The Research On Methods Of Improving The Speed And Accuracy Of Under Frequency Load Shedding

Power plant of modern power system has large capacity and the proportion of large-scale power plant is very big. The distance between power plant and consumer is becoming longer, which makes the electric energy and consumption of some parts of the power system out of balance seriously. Under this condition, if a large capacity power plant or extra-high voltage transmission line is cut, the power will be out of balance seriously and the power shortage of consumer district is very large. In addition, building large capacity power plant to obtain bigger economical weakens the frequency's stabilization under big disturbance and makes malignant frequency accident, leading to the collapse of the power system.This dissertation analyses static and dynamic characteristic of power system frequency, discusses the load frequency adjust modulus as the emphases point and holds the change rule of the dynamic frequency, analyzing the feasibility of estimating steady state frequency. According to the affection of the steady state frequency when active power disturbance occurring at different location, this dissertation defines the sensitivity of system frequency to the disturbance of generator's active power, which is that the same active power impulse in different location will bring different frequency disturb process.This dissertation analyses the principle of traditional UFLS(Under Frequency Load Shedding)method which is based on the absolute value of frequency and UFLS method imported the descending speed rate of frequency, and the disadvantage of the current UFLS principle is pointed out. The modern power system is a complex system, the dynamic process of the falling of system frequency when active power shortage is complex and space distributed, and the frequency variety curve is not a smooth index curve, but a curve which surge continuously, and df /dt won't change monotonously anymore, which has the prodigious difference with the assume of traditional UFLS, and it is one of the main dissatisfied reasons of the current UFLS equipment.This dissertation analyses the estimation of steady state frequency method based on the calculating of real-time dynamic power flow. Calculating the speed up power approximately based on the Jacobi matrix factor table at the time of accident, and then calculating the steady state frequency on line by the algebraic calculation.This dissertation proposes a load shedding method which is based on judging the...