Research On Current-Sharing And Topology Of Resonant MV/lV DC Transformer

In order to cope with the increasingly severe energy crisis and environmental crisis,the development of renewable energy has become a top priority for all countries in the world.As the world's largest developing country and the largest energy demand country,China attaches greater importance to the use of renewable energy.The development of renewable energy has put forward higher requirements on the power grid.Traditional AC power grids are confronted with a series of problems with large-scale and high-proportion access to renewable energy.The emerging DC transmission and distribution network can better adapt to renewable energy,therefore can serve as the most promising renewable energy interface.As the core device in the DC distribution network,the performance of the DC transformer directly affects the development of the DC grid,which is causing widespread concern in academia and industry.To withstand the medium and low voltage of the DC distribution network,DC transformers often degrade the voltage and current stress of power semiconductor devices by adopting different topologies.Currently,the widely used two solutions are power module cascaded structure and modular multilevel structure,which,however,also face problems in practical applications:(1)the power module cascaded structure requires a better voltage balance and current sharing scheme;(2)the topology and control method of the modular multilevel structure are not yet perfect.Therefore,this dissertation conducts studies on these two issues.In view of the voltage balance and current sharing problem in the power module cascaded structure,this paper analyzes the cascaded IPOS LLC resonant converter.First of all,to optimize the efficiency of the cascaded converter,the magnetic intergration sheme is proposed to reduce the cores volume of multiple isolated transformers.The window utilization of the transformer is increased,and the output voltage balance is achieved simultaneously.Then,the current sharing problem in the cascaded LLC resonant converter is developed.The cause of current unevenness is discussed and analyzed.Solutions to current sharing between modules and current sharing within modules are presented,respectively,which can improve the current sharing performance of the converter without increasing the control complexity.Finally,the simulations and experiment results verify the proposed transformer magnetic integration and current sharing strategy.In view of the current research status of the topology and control methods of the modular multilevel structure,this paper proposes a new modular multilevel resonant(MMR)DC/DC converter topology,and the principle,modulation method and voltage balance principle of submodules are analyzed.The proposed topology combines the advantages of MMC and LLC resonant converters,and can adapt to a wide range of input / output voltage requirements under the corresponding modulation mode,while maintaining a small range of frequency changes of the converter.Then a high-voltage start-up strategy for MMR is proposed.The characteristics and advantages of MMR are summarized.Finally,the principle and practicability of the proposed topology are verified by simulations and experiments.