Stability of Single-Loop Voltage Controllers for Grid-Forming with Optimized LLCL Filter

Future more-electronic grids, with growing penetration of controlled switched-mode power supplies, are expected to need grid-forming inverters and related control techniques in renewable energy sources and energy storage systems. LC filters are typically used to connect inverters and the grid with a well-defined current. The LC resonance frequency is typically selected at a value more than one third of the sampling frequency to stabilize the system and prevent oscillations. The effective filter resonance frequency shifts during operation due to the grid impedance, which varies from site to site and can even change over time. Such resonance-frequency shifts might lead the system into unstable operation regions. Conversely, selecting high-resonance LC filters cannot comply with grid codes in suppressing switching distortion. This paper introduces an algorithm for tuning a single-loop voltage controller that employs negative proportional gain for a higher-order alternative LLCL filter with low resonance frequency to drastically attenuate switching distortion. Moreover, the proposed method offers better stability and robustness for weak grids, which are the most important application of grid forming. We analyze the stability of the proposed controller and, finally, verify the robust and stable operation of it in detail with varying grid impedance by simulation and experimental results.