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CAPACITOR VOLAGE BALANCING OF A SPACE VECTOR MODULATION BASED ON THREE LEVEL INVERTER
Features of Multilevel inverter-
1- Multilevel inverter can eliminate need for step up
transformer.
2- Reduce harmonics produced by the inverter
3- Output voltage and power increase with number
of levels.
4- Harmonic content decreases as number of levels
increases and filtering requirements are reduced.
5- In absence of any PWM techniques , switching
losses can be avoided .
TYPES OF MULTILEVEL INVERTER
There are different configurations of multilevel inverter , namely -
1- Diode Clamped Multilevel Inverter,
2- Flying Capacitor Multilevel inverter,
3- Cascaded H-Bridge Multilevel Inverter
1-Diode Clamped Multilevel Inverter
Features -
1- High voltage rating for blocking diodes
2- Switching Device
3- Capacitor voltage unbalance
Diode Clamped Multilevel Inverter
Advantages :
 When number of levels is large in a multilevel inverter, harmonic content is low which avoids the need of harmonic filters.
Efficiency is high since all devices are switched at the fundamental frequency.
Reactive power flow can be controlled.
The control method is simple for a back-to-back intertie system.
Diode Clamped Multilevel Inverter
Disadvantages :
1- Excessive clamping diodes are required when the
number of levels is high.
2- It is difficult to control the real power flow of the
  individual inverter in multi-inverter systems.
Diode Clamped Multilevel Inverter
Switching states for NPC inverter 
Flying Capacitor Multilevel inverter
Features of FCMLI–
1-Large Number of capacitor
2-Balancing capacitor voltage
Flying Capacitor Multilevel inverter
Advantages of FCMLI:
1- FCMLI provides switch combination redundancy
for balancing different voltage levels.
2- Like NPC inverter with more levels , harmonic
content is also low enough to avoid the need of
harmonic filters in FCMLI.
3- Both real and reactive power flow controlled .
Flying Capacitor Multilevel inverter
Disadvantages of FCMLI :
1- A large number of storage capacitors are required
when the number of inverter levels is high .
2- The switching frequency and switching losses are
high .
Flying Capacitor Multilevel inverter
Switching states for FCMLI
Cascaded H-Bridge Multilevel Inverter
Features-
1- For real power conversions,(ac to dc and dc to ac)
cascaded-inverter needs separate dc sources.
2- Circuit will be introduced when two back-to-
back converters are not switching synchronously.
Cascaded H-Bridge Multilevel Inverter
Advantages -
1- It requires least number of components among all
multilevel inverters.
2- Optimized circuit layout and packaging are possible
3- Soft - switching techniques used in for reducing
switching losses and device stresses.
Cascaded H-Bridge Multilevel Inverter
Disadvantage-
 Needs separate DC sources for real power
conversion, thus its applications are limited
Cascaded H-Bridge Multilevel Inverter
PWM – Pulse Width Modulation
-> Also known as Pulse Duration Modulation
-> PWM- There width of pulses of carrier pulse train is
varied in accordance with modulating signal
Characteristics-
-> Controls power received by a device
-> The voltage seen by the load is directly proportional to
source voltage.
-> Instantaneous power of transmitter varies
-> Width of pulse is proportional to amplitude of modulating
signal
Multilevel PWM method
Multilevel carrier based PWM
Selective Harmonic Elimination
Multilevel Space Vector PWM
Sinusoidal PWM
 Multilevel Converter Modulation Strategies-
Multilevel carrier based PWM
DC utilization of THPWM & SVM in linear modulation
region is better than SPWM.
DC utilization = output fundamental voltage
dc page link voltage
Other Types - Sub harmonic PWM
- Switching frequency optimal PWM
Sub Harmonic PWM –
Reference signal continuously compared with each of carrier signal.
Case -1- Reference signal > Carrier signal => active device switched ON
Case -2- Reference signal< Carrier signal => active device switched OFF
Selective Harmonic Elimination -
- The method is implemented based on optimization techniques.
- The optimization starting point is obtained using a phase-shift
harmonic suppression approach
- It provide the highest quality output among all the PWM methods.
Advantage-
- It allows the achievement of output waveforms at a reduced.
switching frequency
- used for high-power applications
Disadvantage – Its complicated hardware implementation.
Selective Harmonic Elimination-
Virtual Stage PWM
Virtual stage PWM = Unipolar Programmed + Fundamental Frequency
PWM Switching Scheme
- Used for low modulation indices
Produce output waveform with lower THD
No. of switching angle not equal to No. of dc source
COMPARISON
Why Use PWM Techniques??
To control inverter output frequency (fundamental
To control inverter output voltage (fundamental)
To minimize harmonic distortion
Advantages –
By this power loss in the switching devices is very low
It doesn’t require synchronization between transmitter and receiver.
Signal and noise separation is easy .
Disadvantages-
- Large bandwidth required