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This paper describes a theoretical and experimental study on a control strategy for the parallel operation of three-phase voltage source inverters (VSI), to be applied to UPS. The proposed 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 currents from the output filter to modify the input voltages of the same filter and thereby control the power flow of each VSI to the load. Additionally, the second loop (voltage control) is responsible for controlling the output voltages of the LC filter, which coincides with the output voltages of the VSI. 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. The VSI and the proposed control strategy are analyzed in an orthogonal (αβ) stationary frame, and as a result, simple and effective models were achieved. The proposed control system was digitally implemented in a TMS320F2812 DSP and was verified through experimental results with a 10 kVA prototype, which has the parallel operation of two three-phase VSIs.
Date of Publication: May 2013