Impact of converter control strategy on low- and high-frequency resonance interactions in power-electronic dominated systems

The transient stability of power-electronic dominated systems such as wind farms and photo-voltaic power plants depends on the control settings of the involved converters, the grid resonances and the system's operating conditions. This paper focuses on investigating the impact of converter control algorithms on possible interactions that could endanger the overall stability of these kinds of systems. Mainly, two control approaches namely the conventional cascaded control structure, consisting of an inner current-control loop and various outer-loop controls, and the immediate voltage control structure which generates the converter voltage directly from the power and voltage controllers without necessarily including a current-control loop, are compared. The latter control structure falls in the class of grid-forming control strategies and the impact of different variants of this control structure will also be investigated. For this purpose, the ac-side input admittance of a grid-connected converter is derived for the two approaches and passivity-based frequency-domain stability analysis is used to assess their impact on system stability. Finally, the analytical findings are verified through time-domain simulations and recommendations to increase system stability are given accordingly.