Research And Design Of Control Platform Of DC-DC Converter For LCC- And VSC-HVDC Grids Interconnection

The DC-DC converter for connecting LCC-HVDC grid and VSC-HVDC grid is based on MMC(Modular Multilevel Converter).It consists of 2 MMC and one medium frequency transformer,which are used for DC-AC-DC conversion and the electric isolation between primary side and secondary side.One MMC connected to VSCHVDC uses Half-Bridge SM(Sub-Module),the other MMC connected to LCC-HVDC uses Full-Bridge SM.The Full-Bridge SM have ability to work in the reversal voltage condition.The MMC-Based DC-DC converter needs a new modulation strategy and balancing control method.The traditional modulation and balancing control is used for grid-connected converter application.The novel modulation strategy and balancing control is used for DC-DC application,reducing transformer volume and bi-direction power flow.The proposed novel modulation strategy is based on quasi-2-level modulation and fundamental frequency modulation,which can increase AC voltage frequency,reduce volume of passive-devices and voltage stress.In simulation environment,many verification experiments are done to verify the proposed method and theory.Experiments include: current control,modulation,balancing control,voltage polarity reversal,etc.Also,this paper proposes a modular power electronics control platform which is based on OMAP-L137 and FPGAs.The control platform architecture is Master-Salve.The master node of this platform has an OMAP-L137 processor and a large capacity FPGA.The slave node has a medium capacity FPGA.All slave nodes are designed in the same regulation.Data bus and address bus are designed in the master node which is good for slave nodes' extension and modular connection.The OMAP-L137 has ability to exchange data with external memory in fast rate.The communication between nodes is based on optical fiber which is very fast as well.So it is easy to realize realtime control.The IP cores and advanced DSP cores development tools are good for designers to realize distributed and real-time control requirement.A bi-polarity 60 kW MMC-based isolated DC-DC converter is built to verify the proposed method and theory.Experiments include: current control,modulation,balancing control,voltage polarity reversal,etc.Finally,this provides a solution for connecting LCC-HVDC and VSC-HVDC.