Research On Differential Protection Based On Super-imposed Fault Component For T-type Transmission Line

With the construction and development of large power system, T-type transmission lines are widely used in extra-high voltage power system in China. Compared with two-terminal transmission lines, there are many different characteristics in T-type transmission lines. Because load current may flow out in the case of internal fault and CT is apt to saturate in the case of external fault in T-type transmission line, there are some unavoidable defections for the principles of two-terminal transmission line differential protection when it is used in T-type transmission line. Therefore, it is important to research on differential protection of T-type transmission line.Conventional current differential protection use the percentage restraint principle, it can improve the sensitivity while ensure the security of differential protection with the set of brake. However, the traditional percentage restraint criteria have a series of inherent disadvantages, in which the main shortcomings is distribution by the long-term impact of a large capacitive current, the failure caused by the load currents and transition resistance in internal fault, be apt to fail when the transformer saturates in external fault, and so on. In addition, its braking coefficient's selection is also a contradiction, it will lower the sensitivity when the value is too big and will affect the reliability when the value is too small.According to the disadvantages of conventional differential protection of T-type transmission line, the paper proposes a novel principle of differential protection which is based on fault component. Current phase differential phase is introduced into realization of current differential protection. Restraint unit of the proposed principle takes the role of drive and restraint respectively in the case of internal and external faults. Therefore, the sensitivity and reliability of differential protection are improved simultaneously. It can meet the demand of the sensitivity for internal fault and security for external fault at the same time. The coefficient of braking can be chosen infinitely. By the detail discusses of polar coordinates systems and rectangular coordinates, it is proved that the new theory also has high sensitivity and reliability. Meanwhile, the compare with the conventional percentage restraint principle has proved that the new criteria have a good performance when the transformer saturates and when the transition resistance have a big value. In order to further enhance the performance of protection, the paper also proposed a comprehensive protection program and proved that it has good performance. Finally, the paper has lots of simulation tests using EMTDC/ PSCAD and MATLAB, the simulation results verify that the new criterion and the comprehensive protection program have good action performance.