Complex state-space current controller for grid-connected converters with an LCL filter

Grid-connected voltage source converters using an LCL filter offer reduced harmonics in the grid current. The system modeling results in a multivariable system with cross coupling. Additionally, the system contains three state-variables instead of the one occurring in the case of grid connection using only one inductance. All the three variables must be measured or estimated. Also, the LCL filter has a resonant frequency that can cause unstable behavior. This variable resonant frequency is usually considered using a damping resistor or active damping. The main purpose of the paper is to present a methodology to design a current controller based on the concept of a controller in the complex domain. The complex controller is obtained using the pole placement method and it uses a state observer in order to only measure the grid current as state variable. The simulation results presented clearly demonstrate the validity of the design approach.