10-11-2012, 04:03 PM
Multilevel Inverters: A Survey of Topologies, Controls, and Applications
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Abstract
Multilevel inverter technology has emerged recently
as a very important alternative in the area of high-power
medium-voltage energy control. This paper presents the most
important topologies like diode-clamped inverter (neutral-point
clamped), capacitor-clamped (flying capacitor), and cascaded
multicell with separate dc sources. Emerging topologies like
asymmetric hybrid cells and soft-switched multilevel inverters are
also discussed. This paper also presents the most relevant control
and modulation methods developed for this family of converters:
multilevel sinusoidal pulsewidth modulation, multilevel selective
harmonic elimination, and space-vector modulation. Special
attention is dedicated to the latest and more relevant applications
of these converters such as laminators, conveyor belts, and unified
power-flow controllers. The need of an active front end at the
input side for those inverters supplying regenerative loads is also
discussed, and the circuit topology options are also presented.
Finally, the peripherally developing areas such as high-voltage
high-power devices and optical sensors and other opportunities
for future development are addressed.
INTRODUCTION
IN RECENT YEARS, industry has begun to demand higher
power equipment, which now reaches the megawatt level.
Controlled ac drives in the megawatt range are usually connected
to the medium-voltage network. Today, it is hard to connect
a single power semiconductor switch directly to mediumvoltage
grids (2.3, 3.3, 4.16, or 6.9 kV). For these reasons, a
new family of multilevel inverters has emerged as the solution
for working with higher voltage levels [1]–[3].
Multilevel inverters include an array of power semiconductors
and capacitor voltage sources, the output of which
generate voltages with stepped waveforms. The commutation
of the switches permits the addition of the capacitor voltages,
which reach high voltage at the output, while the power
semiconductors must withstand only reduced voltages.
Cascaded Multicell Inverters
A different converter topology is introduced here, which is
based on the series connection of single-phase inverters with
separate dc sources [7]. Fig. 4 shows the power circuit for one
phase leg of a nine-level inverter with four cells in each phase.
The resulting phase voltage is synthesized by the addition of
the voltages generated by the different cells. Each single-phase
full-bridge inverter generates three voltages at the output: ,
0, and . This is made possible by connecting the capacitors
sequentially to the ac side via the four power switches.
The resulting output ac voltage swings from 4 to 4
with nine levels, and the staircasewaveform is nearly sinusoidal,
even without filtering.
Multilevel SPWM
Several multicarrier techniques have been developed to reduce
the distortion in multilevel inverters, based on the classical
SPWM with triangular carriers. Some methods use carrier disposition
and others use phase shifting of multiple carrier signals
[38], [43], [44]. Fig. 13(a) shows the typical voltage generated
by one cell for the inverter shown in Fig. 4 by comparing a sinusoidal
reference with a triangular carrier signal.
A number of –cascaded cells in one phase with their carriers
shifted by an angle and using the same
control voltage produce a load voltage with the smallest distortion.
The effect of this carrier phase-shifting technique can
be clearly observed in Fig. 14. This result has been obtained
for the multi-cell inverter in a seven-level configuration, which
uses three series-connected cells in each phase.
A. Multilevel Rectifier
Traditionally, multipulse rectifiers have been used for the reduction
of harmonics in the line current. These multipulse (12-
pulse, 18-pulse, and so on) rectifiers use transformers for phase
shifting in order to eliminate harmonics. To eliminate the phaseshift
transformers, multilevel rectifiers have been proposed.
For those applications that require no regenerative capability,
simplified (or reduced) multilevel rectifiers have been proposed
in [64]. This specific rectifier, named the Vienna rectifier, has
been used for telecommunication power supplies. Fig. 22 shows
the per-phase leg structure for a three-level Vienna rectifier.
Some reduced-parts-count multilevel rectifiers for the number
more than three levels have been proposed [65].
CONCLUSION
This paper has provided a brief summary of multilevel
inverter circuit topologies and their control strategies. Different
applications using different inverter circuits were also
discussed. As mentioned in Section I, an early patent for the
cascaded multilevel inverter can be traced back to 1975. However,
the commercial products that utilize this superior circuit
topology were not available until the mid-1990s. Today, more
and more commercial products are based on the multilevel
inverter structure, and more and more worldwide research
and development of multilevel inverter-related technologies is
occurring. This paper cannot cover or reference all the related
work, but the fundamental principle of different multilevel
inverters has been introduced systematically. The intention of
the authors was simply to provide groundwork to readers interested
in looking back on the evolution of multilevel inverter
technologies, and to consider where to go from here.