A Simple and Effective Active Damping Design for Three Phase LCL Filters

Power converter circuits with grid interaction are gaining increasing attention especially with emerging renewable energy technologies. Passive filters are inevitable for power electronic devices with grid coupling. In this context LCL filter topologies are a recognized cost and space reducing alternative to simple single coil solutions for grid current harmonic attenuation. Since this type of filter represents a third order plant, a simple PI controller would cause instabilities once applied to control the grid current of the undamped filter. A common approach is the addition of a so called active damping to the PI-controller. Documented solutions refer to the feedback of specific state variables and/or implementation of additional filters. The corresponding parameter determination turns out to be generally quite complicated as it often deals with several variables. This article provides practicing engineers with a simple and effective recipe to determine just one active damping parameter for PI current controllers applied to three phase power converters with grid connection via LCL filters. A simple and generally valid formula is derived via the Hurwitz stability criterion that determines a safe operating area of this single parameter.