Adaptive Optimization of Current-Control Loop for Grid-Connected Inverters

The amount of grid-connected power electronics is rapidly increasing and their effect to power quality becomes important especially in areas with high penetration levels. In weak grids with significant grid impedance, the inverter control performance usually decreases and the impedance-based stability issues may arise. A method to analyze the stability issues and to improve the inverter control performance is to consider the grid as a load effect in the small-signal model of the grid-connected inverter. Studies have shown that the bandwidth of the inverter current-control loop decreases in the weak grids through the load effect, and consequently, the controller ability to mitigate harmonics is weakened. This work introduces an adaptive method that keeps the bandwidth of the control loop constant under varying grid conditions. Additionally, an adaptive Notch filter is added into the current-control loop in order to avoid possible stability issues caused by the interactions with the LC-filter resonance and the grid impedance. Improved control performance and stability under various grid conditions are demonstrated through power hardware-in-the-loop tests with a kW-scale three-phase inverter.