Study On Optimization Of Southwest China's Cascaded Hydropower Plants In Short-term And Real-time Scheduling

During the past two decades,China has been experiencing an unprecedented hydropower boom and has ranked first in the world in hydropower generation.Most of the hydropower is focused on the main stream of the largest rivers in southwestern China.A batch of huge cascaded hydropower plants with super scale,large plant,huge unit,high head and prohibited zones have appeared,occupying an important place in the operations and management of power grids and hydropower systems.With the increasing requirements of reservoir's outflow and water level,hydropower peak shaving and plant's output,southwest China's cascaded hydropower plant face extraordinary challenges.Four aspects are included:the first is hydropower scheduling with complex water delay time,the second is hydropower peak shaving with contractual energy,the third is refined adjustment for hydropower real-time scheduling,and the fourth is hydro unit commitment with multiple prohibited zones.It is urgent to solving these problems that have seriously challenged the scheduling and operation of the hydropower plants on the main stream of the largest rivers in southwestern China.Under this background,the study mainly focuses on optimization of southwest china's cascaded hydropower plants in short-term and real-time scheduling,with real-world case studies of huge cascaded hydropower plants on Wujiang river and Goupitan hydropower project.The main achievements are as follows:(1)It is hard to handle the water release related and real number water delay time in an MILP formulation without destroying the linear structure of the water balance constraints.This paper present an MILP model for short-term hydropower optimal scheduling with water delay time.The focus is mainly on two constraints,namely water release related water delay time and water head related plant performance curve.Water delay time is linearized through the use of binary 0-1 integer variables and a pre-computed table.The table represents the relationship between water delay time and water release.The water release related and real number.water delay time can be simultaneously considered.The plant performance curve is linearized by using a three-dimensional technique so that head effect can be modeled.The practical behavior of the MILP model is demonstrated by cascaded hydropower plants on Wujiang river.The results demonstrate that the proposed model is suitable for solving short-term scheduling problem with dynamic water delay time.(2)With complex demands of cascaded system and single plant,it is a challenging work for hydropower to effectively shave peak load.This paper focuses on peak shaving of cascaded hydropower plants in short-term scheduling with fixed energy.The daily energy of a single plant is optimized by allocating contractual energy of hydro system to every single plant.The coupling of multi-dimensional dynamic programming,load shedding and a heuristic method used for load dispatch is presented to solve the model.Multi-dimensional dynamic programming is used to reshape system load so that power outputs from load shedding can meet complex time coupled constraints.Load shedding is adopted to determine power outputs.The heuristic method is adopted to ensure the solution feasible.Case study on Wujiang mainstream cascaded hydropower plants is used to test the proposed model and method.Numerical results show that peak shaving potentials of huge cascaded hydropower system is fully exploited with meet of complex operation constraints.(3)The key problem of real-time scheduling of hydro cascaded system is how to fast and reliably adjust day-ahead scheduling according to deviation of load forecasting and runoff prediction.This paper presented a real-time scheduling method and strategy for hydro cascaded system with complex constraints,including computation order decision,plant commitment,load deviation smoothing,load deviation dispatch and surplus water adjustment.Load deviation smoothing is adopted to shape load deviation,preventing frequent fluctuation of plants' generation.In load deviation dispatch,plants' generation is modified based on day-ahead scheduling,and plant's prohibited zones can be rapidly avoided.Simultaneously,an adjustment strategy for preventing surplus water is adopted to achieve pre-discharge.Two case studies on Wujiang mainstream cascaded hydropower plants oriented to the coordination of power plants and power grid is used to test the proposed method and strategy,and shows that its validity.(4)A batch of huge hydropower plants with multiple prohibited zones have been developed in China.These prohibited zones varying with net head,i.e.head-sensitive,seriously challenge the hydro unit commitment(HUC).This paper presents a MILP model for HUC with a head-sensitive reservoir and multiple prohibited zones.The focus of the study is mainly on two net head related constraints,namely the unit performance curves and multiple prohibited zones.The unit performance curves are discretized into a set of piecewise curves based on the discretized net head intervals so that the head effect is modeled.The prohibited zones are approximated by several simple polygons that depend on the discretized net head interval so that the head effect is appropriately captured.The practical behavior of the MILP model is demonstrated by the Goupitan hydropower plant,one of the most important power sources in West-East Power Transmission Project in China.The results demonstrate that the proposed model is computationally efficient and suitable for large hydropower plants with multiple prohibited zones,showing its potential practicability and validity for solving the HUC problem.Finally,achievements of the research are summarized and also future work directions are drawn.