The Study Of Transmission Line Differential Protection Based On Non-Periodic Current

Current differential protection is widely used in power system protection for its simple principle, high reliability. And it is a very important protection to ensuring the safety and stabilization of the power system. But there are two problems when it used in power system’s protection. The one is that the electromagnetism TA saturation may easily causes wrong-operation. The other is that the transmission line’s distributed capacitive currents to ground may be also cause wrong-operation, especially in ultra high-voltage long lines. And they will seriously threaten the safe of the power system.For the TA saturation, researchers uses a lot of methods to distinguish the TA saturation and eliminate TA saturation’effection, but these make its arithmetic and criterion more complex and lower its performance, The optical-current transformer has no saturation and it can solve this problem. For the second problem, although it adopt kinds of compensating methods, but it can not solve this problem of the traditional line fundamental current differential protection’s performance. So it need us to study the new principle of differential protection.In this paper, first of all, it introduces the optical-current transformer’s working principle and character, then analying the problems of traditional line fundamental current differential protection. Based on the optical-current transformer, which could measure the non-periodic current accurately, it introduces the transmission line differential protection based on the non-periodic current, Using Laplace transform computes the basic non-periodic currents at two ends of the high voltage transmission line. The results shown that the initial magnitude and direction of basic non-periodic currents at two ends are affected by the fault occurring moment, when the fault occurring internal, it is fault current, but in contrast it is a traversing current. So it can distinguish line external fault from internal fault via calculation.For these characteristics, ratio-differential protection and directional differential protection based on the non-periodic current are introduced. When it is used in line protection, the non-periodic current differential protection can solve the puzzle of the fundamental current differential protection’s performance due to the line distributed capacitive current. At last, construct a simulation system of a transmission line in EMTDC/PSCAD, simulate the relay’s operation and the results shown that the new principle is correct and reliable.