Page 98 - Hybrid-Renewable Energy Systems in Microgrids

82 Hybrid-Renewable Energy Systems in Microgrids

Figure 5.1 Schematic of a seven level inverter in single phase.

According to Fig. 5.2 the multilevel positive voltage is fed to the full bridge con-
verter to generate its polarity. Then each full bridge converter will drive the primary
of a transformer. The secondary of the transformer is delta (∆) connected and can be
connected to a three-phase system.
This topology requires fewer components in comparison with conventional invert-
ers. Another advantage of the topology is that it just requires half of the conventional
carriers for sinusoidal pulse width modulation (SPWM) controller. SPWM for seven
level conventional converters consists of six carriers but in this topology three carriers
are sufficient. The reason is that according to Fig. 5.1 the multilevel converter works
only in positive polarity and does not generate negative polarities. So, it implements
the multilevel inverter with reduced number of carriers which is a great achievement
for inverter control. It is also comparable to single carrier modulation while this topol-
ogy requires the same number of signals for PWM. However, this topology needs one
modulation signal, which is easier to generate as opposed to the single carrier modu-
lation method which needs several modulation signals. Another disadvantage of the
topology is that all switches should be selected from fast switches while this topology
does not need fast switches for the polarity generation part [2].
In the following sections, the principle of operation for this topology with respect
to PWM switching and number of components is explained.