Studies On Power System Flexibility Evaluation And Optimal Allocation Considering Power Network Constraints

With high penetration of renewable electricity,the variability and uncertainty faced by power systems have been increasing significantly.It is necessary for power systems to improve the flexibility to cope with power changes and keep power balance.In the meantime,power flows have been grown rapidly,risks of transmission congestion become remarkable,which cause a serious consequence on flexibility transmission.Therefore,coordination between flexibility and transmission capacity should be taken into consideration in flexibility allocation.Flexibility is related with satety,reliability and economy of the power system closely,which is one of the important attribute that cannot be neglect.By studing the whole mathematical theory and methodology of power system flexibility,enhancing the level of flexibility supply capacity,the safe and stable operation of the power system can be ensured.This thesis is background of power systems combined with large-scale renewable energy and based on the current research,analyzing the definition,characteristics and properties of the flexibility,along with the capacity constraints of the power network.The focus is the flexibility balance principle,quantative evaluation and the optimal allocation.Firstly,based on the flexibility value analysis,the flexibility supply model is established,and the evaluation of flexibility considering dynamic characteristics is clarified.Secondly,the similarities and differences of reliability and flexibility is analysed to build a set of flexibility index base on the reliability index.Thirdly,to balance the economy and flexibility of power systems,as well as the computational accuracy and efficiency,the flexibility allocation methodology based on Benders decomposition is developed.The model of flexibility value is built by the marginal value analysis,the value of flexibility is evaluated by the reduction of expected involuntary corrective control costs,which include the costs of load shedding and wind curtailments.Both frequency dynamic characteristics and power network constraints can significantly affect flexibility value therefore both of them should be taken into account in flexibility evaluation.Frequency dynamics are given by piecewise-linear forms using rotor motion equations and DC power flow is employed to model power network constraints.A set of index to quantitatively assess the flexibility of power grid connected with large-scale wind is built.Based on Monte Carlo simulation and economic scheduling model,a practicable method to compute flexibility index is proposed.In the economic scheduling model the uncertainty of both wind power and load demand are considered.This index can assess the loss of flexibility probability and power shortage of system,which reflects the uncertainty of wind forecast and the physical characteristics of flexibility resource on the flexibility index.The optimal allocation of flexibility resources is the key to coordinate economy and flexibility.Economy dispatch and correction control simulation of massive wind scenarios is needed in flexibility allocation,which lead to large scale computation.Therefore,the flexibility allocation methodology based on Benders decomposition method is proposed to balance accuracy and efficiency.The optimization model of flexibility allocation is constructed under the framework of economic dispatch model.Flexibility resources include supply/demand/storage are taken into consideration.