###### Abstract
References [19; 20] apply a first-order low-pass filtering algorithm to smooth the output power of renewable energy sources. Reference [21] employs wavelet packet decomposition to separate high- and low-frequency fluctuations and conducts economic modeling of the energy storage system. Reference [22] utilizes Discrete Fourier Transform (DFT) to decompose the unbalanced power. Reference [23] conducts spectral analysis of load profiles based on Hilbert spectral transform and empirical mode decomposition (EMD). While considering the charge-discharge constraints of energy storage, the system is divided into frequency bands and configured based on the principle of minimizing energy aliasing. However, the research on the capacity configuration of multi-energy complementary systems involved in the above literature is mostly based on the complementarity of a single leading power station or traditional pure pumped storage plants with wind and PV resources. It lacks the transformation and utilization of existing weakly regulated cascade hydropower and the joint complementary capacity development of surrounding distributed wind-solar resources. The main challenges include the following:  
1. Compared to traditional cascade hydropower, cascade hydropower stations that have been transformed into pumped storage plants gain the added ability of \"valley filling\". However, the regulation capacity of small cascade hydropower stations is limited, so a key consideration is how to make full use of the limited regulation capacity for the capacity configuration of pumped storage units and renewable energy. 2. Compared to a single hydropower station or a pure pumped storage plant, the cascade hybrid pumped storage plant utilizes the existing upper and lower reservoirs of cascade hydropower stations. Therefore, the system's mathematical model needs to account for more complex hydraulic interactions. Based on the research ideas and challenges mentioned above, the main contributions of this paper are as follows:  
1. Based on the geographical conditions and reservoir types of cascade small hydropower, the paper proposes adapting the transformation of eligible small hydropower stations into pumped storage plants according to local conditions, enabling them to achieve complementarity between watershed hydropower and distributed wind and PV resources through the coordinated operation of cascade reservoirs, thereby constructing a cascade small hydropower-pumped storage-wind-PV complementary system. 2. A capacity configuration method is proposed for the cascade hydropower-wind-PV-pumped storage complementary power generation system. The method determines the capacity of pumped storage units based on the maximum regulation capacity of cascade small hydropower after pumped storage transformation. Furthermore, spectral analysis is employed to achieve time-series matching between the pumped storage capacity and the power-energy fluctuation characteristics of distributed wind and PV sources, thereby satisfying the capacity demand for smoothing renewable energy volatility. 3. An optimized scheduling model for the cascade small hydropower-pumped storage-wind-PV complementary system is developed, considering the hydraulic-electricity coupling of cascade small hydropower, the output characteristics of wind and PV, and the operating constraints of pumped storage condition transitions. The model undergoes capacity verification under multiple scenarios, with the commercial solver Gurobi (version 9.5.2) employed for solution computation, demonstrating that after capacity configuration using the method proposed in this paper, the system's stable and economic operation can be achieved.