| In actual system, usually take under frequency load shedding as the last defense line for guaranteeing the safety and stability of power network. The traditional scheme mostly using the analysis to single machine driving concentration load model, but ignore the time and space characteristics embodied by the frequency dynamic characteristics of power system, can not reflect the changing process of the frequency, reducing the accuracy of the UFLS scheme. So, considering the frequency dynamic characteristics is important for under frequency load shedding optimization.In this paper, mathematical description is made for frequency dynamic characteristics of power system, the mathematical models of the generator、speed control system and comprehensive load model were built, it is deduced to laid a theoretical foundation for the study on frequency dynamic characteristics of power system.Based on the research method of frequency dynamic process of multiple machines system, binding the traj ectory sensitivity theory, an analysis method of frequency dynamic process was proposed in this paper, which based on speed model and parameter fitting, it matched the simulation curve and measured curve, which reflect the frequency dynamic process more exactly.According to the UFLS scheme can not ensure the minimum load shedding when the system lose different power, considering the dynamic characteristics of the frequency, this paper use the UFLS methods based on the minimum load shedding, also put forward the indicators which evaluate the UFLS scheme.In island state, the sensitivity of power system frequency is higher, and it is more difficult to control. This paper consider the comprehensive constraint between speed governing system and frequency characteristics of load, and put forward the frequency control methods when the system in island state, the optimization results proved that this method had good practicality, but the traditional UFLS scheme is not suitable for the island system. |
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