A high-speed multi-reclosing system is adopted in most EHV trunk transmission overhead lines. The dead time after the clearance of an unbalanced fault is about 1 second in 500kV systems, which is generally decided from the extinction time for secondary arc and the transient stability.
The power flow of trunk transmission lines will be heavier in the future. It is quite possible that the power system goes to step-out during the dead time after the clearance of an unbalanced fault.
The impedance seen by a step-out relay whose inputs are introduced from sound phases is analyzed by the method of symmetrical coordinates. According to the results, the electrical center whose definition in this paper is that the real part of the impedance is zero can be classified using the condition of the same phases in the other circuit. The results are verified by use of the dynamic stability simulation program. (Y-Method).
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