This paper presents a novel control strategy for three-phase voltage-source UPS inverters with second order L-C filters. The control scheme is based on sensing the current in the filter capacitor and using it in an inner feedback loop. An outer capacitor voltage loop is also incorporated to ensure sinusoidal load voltage. A general model of the power circuit (inverter, filter, and load) is first established. Computer simulation results of the proposed three-phase voltage-source UPS system with an R-L load from "cold" start to full-load is then presented to demonstrate the suitability of the proposed control scheme for UPS applications. The dynamic response of the proposed scheme is investigated using rotating frame of reference and small signal approximation. A control loop regulator is then selected to enhance the dynamic response of the control scheme and achieve the desired performance of the UPS system. It is shown in the paper that the control scheme offers many advantages for UPS applications. It is simple to implement and capable of producing nearly perfect sinusoidal load voltage waveform at moderate switching frequency and reasonable size of filter parameters. The scheme has fast dynamic response and provides high voltage utilization of the DC source. In addition, it provides peak current limiting capability and lends itself to both linear and nonlinear load applications.
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