Discrete-time Direct Pole Placement for Stability Enhancement of LCL-Filtered Inverters in the Synchronous-Reference Frame

For LCL-filtered grid-tied inverters, direct pole-placement techniques can combine the fundamental-frequency current regulation and resonance damping simultaneously. As known, for state-space-based direct pole-placement, to purse sensorless operations, state observers have to be employed, which means heavy computation burden and might limit the switching and sampling frequencies. While for transfer-function-based one, the number of necessary variables decreases and meanwhile it saves the computation load from observers. Therefore, the latter is more suitable in practice. Based on this, this paper extends and summarizes this approach for better accessibility, and it shows that, compared to previous applications in the stationary frame, the transfer-function-based discrete-time domain (z-domain) direct pole placement in the synchronous frame presents better robustness against grid impedance variations, improved phase and gain margins, and simplified feed-forward compensation, presuming the cross coupling is ignored. Simulations have verified the analysis and design.