Observers for discrete-time current control of converters equipped with an LCL filter

This paper studies the current-type observer, the prediction-type observer, and the reduced-order observer for discrete-time state-space current control of grid converters with an LCL filter. The robustness against parameter errors and the dynamic performance obtained with these three observers are compared by means of analysis and experiments. A unified control algorithm framework is developed to compare the structures of these observers. It is found that the reduced-order observer is merely a special case of the current-type observer. With the presented design examples, where the resonant poles are damped by means of radial projection, the use of either the current-type or the reduced-order observer leads to better disturbance rejection against the grid-voltage dip and harmonics as compared to the prediction-type observer. All the observers provide equal reference-tracking performance under nominal conditions. For a large parameter error in the grid-side inductance, the prediction-type observer leads to unstable operation of converter.