Regions of active damping control for LCL filters

The control of a grid connected voltage source inverter (VSI) with an LCL filter is a very challenging task, since the LCL network causes a resonance phenomenon near to the control stability boundary. While many active damping methods have been proposed to overcome this issue, the role that PWM transport delay plays in the effectiveness of these strategies is still not fully resolved. This paper presents a theoretical discrete time analysis frame work that identifies three distinct regions of LCL filter resonance - a high resonant frequency region where active damping isn´t required; a critical resonant frequency where a controller cannot stabilise the system; and a low resonant frequency region where active damping is essential. Suitable controllers are then proposed for the two stable regions, with gain calculations that allow for the greatest system bandwidth and damping. Simulation and experimental results verify the presented analysis.