Improved power converter line synchronisation using an adaptive Discrete Fourier Transform (DFT)

The problem of line synchronisation for grid connected power converters where the grid is weak or comes from a remote area power supply with poor frequency governing, is now well recognised. Such supplies can suffer a significant amount of line voltage distortion due to notches caused by power device commutation and/or low frequency harmonic content. This distortion can easily corrupt the output of a conventional zero crossing detector. In this paper a method of filtering the incoming supply voltage using a recursive discrete Fourier transform (DFT) is presented. This filter shows a high degree of noise immunity but produces a phase shift between the incoming supply voltage and filtered output voltage when the time window of the DFT does not match the system period. Two methods of compensating for this phase shift are presented based on tracking the drift in the phase predicted by the recursive DFT. The first method makes a deadbeat adjustment to the time window (thereby changing the sampling rate) while the second approach calculates the phase error based on the linear phase response of the DFT. These compensation algorithms can correct for discrepancies of at least 25% between the DFT time window and the system period, and can track grid frequencies with slew rates as high as 40 Hz/sec with negligible phase shift between the line voltage input and the filtered output waveforms.