 Open Access
 Total Downloads : 203
 Authors : Pramod Kumar Verma, Ahmad Faiz Minai, Mohammad Naseem
 Paper ID : IJERTV6IS060159
 Volume & Issue : Volume 06, Issue 06 (June 2017)
 DOI : http://dx.doi.org/10.17577/IJERTV6IS060159
 Published (First Online): 06062017
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Topological Analysis of Multilevel Inverter for Photovoltaic System
Pramod Kumar Verma1*, 1Department of Electrical Engineering, Integral University,
Lucknow, India
Ahmad Faiz Minai2 , Mohammad Naseem3 2,3Department of Electrical Engineering, Integral University,
Lucknow, India
Abstract – The multilevel concept is used to decrease the harmonic distortion in the output waveform without decreasing the inverter power output. This paper presents the important topologies of multilevel inverter like diodeclamped (neutralpoint clamped), flying capacitor converter (FLC) and cascaded Hbridge with separate dc sources. This paper compares three different topologies of inverters with respect of power losses, Cost, weight and THD. A five level flying capacitor converter (FLC) and cascaded Hbridge inverter will be simulated with the implementation of PWM techniques and its effect on the harmonic spectrum will be analyzed. Each topology has their own features with corresponding advantages and disadvantages when used in a Renewable Energy Power system.
Keywords: THD, DiodeClamped (NeutralPoint Clamped), Flying Capacitor Converter (FLC), Cascaded HBridge Multilevel Inverter.
order harmonics which is considered as a main drawback [10][11].

TOPOLOGIES OF MULTILEVEL INVERTER
The three common topologies of multilevel inverters DiodeClamped Multilevel Inverter (DCMI, Flying Capacitor Multilevel Inverter (FCMI) and Cascaded H Bridge Multilevel Inverter are known as Modular Structured Multilevel Inverters [8].
MULTILEVEL INVERTER
MULTIPLE DC SOURCES
SINGLE DC SOURCES

INTRODUCTION
In heavy duty industries and high voltage applications, the use of multilevel inverters are becoming common since many of the machinery uses electrical drives. The advantages of multilevel converters is their smaller output voltage step, which results in high voltage capability, lower harmonic components, lower switching losses, better electromagnetic compatibility, and high power quality[1], [2]. Also it can operate at both fundamental switching frequency and high switching frequency PWM. It must be noted that lower switching frequency usually means lower switching loss and higher efficiency [3]. Today, multilevel inverters are extensively used in medium voltage levels with highpower applications [4]). The field of applications includes use in laminators, pumps, conveyors, compressors, fans, blowers, and mills Uninterruptable Power Supply (UPS), DC power source utilization, induction heating, high voltage direct current power transmission, variable frequency drive, etc [5][6][7][8].
One clear disadvantage of multilevel power conversion is the great number of power semiconductor switches needed. Another disadvantage of multilevel power converters is that the small voltage steps are typically produced by isolated voltage sources or a bank of series capacitors. Isolated voltage sources may not always be readily available and series capacitors require voltage balance [9]. Multilevel inverters are more popular than conventional twolevel inverters. This is due to twolevel limitation when handling high voltage and power. In additions, the two level inverter also produces a quite numbers of higher
CASCADED H BRIDGE
FLYING CAPACITOR
DIODE CLAMPED
Fig. 1: Classification of MULTILEVEL Inverters

Diode Clamped Multilevel Inverter
The diodeclamped multilevel inverter is the name given to neutralpoint clamped .A diode transfers a limited amount of voltage, thereby reducing the stress on other electrical devices. The maximum output voltage is half of the input DC voltage.
DCMI uses capacitors in series to divide up the DC bus voltage into a set of voltage levels. DCMI have the voltage unbalance problem and difficult to do real power flow control [8].
VDC /2
S1
C1
D1 S2
D2 S3
C2
VDC /4
VDC /2
S
1
C1
Cc S2
VDC /4
S3
C2 Cb
S4
VDC
D3 S4
a
n
VDC
Ca
Ca V
C3
VDC /4
D1
D2
D3
S1
S2
S3
C3 Cb
VDC /4 Cc C4
VDC /2
S1
S2
S3
S4
C4
VDC /2
(a)
S4
(b)
Fig. 4: Five level Flying Capacitor Multilevel Inverter
Fig. 3: Five level diode Diode Clamped (NPC) Multilevel inverter
Fig. 2: Simulink model of five level Diode Clamped (NPC) Multilevel Inverter

Capacitor Clamped multilevel Inverter
Capacitor Clamped Multilevel Inverter also known as Flying capacitor Multilevel Inverter (capacitors are arranged to float with respect to earth) .Here the voltage levels are determined by the charging and discharging of the flying capacitors connected to the neutral point. Flying capacitor multilevel inverter (FCMI) is an alternative solution of DCMI. FCMI is capable to eliminate the clamping diode problems present in the DCMI. The method is the same as the diode clamped inverter.
Fig. 5: Simulink model of five level Capacitor Clamped (FLC) Multilevel Inverter
Fig.6: waveform generated by Capacitor Clamped (FLC) Multilevel Inverter

