10-11-2012, 05:20 PM
Hysteresis Modulation of Multilevel Inverters
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Abstract
The hysteresis modulation for power electronic converters
is attractive in many different applications because of its
unmatched dynamic response and wide command-tracking bandwidth.
Its application and benefits for two-level converters are well
understood, but the extension of this strategy to multilevel converters
is still under development. This paper summarizes and reviews
the various hysteresis modulation approaches available in the literature
for multilevel converters. The pros and cons of various techniques
are described and compared for tracking the reference signal
in order to attain an adequate switching optimization, excellent
dynamic responses and high accuracy in steady-state operation. By
using the recently developed multilevel hysteresis modulation approaches,
the advantages of using several accessible dc potentials
in a multilevel inverter have been fully exploited. All of these hysteresis
modulation approaches are tested for tracking a current
reference when applied to a five-level inverter. The relevant simulation
and experimental results are also presented. This study will
provide a useful framework and point of reference for the future
development of hysteresis modulation for multilevel converters.
INTRODUCTION
THE hysteresis modulation for power electronic converters
are preferred for applications, where performance requirements
are more demanding such as to achieve good dynamic
response, unconditional stability, and wide command-tracking
bandwidth [1], [2]. In this approach, the controlled system variable
is compared against hysteresis band(s) to create the switching
commands for the converter. This control has been widely
used to control the conventional two-level converter, showing
its robustness and simplicity in a lot of applications [3]–[13]. A
brief description of the standard two-level hysteresis control for
output current regulation is presented in the following.
The objective of standard two-level hysteresis current control
is to switch the converter transistors in such a manner that
the converter load current tracks a reference within a specified
hysteresis band. Consider a single-phase half-bridge inverter, as
shown in Fig. 1(a) for two-level hysteresis current control. In
Fig.
MB HYSTERESIS MODULATION
TheMBhysteresismodulation scheme for themultilevel converters
uses symmetrical hysteresis bands to control the switching
so that the inner band causes switching between adjacent
levels, while the outer band causes an additional switching level
change whenever necessary. The process, first proposed in [15]
and later used in [22], [26], [31], [32], is shown in Fig. 3 in the
form of current regulation. Whenever the current error crosses
the inner boundary B, the inverter output is decreased or increased
by one level (depending on which hysteresis boundary
has just been crossed). Generally, this voltage change will cause
the current error to reverse its direction without reaching the
next outer band. However, if the error does not reverse, it will
continue through the boundary of B to the next outer boundary
(placed atΔB out ofB). At this point, next higher or lower level
voltage will be switched. This process continues as discussed
earlier until the current error direction reverses.
MOB HYSTERESIS MODULATION
As opposed to the MB scheme, which uses symmetrically
placed hysteresis bands for current error regulation, the MOB
scheme uses the bands placed with an offset around the zerocurrent
error line. The advantage of using the offsets is that
different bands can be easily implemented. Also, the corresponding
logic can also be easily programmed/implemented in
a way that if the voltage appearing at the boundary of a band
is insufficient to force the error back, it is allowed to move to
the other band. As opposed to the previously presented scheme,
fixed voltage levels are applied in MOB scheme as the current
error crosses a boundary of the band with a certain slope. In this
section, first the conventional MOB scheme is presented, and
then, its modified version is presented, which offers improved
performances.
TB HYSTERESIS MODULATION
As discussed earlier, although the MOB schemes are easy
to implement [15], it requires offset compensation signals to
be added to the controlled system variable, since the bands are
not symmetric about zero. The MB scheme, presented earlier
in Section II, does not suffer from this steady-state-tracking
error problem, but may still not have evenly symmetric current
error waveform, especially for nonsinusoidal current references.
In the following, a TB MHM is first described, which works
on the principle of controlling the system variable within a
single band so that any type of current offset can be avoided.
Then, a modified TB approach for MHM is discussed, which
shows much better performances in terms of tracking as well as
can be used with a limit on the maximum allowable switching
frequency.
COMPARISON OF THE MHM SCHEMES
In the previous sections, various methods for hysteresis modulation
of multilevel converters have been described. As discussed
earlier, though the implementation of MOB method is
easier, it introduces a steady-state tracking error due to the offset
placements of the hysteresis bands. This limitation is particularly
more severe in the higher level inverters. To counteract
this dc offset, an offset compensation strategy to ensure zero
average current error within each switching period is required
for improved performance [15]. The MOB technique is robust,
but has the general limitation of requiring increasingly complex
analog circuitry for implementing the multiple hysteresis
bands and offset compensation as the number of voltage level
increases [15]. Further, the conventional MOB method has the
limitation of skipping of the intermediate voltage level between
two successive switching for higher level inverter. The MMOB
method, however, does not suffer from this limitation and can
be applied to higher level inverter system as well. However, due
to its main limitation of introducing the steady-state tracking
error, the MOB method has not found wide applications.
CONCLUSION
This paper summarizes and reviews the various hysteresis
modulation techniques available in the literature for the multilevel
converters. This includes, in general, the MB, MOB, and
TB hysteresis modulation techniques. To generalize the existing
MHM techniques for higher level inverters, their modified
versions have been also discussed. The basic principle of operation
and logical sequence of the design choices has been
described for each of these schemes. The advantages of using
various accessible dc voltage levels have been fully exploited
by using these schemes. The various schemes considered in
this paper have been further investigated using simulation and
experimental studies for a five-level inverter system. However,
these strategies can easily be extended to any multilevel inverter
structure, even in the case of n-level voltage waveforms and
three-phase systems.