Protection Research Of Transmission Line Based On Natural Frequency

The distance of EHV/UHV transmission line is long,its distributed capacitance is bigger.After a fault,the energy in distributed capacitance are exchanged between the distributed capacitance and inductance. Produce a series of high frequency component, which is composed of an infinite number of different frequency components composition, called the natural frequency component. Through theoretical analysis, after the fault the natural frequency component must exist.So it can not be ignored in the study of protection principle. The natural frequency and the transient component will impact the performance of protection. The contradiction between the reliability and rapidity of the traditional protection has arisen. Because the natural frequency component contains a lot of useful fault information, the protection research based on the natural frequency component is necessary. This paper mainly engages in the use of natural frequency characteristics and produces the protection criterion of EHV transmission line. This paper looks for a method to get the natural frequency quickly.The research achievements are as follows.The character of natural main frequency and natural secondary frequency and its impact pactors are analyzed. Based on the equivalent of the long distance transmission lines,we get the character of natural main frequency and natural secondary frequency.The impact factors of natural frequency are also discussed.It puts forward a new analysis method of transient componentIt estimates the expression of natural main frequency simply and realizes the main theoretical analysis of the transient component. After the fault, the natural main frequency makes the difference with the changing of fault point. The nearer the distance of the fault place, the higher the natural main frequency and the farther the distance of the fault place, the lower the natural main frequency. Use the hypothesis which is often-quoted in the relay protection:The impedance angles of aach part of the system are equal. This paper focuses on the size of natural main frequency following the different fault places in the frequency domain. This paper analyzes the range of the natural main frequency.The new analysis method has a better theory basis and more clear physical conception. The new method can also be used in series compensated transmission line.Protection criterion of transmission line based on natural frequency from one end of line is proposed.The spectrum character of natural frequency after fault are uesd to make protection criterion. According to analysis of internal and external fault, use frequency difference between the natural main frequency and natural Secondary frequency to distinguish between internal and external fault. The protection criterion uses only one side of the line, and ensure reliable and fast action at the exit line.It can be used as criterion of reliable operation at the exit line.Protection criterion of transmission line based on the ratio of natural frequency from two ends of line is proposed. According to analysis of internal and external fault, use the ratio difference of natural main frequency to distinguish between internal and external fault.It is not impacted by swing and so on.This paper briefly describes the application of the Prony algorithm in extracting natural frequency. Only using half of the cycle data,we can get natural frequency more accuratly. This method provides the engineering and practical foundation for the protection based on natural frequency.All protection criterion areverified by MATLAB and PSCAD simulation data.