Observer-Based Active Damping of LCL Resonance in Grid-Connected Voltage Source Converters

Voltage source converters (VSCs) with LCL filters are attractive in grid-connected applications because they greatly reduce the pollution of the utility voltage due to switching harmonics. However, LCL networks have very low impedance at frequencies close to the resonance, and therefore, even small voltage magnitudes at those frequencies can excite currents of large magnitudes or even cause instability. In this paper, a novel current controller structure is proposed that uses a Luenberger observer as a sensor replacement and state predictor to predict the filter capacitor current which is passed through a virtual resistor to achieve a damping effect. The predicted filter capacitor current through a virtual resistor is the inner fast loop, while the outer loop containing the appropriate feedforward terms ensures fast command tracking and zero steady-state error. The use of the predicted states alleviates the impact of computational and sampling delays which greatly improves the active damping (AD) capability. The proposed solution eliminates the need for additional sensors. This paper presents simulation and experimental results which demonstrate AD and the effectiveness of the virtual resistance in actively stiffening the input impedance of the AFE drive at frequencies close to resonance.