Research On DC-link Voltage Ripple Suppression And Modulation Strategy Optimization Based On Cascaded H-Bridge Medium Voltage Drives

With the rapid development of industrialization and the continuous increase in the power level of industrial load,improvements in the topology and technology of medium voltage drives(MVD)have gradually become the focus of research.Among various medium voltage drive topologies,the four-quadrant cascaded H-bridge(CHB)topology has been designed with a high degree of modularity,easy implementation with low-voltage switching devices,and the ability to operate in four-quadrant method.Many drive manufacturers apply this topology for their mature products design.However,how to optimize the performance of the four-quadrant cascaded H-bridge drive(CHB-MVD)to further promote its application still requires further research.This thesis takes the four-quadrant cascade H-bridge topology as the research object and completes the following work:Based on the four-quadrant cascaded H-bridge topology,the mathematical model in the two-phase rotating coordinate frame were established for the three-phase PWM rectifier and three-phase asynchronous motor in the power unit.The corresponding network-side voltage vector oriented control strategy(VOC)for the PWM rectifier and rotor flux oriented vector control strategy(FOC)for three-phase asynchronous motor were constructed.With spefified rated parameters of the motor,cascaded H-bridge inverters with corresponding power levels are designed and built,and precise speed adjustment and smooth operation states switching are accomplished based on the aforementioned control strategy.To resolve the problem of large DC voltage ripple in the power unit under the traditional control strategy,the cause of DC voltage ripple is analyzed that the input power and output power of the power unite cannot be completely matched.Based on this conclusion,three knids of DC voltage ripple suppression strategies are proposed: output power decoupling feedforward strategy by injecting specific current components in three-phase ports of power unit,specific 3rd harmonic injection modulation strategy by injecting specific voltage components in single-phase ports and the combined suppression strategy.The additional injected voltage and current components reduce the fluctuating power subjected by the unit's DC-link capacitors,thereby reducing the DC voltage ripple.The impact of the three strategies on the drive system is compared and the feasibility of the proposed control strategy is verified by simulation results.When the PWM rectifier for power unit adopts bipolar SPWM modulation,LCL filter is needed to improve the quality of grid current waveform.The bifurcation FFT analyzation of bipolar SPWM modulation is used to study the switching harmoics of the bridge arm voltage and the primary/secondary currents of the transformer.Based on the concluded results,a carrier phase-shifting modulation strategy was adopted to optimize the original modulation strategy.Simulation results show that the carrier phase-shifting modulation can improve the equivalent switching frequency of the primary current ripple,improve the waveform quality,and provide the possibility of removing the LCL filter.The research carried out in this thesis would effectively promote the performance of the cascaded H-bridge medium-voltage drive and its application in engineering practices.