Study On Power Conversion Of Multi-Terminal Hybrid AC/DC Microgrid

With the rapid growth of power loads and large-scale connection of distributed generation,the medium-voltage high-power microgrid with hybrid AC/DC sub-grids has become the development trend of the future.This thesis focuses on power conversion study of multi-terminal hybrid AC/DC microgrid.The main work is as follows:1)A novel topology of a multi-terminal hybrid AC/DC microgrid is proposed,which can reduce the power conversion stages and decrease the hardware cost.Furthermore,a novel control method is proposed to adjust the interphase active power by injecting zero-sequence voltage.As a result,the balance of the three-phase grid currents can be guaranteed and the output voltages of DC microgrids can be stabilized.The effectiveness of the proposed control method is verified by the three-port and five-port hybrid AC/DC microgrid cases.2)Based on the analysis of the fault condition in the proposed microgrid,an operation method under system internal fault is proposed.When the condition of internal module fault occurs,the zero-sequence voltage is injected to adjust the three-phase voltage references,and the negative-sequence current is injected to regulate the interphase active power.As a result,the DC capacitor voltages are balanced,and the stable operation of the system is guaranteed.In addition,the system operation boundary and the zero-sequence voltage injection component are analyzed.The effectiveness of the proposed control strategy is verified by a three-port hybrid AC/DC microgrid case.3)The submodule fault-tolerant control strategies of the multilevel converters in the proposed microgrid are studied.For the cascaded H-bridge converter,a novel configuration of the faulty submodule and an improved fault-tolerant control method are proposed,which provides the higher output line-voltage and better tolerance ability under the submodule faults.For the modular multilevel converter,a novel fault-tolerant control method is proposed.When there are few submodule faults,the DC capacitor voltages are raised to a reduced value;when there are many submodule faults,the fundamental zero-sequence voltage is injected,thus improving the tolerance ability of the converter.In addition,a virtual-voltage based single-carrier modulation method is proposed to avoid carrier reconfiguration under submodule faults.Simulation and experimental results verify the effectiveness of the proposed control strategies.4)A novel model predictive control method is proposed for the dual active bridge converter in the proposed microgrid.It can automatically adjust the calculation step,compensate for the error,and obtain a good dynamic response.In addition,the output voltage can be regulated and peak current of the transformer can be suppressed.Simulation and experimental results verify the effectiveness of the proposed method.