Abstract
In most uninterruptible power supply (UPS) systems, transformers are incorpo-rated for galvanic isolation to reduce circulating current. However, the use of trans-formers increases cost and size of a system. In addition, heat generated during opera-tion lowers the stability and reduces the longevity of power components. A trans-fomerless UPS system regulates the power flow between dc link and utility and stabilizes the output voltage. Rather than the conventional abc to dq frame transformation, the division-summation(D-Ʃ) digital control is adopted instead so that the calculation process can be greatly simplified. This research presents a transformerless UPS systems which includes a power factor corrector (PFC) and a voltage source inverter(VSI). For the PFC, a three-phase three-wire full-bridge inverter is adopted while a three-phase four-wire full-bridge inverter is adopted for the VSI. Microprocessor RX62T is incorporated in both cir-cuits to implement the proposed control law. The main contributions of this research are summarized as follows: First, The circulation current is suppressed effectively. Large circulating current flowing through the common ground of the PFC and the VSI due to different switching states is observed. To reduce the circulating current, synchronization of: a) the switching states between the two circuits and b) the input voltage of the PFC and the output voltage of the VSI have been implemented. Secondly, both ac harmonic voltage and inductance variation are taken into account in the control law derivation of the D-Ʃ digital control, allowing a sinusoidal output current even in the presence of harmonic-contaminated grid voltage. Meanwhile, core size can be greatly reduced with the consideration of wide inductance variation. Thirdly, total harmonic distortion of the output voltage can be effectively reduced by incorporating a repetitive control in the D-Ʃ digital control based voltage tracking control. Finally, the feasibility of the proposed control scheme has been verified by the measurements from the implemented transformerless UPS system.