modes are UPC; case 3) Parallel-UPC configuration: the DGs are connected in parallel and DGs' control mode are UPC; case 4) Parallel-FFC configuration: the DGs are connected in parallel and DGs' control modes are UPC. According to the analysis, a comparison between the four configurations is presented, advantages and disadvantages of each configuration is then achieved.