Comparison of Low-Frequency-Commutation Modulation Techniques in a
Symmetrical Cascaded H-Bridge Multilevel Inverter
Abstract
Among the power inverters, modular multilevel converters (MMC) have
gained popularity due to their advantages over conventional two-level
inverters. Although the MMC utilizes more power switches than
traditional inverters, the former exhibits notable benefits such as low
total harmonic distortion, low switching losses with lower voltage
stress, high power quality, reduced electromagnetic interface, and
modularity with numerous degrees of freedom to synthesize output
waveforms. Concerning the synthesized voltage quality, several
low-frequency modulation techniques have been proposed to produce output
voltage with low harmonic content. A particular case of study consisting
of a three-phase even-level MMC inverter is taken as a benchmark to
comparatively evaluate four methods for synthesizing high-quality output
voltage. The employed techniques are ( i) selective harmonic
elimination (SHE), ( ii) generalized SHE, ( iii) THD
Voltage Minimization, and ( iv) Optimum Nearest Level Modulation,
selected based on the phase and line-to-line output voltage THD. The
model is formulated to compute the commutating angles to control the MMC
submodules. Simulations show individual performance under the same
operating conditions. A lab-scale prototype is built to corroborate the
theoretical approach. The results allow the selection of the most
convenient modulation technique based on the number of commutations and
harmonic spectrum.