19-06-2014, 01:13 PM
CONCLUSIONS AND FUTURE SCOPE
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CONCLUSIONS
In the field of high power, high performance applications the
multi-level inverters seem to be the most promising alternative. In
multilevel inverters, as the switching involves several small voltages,
the rapid change in voltage is smaller. Further, switching at the
fundamental frequency will also result in decreasing the number of
times these voltage changes occur per fundamental cycle. But
harmonic elimination is the major issue for multilevel inverters. Space
vector pulse width modulation is considered a better technique of
PWM implementation owing to its associated advantages like better
fundamental output voltage, better harmonic performance and easier
to implementation in digital signal process and micro controllers. The
SVPWM scheme is more popular than conventional technique because
it gives 15% more output voltage than SPWM and prevents unnecessary
switching, hence less commutation losses. These space
vector based algorithms generate not only the desired fundamental
frequency voltages, but also eliminate the harmonics up to the
maximum extent and results in reduced total harmonic distortion
(THD).
This thesis, mainly focuses on the various space vector pulse
width modulation based algorithms for multilevel neutral point
clamped inverter fed induction motor are proposed and implemented.
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The results have been presented and analyzed. The effectiveness of
these algorithms have been discussed in terms of inverter output
voltage, current waveforms, total harmonic distortion, speed of
induction motor and torque ripple.
The application of space vector pulse width modulation control
strategy on three-level voltage source inverter is proposed and
analyzed. The main aim of this method is to prove the effectiveness of
SVPWM in the contribution of switching power losses reduction and to
show the advantage of three-level inverter that carry out voltages with
contents of less harmonic injection than the two-level inverter. On the
other hand from the results, it is observed that as the modulation
index is increased the total harmonic distortion (THD) decreases and
also the fundamental RMS value increases linearly. By using this
method, the results obtained for THD in two-level and three-level
inverters are 54.02% and 28.60% respectively.
But the level of inverter increases, the complexity of control
algorithm and execution time increases. Hence, to minimize the
drawbacks of conventional SVPWM algorithm, a new algorithm based
on fractals has been proposed for the generation of space vector pulse
width modulation for multilevel inverters and applied for three-level
and five-level inverters. From the results, it is observed that the
inverter performance is highly improved. The output voltage total
harmonic distortion for three-level and five-level inverters is 5.7% and
3.61% respectively.
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Further, to improve the performance of multilevel inverters a
qualitative space vector pulse width modulation algorithm has been
proposed and applied to three-level, four-level, five-level, six-level and
seven-level inverters. In this proposed scheme, the duty cycles of
reference voltage vectors are corrected to, easily, identify the location
of the reference voltage vector in each region, the appropriate
switching sequence of the region and calculation of the switching ontimes
for each state. The scheme can be easily extended to high-level
inverters. From the results, it is observed that the generated voltage
spectrum is very much improved with increase the level of inverter.
The total harmonic distortion (THD) is highly reduced as the level of
inverter is increased. The input current drawn by the induction motor
is less distorted as the level of inverter increases. From the above
analysis, the total harmonic distortion (THD) with the proposed
method for two-level, three-level, four-level, five-level, six-level and
seven-level inverters are 42.48%, 24.99%, 17.05%, 11.57%, 6.71%
and 4.67% respectively.
In addition to the previous algorithms, an analytical space
vector pulse width modulation (SVPWM) algorithm based on an
intrinsic relationship between multilevel and two-level inverters is
presented for a multilevel inverter fed induction motor drive. By using
a linear transformation, the switching time of vectors for two-level
inverter can be transformed for multi-level inverter. A novel
classification of voltage vectors is proposed to determine switching
sequence. This method is verified by designing a model for N-level
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inverter and simulated as three-level, five-level, seven-level, nine-level
and eleven-level inverters. The line-to-line voltages and harmonic
spectrum of three-level, five-level, seven-level, nine-level and elevenlevel
inverters highly improved compared to the traditional SVPWM
algorithm and it shows that the level of inverter is increasing the line
voltage is nearly standard sine wave. The obtained total harmonic
distortion (THD) with the proposed method for three-level, five-level
seven-level, nine-level and eleven-level inverters are 16.92%, 4.35%,
2.45%, 2.26% and 2.13% respectively. Hence this method is more
flexible and it can be extended for N-level inverter also.
Finally, the space vector pulse width modulation based
algorithm using decomposition method has been proposed and
described for a seven-level inverter. In this method, the space vector
diagram of the seven-level inverter is decomposed into six space vector
diagrams of two-level inverters in a few steps. After decomposition, all
the remaining necessary procedures for the seven-level SVPWM are
proposed like conventional two-level inverter. This then allows the
determination of switching sequence and calculation of the switching
times as for a conventional two-level inverter. Thus, the proposed
method reduces the algorithm complexity and the execution time. It
can be applied to the multi-level inverters above the seven-level also.
The total harmonic distortion (THD) with the proposed method for
three-level, five-level and seven-level are 5.93%, 2.79% and 1.51%
respectively. The results have been presented and analyzed in this
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thesis. The results have been good agreement with the published work
and many new cases have been given.
5.2 FUTURE SCOPE
This thesis work focuses on space vector pulse width
modulation based algorithms for multilevel inverters. The SVPWM
algorithms proposed in chapter-3 and chapter-4 have essentially been
aimed at reducing the harmonic distortion in the output voltage of
multilevel inverters. The performance of all these algorithms can be
evaluated in the over modulation zone. Analytical evaluation of
harmonic distortion for multilevel inverters can also be carried out.
More SVPWM based techniques can be developed for inverter
switching at much higher frequencies and hardware implementation
can also be done. All the proposed algorithms in this thesis are for
time-invariant systems. Therefore, it is recommended to eliminate
harmonics for time-variant systems. Further to reduce the switching
losses of the inverters, discontinuous pulse width modulation
algorithms have to be proposed.