A Multilevel Inverter(MLI) is a power electronic device built to synthesize a desired A.C voltage from several levels of DC voltages. Generally unbalanced voltages will occur at supply side these can be eliminated by using Multi level Inverter. In this paper a closed loop Control system is designed using PI controller in order to maintain load voltage constant for under voltage and Over voltage conditions and MATLAB simulations have been carried out

K. Ramcharan

Nainala Vasanthakumar

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Nainala Vasanthakumar, K.Ramcharan / International Journal of Engineering Research and 
Applications (IJERA) ISSN: 2248-9622   www.ijera.com 
Vol. 2, Issue4, July-August 2012, pp.2203-2209 
2203 | P a g e  
Power Quality Improvement Using 3 Phase Cascaded H-Bridge 
Multi Level Inverter Under Unbalanced Voltage Conditions 
Nainala Vasanthakumar 1, K.Ramcharan2 
1 P G Scholor, Department of EEE, BVC Engineering College, Odalarevu ,East Godavari(Dt); A.P, India. 
2Assistant Professor, Department of EEE, BVC Engineering College, Odalarevu ,East Godavari(Dt); A.P, India. 
 
Abstract :  
A  Multilevel  Inverter(MLI)  is  a  power 
electronic  device  built  to  synthesize  a  desired  A.C 
voltage from several levels of DC voltages. Generally 
unbalanced voltages will occur at supply side these 
can be eliminated by using Multi level Inverter.  In 
this paper a closed loop Control system is designed 
using PI controller in order to maintain load voltage 
constant  for  under  voltage  and  Over  voltage 
conditions  and  MATLAB  simulations  have  been 
carried out. 
Keywords:-  Cascaded  H-Bridge  Multi  Level 
Inverter(CHMLI),Power Quality Issues. 
 
I.INTRODUCTION 
Multilevel  inverters  have  gained  more 
attention in high  power  applications  because  it has 
got many advantages [1-4]. It can realize high voltage 
and  high  power  output  by  using  semiconductor 
switches without the use of transformer and dynamic 
voltage balance circuits. When the number of output 
levels  increases,  harmonic  content  in  the  output 
voltage  and  current  as  well  as  electromagnetic 
interference decreases. 
The  basic  concept  of  a  multilevel  inverter  is  to 
achieve  high  power  by  using  a  series  of  power 
semiconductor  switches  with  several  lower  dc 
voltage sources to perform the power conversion by 
synthesizing a staircase voltage waveform [1,5]. To 
obtain  a  low  distortion  output  voltage  nearly 
sinusoidal, a triggering signal should be generated to 
control  the  switching  frequency  of  each  power 
semiconductor  switch  .In  this  paper  the  triggering 
signals to multi level inverter (MLI) are designed by 
using  the  Sine  Pulse  Width  Modulation  (SPWM) 
technique.  A  three  phase  cascaded  H-bridge  Multi 
(five) Level Inverter has been taken. Fig.1 shows a 
three-phase five-level cascaded Multi Level Inverter. 
It requires a total of six D.C voltage sources. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.1 Conventional three phase 5 level 
cascaded MLI 
 
This  paper  investigates  an  approach  where  the 
reference  signal  is  modulated  by  the  carrier  wave 
resulting  in  multiple  SPWM  signals.  These  signals 
are then used to drive the „on‟ / „off‟ switches for 
each level of the inverter. 
 
II.CONTROL TECHNIQUES FOR 
MULTILEVEL INVERTER 
There  are  different  control  techniques 
available for a CHB MLI [13, 15]. Among all those 
techniques, PWM control technique which produces 
less total harmonic distortion (THD) values is most 
preferable. In PWM technique, modulated signal can 
be of pure sinusoidal, third harmonic injected signals 
and  dead  band  signals.  The  carrier  signal  is  a 
triangular wave. For generating Triggering pulses to 
MLI, pure sinusoidal wave as modulating signal and 
multi  carrier  signal  which  is  of  triangular in  shape 
have  been  considered  [10,  14,  15].  For  a  m-level 
MLI, (m-1) carrier signals are required. 
 
