Closed-loop control of power system transient stability (1): transient instability detection principle of simple power system

With the development of WAMS (Wide Area Measurement System), the trajectory of generator can be obtained in real time, which makes the closed-loop control of transient stability possible. The direction field near the inflection point curve on the angular velocity-power angle phase plane is researched and it is found and verified that, the system is transiently unstable when the trajectory of ideal generator enters the convex area from the concave area. An instability criterion simply expressed by the slope of phase trajectory and calculated in real time is proposed, which is used to initiate the stability control. A single-machine infinite-bus system is built in PSASP and the simulative results show that, the proposed instability criterion is fast and accurate, suitable for different system parameters, fault types and complicated multiple disturbances.