Research On Interline Power Flow Control Of AC/DC Hybrid Network Based On MMC

With the gradual depletion of global fossil energy resources,domestic and foreign experts begin to develop new renewable energy resources on a large scale in order to realize strategic adjustment of energy structure.Because the multi-terminal flexible AC/DC hybrid transmission technology based on modular multi-level converter(MMC)has the advantages of modular multi-level converter and multiterminal transmission network,which can solve the problem of grid connection and consumption of large-scale renewable energy.In the multi-terminal flexible AC and DC hybrid transmission system based on MMC converter,the power flow control in the DC network is one of the major problems.And the power flow calculation of the hybrid transmission system is the basis for studying its stability control and fault analysis.In this thesis,a mathematical model for steady-state power flow calculation of multi-terminal HVDC transmission system(MMC-MTDC)based on modular multi-level converter is established.A single-iteration alternating iteration method for power flow calculation of multiterminal flexible AC/DC hybrid system based on MMC converter is proposed.The traditional Newton-Raphson method is used for AC subsystem and the Gauss-Seidel method based on the node impedance matrix for the DC subsystem.Taking the improved hybrid test system of IEEE 9 and 22 nodes including three-terminal MMCMTDC transmission system as examples,and the simulation results show that the proposed single-iteration alternating iteration method has the advantages of fast convergence and high accuracy.Aiming at the problem of power flow control in DC network,this thesis analyses and compares the topological structure and working principle of several DC power flow controllers,combines the control strategy of MMC converter,improves the control strategy of MMC-IDCPFC,proposes a new DC voltage droop control strategy for MMC-IDCPFC in multi-terminal flexible DC transmission system.An integral element is introduced into the droop control strategy to realize the effective control of power flow in HVDC system by MMC-IDCPFC.Then,a three-terminal MMCMTDC transmission system is built on PSCAD simulation platform.The traditional droop control strategy and the improved droop control strategy proposed in this thesis are used to simulate.And a three-terminal MMC-MTDC system based on MMCIDCPFC is built,the traditional droop control strategy and the improved control strategy are used to simulate the MMC-IDCPFC.The results show that the improved DC voltage droop control strategy proposed in this thesis is correct and effective.