Active Power Filtering or Active Harmonic Compensation
An Active Harmonic Compensator (or Active Filter) has been developed by the University of Canterbury and Metalect Industries (NZ) Ltd . An active filter removes harmonics from the supply current by injecting the opposite harmonics that are produced by the load. An example of the active filter system is shown in the next graphic.
One of the Active Filters has been installed in the electrical sub of the Electrical Engineering building at the University of Canterbury. This building has a large number of computer and fluorescent lights and this results in a non-sinusoidal current being draw. The figure below shows an example of the operation of the Active Filter. Before compensation the current waveform has a THD of 16% and has pronounced peaks. After compensating it with the Active Filter the current is now near being a pure sinusoid and the THD has been reduced down to 4%.
Example of Reduction of Harmonics Currents
Example of Reduction of Harmonics in Neutral Conductors
Since the load current contains PC and fluorescent lighting these loads generate triplen harmonic components that add together in the neutral conductor. This can overload the neutral conductors. The Active Filter removes these triplen harmonic and therefore reduce the level of current in the neutral. The following figures show the time waveform and frequency spectrum of the neutral current before and after active filtering. It is easily seen that the level of third harmonic is reduced significantly.
Time Domain Waveforms and Frequency Spectrum
