Research On Optimal Operation For Flexible Distribution Networks With High Proportion Of Distributed Renewable Energy Sources

In recent years,to meet the urgent demand of the national energy strategy transition,the clean renewable energy sources(RESs)like solar and wind energy have been developed rapidly on the distribution network side.However,with the increasing penetration rate of RESs,the inherent volatility and uncertainty of RESs as well as the diversity of its operation mode increasingly influence the optimal dispatch of the distribution network.In view of these challenges,by optimizing the operation instructions of interface converters coordinately,this paper mainly focuses on solving the problems of source and load power fluctuations in the within-day stage,and three-phase unbalance and multi-agent coordination under market mechanism in the day-ahead stage.The main contributions are as follows:Firstly,a coordinated optimization framework of multi-converter droop parameters is proposed to absorb the fluctuations of source and load power fluctuations.Based on the forecast data of different time scales,the VSC droop reference points and slopes are optimized hierarchically.The traditional slope optimization model ignores the converter transient characteristics,which may lead to system instability.To solve this problem,taking the ring DC distribution network as an example,based on the V-P droop control,the influences of droop slopes and source/load power on system small-signal stability are analyzed.Then based on the system small-signal stability analysis,an optimal control method of VSC droop slopes is proposed.The small-signal stability constraints are constructed based on the system small-signal state matrix,and then added in the slope optimization model,from which the optimal slope command and slope stability range can be obtained.In addition,to improve the optimization efficiency,an improved algorithm based on the combination of sequential nonlinear programming and genetic algorithm is proposed.Secondly,a VSC droop slope robust optimization model is established for AC/DC hybrid distribution networks considering the influence of RES output power uncertainty.When the actual output power of RESs deviates greatly from the forecast scenario,the system security may be endangerd.Thus,the RES power fluctuations are described in the form of uncertainty set.Besides,to solve the problems of submodel non-convexity and second-order cone relaxation inaccuracy,a V_ac²-P and V_ac²-Q dual droop control strategy and a branch current limit strategy are proposed respectively so that the reliability of dispatch commands can be improved.In addition,to quantitatively evaluate the ability to absorb RES output power fluctuations,based on the slope robust optimization model,a fluctuation interval correction model is established to obtain the maximum fluctuation interval of RESs that can ensure the safe operation of the system.Thirdly,a multi-objective optimization model for the soft open point is established based on semidefinite power flow constraints to suppress the system three-phase unbalance caused by high proportion of unbalanced RES.The optimization model considers the coupling relationship between phases,and can flexibly regulate three-phase power dispatch commands of the soft open point to reduce both system loss and three-phase voltage unbalance.In terms of the algorithm,to solve the problem of rank one relaxation inaccuracy,a cutting plane iterative method is proposed to improve the accuracy of optimal power flow calculation results.Finally,a multi-agent coordinated dispatch method for the distribution network with multiple wind,solar and energy storage based independent microgrids is proposed to deal with multi-objective conflict under market mechanism.The method can be separated into the active power dispatch stage and the reactive power dispatch stage.In the active power dispatch stage,a Stackelberg game model is established to determine the optimal time-of-use price and active power exchange plan between the distribution network and microgrids.In the reactive power dispatch stage,to further improve voltage quality,a reactive power optimization model based on the maximum operation cost constraint is proposed.The remaining capacity of VSCs can be fully utilized to provide reactive power support.And without affecting the operation cost of the distribution network and the internal dispatch plan of microgrids,the average voltage deviation can be reduced.