Active harmonic power filter apparatus and method

Abstract

Described is an active filter (10) for removing harmonic currents from a neutral wire (24) to reduce the current In in the neutral wire. The filter (10) typically is located away from the source (26) and near the load (28). The filter (10) comprises generally a multi-phase transformer (12), a neutral current monitoring device (14), and a harmonic current injection device (16). The components of the present invention operate to draw a corrective harmonic current (IN) from the neutral wire (24) at a tap point (T) near the load (28) and inject the corrective harmonic current (IN) through the multi-phase transformer (12) into the phase wires (20). The corrective harmonic current (IN) substantially cancels the harmonic current (In) in the line on the source side of the tap point (T) and therefore eliminates over-current conditions in the neutral wire. The device preferably includes a rectifier (36) connected by the secondary windings (34) of the transformer (12) to power a DC bus (38). An inverter (50), powered by the DC bus (38), is selectively controlled to construct the corrective harmonic current. Embodiments employing both half-bridge and full-bridge inverters are described as are embodiments using both wye-delta and zigzag transformers. The present invention also includes a method consistent with the operation of the filter.