Out-of-step splitting scheme based on PMUs

A splitting scheme based on PMUs for out-of-step protection system to prevent the total system collapse is proposed in this paper. The proposed scheme is composed of three main function modules, namely, identification of coherent generators, detection of out-of-step state and implementation of under frequency load shedding (UFLS). By the identification and aggregation of coherent generators and according to the active power balance, splitting spots are set rationally. The system is judged whether out-of-step using the adaptive separation composite criterion. The criterion includes the main criterion based on the changing track of the voltage of oscillation center and the auxiliary criterion which reflects the changes of three-phase currents. Then use both frequency and rate-of-change to dynamically set UFLS relays, which minimize the generation-load imbalance in each island and facilitate the restoration process. The effectiveness of the new scheme is verified by simulation results of an actual large-scale power network in China.