Hierarchical Coordinated Voltage Control For Active Distribution Network And Microgrids

At present,the increasingly serious global energy shortage and environmental pollution promote the development of distributed generation(DG)greatly.Microgrid technology emerges to deal with the conflict between the operation strategy of conventional active distribution network(ADN)and the large-scale grid-connected DGs.Microgrid can not only improve the utilization efficiency of DGs,but also guarantee the reliability of power supply.Based on the traditional control strategy of inverter interfaced DG and operation mode of microgrid,This article analyses the voltage control strategy and coordinated voltage optimal control method mainly based on the view of grid-connected DG,microgrid composed of some DGs and multi-microgrid ADN.With the development of related technology,the penetration rate of DG in ADN is increasing.The inverter interfaced DG has a certain reactive power capacity and can provide reactive power and voltage support for the grid such as photovoltaic(PV)system.Considering the reliability and stability requirements of the power system,adjusting voltage of grid connection point using PV system has caused enough attention.Based on the traditional inverter control method,this paper presents a dual-loop control structure and builds the constant active power and constant voltage control model in the outer loop.We stabilize the integration voltage by making full use of PV reactive power output.When the set value of PV active power output is close to the maximum power,the reactive power output capacity will be limited.The corresponding control strategy is proposed to coordinate the contradiction between active and reactive power output.When the voltage fluctuation is large,the active power output of PV can be reduced appropriately to give priority to the requirement of voltage regulation.Different from simple distributed generation system,microgrid generally has two kinds of normal operation mode,that is,the isolated and interconnected mode.When the microgrid is connected to the grid,the ADN should make full use of all kinds of DGs of microgrid to participate in the optimal scheduling.Based on the principle of"secondary voltage control",each PCC is taken as a pilot node.The microgrid is equivalent to a generator at PCC in the ADN level.The reactive power output of this equivalent generator is also taken as the control variable of ADN.The reference value of PCC voltage,which is the objective function of microgrid optimization model,is given by the optimization results of ADN.The microgrid incorporates the reactivepower of DGs into the optimal control variables and coordinates the reactive power sources to control the PCC voltage.With the rapid expansion of the scale of grid-connected microgrids,the interaction between ADN and microgrids becomes more significant.So it is necessary to regulate the reactive power equipments of ADN and microgrids coordinately.Otherwise,it will lead to some technical problems.This paper realizes the coordinated optimal control between ADN and microgrids via decomposition and coordinationmethod.The optimal control problem of the overall network is decomposed into several control sub-problems of different subsystems.First of all,we decompose the whole system at PCC which is regarded as the boundary node between ADN and microgrids.In the ADN level,each microgrid is considered as an equivalent load at PCC.Meanwhile,in the microgrid level,the ADN is treated as an equivalent power supply source at PCC.The optimal control of ADN and microgrids are calculated alternately based on optimal power flow.The ADN communicates with eachmicrogrid to exchange coordination variables at PCC,that is,PCC voltage and power in each iteration step.Then the unity of flexibility and coordination is realized.The simulation analysis is carried out hourly for a total time horizon of one day,and the results obtained reveal good performance of the proposed approach.