A Three-phase Hybrid Cascaded Multilevel Inverter

Since the multi-level inverter can make the power switching devices with low cost and pressure resistance apply to medium and high voltage high power occasions,and effectively reduce the harmonic content of the output voltage,it has been widely used in the fields of flexible transmission and transformation,power transformation of wind power generation and high speed train traction.When the diode-clamped multilevel inverter and the flying-capacitor multilevel inverter are applied to a multi-level occasion of seven levels or more,the system control becomes more complicated,so the cascade multilevel inverter becomes the preferred topology of the high power inverter because of its modularization characteristics.Due to the star connection of the traditional H-bridge cascade three-phase inverter,the slight imbalance of the load will cause the displacement of the neutral point,which seriously affects the normal operation of the system,while the triangular connection,the modulation wave and the inverter output voltage has the phase difference,makes its transmission characteristic become nonlinear,must carry on the pre-decoupling control,makes its control more complicated.For this reason,this paper studies a three-phase hybrid cascaded multilevel inverter topology.The three-phase inverter topology is formed by cascading three-phase diode-clamped multilevel inverters and single-phase diode-clamped multilevel inverters,so the inverter has the advantages of small voltage stress and easy modularity of the device.In this paper,the topology structure and working principle of the inverter are analyzed firstly,and then the switching mathematical model of the topology structure is established.Secondly,this paper introduces the modulation strategy adopted by the three-phase hybrid cascaded multilevel inverter,and makes Fourier analysis of the output phase voltage of the three-phase inverter.Aiming at the problem of midpoint potential imbalance of multilevel inverter,by analyzing the working mode of the inverter,this paper summarizes the main causes of the midpoint potential imbalance,and by selecting the redundant switching state of the inverter,makes the capacitance charge and discharge equilibrium in a modulationwave period,and realizes the capacitance voltage equilibrium of each module unit,The simulation and test are verified.Finally,a three-phase hybrid cascade multi-level inverter experimental prototype is built,and the software and hardware design of the experimental prototype are introduced separately.Through the analysis of the experimental results,the correctness and feasibility of the theoretical analysis are verified.