During the calculation of hydraulic transients in pumped storage plants, the speed change of reversible pump-turbine has great relation with the pressure of water conveyance system, and results in the minimum pressure at draft tube does not occur while all sets in the same hydraulic unit reject load simultaneously, but occurs in the combined conditions of load successive rejection. Based on the hydraulic characteristic of reversible pump-turbine, the reason why the minimum pressure at draft tube occur in the combined conditions and the dangerous interval time are analyzed bonding with the water conveyance system layout of pumped storage plants and the practical operation of pump-turbine. The research indicate that when reversible turbines in the same hydraulic unit reject load successively, the maximum pressure at spiral case maybe lower to some extent compared with load rejections simultaneously, but the change of minimum pressure drop at draft tube is great along with different interval time, which endangers the water conveyance system badly, especially in high head pumped storage plants, and should be pay great attention to it. Combining the practical operation of pump-turbine with calculation in theory, it would be the dangerous interval time of load successive rejection of pump-turbines after the first pump-turbine rejects load and its runaway speed achieve speed peak, the pressure of draft tube would drop the minimum once the other set in the same hydraulic unit also reject load at the moment.

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