Study on control strategies of year-end drawdown level in multi-year regulation reservoir under hydro-photovoltaic bundled operation condition

The bundled operation of multiple clean energy, such as hydropower and photovoltaic, presents a promising strategy to improve integrated resource utilization efficiency and enhance power grid stability. However, under this hybrid development mode, photovoltaic generation directly utilizes hydropower transmission channel for electricity export, inevitably disrupting the long-term operational patterns of hydropower systems—especially for multi-year regulation reservoir critical to basin-scale water management. To address this challenge, this study develops a novel long-term optimization model for hydro-photovoltaic hybrid systems that explicitly accounts for inter-source transmission competition, enabling quantitative assessment of curtailment risks arising from channel capacity constraints. Furthermore, two distinct year-end drawdown level control methods are introduced to evaluate the impacts of photovoltaic integration on multi-year regulation reservoirs and derive adaptive control strategies tailored to hydro-photovoltaic bundled operation. Taking the hydro-photovoltaic hybrid system of Beipan River in Guizhou Province, China as a case, the results show that: (1) The derived energy loss function provides an effective quantitative technology for assessing photovoltaic curtailment losses driven by hydropower output dynamics, enabling temporally coupled optimization previously unconsidered in hydro-photovoltaic hybrid system modeling. (2) Hydro-photovoltaic bundled operations significantly alter reservoir management paradigms, reducing year-end drawdown levels in multi-year regulation reservoirs while constraining their power generation capacity. (3) Crucially, under bundled operation, the year-end drawdown level exhibits a strong interdependency with photovoltaic power generation, which is a phenomenon unobserved in conventional hydropower systems. (4) Our proposed dynamic drawdown control strategy outperforms conventional fixed-level approaches by better harnessing reservoir regulation flexibility, yielding a 0.78–1.44% increase in average annual power generation.