AnOptimized Control Technique ofCascaded H-bridgeMultilevel ActiveFront-ends

The paperisdevoted totheinvestigation ofa three-phase multilevel H-bridge PWM-Rectifier operating asafirst stage ofaback-to-back converter inhigh-power ac drives. First, apredictive control technique isdescribed with reference toa 5-level converter. Starting fromproperly defined reference values, thealgorithm pointed outallows thedirect evaluation oftheduty-cycles ofallthevoltage vectors involved intheVSR modulation. Bymeansofthe evaluated switching functions, attheendofeverysampling interval theline-currents assumevalues practically equal to theimposedones.Some unrestrictive hypotheses are introduced inordertoefficaciously simplify theanalytical expressions. As a secondstep, inthepapera specially designed modulation technique isshown. Inaddition tolow valuesofdc-link voltage oscillations andofline-current distortion insteady-state operations together withhigh values ofthepower-factor, this modulation technique allows tokeepbalanced thedifferent capacitor voltages, by properly usingtheintrinsic redundancy oftheconverter topology. Numerical results arepresented withreference to somesignificant operative conditions inorder tovalidate the theoretical considerations.