Harmonic comparisons of various PWM techniques for basic MLI

ByMD SAQUIB MAQSOOD

M.Tech, PED

Multilevel Inverter:

Number of Level = 2m+1m = number of source

PWM Techniques:The natural sampling techniques for a multilevel inverter are categorized as follows: A. Single-Carrier sine PWM (SCSPWM)B. Sub-Harmonic PWM (SHPWM)a. Phase Shifted Carrier PWM (PSCPWM)b. Constant Switching Frequency PWM (CSFPWM)i.   Phase Disposition PWM (PDPWM)ii. Phase Opposition Disposition PWM (PODPWM)iii. Alternate Phase Opposition Disposition PWM (APODPWM)c.  Variable Switching Frequency PWM (VSFPWM)

Single-Carrier sine PWM (SCSPWM):

Triangular carrier wave is compared with the sinusoidal reference signal to generate pulses.

Phase Shifted Carrier PWM (PSCPWM):All the triangular carriers have the same frequency and the same peak to peak amplitude.Phase shift between any two adjacent carrier waves

Øcr = , Øcr = 90° for 5 level

Phase Disposition PWM (PDPWM):All carriers (multiple) are selected with the same phase and frequency.

Phase Opposition Disposition PWM (PODPWM)

Carriers above the zero line of reference voltage out of phase with those of below this line by 180 degrees.

Alternate Phase Opposition Disposition PWM (APODPWM)

Each carrier of this method is phase shifted by 180 degrees from its adjacent one.

Variable Switching Frequency PWM (VSFPWM):

Carrier sets C1 and C4 will have the same frequencies, C2 and C3 will have the same frequencies.

Result:PWM Techniques 3rd Harmonic % 5th Harmonic % THD % V01 volt

SCSPWM 0.81 0.36 67.12 91.96

PSCPWM 1.48 0.35 27.96 139.7

PDPWM 0.48 0.08 26.15 142.1

PODPWM 0.64 0.61 26.10 142

APODPWM 1.03 0.74 26.48 141.2

VSFPWM 0.51 0.21 26.33 141.2