Modeling And Solving Of N-0 And N-1 Security Region Of Flexible DC Distribution Network

In recent years,the proportion of DC sources and DC loads in distribution network is increasing,and the flexible DC distribution network with many advantages has been widely concerned.At present,the flexible DC distribution network is still lack of effective means in security situation awareness and preventive control.Due to the differences in network forms and operation modes,the security region models for the AC distribution network cannot be applied to the flexible DC distribution network.In view of this,this thesis extends the existing security region theory of the AC distribution network to the flexible DC distribution network,and studies the modeling and solving of the security region of flexible DC distribution network,aiming at describing the security operation range of flexible DC distribution network,determining its dispatching operation boundaries and improving its level of security operation.The main work of this thesis is as follows:1.The steady-state operation of the flexible DC distribution network is analyzed.The typical connection modes,closed-loop operation mode and power flow control strategies of the flexible DC distribution network are analyzed in turn.The accurate power flow model of the flexible DC distribution network is given.The linearized DistFlow branch power flow models of the flexible DC distribution networks with typical connection modes are established,which includes the feeder power expressions and the node voltage expressions.2.The N-0 and N-1 security region models for the flexible DC distribution network are proposed.Firstly,the reasons why the N-0 and N-1 security region models for the AC distribution network are unsuitable for the flexible DC distribution network are analyzed.Then,the N-0 and N-1 security region models for the flexible DC distribution network are established.The established models take into account the closed-loop operation mode and power flow control strategies of the flexible DC distribution network,so they are suitable for the security analysis of the flexible DC distribution network.3.The analytical solving method for the N-0 and N-1 security region of the flexible DC distribution network is proposed.Through the linearization of DistFlow branch power flow of the flexible DC distribution network and the treatment of droop nodes,the security region models of the flexible DC distribution network with nonlinear constraints are transformed into linear inequality systems on load power which can be solved analytically.The analytical expressions of the N-0 and N-1 security region of the flexible DC distribution network are obtained by combining with the numerical examples,and the security operation range of the flexible DC distribution network under the premise of normal operation and N-1 criterion is fully described respectively.4.The rules of the security region size under different control strategies of the flexible DC distribution network are explored.According to the analytical expressions of the security region,the rules are obtained by visualized comparison: the size of the N-0 security region under master-slave control and voltage margin control is the same,which is larger than that under droop control;the size of the N-1 security region under voltage margin control is smaller than that under master-slave control,and the size of the N-1 security region under droop control is smaller than or equal to that under masterslave control.