To design a new type of Multi Level Converter known as Modular Multilevel Converter (MMC) integrated with three winding transformer, then it is used for Power Quality Improvement and Voltage Balancing. This project proposes a new type of Modular Multilevel Converter (MMC) applying a three- winding transformer. In general, MMC requires a buffer reactor in each arm, which increases number of components and converter footprint. The proposed MMC with three winding transformer does not require the buffer reactors. We describe mathematical properties of the proposed MMC. We also confirmed it operated as same as typical MMC (with reactor topology) In MATLAB / SIMULINK SOFTWARE.

1. International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
Issue 08, Volume 4 (August 2017) www.ijirae.com
DOI: 10.26562/IJIRAE.2017.AUAE10080
_________________________________________________________________________________________________
IJIRAE: Impact Factor Value – SJIF: Innospace, Morocco (2016): 3.916 | PIF: 2.469 | Jour Info: 4.085 |
ISRAJIF (2016): 3.715 | Indexcopernicus: (ICV 2015): 47.91
IJIRAE © 2014- 17, All Rights Reserved Page -1
POWER QUALITY IMPROVEMENT USING MODULAR
MULTILEVEL CONVERTER APPLYING THREE
WINDING TRANSFORMER
Kanuri Venkatesh1, Ponasanapalli Raviteja2
EEE Department, Jawaharlal Nehru Technological University,
Kakinada - 533 003, Andhra Pradesh, India
Kanurivenkatesh.vsp@gmail.com,p.tejaeee@gmail.com
Manuscript History
Number: IJIRAE/RS/Vol.04/Issue08/AUAE10080
DOI: 10.26562/IJIRAE.2017.AUAE10080
Received: 20, July 2017
Final Correction: 30, July 2017
Final Accepted: 05, August 2017
Published: August 2017
Citation: Kanuri, V. & Ponasanapalli, R. (2017). Power Quality Improvement Using Modular Multilevel
Converter Applying Three Winding Transformer. IJIRAE:: International Journal of Innovative Research in
Advanced Engineering, Volume IV, 01-09. DOI: 10.26562/IJIRAE.2017.AUAE10080
Editor: Dr.A.Arul L.S, Chief Editor, IJIRAE, AM Publications, India
Copyright: ©2017 This is an open access article distributed under the terms of the Creative Commons Attribution
License, Which Permits unrestricted use, distribution, and reproduction in any medium, provided the original author
and source are credited.
Abstract— To design a new type of Multi Level Converter known as Modular Multilevel Converter (MMC)
integrated with three winding transformer, then it is used for Power Quality Improvement and Voltage Balancing.
This project proposes a new type of Modular Multilevel Converter (MMC) applying a three- winding transformer. In
general, MMC requires a buffer reactor in each arm, which increases number of components and converter footprint.
The proposed MMC with three winding transformer does not require the buffer reactors. We describe mathematical
properties of the proposed MMC. We also confirmed it operated as same as typical MMC (with reactor topology) In
MATLAB / SIMULINK SOFTWARE.
Keywords — Modular Multilevel Converter, Transformer, Flexible AC Transmission Systems, Voltage Source
Converter, High Voltage Direct Current Transmission
I. INTRODUCTION
In the last decade, the electrical power quality issue has been the main concern of the power companies. Power
quality is defined as the index which both the delivery and consumption of electric power affect on the
performance of electrical apparatus.
Fig. 1. Circuit configuration of an MMC with three winding- transformer

