A new harmonic suppression scheme for three-phase four-wire distribution systems

Three-phase four-wire distribution systems have been widely used in commercial and industrial installations. The neutral conductor carries the zero sequence current due to the unbalanced loading among the phase conductors. However, as electronic loads increases, their rectifier front-ends produce significant harmonic current. The triplen (3^rd, 9^th, …) harmonic current produced by these loads tends to accumulate in the neutral conductor due to its zero sequence nature, thus resulting in overloading of the neutral conductor and the distribution transformer. Most of the existing harmonic mitigation schemes focus on re-directing zero sequence current harmonics back to the phase conductors. Such schemes can prevent overloading of the distribution transformer, but the neutral conductor is still burdened by the excessive current harmonics. This paper proposes a new harmonic suppression scheme for the neutral conductors of three-phase four-wire distribution systems. In the proposed scheme, an active filter is connected in series with the neutral conductor. The inverter acts to suppress harmonic current in the neutral conductor to prevent overloading. The active filter operation will not affect the fundamental component due to the unbalanced loading, which the neutral conductor is sized for. The proposed scheme can eliminate current harmonics overloading on both the neutral conductor and the distribution transformer with only one active filter installation