New Ufls Program

With the enlargement of power system scale, power system economy has gained remarkable improvement. At the same time, the ability of resisting disturbance is weakened. This paper introduces primary characteristic of modern power system, expounds the concept of frequency and frequency deviation, and enumerates the influence of frequency deviation on every aspect in power system. Accordingly, it is indicated that frequency is an important index of power system, and frequency control is necessary to power system operation. Then, this paper explains the meaning of Under Frequency Load Shedding (UFLS), design request and development actualities. Besides, traditional method, semi-adaptive method, self-adaptive method and computer assistant arithmetic are compared.This paper analyzes frequency characteristic of power system, and points out that it is classified into static and dynamic. In the analysis of dynamic frequency characteristic, simole power system and complicated power system are considered. For simple power system, after disturbance the frequency change course is a smooth index curve, and the maximal frequency change rate happens at the instant of disturbance beginning. For complicated power system, frequency change has spatial distribution characteristic. Frequency change is not synchronous between disturbed area and non-disturbed area. It is an oscillatory course, and no longer simple index curve. By comparing complicated system with simple system in emulation experiment, this paper indicates that traditional UFLS scheme has great limitation.Therefore, this paper tries to improve the traditional UFLS scheme. Firstly, it proposes the measure that can avoid over-shedding, namely, the mode of "urgent step + basic step + reserve step". It effectively reduces three over-shedding described in this paper. Secondly, a theory that can automatically adjust the action frequency of UFLS device is applied. UFLS device can adjust setting value by maximal and minimum final value of local frequency. By emulation experiment, it effectively solves the question that frequency descends too low when large power shortagehappens again in the process of frequency ascending. Finally, a new System Load Shedding Scheme adapted to large interconnected system is proposed. It can decide the amount and the position of load-shedding by variation of the voltage and shed a certain number of loads to keep frequency steady. By emulation experiment, it excels traditional UFLS scheme obviously.According to theories hereinbefore, this paper designs a new UFLS Control System based on the mode of "area load-shedding +system load-shedding". Integral structure, information flow and principle of work are introduced in detail. In this paper, a new UFLS device based on TMS320C32 DSP is emphatically developed. The principle of work, hardware structure and software design are introduced. The device can shed load quickly according to both the frequency decaying rate and the instruction from upper device. It is beneficial to the stable and secure operation of power system.