Research On High Voltage High Power DC/DC Converter And Control Strategy Based On Hybrid MMC

With HVDC transmission lines and capacity is further increased and the development of multi-terminal DC network,the research of DC/DC converter topology and control strategy suitable for DC grid has important theoretical value and engineering significance.DC/DC converter connecting different voltage levels requires bidirectional power flow control,highpower,high-efficiency and low-loss performance.And it also needs fault isolation capability to avoid the propagation of DC faults.However,isolation transformer is usually used in the middle part of high voltage high power DC/DC transformer,which has problems such as large volume,high loss and difficult manufacturing.Therefore,this paper studies an non-isolated hybrid modular multilevel DC/DC Converter with DC fault current blocking capability.Firstly,the two-arm non-isolated hybrid DC-MMC is studied.It uses the full-bridge which has fault self-cleaning ability to avoid any high capacity medium frequency transformers at DC sides.Respectively,the state-space average model and the phasor model of DC-MMC are both deduced,which provides a mathematical basis for the solution of the steady-state operating point of the system and the design of the controller.Considering the special unbalance of the capacitance voltage of the submodules,the converter needs to introduce an ac current.A minimized AC circulating control strategy is proposed,which ensures the power balance of the bridge arm,reduces the AC component of the bridge arm current and the loss of the converter.By means of simulations in MATLAB/Simulink,the effectiveness of the proposed strategy are validated.The maximum power of DC-MMC converter with only half bridge sub-modules is limited by variable ratio.The full bridge submodule ratio and the submodule capacitance voltage lift coefficient are designed in the non-isolated hybrid DC-MMC converter,so that when the DC-MMC changes the voltage ratio,this converter still retains a large transmission capacity.Because the two-arm non-isolated hybrid DC-MMC still needs inductance filter device composed of high voltage and large capacity inductive elements at DC sides,it has problems such as large volume,high loss,difficulty in manufacturing and DC magnetic bias in the inductor in the operation process.A three-arm non-isolated hybrid DC-MMC converter is proposed to replace the large inductance filter device with a horizontal bridge arm composed of full bridge sub-modules.The working principle of the new converter is analyzed,and the equivalent mathematical model of the converter is deduced.In addition,the simulation models are built up based on the MATLAB/Simulink,the simulation results verify the effectiveness and feasibility of topology and the closed-loop control.Finally,an experiment platform is set up.The design of system parameters and selection of component are analyzed,and the hardware architecture and program framework of controllers are introduced.The experiment results verify feasibility of the topologies and control strategies.