Advanced Synchronous Reference Frame Controller for Three-phase UPS Powering Unbalanced and Nonlinear Loads

This paper describes a high performance voltage controller for 3-phase 4 wire UPS (uninterruptible power supply) system, and proposes a new scheme of synchronous reference frame controller in order to compensate for the voltage distortions due to unbalanced and nonlinear loads. Proposed scheme in this paper is able to completely eliminate the negative sequence voltage distortion due to unbalanced loads and also reduce the harmonic voltage distortion due to non-linear loads, even if the bandwidth of voltage control loop is a very narrow. Therefore, the proposed scheme is especially appropriate for a fully digital controlled UPS or a high power UPS with limited voltage control bandwidth. In order to compensate for the effects of unbalanced loads, the synchronous reference frame controller with the positive and negative sequence computation block is proposed, and the synchronous frame controller with a bandpass filter is proposed to compensate for the selected harmonic frequency of output voltage. The effectiveness of the proposed scheme has been investigated and verified through computer simulations and experiments by a 30 kVA UPS