Research On Phase Correlation Current Differential Protection Of Transmission Line

In order to achieve full line high speed, current differential protection is widely used on the transmission line of high-voltage, super-voltage systems as main protection. With the development of modern power systems, power grids is expanding, power system capacity is increasing, electric voltage level is rising. Thus transmission line operating mode, fault through resistance, and current transformer saturation and other issues raise higher requirement on the current differential protection.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.In order to solve the contradiction of the braking coefficient's selection, the paper proposes a novel principle of phase correlation current percentage differential protection which introduces current phase difference into the criterion, endows restraint unit of protection with the role of both drive and restraint respectively in the case of internal and external faults. Therefore, the sensitivity and reliability of differential protection are improved simultaneously. By the detail discusses of rectangular coordinates and polar coordinates systems, it is proved that the new theory also has high sensitivity and reliability. Meanwhile, the compare with the conventional percentage restraint principle and product-restraint principle have 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 variable-slope phase correlation criteria and proved that it has good performance with the phase correlation current differential criteria. Finally, the paper has lots of simulation tests using EMTDC/ PSCAD and MATLAB, the simulation results verify that both of the new criterions have good action performance.