Studies On Power System Flexibility Evaluation And Optimal Allocation

With the large-scale development of renewable energy sources such as wind power and solar power,the variability and uncertainty faced by power system have increased significantly,the ability of power system to quickly response to power changes and keep power balance,namely power system flexibility has drawn more and more attention.In the research of the evaluation and optimal allocation of power system flexibility,the reliable basis of operational flexibility evaluation,the deep understanding of the connotation of flexibility,and the optimal utilization of new flexibility resources such as energy storage need to be further studied.This paper focuses on these questions,the main work is as follows:First of all,the existing flexibility evaluation methods are analysed and compared.Based on the brief introduction of existing methods for power system flexibility evaluation,they are clarified into different categories according to the application situation and the nature of the metric.The emphasis of each method is revealed,and the advantages and disadvantages of each method are analysed.It is pointed out that there are significant differences among different methods,and it is difficult to make direct quantitative comparisons between them.The flexibility evaluation method,which is clear in principle,simple in calculation and practical,needs to be further researched.Secondly,in order to establish a reliable basis for the evaluation of operational flexibility,the characteristics of day-ahead wind power forecast error is analysed.The probability distribution of the forecast error at each time period is affected by the forecast output level,and there is significant serial-correlation characteristic between the forecast errors of adjacent time periods.Aided by the advantages of the Copula function in describing the correlation of variables,a simulation method of wind power randomness is proposed which can take aforementioned aspects of forecast error into consideration.The forecast error time series generated by the method can provide a reliable reference for the evaluation of operational flexibility.Thirdly,in order to understand operational flexibility in depth and establish decision-making principles for it,the similarities and differences between operational flexibility and generation reliability in generation schedule is discussed and compared from the aspects of concept definition and mathematical expression,and the interaction between them is discussed quantitatively.Besides,for two kinds of decision-making principles used in generation reliability,namely equal risk method and cost benefit analysis method,the feasibility and effect are analysed when they are used to guide the decision-making of operational flexibility.Finally,ramping resources are usually allocated roughly in traditional day-ahead unit commitment,which is difficult to meet the sub-hourly flexibility requirement in real-time operation.Aiming at this problem,a unit commitment model which considering the flexibility provided by energy storage is proposed.Based on the depth analysis of the operation law of power system in the sub-hourly time scale,the critical scenes are identified and related operational flexibility constraints are set,the flexibility provided by energy storage is also analytical represented.The relationship between the hourly day-ahead power arrangement and the sub-hourly flexibility provision of energy storage is established,thus the maximum benefit of energy storage under the limitation of limited energy storage space can be achieved.The enhancement of sub-hourly ramping resources in day-ahead unit commitment is beneficial to the improvement of operational flexibility in real-time operation.In a word,power system flexibility is the typical performance of active power balance problem under the background of large-scale development of renewable energy sources,and the maintance of active power balance is always the core problem of power system operation.This paper launches studies on the evaluation and optimal allocation of power system flexibility,which is positive to the consumption of renewable energy sources.