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Multilevel inverter
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Introduction of Power Electronics
• It is primarily concerned with application ofsolid state devices for conversion and control ofelectrical power.
• With the development of power semiconductortechnology, power handling capabilities andswitching speed of the power devices increasestremendously.
• And with the development of digital signalprocessor, we can easily achieved smoothcontrol over power semiconductor switches .
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Introduction of PowerElectronics
• Inverter fed AC motor drives is themaor application of power electronicscircuits !nown as adustable speed
drives "A#$%.
• PWM based inverter reduces theamplitudes of lower order harmonics in
the terminal voltage by shifting thedominating harmonics towards higherfre&uencies.
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Introduction of 'wo(levelinverter
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Waveform of two-levelinverter
• Produce pulsed outputvoltage waveform,which contain afundamentalcomponent and other
harmonics centre onthe switching fre&uencyand its multiples.
• )igher switchingfre&uency leads tobetter output voltageand current waveform,reduced harmonicscurrents and fastercontrol of drive.
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$isadvantage of 'wo(levelinverter
• #witching losses
• )igh d*+dt due to high switching fre&uency
•
Electromagnetic interference "EMI%
• #tress on the insulation of the machine windings
• Each devices has to be rated to bloc! entire $C
lin! voltage during its o time – #o application of two(level inverter is restricted to
low and medium power application.
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$e-nition of Multi(levelinverter
• eduction of disadvantage of two(levelinverter has been made possible by the useof dierent class of PWM inverters, termed
as Multi(level Inverter.
/Multi(level inverter are power converterscomposed by an array of semiconductor and
capacitor voltage sources, that when properlycontrolled, can generate stepped waveformoutput voltage with adustable fre&uency andamplitude0
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Advantage of Multi(level inverterwith respect to two(level inverter
• It is possible to obtain lower d*+dt across powersemiconductor devices.
• It is possible to reduce the EMI problems.
•
2ess stress on the insulation of machine phase windings.• It is possible to obtain machine current with reduced
harmonics, resulting into reduced tor&ue pulsations in thedrive system.
–
3nfortunately, multilevel converters do have somedisadvantages. 4ne particular disadvantage is the greaternumber of power semiconductor switches needed. Althoughlower voltage rated switches can be utili1ed in a multilevelconverter, each switch re&uires a related gate drive circuit. 'hismay cause the overall system to be more e5pensive and
comple5.
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66' Analysis
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')$ Comparision
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Applications
• )igh power drives li!e – 2aminators
– Mills
– Conveyors
– Pumps
– 6ans
– 7lowers
– Compressors etc8.
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Concepts of Multi(levelinverter
• Considering that m is the number ofsteps of the phase voltage with respectto the negative terminal of the inverter,
– 'hen the number of steps in the voltagebetween two phases of the load is ! 9 :m;
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Concepts of Multi(levelinverter
• 'he term multilevel starts with the three(level inverter introduced by =abae et al.
• 7y increasing the number of levels in theinverter, the output voltages have moresteps generating a staircase waveform,which has a reduced harmonic distortion.
• )owever, a high number of levelsincreases the control comple5ity andintroduces voltage imbalance problems.
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7asic 4perating Principle
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Multilevel Inverter 'opologies
• 'hree dierent maor multilevelconverter structures have beenapplied in industrial applications>
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Multilevel $iode Clamped +=eutral Point Inverter, =PCM2I
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Comparison of three(level inverterwith two(level inverter
• *oltage stress on the switches of a three(levelinverter is e&ual to half to that of a conventionaltwo(level inverter or e&ual to voltage across onecapacitor which is *dc+:.
• 'wo switches of a pole is always open for anyvoltage level and thus share the voltage stresse&ually.
• 'hus voltage rating of the switches is half the $Clin! voltage and the bloc!ing voltage rating of theclamping diode is also half the $C lin! voltage.
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• $iode in the -ve level con-guration has to berated for dierent voltages, which can be higherthan the bloc!ing voltage of the switching devices.
• 7loc!ing voltage of each clamping diode dependson its position in the structure.
• 6or n level inverter, voltage across clamping diodeis
*diode 9 *dc "n(
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#witching states of one -ve(level
phase leg
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E5planation
• $iode clamped multi(level inverter is also !nown asneutral point clamped because of mid voltage levelis formed by clamping the switching devices to theneutral point of the $C bus.
• 'herefore , the ma5imum voltage stress on theswitch is limited to the voltage across each
capacitor *dc+"m(
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• 6or an = level "between the phase and thenegative rail% $iode clamped inverter,
–
'he number of levels in the line(to(line voltagewaveform will be ! 9 :=(<
– 'he number of levels in the line to load neutralof a star or wye load will be p 9 :!(<
–
'he number of capacitors re&uired,independent of the number of phase, is =cap 9
=(<
– While the number of clamping diodes of
une&aul voltage rating per phase is $clamp 9:"=(
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6eatures
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Advantages
• When the number of levels is high ,the harmonics content is low enoughto avoid the need for -lters.
