"Supplementary Material for the Paper \u201cNonlinear Control to Improve Grid Integration of Grid-Forming Converters\u201d"

"This report provides supplementary material and additional information to complement the data in the paper \u201cNonlinear Control to Improve Grid Integration of Grid-Forming Converters\u201d.Abstract: This paper presents a nonlinear control strategy for grid-forming converters (GFCs) that facilitates the integration of converter-interfaced systems into the grid. For large-scale implementation, GFC control must ensure that the converter remains connected during disturbances and support grid stability. GFCs have inner loops that control output voltage and outer loops that calculate voltage references to provide grid services, such as frequency regulation, inertia emulation, and power oscillation damping. Designing the outer control loops is challenging due to nonlinearities in the power transfer equations. To solve this problem, this paper introduces a nonlinear transformation based on exact feedback linearization, which cancels nonlinearities and yields a decoupled linear system. This approach achieves high-performance control over a wide operating range and eliminates the need for small-signal linearization around an operating point. To ensure reliable grid connection, GFCs must meet requirements such as fault ride-through, damping provision, and current limitation during faults. The control strategy presented here addresses these issues and improves GFC performance in multi-machine, multi-converter systems. The effectiveness of the proposed control strategy is validated on the IEEE 39-bus and 118-bus test systems. Performance is compared with existing control schemes using eigenvalue and transient-response analyses, phase portraits, and the integral of absolute error (IAE) metric."