Freezing Grid-Forming Converter Virtual Angular Speed to Enhance Transient Stability Under Current Reference Limiting

Grid-forming voltage source converters have been envisioned to play a major role in future power systems, but they have limited overcurrent capability. Therefore, it is important to investigate their ability to ride through faults and maintain system stability under strict overcurrent limits. Different current priority limiting controls based on conditional anti-windup integration are formulated, and transient stability is analyzed using voltage phasor diagrams. Subsequently, a practical solution is proposed which enhances transient stability, by freezing the virtual angular speed to the pre-fault value during faults, once current saturation occurs. Additionally, temporarily freezing the post-fault angular speed to a value slightly less than pre-fault value ensures recovery from the saturated current state. Simulation results, based on a modified IEEE 39-bus system validate the efficacy of the proposed solution.