| Frequency is one of the most important state variables of the power system safty and stability.If the active power is deficient in the system,the system frequency will drop, severely it will lead to the system frequency snowslide. As the last defense line of the power system safty and stability ,the under frequency load shedding will shed load according to the frequency when the power deficiency is large,then the frequency after disturbance will recover to allowale range.So, it's very meaningful to prevent power system large area blackouts .The paper analysed and summarized the research status of the under frequency load shedding,pointed out the crucial issues. It analyzed the algorithm and characteristics of traditional scheme,semi-adaptive scheme and adaptive scheme and some other schemes. Using the power system simulation software PSS/E, it has established a practical power system, completed the system dynamic simulation, analysised the dynamic characteristics of the complex system frequency and it's factor. Using PSS/E to establish some different under frequency load shedding program use-defined models, and has finished some dynamic simulation in the actual system, and then analyzed the performance of these programs on the system and discussed the effectiveness of these programs the performance. It has adopted a high-order frequency response model ,which based on the Anderson's low-order frequency response model , this model adds the UFLS link closed-loop control. Based on the SFR-UFLS ( System Frequency Response-Under Frequency Load Shedding) frequency response model, it has desingned the under frequency load shedding scheme of the above actual system,and completed the control simulation. The paper has established the single machine model of the above actual system, using the MATLAB software to carry out the under-frequency load shedding program designing and optimization; Through the PSS / E software, the results show that this under frequency load shedding scheme is effective.
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