Abstract: A hybrid active filter shunts three-phase power with passive elements tuned to have low impedance at a frequency of harmonic distortion. The passive elements are in series with controlled voltages sources providing cancelling currents to the harmonic currents. Rapid determination of the harmonic currents is provided in real-time through the use of a Park transformation reducing the variables which need to be manipulated. Measured currents as transformed are used to model undistorted current based on an average power consumed by the load. When the undistorted current is subtracted from the actual current, a harmonic current signal is produced that may be used to drive the voltage sources. The time domain method requires rapidly computed running averages and thus may be performed in-between the acquisition of the measured current samples.