Heavy Loaded Very High Voltage Lines and Distance Protections

Heavy loaded lines (up to 9000 A) raise a difficulty for distance protections because the load equivalent impedance is smaller than some short-circuit impedance. Moreover, the load transfer during single-phase reclosing cycles activates the phase-ground measurement loops. An electrotechnical analysis of the risks of spurious operation or failure and a comparative study of the solutions by impedance blinders and power blinders are presented, this last method being eliminated because not compatible with loads between very great short-circuit power substations. The heavy load requests to have a slope of the zone 1 reactance reach of about 45deg. The corollary is that the single-phase faults with a resistance of about 10 ohms in the middle of the line are detected in zone 1 by none of the ends. In the same way, protections of a strongly importing substation but of low short-circuit power do not detect the single-phase faults. The load transfers during single-phase cycles are made on the parallel system if the short-circuit power of one substation is weak. The risky case is that of a resistive load with capacitive compensation shunt, which authorizes high transit while remaining within the contractual voltage limits: the corresponding phase of the parallel system sees its single-phase loop activated. The use of blinders is effective but does not allow a correct detection of the single-phase faults with a resistance of about 10 ohms. The tests carried out with a distance protection confirm these results