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15～18年和7～10年。 A dynamic planning model for single-unit pump variable speed optimal operation was developed in which peak-valley electricity prices and variable water levels in down-stream were considered. In this model, the minimum electricity consumption cost in one-day operation was the objective function. The speed of pump was a decision variable and specified pumping water volume was the constraint. A comparison between optimized variable speed and fixed speed operations of a single-unit in Jiangdu No. 4 Pumping Station was made. The results show that within the head amplitude and with full-load operation, regardless of considering peak-valley electricity prices, the benefits of variable speed operation was not enough to compensate for energy losses of the VFD and considering the peak-valley electricity prices and part-load operation, the benefit was excellent while without considering peak-valley electricity prices and single-unit operates at 80% and 60% loads, the saving 15 to 18 years and 7 to 10 years can compensate respectively for the VFD investments.