2. International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
Issue 08, Volume 4 (August 2017) www.ijirae.com
DOI: 10.26562/IJIRAE.2017.AUAE10080
_________________________________________________________________________________________________
IJIRAE: Impact Factor Value – SJIF: Innospace, Morocco (2016): 3.916 | PIF: 2.469 | Jour Info: 4.085 |
ISRAJIF (2016): 3.715 | Indexcopernicus: (ICV 2015): 47.91
IJIRAE © 2014- 17, All Rights Reserved Page -2
From a customer point of view, power quality problem can be defined as any problem is manifested on voltage,
current, or frequency deviation that results in power failure. The power electronics progressive, especially in
flexible alternating-current transmission system (FACTS) and custom power devices, affects power quality
improvement. MMC consists of series half-bridge converter and does not require AC filters and can be generating
the sine wave output AC terminal by reducing the switching frequency and number of levels of individual power
devices. In this paper, we have to discuss that MMC has to eliminate the buffer reactor by reducing the number of
components and we use the SVC (TCR) in the MMC also. So, we achieve the magnetic saturation and removing the
lower cost. By applying three winding transformer the voltage and current equations are derived analytically.
II. PROBLEM FORMULATION
A. Topology of modular multilevel converter:
The cascaded half bridge converter consists of three winding- transformer and the primary winding is connected
to the AC grid. The upper and lower arm of each leg is connected to the secondary and tertiary windings of the
converter. The two windings of secondary and tertiary is connected to the center of each leg and reverse direction,
negative side of each winding is connected with each other. The upper and lower arm of the MMC consists of two
components i.e; common component and other is distinct component. The magneto motive force of secondary and
tertiary winding of the common component of the reverse direction and should be cancelled. Generally, the arm
reactor of leakage inductance behaves like secondary and tertiary windings of the transformer. Moreover, the DC
link of the neutral point should be connected to symmetrical monopole applying the HVDC transmission system.
B. Description of 3- winding transformer:
In this paper, we discuss analytically and mathematically of a three-winding transformer and shows flux flow of
the transformer. The magnetic path of the iron core of each winding interlinks between individual windings.
Leakage flux penetrates not only flux but also individual windings. Therefore, number of flux linkage of each
winding Nps  ,, is given by equation (1).




















































N
P
S
NNN
PPP
SSS
lN
lP
lS
N
P
S
N
P
S
NNN
NNN
NNN
N
N
N









00
00
00
(1)
Where, Nps ,, are main flux, NPs 1,1,1 are leakage flux, NpsN ,, are number turns of each winding. When we added
R to the magnetic circuit of the magnetic resistance of the main flux in the iron core, of each winding is expressed
by (2), using the line current is, and the winding current ip, iN..













































NNppNssN
NNpppssp
NNsppsss
lN
lP
lS
N
P
S
N
P
S
iNiNNiNN
iNNiNiNN
iNNiNNiN
R
N
N
N
2
2
2
/1
00
00
00






(2)
is
iP
iN
Vs
VcP
VcN
Ns
NP
NN
Primary
winding
Secondary
winding
Tertiary
winding
ɸs ɸP
ɸN
ɸ1s
ɸ1P
ɸ1N
Fig.2. Flux flow of three winding transformer

3. International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
Issue 08, Volume 4 (August 2017) www.ijirae.com
DOI: 10.26562/IJIRAE.2017.AUAE10080
_________________________________________________________________________________________________
IJIRAE: Impact Factor Value – SJIF: Innospace, Morocco (2016): 3.916 | PIF: 2.469 | Jour Info: 4.085 |
ISRAJIF (2016): 3.715 | Indexcopernicus: (ICV 2015): 47.91
IJIRAE © 2014- 17, All Rights Reserved Page -3
The magnetic resistance of each winding of self and mutual inductance depends on number of windings by
equation (3).
R
N
L
R
N
L
R
N
L
N
N
p
P
s
S
2
2
2



,
R
NN
M
R
NN
M
R
NN
M
Np
pN
Ns
sN
ps
sP



(3)
Leakage flux depends on the leakage inductance and winding current. Accordingly, equation (2) is convert to
equation (4) using equation (3).


































N
P
S
NNpNsN
pNppsP
sNsPss
N
p
s
i
i
i
LlMM
MLlM
MMLl



(4)
Relation between the voltage and winding current is obtained by equation (4).


































