Frequency Adaptive Digital Filter Implementation of Proportional-Resonant Controller for Inverter Applications

The Proportional-Resonant control is used in control schemes that directly act on AC variables avoiding synchronous frame transformation. In this paper, the PR controller is realized using an IIR digital filter and implemented using direct form II transposed structure. It has a simpler discrete time implementation, however, the disadvantage of being difficult to make them frequency adaptive, i.e. change the filter's resonant frequency. Simple closed loop expressions are derived for the coefficients in terms of resonant frequency, sampling time and damping factor. Based on this, a frequency adaptive proportional resonant controller is proposed. It is shown that by modifying a single coefficient in the IIR realization, the controller adapts its resonant frequency to grid frequency fluctuation resulting in a simple and efficient frequency-adaptive proportional-resonant controller.