Research On Dynamic Reconfiguration Method Of Three-Phase Unbalanced Distribution Networks Based On Linear Power Flow Equation

Conventional distribution network reconfiguration(DNR)strategies are the static reconfiguration method for a single time period,which assumes that the load demand is constant throughout the reconfiguration process.In recent years,with the environment issues becoming more prominent,more and more distributed generators(DGs)and electric vehicles have been integrated into distribution networks(DNs),the change of load demand on DN is further aggravated.Thus,the topologies of DN are expected to become more flexible to adapt to time-varying load demand and DG output.Therefore,the dynamic reconfiguration method which means that the network topology can be adjust continuously over specified look-ahead time period is more in line with the operation of DN.As such,the dynamic DNR is attracting increasing attention from both the academic and industrial communities.Currently,most existing approaches for DNR assume that the DN is three-phase balanced and a single-phase equivalent is used.However,unlike transmission networks,DNs are usually unbalanced.Hence,a single-phase network model is insufficient to represent three-phase unbalance characteristics of DNs.In addition,research shows that DN under unbalanced operation will result in a significant increase in the cost of power loss and system investment.Furthermore,three-phase DGs under unbalanced operation conditions should satisfy the current unbalance constraint to protect DGs from tripping.Therefore,it is of great theoretical and practical values to study DNR strategy of three-phase unbalanced DNs.In view of the above problems,this paper presents a dynamic reconfiguration approach for a three-phase unbalanced DN and it is the further development of DNR research which considering the time-varying nature of load demand and unbalance characteristics of DNs.The main works of this paper are as follows:First of all,the rapid calculation of power flow(PF)is of great significance for reconfiguration,restoration and optimization of DN.To this end,a linear three-phase PF equation was proposed based on the radial DN.This equation can be easily used for DNR problems,as well as restoration and optimal PF.Thereby the computation efficiency can be improved and achieve a high accuracy of optimization problems.Then,based on the proposed linear three-phase PF equation,taking the minimum of the sum of switching cost and network loss cost as the goal,a dynamic reconfiguration model which taking into account the three-phase network unbalance and the voltage imbalance constraints is proposed to optimize the topology structure of the network.The purpose of this study is to obtain a flexible network operation topology,which can realize the safety and economic operation of the DN by considering the time-varying nature of load demand and DG output.Finally,a dynamic reconfiguration method of three-phase unbalanced DNs considering three-phase equilibrium generation requirements of DG is proposed in this paper.To minimize the sum of switching costs and power loss costs,a dynamic reconfiguration model,which takes the fact of three-phase network unbalance as well as the unbalanced operation constraints of DGs into account,is built.Meanwhile,three-phase energy storage(ES)is incorporated into the model to achieve the coordinated operation of DG and ES,so that the combination of the balanced outputs of DG and the unbalanced outputs of ES can meet the unbalanced load demands.The proposed method achieves the coordination between the security and the economy of the DN operation,as a result,the power loss can be reduced and the economic benefits of the DN operation can be effectively improved using the proposed methods.In addition,the accuracy of the proposed linear three-phase PF equation is verified by the calculation and analysis of several IEEE standard test feeders.The effectiveness and computation efficiency of the proposed approach is verified by the calculation and analysis of a modified IEEE 34 bus test feeder and IEEE 123 bus test feeder.It is shown that the proposed dynamic reconfiguration approach can meet the application requirements of practical use.