Virtual Circuit Control for Active Damping of LCL resonance in Grid-Connected Voltage Source Converters

This paper presents a virtual circuit controller (VCC), which provides a systematic structured method to stabilize the resonance of an LCL filter in a grid-connected voltage source converter. The semiconductor switches are controlled to mimic the behaviour of a virtual reference circuit. The properly-damped circuit by a resistor in parallel with a filter capacitor is focused on and selected as the reference circuit. A linear transformation matrix is introduced indicating the resemblance between the physical and virtual circuits, and is employed to map the states between both circuits. Accordingly both analog and digital control laws are systematically computed. The insight into the feedback of the capacitor current and voltage through proportional gains is clarified. While the consequence of the former is conventionally known to emulate a resistor for damping purpose, the latter is newly used to emulate an inductor for adjusting the inner-loop control bandwidth and improving the robustness. A Kalman filter is used to deal with the inherent time delay. The detailed discrete-time state-space model of the complete system is derived. The design example of the proportional current controller is demonstrated. The simulation results illustrate the effectiveness of the damping and the robustness of the system against component-value variations.