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Selective harmonic cancellation has become of primary importance in a wide range of power electronics applications, for example, uninterrupted power systems, regenerative converters, and active power filters (APFs). In such applications, the primary objectives are an accurate cancellation of selected harmonics and a quick speed of response under transients. This paper provides a novel signal-processing algorithm for selective harmonic identification based on heterodyning, moving average finite-impulse response filters, and phase-locked loop (PLL). The algorithm is applied over the current of a nonlinear load in the feedforward-based control of an APF. The PLL tracks the phase and frequency of the fundamental component. Then, the fundamental phase is multiplied by the order of the selected harmonic, and two random unitary orthogonal "axis waves" are generated. These unitary waves, rotating at the harmonic frequency, are multiplied by the input load current, thereby "moving" the Fourier series coefficients of the selected harmonic to DC (heterodyning). Moving average FIR filters are used to filter the harmonics generated in the heterodyning process from the DC signal; moving average FIR filters are very suitable for most of the power quality applications, thanks to their "comb-type" frequency response and their quick transient response. Experimental results confirm good performance for steady-state harmonic cancellation and an optimal system response to load transients. The theory of the algorithm has been developed for single- and three-phase systems.
Date of Publication: Aug. 2009