Improved Capacitor Current Feedback Active Damping Strategy for Enhancing the Robustness of LCL Grid Connected Inverters

When the LCL-type inverter adopts digital control, the traditional active damping which is based on capacitor current feedback will be equal to a frequency dependent impedance, and in the frequency band greater than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1 / 6$</tex> of the sampling frequency, this impedance will exhibit negative damping characteristics. Under weak power grid conditions, the time-varying impedance of the power grid over a wide range will cause changes in the LCL resonance frequency. If the resonance frequency of LCL filter is greater than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1 / 6$</tex> of the sampling frequency, robustness will be seriously threatened. In response to the above situation, an improved active damping which is based on capacitor current feedback is proposed. A phase lead compensation link is added to the capacitor current feedback branch to reduce the impact of phase lag caused by control delay, thereby improving the effective damping region of active damping. The improved system is analyzed in the dispersion domain. Finally, simulation verification shows that this strategy enables LCL-type inverters to have great robustness in weak grids. Compared with existing compensation strategies, this strategy introduces less highfrequency noise which is more conducive to the control of LCLtype inverters in weak power grids.