## 8 Conclusions  
In order to improve the power generation stability of multiple strong, uncertain power sources in remote areas, this paper analyzes the complementary characteristics of cascade small hydropower and distributed wind-PV resources. Based on this analysis, a site-specific approach is adopted to select cascade small hydropower for pumped storage transformation as the energy storage method. It also proposes research on the capacity configuration of a cascade small hydropower-pumped storage-wind-PV complementary system. Through simulation, the following conclusions are drawn. 1. Contributions of this study are as follows: 1. The capacity configuration method proposed in this study maximizes the utilization of existing cascaded small hydropower stations by allocating the capacity of pumped storage units based on the total available regulation capacity of the cascade system. In addition, the installed capacities of wind and PV power are determined using a spectral analysis approach, ensuring that the fluctuation rate of their combined output remains within 10%. 2. In terms of operational stability, the proposed system achieves deviation power of 0 MWh and water spillage of 0 m\({}^{3}\) under all four typical day scenarios. In contrast, the system without pumped-storage retrofitting shows deviation power ranging from 40.58 MWh to 67.51 MWh and water spillage ranging from 626.4 \(\times\) 104 m\({}^{3}\) to 928.4 \(\times\) 104 m\({}^{3}\) under the same conditions. These results demonstrate that the proposed system can operate stably according to the scheduled output and significantly reduce wind, PV, and hydropower curtailment, thereby enhancing the system's ability to accommodate renewable energy. (3) In terms of economic performance, the proposed system increases power generation revenue by 5.13% to 6.22%. Over a 10-year period, the total system revenue increases by CNV 116.709 million, demonstrating strong overall economic viability. 2. Inspiration for future study is as follows: (1) In this study, the capacity configuration was carried out under ideal forecasting conditions, without considering prediction errors in wind and PV output. However, in practical applications, forecasting errors are inevitable due to the stochastic nature of renewable energy sources. Therefore, conducting a sensitivity analysis on forecasting errors in future work will be beneficial for evaluating the robustness and adaptability of the system's capacity configuration scheme. (2) In this study, the capacity configuration assumes that the system maintains constant efficiency throughout its entire operational life. However, in practical applications, the output power of PV modules gradually degrades over time, and wind turbine efficiency declines due to mechanical wear, blade erosion, and environmental factors. Therefore, incorporating these factors into the capacity configuration method in future work will contribute to improving the long-term validity and robustness of the proposed capacity sizing approach. In this study, a fixed cut-off frequency was used in the frequency-domain analysis to separate the fluctuating components of renewable energy output. Therefore, future research could consider introducing an adaptive method for determining the optimal cut-off frequency based on system dynamic characteristics. This approach would improve the accuracy of power fluctuation smoothing and further enhance the responsiveness and overall performance of the capacity configuration strategy. Conceptualization, B.L. and S.L. ; methodology, S.L. ; software, S.L. ; validation, B.L., S.L. and J.Z. ; formal analysis, S.L. ; investigation, S.L. ; resources, B.L. ; data curation, J.Z. ; writing--original draft preparation, S.L. ; writing--review and editing, J.Z. ; visualization, S.L. ; supervision, B.L. ; project administration, P.L. ; funding acquisition, B.L. and P.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China, grant number 52267006.