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This paper describes a theoretical and experimental study on a control strategy for the parallel operation of single-phase voltage source inverters (VSI), to be applied to uninterruptible power supply. The control system for each inverter consists of two main loops, which both use instantaneous values. The first (parallelism control) employs the feedback of the inductor current from the output filter to modify the input voltage of the same filter and, therefore, to control the power flow of each inverter to the load. Additionally, the second loop (voltage control) is responsible for controlling the output voltage of the LC filter, which coincides with the output voltage of the VSI. Due to the fact that there is no exchange of information among the VSIs regarding their operation points, it is easier to obtain redundant systems. Furthermore, the connection (or disconnection) of inverters in a parallel arrangement is carried out directly, without connection impedance, and can occur at any operation point of the system. The proposed control strategy ensures the proper sharing of the load current and avoids current circulation among the inverters during transient and steady-state operation. Moreover, its design and implementation are very simple. The control technique was verified through experimental results with a maximum load of 10 kVA supplied by three parallel-connected inverters.
Date of Publication: June 2013