• Inverter eciency is high because alldevices are switched at thefundamental fre&uency.
• 'he control method is simple.
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$isadvantage
• Increase in the number of clamping diodesleads to a comple5 bus structure andma!e the =PC inverter system impractical
to implement when the number of levelsis suciently high.
• Capacitor voltage can uctuate over afundamental cycle "depending on the load
current drawn from the $C lin!% and thismeans that devices are subected bloc!higher voltages than the ideal condition.
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Multilevel Capacitor Clamped+6lying Capacitor Inverter, CCM2I
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#witching 'able for ? level 6C
inverter
PoleVoltage
Vao
Voltage
Level
State if theSwitch
Eect on ying capacitorvoltage
#< #: #? #B
*dc+: ; < < D D =o EectD D < D < D Charging
D D D < D < $ischarging
(*dc+: ( D D < < =4 Eect• #< and #B, as well as #: and #? are
two dierent complementary pairs ofthe switch.
6i 2 l 6l i C it
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6ive 2evel 6lying CapacitorInverter
4 l f 6l i C it
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4ne leg of 6lying CapacitorInverter
#witching state for level 6C Inverter
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#witching state for level 6C Inverter
Polevolta
ge
State of the switch
S1 S2 S3 S S! S" S# S$
V%c&2
1 1 1 1 ' ' ' '
V%c&
1 1 1 ' 1 ' ' '
' 1 1 1 ' ' ' 11 ' 1 1 ' ' 1 '
'
1 1 ' ' 1 1 ' '
' ' 1 1 ' ' 1 1
1 ' 1 ' 1 ' 1 '
1 ' ' 1 ' 1 1 '
' 1 ' 1 ' 1 ' 1
' 1 1 ' 1 ' ' 1
-
V%c&
1 ' ' ' 1 1 1 '
' ' ' 1 ' 1 1 1
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Advantage of edundancy
• As before with the =PCM2I only one switch need
to be opened and one to be closed to changeone state to another. 'his leads to that theinverter can be modulated at low "fundamental%switching fre&uency since a stepped sinusoidal
waveform can be created when every switch is
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E5planation
• 'his circuit is called the yingcapacitor inverter, as independentcapacitors clamp the device voltage
to one capacitor voltage level.• 'he switches in the pair always be in
complementary states.
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6eatures
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6eatures:. 7alancing capacitor voltage
– As we already !nown that, the switching combination for voltagelevel e5cept upper and lower one is greater than one.
– #uch redundancy of the multi(level con-guration is a uni&uefeature and it is an advantage to maintain the capacitor voltagebalance.
–
7ut redundant switching states, vital to capacitor voltagebalancing, are sacri-ced to achieve a higher number of outputvoltage levels.
– 'o produce same output voltage, the inverter has to switcheddierent combination so that it easier to manipulate thecapacitor voltage and !eep them at their proper value.
– #ince the same current ows through all the active capacitors ina state, energy can be transferred from more charged to lesscharged capacitors, balancing the capacitors voltages betweenthe capacitors that are conducting.
– 'hus by proper selection of switching combination , the 6C multi(level inverter may be used in real power conversion.
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Advantage
• 2arge amount of storage capacitorscan provide capabilities during poweroutages.
• Provides switch combinationredundancy for balancing dierentcapacitor voltage levels.
• )armonic content is low enough toavoid the need of -lters.
• 7oth real and reactive power ow
can controlled.
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$isadvantage
• An e5cessive number of storagecapacitor is re&uired when thenumber of levels is high. #o cost of
the converter becomes high.• 'he inverter control can be very
complicated.
• #witching fre&uency and switchinglosses are high for real powertransmission.
Cascaded Multi cell Inverter
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Cascaded Multi cell Inverter,CMCI
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Cascaded Multi cell Inverter, CMCI
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Introduction
• It is a relatively new converter structure, cascaded(inverters with separate dc sources.
• 'his new converter can avoid e5tra clamping diodesor voltage balancing capacitors.
• 'o comply with the de-nition of the previouslymentioned diode(clamp and ying(capacitormultilevel converters, the /level0 in a cascaded(inverters based converter is de-ned by m9:s;
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Advantage
• e&uires the least number of components among allmultilevel converters to achieve the same number ofvoltage levels.
• Modulari1ed circuit layout and pac!aging is possiblebecause each level has the same structure, and there areno e5tra clamping diodes or voltage balancing capacitors.
• #oft(switching can be used in this structure to avoid bul!yand lossy resistor(capacitor(diode snubbers.
• 6or real power conversions, "ac to dc and dc to ac%, the
cascaded(inverter needs separate dc sources. 'hestructure of separate dc sources is well suited for variousrenewable energy sources such as fuel cell, photovoltaicand biomass etc.
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$isadvantage
• =eeds separate dc sources for realpower conversions, and thus itsapplications are somewhat limited.