Research On Power System Frequency Stability And Automatic Load Shedding

Frequency is one of the most important operational parameters of power system. Only with the rated frequency can the electrical devices work efficiently and safely. A lot of research work has been done on how to keep the system frequency stable after a certain disturbance. The under-frequency load shedding is capable of dealing with some regular disturbances, but it may not be able to act fast and correctly enough to prevent some emergency disturbance. Automatic load shedding can act much faster. It's a good support to power system stability control.Wide-area measurement system(WAMS) is developing rapidly recently. It's capable of providing the dynamic track of the large-scale interconnected power system. WAMS is now being used within state estimation and failure analysis. It can be predicted that, predictive frequency estimation after a certain disturbance and automatic load shedding through WAMS are coming soon. With WAMS and its stability control, the system can be prevented from frequency collapse and widespread outage by performing automatic load shedding control after a large disturbance.The present status of power system frequency stability analysis and automatic load shedding research is summarized in this paper. A new kind of predictive algorithm of frequency after a disturbance has been proposed in the basis of direct method for frequency stability analysis of power system. This algorithm uses rectangular coordinates to build the incremental equations and calculate the steady-rate frequency with the data provided by WAMS. Because of the structural characteristic of powerflow equations under rectangular coordinates, the incremental equations are accurate without any approximation and the calculation is more precise. The predictive algorithm of frequency and its corresponding algorithm of automatic load shedding are achieved with Matlab. And by doing research and test on WSCC9-bus system and IEEE's 50-generator modified test system with PSS/E, the algorithm is improved and tested. The results of simulation and calculation show that the proposed algorithm is accurate and valid.