부산대학교 도서관

A line-frequency zig-zag transformer is beneficial to integrate ac grid and photovoltaic (PV) source in a unified ac-dc (UACDC) system. In this paper, the zig-zag transformer efficiency is characterized during UACDC operation. For reliable estimation of efficiency, winding resistances and hence copper loss are accurately determined with inclusion of eddy current effects. Furthermore, an optimal power sharing ratio between the ac grid and PV source, is deduced to maximize the transformer efficiency at a given output power. A 1.0 kW, 415/130 V, 50 Hz, delta/zig-zag transformer is used to experimentally validate these attributes of UACDC operation. At rated output of 1.0 kW, a transformer efficiency of 93.55% is achieved, an improvement of 1.34% with respect to conventional ac power transfer. For the same secondary current capacity, UACDC operation allows an additional 17.64% of output power transfer without any thermal overloading of the secondary winding. Following benefits are therefore derived from the UACDC operation e.g. integration of PV with the ac grid, improved efficiency and effective utilization of installed thermal capacity of the zig-zag transformer.