Research On Collaborative Control Of Multi-terminal DC Distribution Network

The vigorous development of power electronics and the large-scale access of distributed generation have promoted the research and development of multi-terminal DC distribution network.Compared with the traditional AC distribution network,the multiterminal DC distribution network has certain advantages in power supply capacity,line loss,and control strategy.Theoretically,the DC system can be developed without many complicated problems such as three-phase imbalance,grid harmonics and reactive power compensation.Therefore,it greatly improves the reliability and controllability of the distribution network and becomes a significant direction for the construction of future distribution system.However,apart from the uncertainty of distributed generation in the system,as DC distribution network can be divided into different complex topologies and operate in diverse modes,it is confronted with new challenges in stable and economic operation.Hence,it is necessary to deeply study the cooperative control strategy of control units in the multi-terminal DC system.Based on this,this dissertation conducts research from the following four aspects:(1)Firstly,the typical topology of DC distribution network is summarized.750 V ring DC distribution network is determined as the research object.The voltage source converter(VSC)is selected as the key device of converter station,and its detailed mathematical model is given.The output characteristics of photovoltaic and energy storage system are analyzed.The typical control strategies of critical devices are given.In terms of the collaborative control of multi-terminal DC distribution network,the hierarchical structure and the current collaborative control method are concluded.(2)Secondly,based on the primary control strategy of VSC-slope deviation control,an improved collaborative control method is proposed.This method divides converter stations into main converter station and auxiliary converter stations.The voltage of the DC system is divided into normal operation intervals and critical operation intervals.The outer-loop control strategy of converter stations and energy storage system in each interval are given.The effectiveness of the proposed strategy is verified through simulations in PSCAD/EMTDC software,and the slope deviation control strategy is compared to verify the superiority of the proposed strategy.(3)Thirdly,based on the limitations of secondary control,a distributed secondary optimal control strategy is proposed.This strategy is based on sparse communication between adjacent converter stations.It mainly adopts a distributed consensus algorithm to achieve voltage control and active power control simultaneously,which improves the utilization of converter stations while reducing the average voltage deviation.The effectiveness and reliability of the strategy are verified through simulations.Droop control is compared to verify the superiority of the proposed distributed secondary optimal control strategy.(4)Finally,based on the proposed distributed secondary optimal control strategy,the small disturbance stability analysis is studied.The idea of modular modeling is employed to establish the small signal model of entire system.Moreover,the eigenvalue analysis method and major participant analysis method are employed to study the impacts on system stability with the variation of important parameters.The design rules of these parameters are obtained,which provides an academic guideline for the selection of the parameters.The simulation verifies the accuracy of the established small signal model.