許諾条件により本文（PDF）は2008-11-01に公開Analysis of voltage instability of power systems is of basic importance for prevention of cascading outages and large blackouts. We propose to use reachability analysis of hybrid automaton for voltage instability analysis of power systems. Hybrid automaton is a well-known mathematical model of hybrid systems. Reachability analysis is performed by computing forward or backward reach sets for unsafe sets of a hybrid automaton. The automaton can represent interaction between continuous voltage oscillation and discrete control of power equipments. The unsafe sets are also subsets of state space in which system voltages show unacceptable levels such as low or high values. For single machine-load bus (SMLB) system with controlled route switching, we show that computation of backward reach sets of hybrid automaton makes it possible to determine voltage instability in the SMLB system. The obtained result implies that voltage instability leading to cascading outages can be detected before its occurrence.