Grid Forming Converter Angular Speed Freezing to Enhance Transient Stability in 100% Grid Forming and Mixed Power Systems

The transient stability of a power system depends on the relative divergence of angular speeds between voltage sources during a fault disturbance. A unified angular speed control strategy is proposed here for grid-forming and synchronous machine voltage sources. Specifically, for a system consisting only of grid-forming converters (GFs), the virtual angular speed of all GFs is proposed to be frozen to the pre-fault value, while for a system with a mix of grid-forming converters and synchronous machines (SMs), the virtual angular speed of the GFs should be frozen to the weighted mean of the SM rotor speeds. The proposed freezing techniques are designed to be effective for short-circuit faults, and to be indifferent to phase-jump events. Simulation studies are performed on 100% GF and mixed GF/SM systems for both 3-phase fault and phase-jump events, for a range of GF control settings and system conditions, to demonstrate the effectiveness of the virtual angular speed freezing techniques.