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It is imperative to accurately consider the effect of external network on internal system's voltage stability for Online voltage stability assessment of a subsystem within a large interconnected power grid. Since the heaviest load condition of a system and a set of credible severe contingencies have to be considered in VSA, traditional network equivalent with respect to a snapshot case cannot meet the requirements. Two different distributed computation modes are proposed to exactly consider the effects of external networks. They are the synchronous iteration (SI) mode and the asynchronous iteration (AI) mode. The continuation power flow methods under both SI and AI mode are developed to calculate the load margins. The numerical results of IEEE118 bus power system indicate that the decomposition and coordination methods can get the better results comparing to the local equivalence methods.