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

Abstract

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.

Robust Active Damping in LCL-Filter-Based Medium-Voltage Parallel Grid Inverters for Wind Turbines

“…This procedure requires additional sensors which depend on the position of the virtual resistor. The utilization of observers has been proposed in [5] to avoid using extra sensors at the expense of more controller complexity and computational load. Careful design of the digital controller must be exercised because of the computational and PWM delays.…”

“…Procedures based on the state-space [9], [10], observers [5], LQR [11], [12] and Kalman filters [16] require matrix operations and cannot meet the requirement 1). Procedures based on virtual resistors [2]- [4] require an extra sensor for each virtual resistor and cannot meet the requirement 2).…”

Variable Structure Control for Three-Phase LCL-Filtered Inverters Using a Reduced Converter Model

“…However, in order to avoid these self-tuning procedures, different solutions can be found in the literature using feedback controllers [10], [11]. These controllers are usually based on a state observer where an accurate power converter model is used to estimate the state variables.…”

Systematic control design for half-bridge converters with LCL output filters through virtual circuit similarity transformations

“…Also an observer is employed to directly tackle the problematic time delay. Reference [9] uses a Luenberger observer to predict the capacitor current for the active damping. However, the feedback gain must be properly tuned, otherwise strong damping gives rise to low-order harmonic distortion on grid currents.…”