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Multilevel inverters: an enabling 4

technology

Jayabal Gayathrimonicka Subarnan
Adhi College of Engineering and Technology, Kancheepuram, India

1 Introduction

Multilevel Inverters are the most preferred choices for electronic power conversion
in high-power applications, and it seeks increased attention in industry and research
areas. It has been accepted globally as a matured and proven technology and has been
entrenched successfully in electronic power conversion system. Currently, they are
being utilized in standard and customized products that enable power to a extensive
range of applications, such as compressors, extruders, fans, blast furnace blowers,
gas turbine starters, mixers, grinding mills, rolling mills, conveyors, crushers mine
hoists, reactive power compensation, hydro pumped storage, wind energy conversion,
railway traction marine propulsion, high voltage direct current (HVDC) transmission
[1], and so on. Researchers over the entire world are trying to further improve energy
efficiency, simplicity, reliability, power density and cost of multilevel converters, and
enlarge their application field as they are competitive and attention seeking than other
topologies. Recently, many publications have presented multilevel inverter technology
and cited the growing importance of multilevel inverters for power quality and high-
power applications.
The unique structure of multilevel inverters allows them to achieve high voltages
with less harmonic content without transformers. The general purpose of the multi-
level inverter is to obtain a desired voltage from several levels of DC voltages. The
synthesized output waveform has more steps; as the number of levels increases, the
produced staircase wave that approaches the required waveform. As the number of
steps increase in the waveform, the harmonic distortion of the output wave decreases,
approaching zero. Multilevel inverter is structured such that no voltage-sharing prob-
lems are encountered by the active devices. Some of the applications these invert-
ers come under the sections: static var compensation, drives for electric motors,
back-to-back high-voltage intertie, and adjustable speed drives (ASD). In static var
compensation, only reactive power flows between the converter and the system but in
the case of motor drives, the converter must handle bidirectional real power flow [2].
Nowadays, industrial needs are demanding high power handling equipments, which
now reaches the megawatt level. In today's scenario, it is difficult to connect a sin-
gle power semiconductor switch directly to medium voltage grids (2.3, 3.3, 4.16, or
6.9 kV). Due to these reasons, a new group of multilevel inverters has emerged as the
solution for working with higher and medium voltage levels [3].

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