Recent surveys of 208 / 120 V three-phase four-wire electric systems, buildings, and industrial plants with computers and nonlinear loads show excessive currents in the neutral. These neutral currents are fundamentally third harmonic, and their presence is tied to wiring failures, elevating of neutral potentials, transformer overheating, etc.In response to these concerns, this paper proposes a new active power filter scheme to cancel neutral current harmonics.
In this paper a new approach to improve power factor and reduce harmonics generated by a three phase diode rectifier is proposed. The proposed approach is passive and consists of a novel interconnection of a staddelta transformer between the ac and dc sides of the diode rectifier topology (see Fig. 1). This novel interconnection is shown to generate a circulating third harmonic current between the ac and dc side of the rectifier bridge. The circulating third harmonic current is then shown to drastically improve the performance of the diode rectifier type interface. The resulting input current is near sinusoidal in shape with significant reduction in line current harmonics. Experimental results from a 6kW laboratory proto-type are detailed.
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