Nonlinear Control to Improve Grid Integration of Grid-Forming Converters

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 the output voltage and outer loops that calculate 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 results in a decoupled linear system. This approach avoids small-signal linearization around an operating point, achieving high-performance control over a wide operating range. To ensure a 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 characteristics such as electromechanical oscillation damping and critical clearing time. The effectiveness of the strategy is validated using the IEEE 39-bus and 118-bus test systems. Performance is compared with existing control schemes through eigenvalue and transient-response analyses, phase portraits, and the integral of absolute error (IAE) metric.