Systematic design method for PI controller with Virtual Resistor-based Active Damping of LCL filter

Abstract

The Virtual Resistor based Active Damping (VR-AD) is widely employed in converters connected to the grid via LCL filters in order to mitigate the inherent resonance of the filters. Nevertheless, in digitally controlled systems, the PWM and the calculating delays modify the system characteristics in terms of frequency and phase, thus destabilizing the system and degrading the VR-AD performances, mainly in low switching frequencies. Moreover, the stability of the system is greatly affected under weak grid operation characterized by large grid impedance variation. This paper solves these problems by proposing a systematic, robust and optimized design procedure of voltage oriented PI control (VOC) with VR-AD. The considered design procedure ensures robust control (sufficient stability margins) and high quality of grid current (reduced steady-state error and minimized THD value) despite the negative impact of digital time delay, grid impedance variation and filter parameters change. Simulation and experimental results are presented to show robustness and efficiency of the suggested design procedure. Keywords: Digital controllers, LCL filter, Time delay, Grid-connected converters, Virtual Resistor, Real Resistor, VOC, Grid impedance variation, Controllers design