Transient torsional vibration induced on modern large turbine and generator shafts by high-speed reclosing is investigated qualitatively and quantitatively, assuming linearity and no damping as the first approximation. The effect of repeated impositions is analyzed both for sinusoidal torque fluctuation and for stepwise torque fluctuation. It is shown that the danger of high-speed reclosing is attributable to repetitive imposition (as many as four times); furthermore, the severity of transient vibration is decisively influenced by the timing of repeated impositions. The effect of three-phase high-speed reclosing on a typical 600MW machine is calculated both by computer and by modal analysis. The large response predicted, which is far beyond the elastic limit, is possible to be induced under actually feasible timing. Absolutely safe three-phase high-speed reclosing could not be achieved by strengthening the shaft because the strengthened shaft becomes sensitive to sinusoidal torque fluctuation.

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