Impacts Of Cascade Reservoirs On The Flood Control Operation And Power Generation Of Downstream Reservoirs

With the rapid growth of power demand in national economic development,the development and utilization of hydropower resources has gradually accelerated.A tremendous amount of large-scale water conservancy projects with large regulating storage capacity and strong regulating storage capacity have been built in the upper reaches of the Yangtze River,and the joint operation pattern of large-scale mixed reservoirs has gradually formed in the basin.However,while the operation of cascade reservoirs has brought huge economic and social benefits,the hydrological situation of the downstream river has be changed,which brings new challenges to the flood control operation and power generation of downstream reservoirs.The upstream and downstream of cascade reservoir were closely related with each other.The operation of upstream reservoirs results in the increase of inherent randomness of downstream runoff and uncertainty of hydraulic elements,which affect power generation,fading,water storage and flood season operation of downstream reservoir in the medium and long-term time scale,and reservoir operation safety of water level control,over-limit water level,limit water of flood period in the short-term time scale.At present,water conservancy projects in China have been in the critical transition period from planning and construction stage to operation and management stage.How to quantitatively analyze the impact of cascade reservoirs operation on the flood control operation and power generation of downstream reservoirs is a key scientific and engineering technical problem to be solved urgently on the basis of understanding the operation rules of reservoir group operation in the watershed.Therefore,taking the upstream reservoirs of the Yangtze River as the research object on this paper,supported by the existing theoretical system and emerging technical methods such as reservoir operation,hydroelectric energy,operation research,artificial intelligence,the idea of combining problem abstraction with mathematical modeling,simule-optimization analysis and engineering practice verification was adopted,the impacts of the upper reservoirs on the flood control operation and power generation of downstream reservoirs on the point of flood risk and power generation capacity has been studied in depth.The research results have been successfully applied in the production dispatching system of Three Gorges cascade reservoirs dispatching communication center,providing reference for other reservoir group dispatching decisions.The main contents and innovative achievements of this paper are as follows:(1)Based on the historical operation data of reservoirs and the principle and characteristics of reservoir operation,the water level,inflow,outflow in the early stage and inflow of current period of reservoir were selected as the influencing factors to form the input factor set.Considering the characteristics of operation data and the principle of decision tree algorithm,the water level at the end of time period was determined as model output,and the corresponding evaluation index of simulated water level based on the reservoir regulation storage was proposed.Further,in order to solve the difficult problems of selecting input factors and calibrating model hyperparameters,correlation coefficient and mutual information were used as correlation evaluation indexes of model input factors,and tree Parzen evaluator was introduced to optimize the number of input factors and algorithm hyperparameters.Finally,an extraction model of reservoir operation rules based on decision tree and its integration model was established,which combined historical dispatching process and expert experience.The experimental results indicated that the decision tree and its integration model had be strongly applicable in the field of reservoir dispatching rules extraction.(2)In light of the problem of analysis of accumulated error in time and space of reservoirs simulation operation,a rolling simulation method for reservoir operation was proposed by combining the characteristics of quantile regression forest and reservoir operation,which effectively reduces accumulated error in time of reservoir rolling simulation.At the same time,a cascade overall simulation model was established based on the multi-output random forest and the multi-output integrator respectively,and the accumulation rule of spatial error in cascade reservoirs simulation was analyzed.Furthermore,the coupling effect between time cumulative error and space cumulative error of reservoirs simulation was analyzed by step-by-step rolling simulation.Based on this,the simulation operation model of giant reservoir group in the upper reaches of the Yangtze River was established.The experimental results showed that the simulation model of reservoir group dispatching had high precision and could provide reference for reservoirs simulation in other catchments.(3)Starting from the risk analysis of reservoir flood control operation under uncertain conditions,an improved flood control risk analysis method based on stochastic differential equation theory was put forward,which comprehensively considered the uncertainties of forecasting error,discharge and storage capacity curve.At the same time,combining with the reservoir group simulation dispatching model in the upper reaches of the Yangtze River,the origin moment recursion formula of each order of the Three Gorges inflow flow was deduced,and the seamless connection between the quantile regression forest theory and stochastic differential equation for reservoir flood dispatching calculation was realized.Then,the recursion formula of origin moment of each order of the flood level of the Three Gorges reservoir was deduced,and the maximum entropic distribution of water level at each time was given.Finally,a risk analysis method for flood control operation of reservoir groups considering uncertainties of simulation model was proposed,which overcame the shortcomings of the assumption of normality of uncertain factors in traditional stochastic differential equation theory.Based on the mentioned aboved,the effects of flood control operation of upstream reservoirs on flood level and flood risk of Three Gorges Reservoir under different interval forecast accuracy and different water level scenarios were analyzed.The experimental results showed that water level of the Three Gorges Reservoir fluctuated around the mean process,and the range of water level distribution gradually increased by the gradually accumulated uncertainties in time,affected by uncertainties of simulation operation model of upstream reservoir group and uncertainty of interval forecast error.(4)Following the idea of the regulation streamflow volume of upstream reservoirs to the regulation generation volume of downstream reservoirs,a ten-day scale simulation operation model of upstream reservoirs of the Yangtze River was established by adopting the rolling simulation method base on mutual information theory,bayesian hyperparametric optimization algorithm and quantile regression forests algorithm and the annual distribution changes of water inflow from upstream reservoir group to Xiluodu and Three Gorges Reservoir was analysed.Then,taking the output of simulation operation model as input flow and maximum generation capacity as target and considering water level,flow and output constraints comprehensively,an optimal operation model for medium and long-term power generation of cascade reservoirs of Jinsha River downstream-Three Gorges was established.Finally,the effects of cascade reservoirs in the middle reaches of Jinsha River,Yalong River,Minjiang River,Jialing River and Wujiang River on regulation streamflow volume,regulation generation volume and combined regulation generation volume of Jinsha River downstream-Three Gorges under different typical incoming water conditions were quantitatively analyzed.The experimental results showed that the cascade regulation and storage volume of each sub-basin in the upper reaches of the Yangtze River was basically in line with the trend of regulating reservoir capacity under each typical inflow condition.And as the Yalong River cascade was located in the upstream of the main stream cascade,the regulation and storage volume were ahead of other sub-basin cascade reservoirs.