Aiding Effects of Interleaving to Improve Design Performances of a Line-frequency Zig-Zag Transformer Employed in a Unified AC-DC System

A line-frequency zig-zag transformer is beneficial in a unified ac-dc (UACDC) system to integrate ac and dc power from three-phase ac grid and photovoltaic (PV) sources, respectively. Leakage inductances of the zig-zag transformer can be utilized as the integrated boost inductances for the PWM converter operation. It eliminates the additional line-frequency inductors and reduces the component count, thereby improving the overall power density. This paper presents analysis on different winding arrangements and their effects on the zig-zag transformer operation. It concludes that in case of the UACDC system, interleaved winding arrangement is best suited where the primary winding is placed between the zig-zag winding sections. Benefits of the interleaved winding strategy are two-folds. It augments the integrated boost inductance, offered by the zig-zag windings, and helps useful integrating low voltage (LV) PV with three-phase ac grid using a single PWM converter. Interleaving technique also keeps the eddy current effects minimal in the multi-layer windings. It reduces the ac resistance and copper loss, thereby improving the transformer efficiency. 2-D FEM analysis and experimental results obtained from a 1 kW, 415/130 V, 50 Hz, zig-zag transformer prototype are furnished to validate the theoretical analysis and practical feasibility of the interleaved winding strategy for the zig-zag transformer, employed in the unified ac-dc power conversion system.