Coordinated control strategy for load variations in hydropower plants cascaded by regulating reservoirs

Hydropower plants cascaded by regulating reservoirs (HPCR) are effective for utilizing ultra-high head hydropower resources. The operation of an HPCR requires precise discharge coordination between upstream and downstream plants to maintain system stability and prevent fluctuations in reservoirs levels, which could otherwise disrupt the consistent output of the turbines. In this context, the present study proposes a coordinated control strategy to manage load variations while ensuring discharge coordination. This strategy addresses both scheduled load adjustments and unscheduled small load disturbances. For scheduled load adjustments from the power grid, this study presents a load distribution strategy that allocates the total load across all units in the HPCR, providing the target guide vane opening degree that ensures discharge coordination. For unscheduled small load disturbances, state-space models were developed to represent the HPCR's behavior under isolated and interconnected operations. The stability and dynamic response were analyzed. Additionally, a novel proportional-integral-derivative (PID) governor control strategy was introduced that incorporated discharge deviation between upstream and downstream plants. Simulation results demonstrate that, under scheduled load adjustment conditions, the proposed method reduces discharge deviation by approximately 3.8% of the rated flow compared with the conventional proportional distribution mode. Furthermore, under unscheduled small load disturbances, the attenuation ratio of oscillations is reduced by up to 15.1%, indicating significantly enhanced damping performance and faster decay of dynamic responses. This approach effectively maintains system stability and regulation quality, enabling the HPCR to achieve rapid and coordinated stabilization in response to unscheduled small load disturbances.