Cascaded inverters are ideal for connecting renewable energy sources with an AC grid, because of the need for separate dc sources, which is the case in applications such as photovoltaic or fuel cells. The inverter could be controlled to either regulate the power factor of the current drawn from the source or the bus voltage of the electrical system where the inverter was connected. The modulation techniques are crucial in operating any inverter at desired conditions. In this work different PWM techniques are implemented for five level cascaded multilevel inverter and THD variation is analyzed.

1. Harmonic comparisons of various
PWM techniques for basic MLI
By
MD SAQUIB MAQSOOD
M.Tech, PED

2. Multilevel Inverter:
Number of Level = 2m+1
m = number of source

3. 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)

4. Single-Carrier sine PWM (SCSPWM):
Triangular carrier wave is compared with the sinusoidal reference signal to
generate pulses.

5. 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 =
360°
𝑚−1
, Øcr = 90° for 5 level

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

7. 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.

8. Alternate Phase Opposition Disposition PWM
(APODPWM)
Each carrier of this method is phase shifted by 180 degrees from
its adjacent one.

9. Variable Switching Frequency PWM (VSFPWM):
Carrier sets C1 and C4 will have the same frequencies, C2 and C3
will have the same frequencies.

10. 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