CHAPTER has advantages over the conventional two-levels inverters

                                                          
CHAPTER 1

                                             
INTRODUCTION

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INTRODUCTION

         Multilevel
inverter continues to receive more attention because of their high voltage
capability and efficiency, high EMI. Low switching
losses. Nowadays multilevel inverters are becoming increasingly popular in
power applications , as multilevel inverter are becoming increasingly popular
in power applications, as multilevel inverters have the ability to meet the
increasing demand of power rating and power quality with reduced harmonic
distortion and lower EMI

       Multilevel inverter has advantages over
the conventional two-levels inverters that uses several switches to achieve for
high switching frequency Pulse Width Modulation(PWM).
Multilevel
inverter uses number of power semiconductor switches, dc
sources(batteries/capacitors) to synthesize staircase output voltage waveform.
By increasing number of levels the output voltage waveform approaches near to
sine wave improving its quality. Due to these advantages they found wide
applications in adjustable speed drives, HVDC, FACTS, wind farms, photovoltaic
systems, electric vehicles and so on

    
Multilevel inverter has advantages over the conventional two levels
inverter that uses several switches to achieve  for high switching frequency Pulse width modulation(PWM).Multilevel
inverter has some features are as follow:

 Low
distortion and lower dv/dt  can be
generated. Multilevel
inverter can draw input current with very low distortion.Multilevel inverter generate small
common mode voltage.Multilevel inverters can be operate with
lower switching frequency 

MLI
STRUCTURES

                Multilevel inverters have an
arrangement of power switching devices and capacitor voltage sources.
Multilevel inverters are suitable for high-voltage applications. It have
ability to synthesize output voltage waveforms with a better harmonic spectrum
and it  attain higher voltages with a
limited maximum device rating. 

There
are three main types of multilevel inverters:

Diode-clamped (neutral clamped)Capacitor-clamped (flying capacitors)Cascaded H bridge inverter 

                                                                                                                    

 

 

   

 

    

 

              Fig.1.1 Classification  of multilevel inverters

                                                                                                                                                                                                                                                               
                                                                                                                                                                                                                                                                  1.2.1Diode Clamped

               The diode –clamped inverter is
also known as the neutral-point clamped inverter(NPC) which was introduced by
nabe at al(1981).The diode-clamped inverter consist of two pairs of series
switches (upper and lower)in parallel with two series capacitors where the
anode of the upper diode is connected to the midpoint(neutral) of the
capacitors and its cathode to the midpoint of the upper pair of switches; the
cathode of the lower diode is connected to the midpoint of the capacitors and
divides the main DC voltage into smaller voltages. The middle point of the two
capacitors can be defined as the “neutral point”. The NPC uses  single DC bus that is subdivided into a
number of voltage levels by series strings of capacitors.

1.2.1.1Operation
of Diode-clamped

              Three
level diode-clamped converter in which the dc bus consist of two capacitor,. For dc-bus voltage , the voltage across
each capacitors is  and each device voltage stress will be limited
to one capacitor voltage level  through clamping diodes. To explain how the
staircase voltage is synthesized, the neutral point n is considered  as the output phase  voltage reference  point. There are three switch combination to
synthesize three-level voltages across a and n.

Voltage level  turn on the switches  and Voltage level, turn on the switches  and Voltage level  turn on the switches

           For a three-level diode-clamped
inverter if point 0 is taken as the ground reference, the output voltage has
three states 0, +   and Vdc . The
line-line voltages of two legs with the capacitors are  ,, -. Three
phases are necessary to generate a three-phases voltages.

                                                                                                                        

       

                              Fig.1.2 Diode
clamped MLI  

1.2.1.2
Features of Diode-clamped

High voltage rating required for
blocking diodesUnequal device ratingCapacitor voltage unbalance

1.2.1.3
Diode- Clamped MLI Application

Static var compensationVariable speed motor drivesHigh voltage system interconnectionsHigh voltage dc and ac transmission lines

1.2.1.4Advantage
of Diode-Clamped

All of the phases share a common dc bus,
which minimize the capacitance requirements of the converter. For this reason,
a back-back topology is not possible but also practical for uses such as a
high-voltage back-back inter- connection or an adjustable peed drive.The capacitors can be pre-charged as a group.Efficiency is high for fundamental
frequency switching. When the number of levels is high
enough, harmonic content will be low enough to avoid the need for filters .

