Evaluating variable-speed operation for improved hydropower plant operation

Abstract

The accelerating transition toward renewable energy demands innovative solutions for grid stability and reliability. This work investigates partial and total conversion of existing large hydropower plants to VSO as a means to increase operational flexibility, reduce start–stop cycles, improve efficiency and enhance energy production. Among available technologies, the MMC stands out for its modular, scalable design and suitability for high voltage and power ratings, enabling VSO retrofits without replacing existing synchronous machines. A general methodology is proposed that combines universal hill charts, similarity laws and a power-redispatch algorithm to exploit the extended H–Q–N operating domain under VSO. Simulations using historical operating data compare FSO and VSO for several partial- and full-conversion scenarios. The results show a consistent reduction in machine starts and stops, from about 19% with only three units converted to VSO up to 32% for full-plant conversion. At the same time, VSO combined with optimized dispatch systematically increases annual energy production: for the representative hydrological year, the gain is of the order of 2.5% relative to the historical fixed-speed baseline, whereas optimization alone (without VSO) yields negligible additional energy. The optimized redispatch further indicates that most of these flexibility and energy-gain benefits can be captured with partial conversion, outlining a more cost-effective modernization path than converting all units. In a second step, the same framework is extended to cavitation mitigation using a σ–Ns criterion and VSO-based adjustment strategies in discharge and speed. For a modest power-tolerance band that can be compensated by other units, up to approximately 75% of initially cavitating operating hours are transferred to a safe region with limited deviation from the original dispatch. Overall, the study shows that VSO conversion of existing hydropower plants is technically feasible and highly beneficial, offering an integrated strategy to reduce start–stop cycles, increase energy production and firm energy, and mitigate cavitation, thereby supporting the reliable integration of high shares of variable renewable generation.