## 3 Analysis of Pumped Storage Transformation
### Ransformation Conditions  
The essence of the site-specific pumped storage transformation of cascade small hydropower stations is the utilization of existing reservoirs as energy storage units. Compared to pure pumped storage power stations, retrofitting existing hydropower facilities for pumped storage alleviates some of the stringent geographical and engineering requirements. However, several objective constraints must still be carefully considered [29; 30; 31], including the geological conditions of the hydropower stations, the regulation capacity of the existing reservoirs, the water head, and the elevation-to-distance ratio between cascade hydropower stations; this ratio, defined as the vertical elevation difference divided by the horizontal distance, directly affects site selection feasibility and the effective utilization of water head.  
Figure 1: System structure diagram.  
The geological conditions should be selected as much as possible to facilitate the construction of water conveyance systems, powerhouses, and related facilities. It is necessary to refer to the geological conditions of the original conventional hydropower stations and select the hard rock mass with high seismic stability and complete structure to carry out the transformation of the pumped storage power station to avoid damage to the original conventional hydropower equipment during the transformation process.  
The upper reservoir is given priority to regulation performance. The better the regulation performance, the higher the power generation benefit resulting from changes in water head and volume due to pumping and storage. However, most of the cascade small hydropower stations are daily regulating reservoirs, so it is necessary to heighten the dam to increase its regulating capacity. The site selection will be abandoned if there is no condition for increasing the regulation capacity.  
The lower reservoir is given priority to the storage capacity. The storage capacity of the lower reservoir should be large enough to meet the demand of continuous pumping for 6 h with the full power of the pumped-storage unit to ensure that the upper reservoir of power generation does not cause excessive fluctuation of the lower reservoir water level when it is operated under pumping conditions.  
The water head should not be too low. Because the transformation is based on the original conventional hydropower station, the adjustable storage capacity of the reservoir is basically determined, so the higher the water head, the greater the installed capacity of the reservoir, and the development value is relatively high; under the same output, the water flow required for the high head is small, the size of the equipment structure purchased in the construction process is also smaller, and the transformation investment and engineering quantity are reduced accordingly. Considering the natural drop limit of the upper/lower reservoir of the conventional cascade hydropower, where the maximum head generally does not exceed 250 m, selecting hydropower stations with a head between 50 m and 250 m for transformation is advisable.  
The space-depth ratio should not be too large. Under normal conditions, the space-depth ratio of a pure pumped storage power station is generally not more than 12. Due to the limitation of the lower water head, the space-depth ratio of cascade small hydropower stations is slightly larger than that of pure pumped storage power stations. An excessive space-depth ratio will increase the length of construction cost of water diversion structures, so the space-depth ratio of transformable hydropower stations should be relatively small.