1.2.1.5
Disadvantage of Diode-clamped

Real power flow is difficult for a
single inverter because the intermediate dc levels will tend to overcharge or
discharge without precise monitoring and control.The number of clamping diodes is
quadratically related to the number of levels, which can be cumbersome for
units with a high number of levels.

1.2.2
Capacitor-clamped

            The capacitor clamped inverter
alternatively known as flying capacitor was proposed by meynard and foch in
1992. The structure of this inverter is similar to that of diode clamped
inverter except that instead of using clamping diodes, the inverter uses
capacitor involves series connection of capacitor clamped switching cells. This
topology has a ladder structure of dc side capacitor, where the voltage on each
capacitor differs from that instead of next capacitor. The voltage increment
between two adjacent capacitor legs gives the size of the voltages steps in the
output waveform.

1.2.2.1
Operation of FCMLI

               
In the operation of flying capacitor multi-level
inverter, each phase node( can  be
connected to any node in the capacitor bank, . Connection of the
a-phase to positive node  occurs when  and  are turned on and to the neutral point voltage
when  and  are turned on. The negative node  is connected when  and  are turned on. The clamped capacitor c1 is
charged when , ans  are turned on and is discharged when  and  are turned on the charge capacitor can be
balanced by proper selection of the zero states. In comparisons to three-level
diode-clamped inverter, an extra switching state is possible. In particular,
these are two transistor states which make up the level. Considering the
direction of the a phase flying capacitor current  for the redundant states, a decision can be
made to charge or discharge the capacitor and therefore , the capacitor voltage
can be regulated to its desired value by switching within the phase. As with
the three-level flying capacitor and therefore, the capacitor voltage can be
regulated to its desired value by switching within the phase.as with the
three-level flying capacitor inverter, the highest and lowest switching states
do not change the charge of the capacitors. The two intermediate voltage level
contain enough redundant states so that both capacitors. The two intermediate
voltage levels contain redundant states so that both capacitors can be
regulated  to their ideal voltages. Similar
to the diode clamped inverter clamping requires a large number of bulk
capacitor to clamp the voltage. Provided that the voltage rating of each  capacitor used is the same as that of the
main power switch, an N level converter will require a total of  clamping 
the flying-capacitor inverter does not require all of the switches that
are on (conducting) in a consecutive series. Moreover, the flying-capacitor inverter
has phase redundancies 1, 3. These redundancies allow a choice of  charging /discharging specific
capacitors  and can be incorporated in
the control system for balancing the voltages across the various levels. The
voltage synthesis in a five level capacitor-clamped converter has more flexibility
than a diode-clamped converter. Capacitor-clamped multilevel inverter
topologies are relatively new compared to the diode-clamped or the cascaded
H-bridge cell inverter topologies. 
Redundancy in the switching states is available by using flying capacitors
instead of clamping diodes. This redundancy can be used to regulate the
capacitor voltages and obtain the same desired level of voltage at the output.
Figure 1.3 shows a single-phase five-level capacitor-clamped multilevel
inverter topology. The voltage across the capacitors is considered to be half
of DC source voltage Vdc . The output voltage consists of five
different voltage level  Vdc,-Vdc,0,Vdc,
 Vdc .

 

                       Fig.1.3 Topology of
capacitor clamped MLI

1.2.2.2
Features  of FCMLI

              The major
problem in this inverter is the requirement of a large number of storage
capacitors. Provided that the voltage rating of each capacitor used is the same
as that of the main power switch, an m-level converter will require a total of  auxiliary capacitors per phase leg in addition
to ) main dc bus
capacitors. With the assumption that all capacitors have the same voltage
rating, an m-level diode –clamp inverter only requires  capacitors. In order to balance the capacitor
charge and discharge, one may employ two or more switch combinations for middle
voltage levels(i.e.,  and ) in one or several
fundamental cycles. Thus, by proper selection of switch combinations, the
flying-capacitor multilevel converter may be used in real power conversions.
However, it involves real power conversions, the selection of a switch
combination becomes very complicated, band the switching frequency needs to be
higher than the fundamental frequency.

