An AHF is installed in parallel to the harmonic generating loads. ACT first measures the current being drawn momentarily by the load. Then filter control unit analyses this current for amplitude & harmonic order. Then it feeds a compensation current into system whose amplitude and individual harmonic order is exactly equal to the current drawn by the load but which is 180 degree out of phase with it. So the harmonic currents cancel each other and the supply network only has to supply the fundamental frequency and is not contaminated with harmonics at the point of connection.