 
 
 
 Nainala Vasanthakumar, K.Ramcharan / International Journal of Engineering Research and 
Applications (IJERA) ISSN: 2248-9622   www.ijera.com 
Vol. 2, Issue4, July-August 2012, pp.2203-2209 
2204 | P a g e  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.2 Control techniques for a cascaded H-bridge 
MLI 
 
Sinusoidal PWM   
For  generation  of  triggering  pulses  to  the 
MLI,  carrier  signals  are  constructed  for  different 
modulation  indices  like  APOD,  POD,  PD,  PS  and 
Hybrid control techniques. Output phase voltage has 
been  measured  using  all  the  techniques.  THD 
analysis  for  the  PS  control  techniques  in  Bipolar 
mode of operation have been presented in this paper. 
Multilevel  sinusoidal  PWM  can  be  classified  as 
shown in Fig.3 [14-19].   
Multi carrier PWM techniques have sinusoidal signal 
as reference wave and triangular as carrier signals [6-
7]. 
Amplitude 
Modulat
ion 
Ma=Am/(
(m-
1)*Ac). 
Frequency modulation 
Mf=FC/Fr 
Here Am =Amplitude of modulating wave 
(sin wave) Ac=Amplitude of carrier wave 
(triangular wave) 
Fc =Carrier Frequency, Fr=Reference Frequency 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.3 Classification of Sinusoidal PWM 
 
Modes of Operation   
For generating triggering pulses in Bipolar mode, 
four  carrier  signals  of  triangular in nature  and  one 
sine wave are used. In the case of Unipolar mode of 
operation, two reference sine waves and two carrier 
signals (level-1)/2 which are triangular in nature are 
used  to  generate  the  pulses  to  MLI[6,15].  For 
Unipolar mode  of  operation  the  formulae has  been 
changed to Ma= (Am/ ((m-1)/2*Ac), 
Mf= Fc /Fr. 
Phase shifted carrier control technique (PS) 
. 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.4  Carrier  arrangement  for  Bipolar  mode  in  PS 
technique Nainala Vasanthakumar, K.Ramcharan / International Journal of Engineering Research and 
Applications (IJERA) ISSN: 2248-9622   www.ijera.com 
Vol. 2, Issue4, July-August 2012, pp.2203-2209 
2205 | P a g e  
For  a  five  level  MLI,  in  PS  technique  the  carrier 
signals which are phase shifted by 90 degrees (360/4) 
and the reference signal is of sinusoidal is taken and 
shown in Fig.4. 
 
III. POWER QUALITY 
Power  Quality  is  the  concept  of  powering 
and grounding sensitive equipment in a matter that is 
suitable to the operation of that equipment according 
to IEEE Std 1100. 
Power quality is mainly concerned with deviations of 
the voltage from its ideal waveform (voltage quality) 
and deviations of the current from its ideal waveform 
(current  quality).Power  quality  phenomena  can  be 
divided  into  two  types,  they  are  1)Variations 
2)Events. 
Voltage  and  Current  variations  are  relatively  small 
deviations of voltage or current characteristics around 
their nominal or ideal values. The two basic examples 
are voltage magnitude and frequency. 
Events are phenomena which only happen every once 
in  a  while.  An  interruption  of  the  supply  voltage 
[IEEE Std.1159] is the best-known example. 
impedance,  fault  distance,  system  characteristics 
(grounded or ungrounded) and fault resistance. The 
duration of the sag depends on the time taken by the 
circuit  protection  to  clear  the  fault.  High  speed 
tripping is desired to limit the duration of sags. 
 
Over Voltage 
Just  like  with  under  voltage,  overvoltage 
events  are  given  different  names  based  on  their 
duration. Over voltages  of very short duration, and 
high  magnitude,  are  called  ―Transient  Over 
Voltages‖, ―Voltage Spikes,‖ or sometimes ―Voltage 
Surges.‖  Over  Voltages  with  a  duration  between 
about 1 cycle and 1 minute. The latter event is more 
correctly called ―Voltage Swell‖ or temporary power 
frequency  overvoltage.  ―Longer‖  duration  over 
voltages are simply referred to as ―Over Voltages.‖ 
Long and Short over voltages originate from, 
 
lightning strokes, switching operations, sudden load 
reduction,  single  phase  short  circuits,  and 
nonlinearities.  A  resonance  between  the  nonlinear 
magnetizing  reactance  of  a  transformer  and  a 
capacitance (either in the form of a capacitor bank or 
the capacitance of an underground cable) can lead to 
a  large  overvoltage  of  long  duration.  This 
phenomenon  is  called  Ferro  resonance,  and  it  can 
lead to serious damage to power system equipment. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.5 Voltage magnitude events as used in IEEE Std.l 
159-1995 
 
Under Voltage 
Under  voltages  of  various  duration  are 
known under different names. Short duration under 
voltages are called ―voltage sags‖ or ―voltage dips‖. 
Long  duration  under  voltage  is  normally  simply 
referred  to  as  ―under  voltage‖.  Voltage  sag  is  a 
reduction in the supply voltage magnitude followed 
by  a  voltage  recovery  after  a  short  period  of  time. 
When  a  voltage  magnitude  reduction  of  finite 
duration can actually be called a voltage sag. 
 
