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DSP control of a three phase UPS inverter Wicks, Kenneth

Abstract

The demand for three-phase uninterruptible power supplies has increased with the decrease in reliability of power supplied by power utilities. A single three-phase uninterruptible power supply (UPS) can typically supply many loads at a time. A UPS must be able to supply relatively undistorted output line voltages with highly non-linear loads. Many papers have been devoted to introducing new control schemes that can supply these loads effectively. A new control scheme is introduced, in this work, which is implemented with a digital signal processor (DSP). The controller acts in a stationary reference frame as opposed to the commonly used rotating reference frame. Although using a stationary reference frame can create phase errors, the control algorithm is computationally more efficient than using a rotating reference frame. The proposed technique uses two line voltages and the corresponding differential output filter inductor currents for those lines as the control variables. As a result, this technique utilizes fewer sensors compared to techniques using a rotating reference frame. In this work, the plant model is developed and the stability of the controller is analyzed. The controller design is facilitated with the use of a Mathcad workspace, Steady-state simulations and experimental results are presented for a balanced resistive load. Even with relatively slow inner current loops, the performance of the controller is acceptable. Slow current controllers are used because of large noise in the current measurements of the experimental inverter. This work provides the basis to further investigate the feasibility of using the proposed controller to supply non-linear loads.

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