A Capacitor-Current-Feedback Positive Active Damping Control Strategy for <i>LCL</i>-Type Grid-Connected Inverter to Achieve High Robustness
Shaojie Li, Hua Lin
Abstract
To guarantee system stability, proportional capacitor-current-feedback (CCF) active damping control has been widely used in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -type grid-connected inverter. However, when digital control is adopted in a grid-side current-controlled inverter, proportional CCF active damping will show negative damping characteristic in frequency band higher than one-sixth of the sampling frequency ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> /6), causing poor robustness in weak grid. In this article, an unstable second-order phase lead compensator is proposed to insert into the CCF path for eliminating the negative effect resulting from the digital control delay. In doing so, the positive damping region is extended to almost Nyquist frequency ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> /2), which achieves high robustness against grid impedance variation. To ensure system stability, the discrete rule is investigated for the proposed compensator and system performance is analyzed when the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> resonance frequency changes in the range of (0, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> /2) in discrete domain, a particularly simple and practical design is further presented for the proposed compensator and the current regulator. Finally, the effectiveness of the proposed strategy and parameters design is verified by the experimental results.