Implementation of Single-Phase Transformerless Online Uninterruptible Power Supply with Multiple-Resonant Controller

In this paper, the transformerless uninterruptible power supply (UPS) is explained. The use of the DC–DC bidirectional converter eliminates the transformer from the UPS structure. This not only reduces the size, weight, and cost of the system, but also prevents leakage currents due to the saturation of the transformer when the UPS operation is changed. Furthermore, by employing the quasi-half-bridge (QHB) structure in the UPS configuration, in addition to profiting from the advantages of unipolar modulation, the system efficiency is improved due to the switching of auxiliary leg switches in grid frequency. Subsequently, using the multiple-resonant controller in combination with a sinusoidal pulse width modulator leads to less total harmonic distortion in both input current and output voltage, compared to the allowed limit specified by the IEC62040 standard. To evaluate the robustness of the system toward grid voltage and load changes, first, the input voltage was varied by 40%, and then the nonlinear and step loads were applied to the system. Under these conditions, the QHB UPS demonstrated good behavior. This system was implemented experimentally in addition to simulations, and the results are presented.