Investigation for harmonic mitigation in the line and neutral currents of three-phase four-wire system feeding two-pulse rectifiers with balanced and unbalanced load using Zig-Zag transformer

In the commercial sector single-phase, non-linear loads are predominant and their numbers are on the increase. These low-power loads include conventional single-phase power supplies, switch mode power supplies, fluorescent lamps, heating elements and; pumps and motors for ventilation, air-conditioning and elevators. The two-pulse rectifier is the most popular single-phase topology deployed for ac-dc conversion; however, when feeding an inductive load, its line current has a total harmonic distortion (THD) of around 28%, that is a violation of power quality standards. Another concern is the presence of third and other triplen harmonics in the line currents that add up in the neutral conductor. This paper reports THD reduction in the line currents and neutral current decrease, by mitigation of triplen and non-triplen harmonics, using a Zig-Zag transformer between the three-phase utility and the return conductor of the rectifiers with balanced load. The currents are also investigated for harmonic content with the rectifiers feeding unbalanced load. Harmonic estimates are obtained using MATLAB simulation of the rectifiers with and without the Zig-Zag transformer. Experimental results with the utility feeding the rectifiers directly without the Zig-Zag transformer are also presented.