Demystification of Active Damping Design for Three Phase LCL Filters

Passive filters are inevitable for power electronic converters with grid coupling. In this context LCL filter topologies are a recognized cost and space reducing alternative to single coil solutions for grid current harmonic attenuation. Since this type of filter represents a third order plant, a simple PI controller can cause instabilities once applied to control the grid current of such filter. A common approach is the addition of a so called active damping to the PI-controller. Usually specific state variables are fed back via additional filters to overcome the oscillation problem. However, straight forward solutions regarding the parameter design of these filters can hardly be found. Within this article an extensive stability analysis is presented based on the Hurwitz criteria as applied to a simplified continuous plant model. As a result not only clear guide lines can be given to identify impending instabilities. Furthermore, an easy to implement active damping solution can be accurately designed to ensure robust operation.