Abstract: The efficiency of water energy utilization of a conventional fixed-speed small hydropower unit is often very low when its operation deviates from the design conditions. However, by examining the comprehensive characteristic curves of the turbine, a law that for a unit under the optimal guide vane opening and a certain optimal unit speed achieves the highest efficiency of water energy utilization has been found. And, a variable-speed control strategy was proposed to achieve such an efficiency, particularly for the condition of significant water head variations. This paper presents a mathematical model for simulations of a variable-speed small hydropower unit, incorporating the operation of the turbine, governor, generator and converter. A closed-loop variable-speed control system based on this new strategy was designed. Numerical simulation results indicate that compared to the fixed-speed operation scheme, the new scheme achieves a 2.39% increase in power output at the water head of 28 m. Field tests at the Zangtanqiao hydropower station verify that the small hydropower unit is successful in operating under its optimal guide vane opening and optimal rotational speed, and that the new control strategy improves power generation efficiency across high, medium, and low head conditions. The new strategy is easy to apply in the industry of small hydropower and of great significance to the real hydropower projects.

Key words: small hydropower, variable speed operation, optimal guide vane opening, optimal unit speed, optimal rotational speed