Research On Aggregated Power Flexibility Of Active Distribution Network

With the emergence of more and more distributed energy sources(DER)such as renewable energy,energy storage systems,and electric vehicles in the distribution network,the traditional distribution network changes from passive to active through the comprehensive coordination and control of various DER,thus the traditional distribution network can develop toward to the direction of active distribution network(ADN).At the same time,due to the increasing proportion of new energy resources,part of the thermal power units in the transmission grid will be replaced,and the uncertainty caused by wind power generation and photovoltaic units has made the problem of insufficient reserve of the transmission grid more prominent.Therefore,it is important to exploit the potential of flexible regulation of ADN,so as to realize flexible power exchange with the superior network at the root node,and to provide auxiliary reserve services for the upstream power grid.This paper proposes a method to model and quantify the aggregated power flexibility of ADN,i.e.,the net power injection at the root node of the distribution network.Considering the factors of power supply reliability,economy and uncertainty of renewable energy output and load prediction,the effective feasible region of the aggregated power flexibility of the ADN can be obtained.The main research work and results are summarized as follows:(1)Considering the network security constraints and the operational constraints of DER,an evaluation model for the aggregate power flexibility of ADN with multiple types of DERs is established,and the aggregated flexibility of active power,active power and reactive power is realized at the root node.Based on the DistFlow power flow model and a variety of linearization techniques,the established model is transformed into a mixed integer linear programming problem,and the commercial solver CPLEX is used to obtain a solution efficiently.(2)Considering the influence of power supply reliability on flexible aggregation of distribution network,expected energy not supplied(EENS)is introduced as the reliability evaluation index,and a flexible evaluation model of the aggregated power of ADN considering reliability is established.When considering the line outage events,a fast algorithm is proposed to avoid the computational complexity caused by dynamic reconfiguration and to get the solution of the model efficiently.Based on this,considering the power purchase cost,DER operating cost,photovoltaic curtailment cost,load-shedding cost,and flexibility benefits,an aggregation power flexibility evaluation model is established.The results obtained can help distribution system operator make trade-offs between reliability and economy,and make the best flexibility aggregation scheme.(3)To address the increasing uncertainty of renewable energy output and load forecasting,based on the reasonable description of uncertainty,a scenario-based ADN agregation power flexibility evaluation model considering uncertainty is established.And the impact of uncertainty on the result of flexibility aggregation is analyzed.It also considers the reliability of power supply and the uncertainty of photovoltaic and load,and a tradeoff between reliability,uncertainty and flexibility is made.