N
P
S
NNpNsN
pNppsP
sNsPss
N
p
s
i
i
i
LlMM
MLlM
MMLl
dt
d
v
v
v
(5)
By introducing, transformation ratio nsp and nsn of each mutual-inductance are changed excitation inductance by
equation (6).
N
P
PN
N
S
sN
P
S
sP
N
N
n
N
N
n
N
N
n  ,, (6)
C. Applying the transformer to MMC:
In this paper, we applying the voltage and current equations of the transformer to MMC model of the equivalent
circuit of the model (fig 1) . The upper arm current and lower arm current can be obtained by common current
component and distinct current component. The common current and distinct current component is determined
as icir, idef as equation (7). Therefore, lower arm current iN and upper arm current ip is defined as equation (8).
 
Npdef
Npcir
iii
iii


2
1
(7)
defcirN
defcirp
iii
iii
2
1
2
1


(8)
Using the three winding transformer of voltage and current equations of the common current component icir and
distinct current component idef is given by equation (9).












































defcir
defcir
s
NNNpNNsN
P
PN
PPPsP
s
sN
s
sp
ss
N
P
S
ii
ii
i
LlLnLn
L
n
LlLn
L
n
L
n
Ll
dt
d
v
v
v
1
11
(9)

4. International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
Issue 08, Volume 4 (August 2017) www.ijirae.com
DOI: 10.26562/IJIRAE.2017.AUAE10080
_________________________________________________________________________________________________
IJIRAE: Impact Factor Value – SJIF: Innospace, Morocco (2016): 3.916 | PIF: 2.469 | Jour Info: 4.085 |
ISRAJIF (2016): 3.715 | Indexcopernicus: (ICV 2015): 47.91
IJIRAE © 2014- 17, All Rights Reserved Page -4
Now, the transformation ratio of primary to tertiary nsn and primary to secondary nsp are made equal to n.
Therefore, the leakage inductance of tertiary winding IN and secondary winding IP are made equal to l. The
equivalent circuit of three winding transformer is given by,
Sv
Si Sl
SL SN N
defi l Pi
Pv
Nv
Nil
Ideal
transformer
Fig.3. Equivalent circuit of three winding transformer
The equivalent circuit of the three -winding transformer is shown in figure 3. It consists of ideal transformer and
parallel connected to the primary leakage inductance ls, the excitation inductance Ls, of the secondary side of ideal
transformer. An MMC consists of cascaded converter of switching frequency and should be higher than output
voltage; and should be changed by variable voltage source.
D. AC side current of the MMC:
In this paper, AC side current of the MMC is defined. The three phase AC line voltage and AC line current is vs(r,s,t)
and is(r,s,t). The voltage and current equation of primary side is given by equation (10).
































t
s
r
st
ss
sr
s
st
ss
sr
v
v
v
n
i
i
i
dt
d
l
v
v
v
sec_
sec_
sec_
(10)
Where, vsec(r,s,t) is secondary voltage of ideal transformer. The ideal transformer of secondary voltage is given by
equation (11), and it is followed by equation (11) to equation (12).
 






2
_sec
def
cirnparmpN
i
i
dt
d
lvvvv







2
_
def
cirnNarm
i
i
dt
d
lvv (11)
Where, MMC of DC link voltage is VPN. The neutral point voltage Vn of each transformer is connecting between
negative side of DC link, varm_p, varm_N are total output voltage of each cells in each arm.






 defnParmNarmPN i
dt
d
lvvvvv 2
2
1
__sec (12)
Now, the each transformer of neutral point vn of DC link is same voltage and secondary voltage is followed by (13).






 defParmNarm i
dt
d
lvvv __sec
2
1
(13)
Generally, the primary side and secondary side of the transformer of an excitation inductance of a transformer is
approximately equal.

