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Multilevel inverters for 5

photovoltaic energy systems in

hybrid-renewable energy systems

Ehsan Najafi*, Natarajan Prabaharan**
*Faculty of Electrical and Computer Engineering, Qom University of Technology,
Qom, Iran; **Department of Electrical and Electronics Engineering, Madanapalle
Institute of Technology and Science, Madanapalle, Andhra Pradesh, India

1 Multilevel inverter topology

A multilevel inverter topology that is suitable for PV applications should have re-
duced number of switches. One of the techniques to put the idea into practice is to
make hybrid multilevel inverters. In these class of inverters, all inverter switches do
not need to work in high frequency to generate the required output voltage, but they
are separated into two main parts. One part is responsible for generating absolute re-
quired output voltage levels. This part is working in high frequency but only generates
positive levels. In contrast, next part, that is connected to the output, generates the re-
quired output polarity. This part does not need to work with high switching frequency
and only changes with the line frequency.
This theory greatly reduces the number of switches and makes the multilevel
inverters more economic. In 2011, this theory was put into practice as reversing
voltage (RV) topology #1 [1]. The RV topology in seven levels is depicted in
Fig. 5.1. As can be seen, it requires 10 switches and 3 isolated sources. The prin-
ciple idea of this topology as a multilevel inverter is that the left stage in Fig. 5.1
generates the required output levels (without polarity) and the right circuit (full
bridge converter) decides about the polarity of the output voltage. This part, which
is named Polarity Generation, transfers the required output level to the output with
the same direction or opposite direction according to the required output polarity.
It reverses the voltage direction when the voltage polarity requires to be changed
for negative polarity.
This topology easily extends to higher voltage levels by duplicating the middle
stage as shown in Fig. 5.1. Therefore, this topology is modular and can be easily in-
creased to higher voltage levels by adding the middle stage in Fig. 5.1.
It can also be applied for three-phase applications with the same principle. This to-
pology uses isolated DC supplies. Therefore, it does not face voltage-balancing prob-
lems due to fixed DC voltage values. In comparison with cascade topology, it requires
just one third of isolated power supplies used in cascade type inverter. In Fig. 5.2 the
complete three-phase inverter for seven levels is depicted with a three-phase delta
connected system.

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