Nonlinear Models And Nonlinear Optimization Methods For Medium And Long Term Hydrothermal System

In the past 20 years,China has witnessed the great development of its power and hydropower systems.In southwest China,large scale hydrothermal systems,which are dominated by hydropower,have been built.These systems are characterized by large installed hydropower capacity with high proportion.In these systems,main stream cascade hydropower plants and multi-year regulation hydropower plants play an important role on the operation of the entire power grid.Therefore,how to make reasonable plans to operate multi-year hydropower plants and cascade hydropower plants,which lay the basis for optimization of the entire power grid,is extremely important.Considering the real world operation of the Yunnan power grid,this paper studies three key facing problems,including forebay water level control of multi-year regulation hydropower plant,optimization of cascade hydropower plants and joint operation of hydropower plants and thermal power plants within the same power grid.The purpose of this research is to construct new approaches to the problems.The main innovative results are introduced as follows:(1)Hedging rule of end of year stored energy for multi-year hydropower plant is proposed.For a multi-year hydropower plant,the forebay water level control between years and within a year is directly related to the long-term safe and economic operation of power grids.Traditional methods usually pay more attention on the total power generation,but in-depth and detailed research on how to coordinate the multi-year hydropower operation within year and between years lacks.Based on the theory of hedging in economics,this paper developed a hedging model considering annual power generation and end of year stored energy.In the proposed model,annual inflow is assumed following Pearson III distribution,monthly inflow sequences are generated by appling the disaggregation model and hedging rules are achieved by adopting nonlinear programming.The generated hedging rules are compared with the standard operating policy and results demonstrate that in dry conditions,hedging rules could effectively reduce the system benefits loss compared to the standard operating policy,which provides a new way to solve the year-end water level control problem for multi-year hydropower plants.(2)A nonlinear global optimization method for cascade hydropower plants is proposed.Optimizing the operation of southwest main stream cascade hydropower plants is difficult due to large installed capacity,multiple series and highly coupled relationships in time and space.Therefore,a new approach for optimal operation of cascade hydropower plants is proposed.This method applys the branch and bound framework,adopts the McCormick envelop and tangent cuts to bound nonlinear functions and applies interval analysis and constraint propogation to speed up the convergence.This method is compared with gradient based nonlinear programming,nonlinear programming with multiple start initial points,dynamic programming and dynamic programming with successive approaximation.Results demonstrate that nonlinear programming could guarantee the calculation accuracy and improve the solving efficiency,which provides cascade hydropower station optimization problems an efficient and feasible optimization method.(3)A large scale medium and long term coupled operation model for a provincial hydrothermal system is proposed.In southwest China,many provincial hydrothermal systems have been built with the total installed capacity of 100 million kW,in which the installed hydropower capacity accounts for more than 70%of the total installed capacity.How to operate such systems considering uncertain inflows and load demands becomes a new challenge.Therefore,a large scale medium and long term coupled operation model is proposed.The end of month forebay water level from long term model is selected as the boundary for mid-term model,thus guarantee the consistency between long term and mid-term.Updating and re-optimizing strategy is adopted to realize hydrothermal coordination in multiple time series.The performance of the proposed model and solving procedure is compared with historical records.Results indicate that the proposed model and solving procedure could effectively reduce fuel cost and thermal emission,which provides the large-scale provincial hydrothermal system a feasible modelling and scheduling management method.At last,the work and innovation of this research have been concluded and future research is prospected based on the shortcomings of the current research.