Transient Stability Assessment for Current-Constrained and Current-Unconstrained Fault Ride Through in Virtual Oscillator-Controlled Converters

Unified virtual oscillator controller (uVOC) inherits the rigorous analytical foundation offered by oscillator-based grid-forming (GFM) controllers and enables fast overcurrent limiting and fault ride through (FRT). Control design for effective FRT requires transient stability analysis. Existing transient stability analysis methods and studies are limited in either considering only current-unconstrained scenarios or neglecting the simultaneous power-angle and voltage dynamics. Under current-constrained faults, the voltage and power-angle dynamics are strongly coupled and both play critical roles in determining transient stability. Therefore, decoupled analysis of the two, typically used in transient stability studies, does not offer comprehensive insight into the system dynamics. In this work, a comprehensive modeling and analysis method for transient stability in uVOC-based converters is developed under both current-saturated and current-unsaturated symmetrical ac faults. We utilize a phase-plane analysis of the overall system in a single graphical representation to obtain holistic insights into the coupled voltage and power-angle dynamics. The FRT controller and the analysis method have been validated through simulations and hardware experiments. The results demonstrate that uVOC is not constrained by a critical clearing angle unlike droop and virtual synchronous machine (VSM)-type second-order controllers.