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Ekctrical & Computer Eng ince ring: An International Journal < ECU) Volume4. N untber 3. Septeniber 2015

3. Modulation Fundamentals

PuLsc widlii inodulutian (PWM) melhodologics used in lite multilevel eomerter modulation can be classified aeeording to switching frequeney. The three multilevel PWM methods most diseussed in the literaturę havc bcen multilevel carrier-based PWM. selective harmonie eliminalion. and multilevel spacc vector PWM: all are c\tensions of tniditional two-level PWM strategiem to scscral levels. Other mulliletcl PWM methods have becn used to a much lesser extent by researchers 117].

Mullilevcl PWM methods uses high switching freojuency eairier waves in comparison to the reference waves to generale a sinusoidal oulpul wave. To reduce harmonie distortions in the outpul signal phase-shifting techniques are used. There are seteral methods that ćhange disposition of or shift multiple triangular earrier waves. The number of eairier waves used is dependent to the number of switches to be controlled in the interier. In addition to the sinusoidal earrier wave modulation methods presenled there are two alternalivc methods they are Position Opposilion Disposition (APOD)and Phase Opposition Disposition (POD) |18)| 19)|20].+

Inour sludy we will focuson the Phase Opposilion Disposition (POD) technique indetail.

3.1. Phase Opposition Disposition PWM (PODPWM)

In Phase Opposition Disposition (POD), the earrier signal abovc the zero axes is in phase wilh eaeh other having same frequency and same amplitudę. Conseeutivcly helów the zero axis the earrier wavc have phase shifted 180 degrec with the same frequency and same amplitudę as the above zero axis. The figurę demonstrates the PODPWM. The three lcvcl mullilevel eomerter simulation sine wavc and the triangular eairier shown in Figurę 4.

Figuro 4.POD of thrcc lcvcl multi lcvcl contcrtr

4. AdvantagesOfMultilevel Conyerter

Multilevcl eomerters altraetivc features ovcr a eom cntional two-levcl eomerter by usc of high switching frequeney pul.se widlh modulation (PWM).