Research On Modeling And Coordinated Control Of DC Distribution Network

With the industrial application of power electronics and the large scale application of photovoltaic,wind energy and other distributed energy,compared with the traditional AC power distribution network,DC power distribution network has many advantages,such as high reliability of power supply,low line loss,low harmonic content,narrow transmission corridor,low line cost,convenient access to new energy power and energy storage,etc.,which make it has broad application prospects in the realization of distributed energy Grid connection,urban distribution network,offshore platform power supply and other fields,etc.However,there are many nodes in the DC distribution network,and the load and power source types are complex and diverse.Futhermore,with the expansion of the scale of the distribution network,the further research of the DC distribution network is restricted by the subsequent problems of system simulation,coordinated control,and fault recovery.This paper focuses on modeling and coordinated control of the DC distribution network.Firstly,the research background is expounded.The development and the research actuality of DC distribution network are analyzed and summarized.Then the research actuality and problems of modeling,coordinated control and fast fault recovery of DC distribution network are analyzed.Futhermore,the basic topology and principle of DC distribution network are expounded,and the control strategy of VSC-the basic control unit in the system is expounded.In terms of limited simulation speed caused by the use of a large scale of power electronic devices in the current DC distribution network simulation,the two sides of the flexible DC device are decoupled and a fast simulation model based on the external characteristics is proposed,including AC/DC converter and DC/DC converter.By the corresponding droop controller and power controller,the characteristics of flexible DC devices can be simulated accurately,which greatly improves the simulation speed.Furthermore,based on the fast simulation model,the simulation model of Suzhou Middle and Low Voltage DC Distribution Demonstration Project is established,and its operation modes,power flow distribution,load and photovoltaic fluctuation etc.,are studied and analyzed,which provides the simulation model and theoretical support for the engineering design and operation of the demonstration project.According to the different types of load and power connected by different control units in the DC distribution network,the control purpose is stabilizing the DC bus voltage.Aiming at different operation states and control characteristics of different control units of the system,a hierarchical coordination control strategy for the DC distribution network is proposed to improve the stability and reliability of the DC distribution network system.Finally,aiming at the problem of long restart time and slow recovery after fault in the flexible DC distribution network,a fast recovery strategy for the flexible DC distribution network after fault is proposed.An energy storage built-in converter(ESBC)topology is proposed and the recovery time sequence is improved to shorten the restart time of the converter.A staged charging strategy is proposed to suppress the impulse current.This strategy can realize the uninterrupted power supply of the important load during the fault period and the fast recovery after the fault.The simulation model of two terminal DC distribution network is used to verify the effectiveness of the strategy.