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Increased attention has been paid to the high-capacity variable-speed pumped storage to stabilize the large-scale renewable energy. Nevertheless, the optimum reference values of variable-speed units (i.e., the optimum speed and optimum gate opening) come from the actual operating data from power plants or the model pump-turbine test from manufacturers. It is generally a commercial secret, and no historical data is available for a new power plant. For general researchers, it is not easy to obtain relevant data. Therefore, this paper proposes an optimization method for the variable-speed unit and establishes a simplified steady-state model that can be used for medium and long-term power system research. First, based on the pump-turbine's performance curves, this paper presents an optimization method for the variable-speed unit in turbine and pump mode without additional tests, aiming to optimize the turbine's efficiency and track the input power of the pump. Second, the concept of &#x201C;steady operation area in non-optimum condition&#x201D; is proposed based on the non-linear characteristics of the optimum operating condition curve. Finally, simplified steady-state models of the variable-speed unit are established, and the model validation has been tested in typical scenario analysis. The results show that the variable-speed unit's steady-state operating characteristic can be described by this simplified linear model, based on the power, the head, and the optimum speed (or the gate opening). This simplified model's percentage error is less than 5%, which meets engineering accuracy requirements. The error mainly comes from the linear correlation coefficient of the optimum operating condition curve and the operating head's deviation from the design head. This model is available for power system planning and dispatch.

Optimization Method and Reduced-Order Steady-State Model for Variable-Speed Pump-Turbine Unit