Frequency-doubling Modulation Method for Hybrid Cascaded Eleven-level Inverter Power Equalization
Hybrid cascaded multilevel inverters can achieve higher level output with fewer switching devices and have been widely used,but they cannot achieve power balance between high-and low-voltage units.When the bat-tery pack module is used for power supply on the DC side,if the power is not balanced,the battery pack module corresponding to each H-bridge unit will be in different discharge characteristics,which increases the failure proba-bility and maintenance difficulty and increases the maintenance cost in the later period.To solve this problem,two power equalization methods based on frequency doubling modulation are proposed.Firstly,the two methods use tri-angular carrier and reconstructed carrier to perform phase shift for the three low-voltage units,respectively,so as to increase the equivalent switching frequency of the inverter from fc to 6fc/12fc within the full modulation range.The switching times of each switching device in the low-voltage unit is around 60 times/174 times,and the switching loss balance in the low-voltage unit is realized.When the modulation degree M=0.9/0.6/0.3,the output power of the low-voltage unit fluctuates around 657.5 W/293.5 W/73.5 W,and the power balance between the low-voltage units is realized.Secondly,the step wave low-frequency modulation is adopted for the high voltage unit,and the conduction angle is dynamically adjusted with the change of modulation degree,so that the switching times of each switching device in the high voltage unit is 1,and the switching loss balance in the high-and low-voltage units is realized.When the modulation degree M=0.9/0.6/0.3,the output power of the high-voltage unit and the low-voltage unit fluc-tuates around 1 317 W/657.5 W,586 W/293.5 W,and 146.5 W/73.5 W,and the power balance between the high-and low-voltage units is realized.Then,the advantages and disadvantages of the two methods and their application occa-sions are compared.Finally,the correctness and feasibility of the method are verified by experiments,and a general method for double equalization of cascaded H-bridge inverters with a DC voltage ratio of 1:1:1:…:x is given.
inverterfrequency doubling modulationreconstruct carrierequivalent switching frequencyswitching loss balancedpower balance between high and low voltage units
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