Research Of Electric Transmission Line Longitudinal Current Differential Protection

In recent years, with rapid development of China’s electric power system, the increasing EHV(extra-high voltage) and UHV(ultra-high voltage) electric transmission lines have become the largest number of power equipments in power system of our country. Operating experience shows that most of the power system faults are the faults of the transmission line. Therefore, the electric transmission line has enormous influence on the safety and stable operation of the power system. So, it is particularly important to improve the selectivity, quick action, sensitivity, reliability of the line protection.Longitudinal current differential protection has become the first selection of the main transmission line protection with its unique advantages such as absolute selectivity, high sensitivity, no mal operation in system shaking, natural selection phase, and so on. However, in practice, many inevitable factors which are bad to the safety and steady operation of the power system, such as load current and distributed capacitive current, critically lower the sensitivity of the protection. This paper is based on the problems existing in the current protection of transmission lines, significant for further improving the safety, stability and sensitivity of the China’s electric power system. The main contents are as follows:(1) On the basis of reading related works and references, the basic principles and related concepts of longitudinal differential protection are introduced. Here we have focused on the current differential protection, especially the research of analysis of the action criteria.And the further analysis and summarize of the influence factors of differential protection’s right action and the corresponding measures are described.(2) On the basis of analyzing the conventional differential protection, the variation differential protection is compared to be studied. And further research on the variation extract method is carried on. It is found that variation differential protection can effectively reduce the influence of load current, and make the current differential protection more sensitive.(3) The capacitance current compensation is studied. Based on the two kinds of compensation mode under steady and transient state, this paper puts forward the single terminal voltage compensation method suitable for limited channel capacity or TV break line, and the innovative capacitance current compensation method of T circuit under steady and transient state is introduced. Adopting the capacitive current compensation method, the unbalance current of the protection line can be lowered greatly when internal and external fault happens. Thus, action thresholds is reduced, and the sensitivity and movement speed of protection are to greatly improved.(4) The simulation about capacitive current compensation of current differential protection has been carried on by using EMTP and MATLAB tools according to Fourier algorithm.