Control Strategy Of Power Converters With Unbalanced Voltage In Hybrid Micro-Grid

AC/DC hybrid microgrid takes the advantages of both AC microgrid and DC microgrid,and will definitely become an important part of future smart grid.The three-phase AC/DC converter,as a bridge connecting AC and DC buses,can realize the bidirectional flow of energy between DC and AC buses.Therefore,it is of great significance to investigate the power converter in the hybrid AC/DC microgrid with three-phase AC/DC converter as the main component.The specific research contents are as follows:Firstly,the development process and trend of microgrid and the current status of domestic and international research are briefly introduced,and the architecture and control structure of AC/DC hybrid microgrid are introduced and analyzed.The mathematical models of photovoltaic cells,wind power generation system,energy storage system,and three-phase AC/DC converter are established respectively for the structural characteristics of hybrid microgrid,and the control methods of power converters in each micro source are introduced and analyzed.Finally,a hybrid AC/DC microgrid system is established,and the microgrid system is simulated and verified in two states: grid-connected and off-grid.Secondly,this paper introduces the most commonly used synchronous rotating frame-PLL and the conventional sinusoidal amplitude integrator(SAI)phase-locked loop.By deriving the mathematical model of the conventional SAI phase-locked loop,a two-step optimization upgrade is performed.A new phase-locked structure based on the multiple variable filter(MVF)and SAI with a simpler structure for unbalanced and distorted grids is proposed.Finally,a comparative analysis of the three phase-locked loops is conducted under multiple operating conditions,and it is verified that the new phase-locked loops can achieve good phase-locking effects under multiple operating conditions with simple structures.Thirdly,when the three-phase AC/DC converter operates in an unbalanced state,the unbalanced voltage will lead to dc-bus voltage double frequency fluctuation and ac-side current asymmetry,which will affect the normal operation of the three-phase AC/DC converter.In order to solve this problem,a hybrid control system is designed in this paper,which adopts a negative sequence current suppression strategy based on a new phase-locked loop on the AC side and a DC-APF DC bus voltage ripple suppression method on the DC side,and the effectiveness is verified by simulation,which can significantly reduce the total harmonic distortion(THD)of the AC side current and suppress the DC bus fluctuation to less than 0.1%.Finally,the Si C MOSFET-based three-phase AC/DC experimental platform was built.Firstly,the Si C MOSFET switching performance is simulated and analyzed in LTspice,and the driver design of Si C MOSFET is carried out based on the simulation results.A driver circuit is designed and a double-pulse test is conducted in LTspice to verify the feasibility of the driver designed in this paper by comparison.Finally,experimental verification of the phase-locked loop and three-phase AC/DC converter control strategy was conducted.