Capacity planning for large-scale wind-photovoltaic-pumped hydro storage energy bases based on ultra-high voltage direct current power transmission

To address the mismatch between renewable energy resources and load centers in China, this study proposes a two-layer capacity planning model for large-scale wind-photovoltaic-pumped hydro storage energy bases integrated with ultra-high-voltage direct current transmission lines. The model introduces a multi-mode operational framework, enhancing its adaptability to diverse regional conditions and operational scenarios. Additionally, it explicitly incorporates ultra-high-voltage direct current operational constraints, ensuring realistic and robust transmission planning. The outer-layer focuses on capacity optimization, while the inner-layer employs an 8760-h time-series simulation to comprehensively evaluate operational performance under varying conditions, offering a practical and generalizable solution for renewable energy base planning. The case study shows that: (1) Integrated operation of wind and photovoltaic power with pumped hydro storage enhances transmission stability and efficiency, achieving a power supply guarantee rate over 90 % and curtailment rate below 15 %. (2) Under free transmission mode, the transmission curve is smooth and stable, with power supply guarantee rate surpassing 99 % and curtailment rate under 4 %. (3) Due to temporal mismatches between renewable generation and load demand in Northwest China, the agreed transmission curve mode reveals that pumped hydro storage alone is insufficient to meet external transmission needs, requiring gas turbine units for collaborative regulation.