Design of a neutral harmonic mitigator for three-phase four-wire distribution systems

In a three-phase four-wire distribution system, the current in the neutral conductor is expected to be within 20% of the current in the phase conductors if the loads are reasonably balanced. However, as more and more electronics loads, such as computers, uninterruptible power supplies (UPSs), etc. are being used, their single-phase rectifier front-ends draw significant zero sequence triplens (especially the 3/sup rd/) harmonics current from the line. These harmonics components accumulate in the neutral conductor due to their zero sequence nature, and result in overloading of the conductor and the distribution transformer of the three-phase four-wire system. Most of the existing harmonic mitigation schemes for three-phase four-wire systems focus on re-directing zero sequence current harmonics from the neutral conductor back to the phase conductors. However, the portion of the neutral conductor which lies between the loads and the active filter is still burdened by the excessive current harmonics. In this paper, a series active filter system for three-phase four-wire distribution systems is proposed. An active filter inverter is placed in series connection with the neutral conductor to suppress the excessive current harmonics, thus prevent the overloading problem of the neutral conductor and the distribution transformer. Detailed analysis of the proposed system is presented, and laboratory test results are provided to validate the filtering performance of the proposed system.