Investigations on Three Phase Five Level Cascaded type ... - IOSRJEN

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IOSR Journal of EngineeringMay. 2012, Vol. 2(5) pp: 1056-1063hybrid topologies and multicarrier SPWM applied tomultilevel inverter, the paper makes a deep study onvarious modulation strategies applied to hybridtopologies.Fig. 1 shows a configuration of the three phase fivelevel H bridge type FCMLI. The concept of thisinverter is based on connecting H bridge inverters inseries to get a sinusoidal voltage output. The outputvoltage is the sum of the voltage that is generated byeach cell. The number of output voltage levels are2n+1 where n is the number of cells. The switchingangles can be chosen in such a way that the THD isminimized. The output voltage obtained from FCMLIcascade is E/2. An n level cascaded H-bridgemultilevel inverter needs 2(n-1) switching devicesSimilarly for FCMLI cascaded 2(n-1) switchingdevices are used. The gate signals for chosen fivelevel cascaded type FCMLI are simulated usingMATLAB-SIMULINK. The gate signal generatordeveloped is tested for various values of modulationindex m a and for various PWM strategies. Fig.2shows a sample SIMULINK developed for PDPWMtechnique. The simulation results presented in thispaper in the form of the outputs of the chosen FCMLIcascaded are compared and evaluated.Fig. 1 A three phase five level H-bridge type FCMLIFig. 2 Sample PWM generation logic usingSIMULINK developed for PDPWM techniqueIII. MODULATION STRATEGIESDevelopment of concerns PWM control strategies thedevelopment of techniques to reduce the THD of thecurrent. It is generally recognized that increasing theswitching frequency of the PWM pattern reduces thelower frequency harmonics by moving the switchingfrequency carrier harmonics and associated sidebandharmonics further away from the fundamentalfrequency component. The modulating/referencewave of multilevel carrier based PWM strategies canbe sinusoidal. In general for a five level inverter fourcarriers are needed for symmetrical multilevelinverter. As far as the particular reference wave isconcerned, there is also multiple Control FreedomDegree (CFD) including frequency, amplitude, phaseangle of the reference wave. This paper focuses onhybrid power circuit with sinusoidal reference andtriangular carrier which have been used in chosenthree phase cascaded type FCMLI. The followingstrategies are employed in this study which have usedthe intersection of a sine wave with a triangular waveto generate firing pulses.The amplitude modulation index forPD/POD/APOD/VFPWM = 2A m /(m-1)A c.The amplitude modulation index forPSPWM = A m / (A c /2).The amplitude modulation index forCOPWM = A m /(m/4) A C.m f = f c /f mwheref c – Frequency of the carrier signalf m – Frequency of the reference signalA m –Amplitude of the reference signalISSN: 2250-3021 www.iosrjen.org 1057 | P a g e
IOSR Journal of EngineeringMay. 2012, Vol. 2(5) pp: 1056-1063A c – Amplitude of the carrier signalm – number of levels.III. (a) APOD strategyAs can be seen in the Fig.3 for a five level inverter atotal of four carrier waves are used.i) They are arranged in such a manner that eachcarrier is out of phase with its neighbor by 180degrees.ii) The converter switches to + V dc / 2 when the sinewave is higher than all carrier.iii) The converter switches to +V dc / 4 when the sinewave is lower than the uppermost carrier waveformand greater than all other carrier.iv) The converter switches to 0 when the sine wave islower than the two uppermost carrier waveform andgreater than two lowermost carrier.v) The converter switches to - V dc / 4 when the sinewave is higher than the lowermost carrier waveformand lesser than all other carriers.vi) The converter switches to - V dc / 2 when the sinewave is lesser than all carrier.e) The converter switches to - V dc / 4 when the sinewave is higher than the lowermost carrier waveformand lesser than all other carriers.f) The converter is switched to - V dc / 2 when the sinewave is less than both carrier waveforms.In the PWM scheme there are four triangles, uppertwo triangle have magnitude from 2 to 0 and thelower two triangles exist from 0 to –2 and these twotriangle waveforms are out of phase.Fig. 4 Carrier arrangement for PODPWM strategy(m a = 0.8 and m f = 40)Fig. 3 Carrier arrangement for APODPWM strategy(m a = 0.8 and m f = 40)III. (b) POD strategya) Four carrier waveforms are arranged so that allcarrier waveforms above zero are in phase and are180 degrees out of phase with those below zero.b) The converter is switched to + V dc / 2 when thesine wave is higher than both carrier waveforms.c)The converter switches to +V dc / 4 when the sinewave is lower than the uppermost carrier waveformand greater than all other carrier.d) The converter is switched to zero when the sinewave is greater than the lower carrier waveform butless than the upper carrier waveform.III. (c) PD strategyThe rules for phase disposition method for amultilevel inverter area) 4 carrier waveforms in phase are arranged.b) The converter is switched to + V dc / 2 when thesine wave is greater than both carriers.c)The converter switches to +V dc / 4 when the sinewave is lower than the uppermost carrier waveformand greater than all other carriers.d) The converter is switched to zero when sine waveis lower than upper carrier but higher than the lowercarrier .e)The converter switches to - V dc / 4 when the sinewave is higher than the lowermost carrier waveformand lesser than all other carriers.f) The converter is switched to - V dc / 2 when the sinewave is less than both carrier waveforms.As can be seen from the Fig.5 in the PWM schemethere are four triangles, the upper two trianglesranges from 2 to 0 and the lower two triangle rangesfrom 0 to –2.ISSN: 2250-3021 www.iosrjen.org 1058 | P a g e
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Investigations on Three Phase Five Level Cascaded type ... - IOSRJEN

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