  Unified virtual oscillator controller (uVOC) inherits the rigorous analytical
foundation offered by oscillator based grid-forming (GFM) controllers and
enables fast over-current 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, the overall FRT method for uVOC is presented and
a comprehensive modeling and analysis method for transient stability is
developed under both current-saturated and unsaturated symmetrical AC faults.
We utilize 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.
