Design on digital LPF for time domain harmonic detecting algorithm

The time domain harmonic detecting algorithm is the most widely used algorithm in industrial applications. The most of research works was concentrated on interpretation of physical meaning but the digital LPF (low-pass filter) having a great effect on detecting results was almost not investigated, especially the LPF for selective harmonic was not researched in detail. In this paper, Butterworth LPF and mean filter commonly used in time domain algorithm were analyzed by mathematical methods in detail at first, and the amplitude-frequency characteristics were acquired individually. The arbitrary response time could be obtained by setting up the quantity of accumulating point in mean filter. It can be seen from theoretical analysis and simulation results that mean filter with response time of half line frequency period is the preferred LPF in selective harmonic detecting and total harmonic detecting composed of several selective harmonics. Because mean filter not only has great advantages on AC component depression and response time but also it is insensitive to grid frequency drift. A prototype 30 kVA shunt three-phase active power filter was set up for the proof of concept. The prototype system was operated at 12.8 kHz sampling frequency and was controlled by TMS320F28335 DSP. The experimental results proved the validity of the proposed scheme.