Active zero-sequence cancellation technique in unbalanced commercial building power system

In this paper, active zero-sequence cancellation technique is proposed for unbalanced commercial building power system. A grounding zig-zag transformer is used to flow the equal harmonic currents through all transformer windings and to allow a fundamental current into utility neutral line. Lower voltage and high current rated single-phase inverter regulates only a single phase current. Then, other phase currents are equally distributed due to the zig-zag combination of the transformer. The fundamental neutral current flow in utility side is achieved by circulating harmonics into the zig-zag transformer. And the proposed control technique based on mathematical analysis is developed for unbalanced nonlinear loads normally shown in commercial office buildings. The technique requires sensing only a neutral load current and a single transformer winding current The algorithm is implemented by a digital signal processor (DSP) without any line voltage sense. Experimental results are presented to verify the analysis and to show that the proposed technique is capable to be used for zero sequence cancellation under unbalanced as well as balanced system.