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In order to implement a distributed calculation of power flow and static security analysis in distributed EMS (DEMS), a new distributed dynamic power flow algorithm based on asynchronous iteration for multi-area interconnected system is presented in this paper. Based on online tracing Thevinan equivalence for exterior-network, the proposed algorithm can follow the change on system state and give a consistent result with power flow calculation for whole power system. Constructing a fix-point iteration scheme about boundary state variable, the proposed method gradually corrects equivalent injection power of exterior-network to match the power flow result for each sub-system with that for whole system. Some measures are adopted to assure that the accustomed modeling methods for sub-systems can be maintained. The outstanding feature of this method is the independence of load flow calculation for each sub-system. That means, a sub-system can give an acceptable local power flow result even when the distributed calculation can not be performed. Any sub-system can join into or move back from the distributed calculation scheme. So, this method is very suitable for the application of DEMS under recent condition of data exchanging between control centers. Since the network equivalent information has been tracked during the calculation, the algorithm has a good convergence, which has been testified by an example of IEEE-118 system and a real-life system.