An active power filter with harmonic detection method based on recursive DFT

In this paper, an active filter with a harmonic detection method based on the recursive DFT is proposed. The main features of the recursive DFT are that: (1) it gives directly the harmonic waveform to be compensated; (2) it has a fast response suitable for the tracking of time-varying harmonics; (3) it can be used to detect only specific or all harmonic components with high accuracy; and (4) it requires only a few calculations and can be implemented in real-time. As for a balanced three-phase three-wire system, a new recursive DFT which uses only 1/6 of the data points of the usual DFT is also developed to reduce the response time further. An instability or loss of detectability phenomena in the implementation of the recursive DFT, caused by the coefficient quantization, is pointed out and a remedy is also given. The good filtering and tracking characteristics of the proposed active filter are verified by experiments.