Control System Design Of An MMC-based Medium-voltage DC/DC Converter For Undersea Observatories

The cabled undersea observatory network is one of the key technologies in marine scientific research.In a cabled ocean observatory network,shore stations connect deep-sea scientific devices via submarine cables to provide continuous power and obtain scientific data.Shore stations generally use medium voltage DC to supply the undersea observatory.Therefore,an undersea junction box is required to convert the medium voltage DC to low voltage DC for the scientific devices.As the scale of the observation network expands,the number of scientific devices increases and the total required power rises.Thus,a high-power,high-reliability medium-voltage to low-voltage DC converter is required.This thesis mainly studies the application of a resonant modular multilevel DC/DC converter(RMMDC)in undersea junction box power converter.The main contents of this thesis are as follows:1.The operating principle of RMMDC is analyzed.The first stage of RMMDC is a singlephase modular multilevel converter(MMC)based on half-bridge submodules,and the second stage is an LLC resonant converter.The proposed control strategy combines input voltage feedforward with output voltage feedback to ensure that the converter operates over a wide input voltage range while restricting the operating frequency to a narrow range.The proposed control strategy could improve the overall conversion efficiency.The operating principle and control strategy of the converter are verified by simulations.2.A two-stage grouping control system suitable for MMC-structure converters is proposed,which includes a main controller and several submodule controllers.Multiple series-connected submodules are considered as a group,and each group shares a submodule controller,thereby reducing the number of required controllers and optical fibers,simplifying the structure of the control system.The hardware design of the two controllers are introduced.To make the controllers cooperate effectively to realize the control strategy,the control tasks are divided and defined for each controller,and the sequence for the control tasks performed in each control cycle is designed.3.The distributed self-powered auxiliary power supply system of the converter is designed.In each submodule,the auxiliary power supply directly draws power from the submodule capacitor and supplies power to the drive circuit and controller of the submodule,so that modular design is realized.The submodule controller is redundantly powered by the auxiliary power supplies of multiple submodules in the same group,which improves the reliability of power supply.A simple power supply structure is designed for the main controller: during the initialization of the converter,the main controller takes power from the startup circuit and completes the soft start of the converter.After the output voltage is stable,the main controller takes power from the output side.In addition,in view of the problem that the submodule capacitor voltages cannot be balanced during the initialization process,a voltage-balancing circuit is designed,which,by adjusting the power consumption of the self-powered auxiliary power supplies,effectively balances the voltages of each submodule capacitor to ensure the stable operation of the system.The designed scheme was applied to a 40-kW undersea junction box power converter prototype based on RMMDC,and the designed control and auxiliary power supply systems were verified through experiments.