  Multilevel inverters (MLIs) are popular because of their advantages such as
improved output voltage quality, lower switching losses, low EMI, and ability
to handle higher voltage and power levels. To generate the desired output
voltage in the MLIs, there are different switching methods including
multicarrier pulse-width modulation (PWM), selective harmonic elimination
(SHE), and multilevel space-vector PWM (SVPWM) methods. The SHE method
minimizes the number of switching and hence the switching losses. Also, the
method eliminates or attenuates specified low-order harmonics and adjusts the
fundamental output voltage to the desired value. However, problems arise in the
low modulation indexes where the harmonic distortion of the output voltage
increases considerably. To solve the problem, this paper proposes a method to
adjust the DC voltage when the low output voltage is required. Therefore, the
modulation index will increase and the output voltage quality will be improved.
The proposed method is verified on a 7-level Cascaded H-Bridge (CHB) inverter
using the SHE method solved by particle swarm optimization (PSO) algorithm.