For the IEEE voltage drop is only a sag if 
the during sag voltage is between 10% and 90% of 
the nominal voltage. Voltage sags are mostly caused 
by short circuit faults in the system and by starting of 
large motors. 
Voltage sag is generally characterized by depth and 
duration. The depth of the sag depends on the system 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 6 Closed Loop Block Diagram 
 Nainala Vasanthakumar, K.Ramcharan / International Journal of Engineering Research and 
Applications (IJERA) ISSN: 2248-9622   www.ijera.com 
Vol. 2, Issue4, July-August 2012, pp.2203-2209 
2206 | P a g e  
In this paper the three phase AC Voltage Supply is 
taken directly and is given to three phase controlled 
rectifier  which  converts  AC  supply  to  controlled 
pulsated DC Voltage and this is given to Low pass 
filter.  Low  pass  filter  is  a  device  which  converts 
Pulsated  DC  voltage  to  Pure  DC  Voltage  and  this 
pure DC Voltage is given as a input to Cascaded H-
bridge 5 level Multilevel Inverter and the output of 
MLI is given to Load. 
 
In closed loop Control the supply voltages and load 
voltages are both compared and error value is given 
to PI Controller. Output of PI controller is imposed 
on the phase shifted carrier so as to get pulses. These 
pulses are given to MLI for each phase. Here we are 
using three PI controllers for three phase MLI. The 
desired voltage at the load bus is maintained at 1 pu 
.To regulate the load-bus voltage, a PI controller is 
employed  that  contains  a  feedback  signal  derived 
from the voltage at the load bus, V.   
Zc  = kpe + ki  , 
 
 
Kp= Proportional constant, Ki= Integral constant, e = 
Error constant 
 
Design of Filter   
Here LC Filter is designed and LC Filter is 
the combination of two filters provides a lower ripple 
than is  possible  with  either  L  or  C alone.  As it  is 
known, in an inductor filter, ripple increases with RL 
but decreases in a capacitor filter. The combination of 
L and C filter makes the ripple independent of RL. 
 
Here C = 
 
 
VRF =  , 
 
where VRF is Voltage Ripple Factor 
 
IV.SIMULATION STUDIES 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig 9 Output Phase Voltage Waveforms of MLI Nainala Vasanthakumar, K.Ramcharan / International Journal of Engineering Research and 
Applications (IJERA) ISSN: 2248-9622   www.ijera.com 
Vol. 2, Issue4, July-August 2012, pp.2203-2209 
2207 | P a g e  
   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.11 Open loop output voltage(2000V) waveform at rated voltage      Fig.14 Closed loop Simulink diagram of     
                                                                                                                       Cascade H-Bridge 5 Level MLI 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 12 Open loop Output Voltage when Under Voltage(1600V) 
                                  is created                                                                  
                                                                                                             Fig 15.Closed loop output voltage(2000V) waveform at rated voltage Nainala Vasanthakumar, K.Ramcharan / International Journal of Engineering Research and 
Applications (IJERA) ISSN: 2248-9622   www.ijera.com 
Vol. 2, Issue4, July-August 2012, pp.2203-2209 
2208 | P a g e  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 13 Open loop Output Voltage when Over Voltage(2500V)  
is created                                                                                    Fig. 16 Closed loop Output Voltage when Under Voltage is created 
 
 
   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 17 Closed loop Output Voltage when Over Voltage is 
created 
 
S.No  Parameter  Value 
 
1  Amplitude Modulation  0.83 
 
Index(Ma) 
 
   
 
2  Number Of Levels(M)  5 
 
3  Reference Wave  2V 
 
Amplitude(Am) 
 
   
 
4  Frequency  50Hz 
 
5  Carrier Wave Amplitude(Ac)  0.6V 
 
6  Frequency Modulation  20 
 
Index(Mf) 
 
   
 
7  Carrier Frequency(Fc) 
1KHz 
 
 
8  Voltage Ripple Factor(VRF)  10%(Assume) 
 
 
 
9  Filter Capacitor (C)  1054.2µF 
 
10  Filter Inductance(L)  13.72mH 
 
 
V.CONCLUSION 
In this paper three phase cascaded H-Bridge 
multilevel inverter is simulated and observed under 
various  unbalanced  voltage  conditions  like  Under 
Voltage and Over Voltage. In the open loop system 
Under Voltage and Over Voltage were introduced at 
the  Supply  side,  and  the  waveforms  of  those  were 
observed under unbalanced conditions. 
In  the  closed  loop  system  with  the  help  of  PI 
Controller the load voltage is maintained constant for 
unbalanced  voltage  conditions  and  these  can  be 
observed from the above simulations. 
 
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Multi Level Inverter Under Unbalanced Voltage Conditions

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Multi Level Inverter Under Unbalanced Voltage Conditions

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