| Wind and solar energy are the inevitable choice to support the decarbonization of power systems and achieve the carbon peaking and carbon neutrality goal of China,but the volatility and intermittency of wind and photovoltaic(PV)power output severely limit their grid-connected consumption.Cascade hydropower and energy storage have strong regulation ability and fast response speed properties,and integrating hydro,wind,solar,and energy storage as a complementary system currently one of the effective ways to promote the consumption of new energy.For the short-term coordinated operation of complementary system,fully considering the forecast uncertainties of wind and PV power,and formulating the day-ahead generation schedule can effectively improve the reliability of the system.Furthermore,with the development and construction of large-scale watershed-type clean energy base,UHV DC power delivery has been widely applied to achieve clean energy external delivery consumption and solve the issue of the uneven distribution of power sources and loads in China,and meanwhile,it also puts forward higher requirements for the short-term operation of hydro-wind-PV-battery complementary system.Therefore,it is of great significance to study the coordinated operation of mutli-energy complemenatry system adapting cross-regional external delivery consumption for ensuring the safe and stable operation of complementary system and UHV DC,and improving the complementary performance of the system.The clean energy base in the upper Yellow River basin and the "Qinghai-Henan" UHV DC power transmission project are selected as a case study.Focusing on the short-term coordinated operation of the HWPB complementary system,this study quantifies the complementarity between wind and PV power and reveals its changing rules.Considering the steadiness of UHV DC power delivery,a day-ahead risk scheduling model of hydro-wind-PV(HWP)/hydro-wind-photovoltaic-battery(HWPB)complementary system is established and solved,the short-term risk-benefit conversion rules of complementary system and the improvements of battery storage on complementary performance of HWP system are elucidated,and intraday joint operation strategies of battery storage and cascade hydropower plants is proposed.Furthermore,the flexibility supply and demand reliability of the clean energy base in the upper Yellow River basin under different consumption ways is evaluated.Main findings of this study are shown as follows:(1)The hydro,wind,and solar data of the clean energy base in the upper Yellow River basin from 2007 to 2016 are compiled,and the basic statistical characteristics of runoff,wind and PV power output are clarified.Results indicate that the seasonal characteristic of runoff in the upper Yellow River basin is significant,with the highest average runoff in July and a large variation range,while the lowest average runoff in January and a small variation range.Annual utilization time of wind and PV power are 1796 h and 1476 h,that is,the capacity factor are 0.21 and 0.17,respectively,which are basically consistent with the actual statistics results of wind and PV power in Qinghai Province for many years.(2)The variation description-based multi-energy complementarity assessment metrics system is established,which is used to quantify multi-timescale complementarity between wind and PV power output,and reveal the changing rules of the complementarity.Results indicate that wind and PV power output show certain complementarity on intraday,daily,and annual timescale,in order of complementarity from strong to weak:daily complementarity>annual complementarity>intraday complementarity.The optimum intraday complementarity is found when the wind and PV installation ratio is 1:0.7～1:0.5,the optimum daily and annual complementarity are found when the wind and PV installation ratio is 1:3～1:2.(3)Day-ahead risk scheduling model of the HWP complementary system while considering the steadiness of power delivery is established,a two-layer nested optimization framework is designed to solve the model and the short-term risk characteristics and risk-benefit conversion rules of the HWP complementary system is clarified.Results indicate that solving the risk scheduling model of the HWP complementary system can effectively achieve a balance of risk,peak shaving,and power generation,the obtained "five segment line"day-ahead generation schedule can meet the steady requirement of UHV DC power delivery,while track the load process of the receiving-end power grid as much as possible.There is a complex conversion relationship among risk,peak shaving,and power generation of the system:in winter and spring,risk rate is sensitive to the variations in the system’s peak shaving performance when risk rate is high;in summer and autumn,generation schedules with high delivered power or high peak shaving performance always have high risk rate.In spring,summer,and antumn,when the runoff is relatively sufficient and the system’s planned delivered power is high,the system’s power shortage risk is high;while in winter,when the runoff is relatively scarce and the system’s planned delivered power is low,the system’s power curtailment risk is high.Compared with a traditional day-ahead scheduling method,the day-ahead risk scheduling can reduce the system’s operational risk by more than 70%,but meanwhile,it needs to be at the cost of losing 0.5%～14.4%peak shaving performance or 0.2%～2.4%power generation.(4)Coupled with the operation module of battery storage,the day-ahead risk scheduling model of the HWPB complementary system is established and solved,the improvements of battery storage on complementary performance of HWP system is quantified,and intraday joint operation strategies of battery storage and cascade hydropower plants is proposed.Results indicate that the risk-benefit conversion rules of the HWPB complementary system is similar to the HWP system’s,but its characteristics and rules are more prominent.Conservatively,battery storage with 10%capacity configuration ratio can improve the complementary performance of the system by 23.08%,the operational risks are reduced by 98.18%,and the peak shaving performance and daily average delivered power are increased by 3.74%and 0.55%,respectively.Intraday joint operation strategies of battery storage and cascade hydropower plants can effectively maintain the state of charge of battery storage,and ensure the sustainable operation of the system.The joint operation may reduce the hydropower efficiency in spring,summer,and autumn,setting the planned charge time period of battery storage at a later valley load period can reduce or avoid this negative effect;the joint operation can improve the hydropower efficiency in winter,which can be further increased by setting the planned discharge time period at a later peak load period.(5)The wind and solar flexibility demand optimization models under different consumption ways are established,the flexibility supply and demand reliability of the clean energy base in the upper Yellow River basin is evaluated,and the optimal installed ratio of wind and PV under the criterion of minimizing flexibility demand is determined.Results indicate that whether it is local consumption or external delivery consumption,the stronger the peak shaving performance of the system’s generation schedule,the higher the flexibility demand of wind and solar energy.On average,the flexibility demand of wind and solar energy of external delivery consumption is 33.7%higher than local consumption’s.For the clean energy base in the upper Yellow River basin undertaking the cross-regional external delivery consumption,the flexibility supply guarantee rate of HWP/HWPB complementary system are 82.8%and 91.1%,respectively;when the installation ratio of wind and PV is 1:2～1:1.4,the flexibility demand is the smallest,and wind and PV power output also has a preferable daily and annual complementarity. 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