Research On Efficiency Optimization And Voltage Balancing Technologies Of A Modular Multilevel DC Transformer

The energy and environmental crisis is a common challenge for all mankind currently.It is also one of the barriers that affect the social development of our country.Energetically promoting energy innovation and developing renewable energy resources(RES)have become an urgent task for China and the world.Due to the random volatility of RES such as wind and solar energy,high penetration of RES will disturb the conventional AC grid.The energy storage device,which needs to be used together with RES,can decouple electric energy production from time,and make the RES adjustable and controllable.The output of most of the RES(such as solar energy)and energy storage device(such as batteries)are DC.Their integration into DC power grid will reduce the transformation and improve the efficiency.In addition,no frequency and phase synchronization and reactive power compensation are required in DC power grid,which simplifies the control strategy.With the rapid development of RES,DC power grid has a great application prospect.As one of the fundamental devices in a DC grid,the performance of a DC transformer will greatly affect the development of DC distribution network,which has aroused extensive attention and research by domestic and abroad scholars.Since the DC transformer is put forward,various research institutions have proposed different structures.In order to solve the problem of insufficient current and voltage stress in power semiconductor devices,the Input-series-output-parallel(ISOP)structure is one of the popular solutions.However,the multiple transformers in the structure make it difficult to design and increase the cost.In this paper,a modular multilevel DC transformer(MMDC)is considered,which only uses a high frequency transformer to simplify the design of the transformer.The DC transformer is based on the dual active bridge(DAB)and modular multilevel converter(MMC)structure,and it inherits the desirable features of the two structures,namely bidirectional flow of energy,soft switch,fault tolerant control and working in a wide range of voltage.But there are also problems with the two structures:(1)too large current stress and reflux power reduce the efficiency;(2)the submodule capacitor voltage need to be balanced.Hence,this paper has carried out the research on these two problems.In view of the problem of reflux power,a Tripple-phase-shift(TPS)control is proposed in this paper.The optimal parameters for the minimal current stress are derived by combining the mathematical deduction and graphic analysis.The soft switching condition of the power semiconductor device under this control method is also analyzed,which ensures the ZVS of each device.A unified control strategy based on three phase shifting is proposed to minimize the current stress under all working conditions.The experimental results under the control method are given to verify the algorithm.The control strategy can be also applied to the DC transformer to reduce the circulating power.In view of the problem of balancing of the submodule capacitor voltages,a voltage balancing method based on capacitor voltage is proposed in this paper.Different from the existed scheme,the proposed method is not limited by voltage gain or load conditions and it can produce more step waves on the medium voltage side,which reduces the dv/dt stress of the transformer and ensures the stable operation of the system.At last,the simulation and experimental results are given to verify the feasibility of the control strategy.