Observer based active damping of LCL resonance in grid connected voltage source converters

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