5. International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
Issue 08, Volume 4 (August 2017) www.ijirae.com
DOI: 10.26562/IJIRAE.2017.AUAE10080
_________________________________________________________________________________________________
IJIRAE: Impact Factor Value – SJIF: Innospace, Morocco (2016): 3.916 | PIF: 2.469 | Jour Info: 4.085 |
ISRAJIF (2016): 3.715 | Indexcopernicus: (ICV 2015): 47.91
IJIRAE © 2014- 17, All Rights Reserved Page -5
 



































tparmtNarm
sparmSNarm
rparmrNarm
st
ss
sr
s
st
ss
sr
vv
vv
vv
n
i
i
i
dt
d
lnl
v
v
v
____
____
____
2
2
1
2
(14)
The subtraction of upper arm and lower arm is varm_N, and varm_p is defined by equation and is converted to dq
component of AC line voltage using matrix transformation C.
























tparmtNarm
sparmSNarm
rparmrNarm
ct
cs
cr
vv
vv
vv
n
v
v
v
____
____
____
2
(15)
Where,
      


















































cq
cd
ct
cs
cr
sq
sd
st
ss
sr
sq
sd
st
ss
sr
v
v
C
v
v
v
i
i
C
i
i
i
v
v
C
v
v
v
,, (16)
 




















)
3
4
cos(
)
3
2
cos(
sin
)
3
4
cos(
)
3
2
cos(
cos




wt
wt
wt
wt
wt
wt
C
(17)
Finally, dq coordinates of AC line voltages by inverse transformation matrix C is obtained by,
 


















)
3
4
cos(
)
3
4
cos(
)
3
2
cos(
)
3
2
cos(
sin
cos
3
21




wt
wt
wt
wt
wt
wt
C (18)
    




 


01
101
wC
dt
d
C (19)
The equivalent model of the transformer of AC line current of voltage of converter arm is given by equation (18).
The upper and lower arm current of AC line current is same as MMC converter. The secondary and tertiary
winding of leakage inductance is connected in parallel of AC line reactor. It can be operated as MMC with three
winding transformer as same as typical MMC.
E. DC side current of the MMC:
In this paper, DC link side current of the MMC is defined. The voltage and current of the DC link current is given by
equation (20).
NNarmpparmpN i
dt
d
lvi
dt
d
lvv  __ (20)
It consists of common current component icir and distinct current component idef of the arm current iP and iN is
mentioned before paper. Using the voltage and current equations is expressed by (21).
cirNarmparm
def
cirNarm
def
cirparmPN
i
dt
d
lvv
i
i
dt
d
lv
i
i
dt
d
lvv
2
22
__
__














(21)
Finally, the common current component is determined by,
 dtvvv
l
i NarmparmPNcir __
2
1
 (22)

6. International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
Issue 08, Volume 4 (August 2017) www.ijirae.com
DOI: 10.26562/IJIRAE.2017.AUAE10080
_________________________________________________________________________________________________
IJIRAE: Impact Factor Value – SJIF: Innospace, Morocco (2016): 3.916 | PIF: 2.469 | Jour Info: 4.085 |
ISRAJIF (2016): 3.715 | Indexcopernicus: (ICV 2015): 47.91
IJIRAE © 2014- 17, All Rights Reserved Page -6
2
n
abc to
dq
conv.2
n
2
n







2
1
2
ln
lw s







2
1
2
ln
ls s







2
1
2
ln
lw s







2
1
2
ln
ls s
dq to
abc
conv.
rparmv __
rNarmv __
sparmv __
sNarmv __
tparmv __
tNarmv __
wt
crv
csv
ctv
cqv
sqv
cdv
sdv
sdi
sqi
sri
ssi
sti
Fig.4. AC line current between voltages of converter arm
The upper and lower arm of common current component carries DC link side converter. The secondary and
tertiary windings of IP and IN of leakage inductance behaves buffer reactor.
TABLE I - PARAMETERS OF THE MMC PROTOTYPE
Meaning Value
AC Voltage 200V, 50Hz
DC Voltage 400V
Power rating 10KVA
Cell capacitor voltage 100V
Number of cell in each arm 4
Switching frequency (individual devices) 1KHz
Leakage inductance (primary winding) 1.95MH (15.3%Z)
Leakage inductance (secondary, tertiary winding) 1.3MH (8.37%Z)
Capacitance in each cells 3960uF
Transformation ratio 115V (P)/127V (S,T)
III.SIMULATION RESULTS AND DISCUSSION
The parameters of the MMC as shown in the table 1, The AC grids of the prototype converter are connected to the
DC lines. The experimentation of the normal transformer, its iron core of saturation magnetic density is 1.85[T] at
116% (233V, 50Hz) of rated voltage. The magnetic density of iron core is 1.59[T] and rated voltage is (200V,
50Hz).
IV. DIRECTION POWER FLOW IS AC TO DC AND DC TO ACCONVERSION
The experimental results of figure 5 shows in grid connected operation. In all results, capacitor voltages of each
cell and sinusoidal waveforms of ac line current are obtained and balanced with each other. This is AC to DC
conversion is obtained by varying the duty cycle in the control circuit. Duty cycle< 0.5 is varied in control circuit.
Fig.5.When the direction power flow is AC to DC