1.2.2.3
Advantages of  FCMLI

Large ‘n’ allows the capacitors extra
energy during long discharge transient.Phase redundancies are available for
balancing the voltages levels of the capacitors Lower total harmonic distortion when the
number of levels ‘n’ is highActive and reactive power flow can be
controlled.Added clamping diode are not needed.The required number of voltage level can
be achieved without the use of the transformer. The assists in reducing the
cost of the converter and again reduces power loses Series string of capacitor clamped share
the same voltage.The capacitor within the phase leg are
charged to different levels. The large number capacitors enables the
inverter for deep voltage sag. 

1.2.2.4
Disadvantage of FCMLI

Large numbers of capacitors are bulky
and more expensive than the clamping diodes used in the diode-clamped
multilevel inverter.Complex control is required to maintain
the capacitors voltage balanceSwitching utilization and efficiency are
poor for real power transmissionPre-charging all of the capacitors to
the same voltage level and start-up are complex.The capacitor have large fraction of the
dc bus voltage,The large number of capacitor are used
in FCMLI so it is  more expensivePackaging the inverter  with number of levels are more difficult.

1.2.3
Cascaded  multilevel inverter

            The cascaded H-bridge inverter has
tremendous interest due to the greater demand of medium–voltage high power
inverter. The high power inverter to construct multilevel phase legs with
separate dc sources. One more alternatives for a multilevel inverter is the
cascaded multilevel inverter or series H-bridge inverter. The series H- bridge
inverter appeared in 1975. Cascaded multilevel inverter was not fully realized
until two researchers, lai and peng. They patented it and presented its various
advantage in 1997. Since then ,CMI 
synthesis its output nearly sinusoidal voltage waveforms by combining
many isolated voltage levels. By adding more H-bridge converter, the amount of
var can simply increase without  redesign
the power stage, and build-in redundancy against individual H-bridge converter
failure an be realized. A series of single-phase full bridge make up a phase
for the inverter. A three phase CMI topology is essentially composed of three
identical phase legs of the series-chain of H bridge converters, which can
possibly generate different output voltage waveform and offers the potential
for AC system phase-balancing. This features is impossible in other VSC
topologies utilizing a common dc link. Since this topology consists of series
power conversion cells, the voltage and power level may be easily scaled. The
dc link supply for each full bridge converter is provided separately, and this
is typically achieved using diode rectifier fed from isolated secondary
windings of a three-phase transformer. Phase-shifted transformer can supply the
cell in medium –voltage system in order to provided high power quality at the
utility connection.

1.2.3.1
Operation of cascaded H bridge inverter

              The
output of each h-bridge can have three discrete level, result in a staircase
waveform that is nearly sinusoidal even without filtering. A single
H-bridge  a three-level inverter, each
single phase full bridge inverter generates three level inverter, each
single-phase full-bridge inverter generates three voltages at the output – ,0 and

 

         

                      Fig.1.4 single H- bridge
topology

The
four switch S1,S2,S3 and S4 are
controlled to generate three discrete output with levels,-, ,
0 and .  When S1 and S2 are on,
the outputs is : when either pair S1
and S3 or S2 and S4 are on, the output is 0 figure
1.5 Shows a single-phase, five-level cascaded H-bridge cell inverter realized
by connecting two three a level conventional full bridge inverters in series
was presented in Tolbert et al (1999). Switch pairs S1and S3
and S2 and S4 are complementary to each other. The
different voltage levels  that can be
obtained at the output terminals are,-, 0, ,.
If the dc voltage sources in both the inverter circuits connected in series are
not equal to each other, then nine levels can be obtained at the output
terminals. The number of levels in the output voltage can be increased by two
by adding an identical inverter in series. The n number of output phase voltage
levels in  a cascaded inverter with
separate dc sources   is 2(n-1)s possible
levels.

                     Fig.1.5 five level
cascaded H-bridge inverter

1.2.3.2
Advantage of cascaded H-Bridge inverter

The series structure allows a scalable,
modularized circuit layout and packaging due to the identical structure of each
H-bridge. No extra clamping diodes or voltage
balancing capacitors are necessary. Switching redundancy for inner voltage
levels is possible because the phase voltage is the sum of the output of each
bridge.