It is of prime importance to introduce a large amount of distributed generators (DGs) in order to realize the sustainable energy society. In distribution systems with many DGs, it becomes important issue to maintain the power supply reliability. It is considered that power supply reliability can be improved in microgrid because it can continue the power supply even in the case of outage in utility grid. In order to clarify the effectiveness for the reliability, the authors have developed so far a new algorithm to modify the protective relay action and power system operation when a fault occurs. However, only the adequacy is treated in those papers. Actually, not only adequacy but also security must be satisfied in order to realize the transition to the islanding operation mode. Hence, in this paper, we develop a new method which can derive "Islanding Security Region (ISR)" representing the set of required conditions to realize the transition successfully. By using the ISR and reliability analysis method, power supply reliability considering dynamics of microgrid can be calculated.
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