7. International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
Issue 08, Volume 4 (August 2017) www.ijirae.com
DOI: 10.26562/IJIRAE.2017.AUAE10080
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IJIRAE: Impact Factor Value – SJIF: Innospace, Morocco (2016): 3.916 | PIF: 2.469 | Jour Info: 4.085 |
ISRAJIF (2016): 3.715 | Indexcopernicus: (ICV 2015): 47.91
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This is DC to AC conversion is obtained by varying the duty cycle in the control circuit. Duty cycle > 0.5 is varied in
control circuit.
Fig.6. When the direction power flow is DC to AC
V. REACTIVE POWER LAGGING AND LEADING OPERATION
When the faults occurs in the power distribution, voltage and currents decreases and at that condition reactive
power lagging operation is occurred.
Fig.7. In reactive power lagging operation
When the faults occur in the power distribution voltage and currents increases and at that condition reactive
power leading operation is occurred.
Fig.8. In reactive power leading operation

8. International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
Issue 08, Volume 4 (August 2017) www.ijirae.com
DOI: 10.26562/IJIRAE.2017.AUAE10080
_________________________________________________________________________________________________
IJIRAE: Impact Factor Value – SJIF: Innospace, Morocco (2016): 3.916 | PIF: 2.469 | Jour Info: 4.085 |
ISRAJIF (2016): 3.715 | Indexcopernicus: (ICV 2015): 47.91
IJIRAE © 2014- 17, All Rights Reserved Page -8
VI. POWER FLOW REVERSAL FROM CONVERTER 1 TO 2 AND 2 TO 1
This is power flow reversal from converter 1 to converter 2 is obtained by rectifier operation in the circuit during
converter charging period.
Fig.9. Power flow reversal from converter 1 to converter 2
This is power flow reversal from converter 2 to converter 1 is obtained by rectifier operation in the circuit during
converter charging period.
Fig.10. Power flow reversal from converter 2 to converter 1
VII. FAULT OPERATION WHEN 1 LINE TO GROUND AND 3 LINES TO GROUND
It shows experimental results in fault operation through AC line fault and magnetic saturation does not appear by
charging the AC line voltage.
Fig.11. fault ride through operation when 1 line to ground fault

9. International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
Issue 08, Volume 4 (August 2017) www.ijirae.com
DOI: 10.26562/IJIRAE.2017.AUAE10080
_________________________________________________________________________________________________
IJIRAE: Impact Factor Value – SJIF: Innospace, Morocco (2016): 3.916 | PIF: 2.469 | Jour Info: 4.085 |
ISRAJIF (2016): 3.715 | Indexcopernicus: (ICV 2015): 47.91
IJIRAE © 2014- 17, All Rights Reserved Page -9
Fig.12. fault ride through operation when 3 lines to ground fault
VIII. CONCLUSION
The paper is successfully implemented using MATLAB/ simulink software. In these control technique 24 cells
have been implemented by using this technique we can reduce the cost of the transformer and can improve the
output efficiency. This paper introduces a new type of Modular Multilevel Converter applying a three -winding
transformer. The topology can merge buffer reactors into a grid connected transformer. In future study the DC
bias magnetism of the buffer reactor into a grid connected transformer and the MMC with three winding
transformer does not require a special transformer.
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