in this paper, a plug'n'play control strategy proposed for modular online UPS system is presented, which allows to plug the UPS modules in or out randomly. This provides a lower difficulty for the maintenance of the whole system. A two level control scheme was proposed, including local controllers to achieve active and reactive power sharing and central controllers to maintain synchronization capability, which allows the online UPS modular system having faster dynamic performance according to

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... e standard IEC 62040-3. A detailed small signal mathematical model was developed in order to analyze the proposed modular online UPS system. Experimental results and data are presented to validate the stability analysis and support the proposed plug'n'play control feasibility.