Zero-sequence harmonics current minimization using zero-blocking reactor and Zig-Zag transformer

In the distribution power system, the third harmonics of zero-sequence caused by nonlinear loads usually result in high-voltage distortion levels throughout the facility, neutral conductor overloading, motor heating, transformer heating, increased losses, and excessive harmonic injection onto the utility supply system. This paper presents a novel method for minimizing the zero-sequence harmonics by using zero-sequence blocking reactor (ZSBR) and Zig-Zag transformer. Zig-Zag transformer is a special connection of three-phase transformer´s windings. The ZSBR is also a special connected transformer, whose three windings are wounded in the same core. The ZSBR has zero reactance for positive and negative-sequence components but giving three times of self-reactance for zero- sequence reactance. The ZSBR placed in series with the source provides high zero-sequence impedance while the Zig-Zag transformer placed parallel with the load provides low zero- sequence impedance. Thus, the zero-sequence harmonics currents tend to flow through the Zig-Zag transformer instead of the source, and the purposes of eliminating harmonic is gained. In this paper, an analysis is carried out; simulations and laboratory tests are used to evaluate the performance of the Zig- Zag transformer and ZSBR under ideal and non-ideal power conditions. The simulation and laboratory test results indicate that the combination of ZSBR and Zig-Zag transformer as filter is a better and effective way to attenuate the neutral current, which also provides an innovational way to improve power quality.