Frequency tracking of digital resonant filters for control of power converters connected to public distribution systems

Resonant filters implemented in the control of grid-connected power converters should adapt their coefficients in real time using a grid frequency estimation, for example, obtained by a phase-locked loop (PLL). Otherwise, perfect reference tracking and total disturbance rejection of ac signals are not achieved. An optimised implementation of resonant filters, which avoids shortcomings such as inaccurate pole placement and excessive resource consumption, is provided in this study. Normal grid frequency ranges of operation according to the Standard EN 50160 have been taken into account to optimise the trade-off between accuracy and resources. A shunt active power filter prototype (1 kVA rated) with frequency adaptive proportional + resonant current controller has been built to obtain the key figures. Main experimental tests prove that zero error in steady-state tracking of ac current references is achieved, even in the presence of significant frequency deviations (up to +/- 2 Hz). The influence of PLL estimations quality on the current controller is also experimentally verified (in practice, grid frequency estimations are not constant, so adaptive resonant filters are non-linear). More specifically, the sensitivity to ripple in estimations has been tested and its acceptable level is established. Finally, robustness in the presence of transients such as load changes, frequency steps and grid faults is proved.