Modular Plug'n'Play Control Architectures for Three-Phase Inverters in UPS Applications

In this paper, a control strategy for the parallel operation of three-phase inverters in a modular online uninterruptable power supply (UPS) system is proposed. The UPS system is composed of a number of dc/acs with LC filter connected to the same ac critical bus and an ac/dc that forms the dc bus. The proposed control includes two layers: 1) individual layer and 2) recovery layer. In individual layer, virtual impedance concept is employed in order to achieve active power sharing while individual reactive power is calculated to modify output voltage phases to achieve reactive power sharing among different modules. Recovery layer is mainly responsible for guaranteeing synchronization capability with the utility and voltage recovery. With the proposed control, improved voltage-transient performance can be achieved and also dc/ac modules can be plugged in and out flexibly while controlling the ac critical bus voltage, thus allowing plug'n'play or hot-swap capability. Detailed control architecture of the individual and recovery layers is presented in this paper. An experimental setup was built to validate the proposed control approach under several case-study scenarios.