Investigation of Control Loop bandwidths in the Automatic Tuning Scheme for Grid Forming Inverter

While the grid-forming control is being adopted across converter electronics in grid-connected renewable sources, the tuning of the state-of-the-art cascaded controllers may pose a challenge, especially while using the proportional-integral controllers. The design of the filter at the converter interface and the subsequent controller to navigate the electrical interactions with the connected grid and loads cannot be overemphasized. In this article, the description of the model inversion principle used to create the inner current and voltage controllers in voltage source converters is presented systematically in conjunction with LCL filter dynamics, highlighting the respective current and voltage loops, which are the benchmark of cascaded controllers. Subsequently, the tuning of parameters of the current and voltage loops is investigated for stable operation modes, which are narrowed down to the current loop time constant and voltage loop time constant. Their variation over a range of values as used in literature is investigated using eigenvalue analysis to graphically reveal stable and unstable instances. Stability analysis and simulation results are provided to verify the model in MATLAB Simulink.