Grid-forming hybrid angle control for power converters in low-inertia power systems

Abstract

This thesis investigates the comparative system-level performance of classic grid-forming converter control strategies in different low-inertia power system models, i.e., the IEEE 9-bus test system and Hydro-Quebec transmission grid models. The extensive electromagnetic transients simulation-based case studies highlight the positive influence of the grid-forming converters on frequency stability. Further, the behavioral differences of several state-of-the-art grid-forming control techniques are uncovered. Moreover, the interactions of grid-forming converters and synchronous machines in low-inertia power systems are explored. Thus, it is observed that the choice of converter control design i.e., a grid-forming or grid-following control concept plays a critical role in achieving high levels of converter-based generation integration. Further, the post-contingency evolution of frequency stability metrics and their effectiveness in low-inertia power systems are discussed. Subsequently, on the basis of several observations made via simulation case studies, a new grid-forming converter control strategy is designed, i.e., the hybrid angle control. A detailed nonlinear stability analysis of the proposed grid-forming control concept is presented that establishes the almost global asymptotic stability of the closed-loop converter dynamics. The almost global asymptotic stability of the gridconnected converter is proved under parametric existence and stability conditions that are solely met by an appropriate choice of control parameters. In addition, a complementary current-limiting controller is designed that is compatible with the hybrid angle control and preserves the closed-loop stability. Next, from a control-theoretic viewpoint, the application and scalability of the hybrid angle control for the interlinking converters in non-synchronous hybrid AC/DC power grids is investigated. It is observed that not only the stability guarantees of the hybrid angle control are fully scalable in hybrid AC/DC power grids, but also they do not require strong assumptions on the underlying dc interconnections. Moreover, the system-level and device-level control concept performances are respectively verified via electromagnetic transients simulation-based case studies and controller-hardware-in-the-loop simulation approach. Additionally, guidelines on the stability analysis of a two-converter system under the hybrid angle control and recommendations on designing several other multivariable grid-forming controls are presented. Finally, this thesis is concluded by presenting the summary and outlook of this doctoral research, and listing the remaining open problem in this research area.