A Reference Frame Rematching Method for Enhancing Large-Signal Stability of Grid-Following Inverter Under Weak Grid

The classical phase-locked loop (PLL)-based vector current control scheme has been widely used in grid-following (GFL) inverter systems. However, GFL inverters with this classical control scheme tend to be unstable under severe grid fault conditions, especially in weak grid cases. This article reveals that the PLL angle's dynamic is the main reason for instability. Specifically, the control <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> frame and the system <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> frame are synchronized easily in strong grid cases where the PLL angle's dynamic is relatively small, but the two <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> frames have a risk of losing synchronization in weak grid cases where the PLL angle's dynamic is relatively large. To address this problem, this article proposes a reference frame rematching method, which can transform the original control <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> frame into a new control <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> frame that is almost identical to the system <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> frame. So, the losing synchronization issue can be addressed. A significant advantage of the proposed method is that it can make sure that an equilibrium point exists during severe grid faults. Besides, it can achieve a seamless transition between normal and grid fault cases, without the need of grid fault detection. Simulations and experiments verify the effectiveness of the proposed method.