| With the rapid development of wind-solar power and the reduction of coal-fired units,the lack of flexibility in the power generation system has become a pressing issue that needs to be addressed in the context of energy transition.With their mature technology,low-carbon and environmentally-friendly features,significant development potential,and high flexibility,hydropower and pumped hydro energy storage(PHES)have become the most reliable sources to assist the power system transformation.However,the urgent issue at present is how to overcome the limitations of their operation and scheduling,and fully tap their power generation flexibility.On the other hand,the release of the flexibility of hydropower and PHES will result in more frequent participation of units in grid power regulation,which will impose a significant burden on the stable operation of the units themselves and potentially hinder the transformation of the flexibility of hydropower and PHES.Therefore,measuring the burden of unit operation stability under the flexibility transformation is the second issue that must be addressed when developing a reasonable transformation path for hydropower and PHES.To address the above two issues,this dissertation first establishes a power system unit commitment model that takes into account the operating characteristics of hydropower and energy storage,and improves the relevant database of the Chinese power system.Then,from the perspective of hydropower,the dissertation explores the effect of hydropower flexibility operation on improving the flexibility and decarbonization of the power system.Furthermore,from the perspective of energy storage,the dissertation comprehensively evaluates the adaptability of different PHES technologies in regional power systems.Finally,the dissertation reveals the relationship between PHES flexibility and unit operation stability.The main research content and results are as follows:(1)Improved power system unit commitment model and Chinese power system database considering hydropower operating characteristicsThe previous power system unit commitment models often lacked sufficient detail when it comes to modeling hydropower.To address this issue,we developed a unit commitment model that considers both the operational characteristics of hydropower units and reservoir operations.The model aims to minimize the total system operating cost.Furthermore,we proposed a novel double-layer clustering principle based on the flexibility characteristics of hydropower,which ensures computational efficiency while also accounting for the operational flexibility of hydropower.Lastly,we summarized the basic information of multiple open-source databases for generation,load,and power grid,and created a new database for the Chinese power system.The database fills in the gaps in data for hydropower,PHES power plants,and run-of-river hydropower units.This model and database provide a solid foundation for following research.(2)The contribution of flexible hydropower operation to power system flexibility and decarbonization in the context of coal-fired power unit optimizationThis dissertation proposed a flexible hydropower operation strategy to address the lack of power system flexibility and high carbon emissions caused by coal-fired power unit reduction.Taking three provinces with significantly different energy structures(Qinghai,Xinjiang,and Shaanxi)as examples,we investigated the impact of hydropower’s flexible operation on power system flexibility and carbon emissions under different coal phase-out scenarios,including the current state,normal retirement,and accelerated retirement.The results show that flexible hydropower operation can significantly reduce system carbon emissions and effectively promote the integration of wind and solar power.The wind and solar power integration rates in Xinjiang,Qinghai,and Shaanxi increased by 15.99%,17%,and 6.7%,respectively.(3)Comprehensive evaluation of different PHES technologies considering the flexibility characteristics of the unitTo compare the flexibility potential of different pumped storage technologies(traditional fixed-speed,doubly-fed variable-speed,full-power variable-frequency,three-winding and separate-speed variable-speed),this study considers in detail the characteristics of the five types of PHES in terms of stable output range,unit efficiency,startup time,ramp rate,and provision of standby capacity based on a unit commitment model,against the background of energy transition.In addition,indicators are proposed from economic,technical,and environmental perspectives,including system operating costs,unit startup frequency,cumulative regulation power,and carbon emissions.The study examines the characteristics of different PHES technologies in the Qinghai power system in 2030 under cost preference and carbon emission preference.The results show that the three-winding PHES unit has the highest score of 64.9 under cost preference,while the separate-speed variable-speed PHES unit has the highest score of 77.4 under carbon emission preference.The fixed-speed pumped storage unit has the lowest score under both preferences.(4)Evaluation of PHES flexibility and stability based on a unit commitment-governing system soft link modelTo explore the burden of unit operation stability under PHES flexibility transformation,a modular modeling method is used to establish a PHES governing system model including the pump turbine,pressure pipeline,generator,and speed governor.Furthermore,based on the water head and unit output parameters from the unit commitment model,which are used as the boundary conditions for the PHES governing system model,a unit commitment-governing system soft-linking model is established.Based on a case study of a wind-solar-thermal-PHES power generation base,this study establishes evaluation indicators for PHES flexibility and unit stability and investigates the impacts of wind-solar ratio,unit operating flexibility parameters,and reservoir regulation methods on system flexibility and unit stability.The results demonstrate that as the wind power ratio increases,the system’s loss of load decreases and the wind-solar curtailment rate increases,while the intensity of flexibility insufficiency increases.The maximum values of head,speed,and flow rate under load-increasing operation conditions occur 0.44 seconds,1.33 seconds,and 0.35 seconds earlier,respectively. |
