Evaluation And Improvement Of Modulation Techniques For Three-level Inverters

ABSTRACT:Re-evaluation of modulation techniques for multilevel converters has been an extensively investigated theme in these recent years.Complexities of multilevel converters however increase with the number of desired voltage levels.It is thus comparably more common to use three-level inverters,especially the neutral-point-clamped(NPC)inverter.At present,the conclusion is that the centered space-vector pulse modulation(CSVPWM)with phase disposition(PD)carrier arrangement generates the least switching harmonics.This conclusion is always true only at the line-to-line output voltage of the inverter or with only a simple L-filter considered.High-order filters are however used increasingly thanks to their smaller value of passive components needed,compared with those of the simple L-or LC-filter.Since filtering characteristics of different filters differ significantly,performances of modulation techniques ought to be re-evaluated,if high-order filters are used for better attenuating the switching harmonics.The intention,at the beginning,here is thus to investigate characteristics of different high-order filters,including LC-,LCL-,LLCL-,LCL-LCI-,and LCL-LC2-filters.And then re-evaluate different modulation techniques when these filters are individually connected to a NPC inverter.In sum,these five different filters have been considered with some of them found to output better current qualities when the sinusoidal pulse width modulation(SPWM)with alternative phase opposition disposition(APOD)carrier arrangement is used.The finding is thus different from common belief,especially with a low switching frequency.The same atypical finding has been found among standard sinusoidal,centered space-vector and discontinuous pulse width modulation techniques when analyzed with the same high-order filters.To prove the finding,simulation and experimental results have been obtained and discussed.However,the modulation technique of SPWM with APOD carrier arrangement generates a large amount of harmonics around the carrier frequency,which constrains the value of passive components required for LCL-filter.That is to say,more value of passive components is needed when the SPWM with APOD carrier arrangement is used.To overcome the drawback,an Alternative Third-Harmonic Injection PWM with APOD(ATPWM&APOD)is proposed to reduce the prominent carrier sideband harmonics,by which hence decreases the total inductance of LCL-filter.The proposal is subsequently found to be effective with the use of LLCL-filter.Performance of the proposed method is finally validated by simulated and experimental results.