Cascaded HBridge Multilevel Inverter (CHBMLI)
CHMI is also known as Modular Structured Multilevel Inverter (MSMI) is based on the series connection of several single phase inverters [13], [14]. CHMI require separate DC source, therefore it is suitable for various applications especially in renewable energy source applications. Inverter Consists of (n1)/2 or h number of singlephase Hbridge inverters (MSMI modules). MSMI output phase voltage
Vo = Vm1 + Vm2 + .. Vmh (1)
Vm1 : output voltage of module 1 Vm2 : output voltage of module 2
Vdc
S1 S2
+
Vout
–
S3 S4
Fig. 7: Single Hbridge topology
Module1
Module2
VDC
VDC
S11 S21
S31 S41
S12 S22
S32 S42
Vm1
Vm2
V Phase
VO
Module n
VDC
S3
n
S1n
S2n
S4n
Vmn
Fig. 11: Simulink model of Cascaded Hbridge (CHB) Multilevel Inverter

RESULT ANALYSIS

Power Losses Calculations
An induction motor is used as a load for balanced three
Fig. 8: Structure of a singlephase nllevel MSMI
phase system. Result analysis is given below; All the Loss calculations is done for IGBT.
Mathematical equations for power losses calculations
Module1
Pci=UCEO.IIav+r I2
(2)
Module2
VDC
VDC
S11 S21
S31 S41
S12 S22
S32 S42
Vm1
Vm2
V Phase
VO
c. Irms
D. Drms
Pcd=UDO.IDav+r I2 (3)
Since the number of switches are same in all the three considered topologies of five level inverters so comparative study of power loss in switches and diode are as follows

5Level Diode Clamped Multilevel Inverter
The RMS current that is passing through one of the switches is 48.19A and the average current that is passing through one of the switches is 15.99A.
IIav=15.99A IIRMS=48.19A
Fig. 9: Singlephase fivelevel CHMI configuration
Fig. 10: Staircase sinusoidal waveform generated by Hbridge cascaded Multilevel converter
No current passes through the anti parallel diodes in full
load, so the conduction losses of anti parallel diodes are equal to zero.
According to (2) and for one switch, the power losses are: Psw= 17.7982W
There are 24 switches for three phases s:
Psw=427.1566W
For the diode clamped multilevel inverter, the diode power losses should be calculated by (3). The power losses for one diode are:
PcD= 7.0697W
There are 36 diodes in the 5level diode clamped multilevel inverter, so the total power losses are:
PcD= 254.5108W
Since the switching frequency is 50Hz, the switching losses are neglected in this paper.

5Level Flying Capacitor Multilevel Inverter
The RMS current that is passing through one of the switches is 49.02A and the average current that is passing through one of the switches is 17.28A.
IIav=17.28A IIRMS=49.02A
No current passes through anti parallel diodes in full load,
so the conduction losses of anti parallel diodes are equal to zero.
According to (2) and for one switch, the power losses are: Psw= 19.1105W
There are 24 switches for three phases so:
Psw=458.6523W

5Level Cascaded HBridge Multilevel Inverter According to (2) and (3) the power losses are: Psw= 1491.2W
PcD= 209.3593W
Power Loss
2000
1500
1000
500
Power loss
(watt)
0
5level 5level 5level
NPC FLC MLI CH MLI
Fig. 12: Power losses comparison for IGBT FD300R06KE3 for 5level Inverters


Weight and Cost Calculations
Weight comparison is done for each topology by calculating the weight of all of the components of the inverters. The same switch is considered for all of the topologies to have a more accurate comparison. The IGBT FD300R06KE3 is used for all of the topologies. In the 5 level diode clamped multilevel inverter, the 5level flying capacitor, the 5level cascaded Hbridge the capacitor C4DEFPQ6380A8TK is used, since the DC input voltages are higher, so a capacitor with higher voltage tolerance is needed. In the 9level diode clamped multilevel inverter, the 9level flying capacitor multilevel inverter and the 9 level cascaded Hbridge multilevel inverter the capacitor FFV34E0107K is used, since the DC input voltages are lower, so a capacitor with lower voltage tolerance is needed. Table.1 shows the total weight and the total cost of all types of inverters. To have a better understanding of different types of inverters, comparison are shown in the charts in Fig.1317.
Table. 1: Weight and Cost Comparison
Type of inverter
Number of switches
Number of capacitors
Number of diodes
Weight(Kg)
Cost(Rs.)
5level NPC
24
12
36
13.8
4493.3
5level FLC MLI
24
30
0
20.73
5894.2
5level CH MLI
24
6
0
10.67
3722.5
30
20
10
0
Number of switches
Cost(Rs.)
5level 5level 5level
NPC MLI FLC MLI CHB MLI
Cost(Rs.)
8000
6000
4000
2000
0
5level 5level 5level
NPC FLC MLI CHB MLI MLI
Number of
switches
Fig. 13: Switch comparison for all types of Inverter
Number of capacitors
40
30
20
10
0
Number of
capacitors
5level 5level 5level
NPC MLIFLC MLI CHB MLI
Fig. 14: Capacitor Comparison for all types of Inverter
Number of diodes
40
30
20
10
0
Number of
diodes
5level 5level 5level
NPC MLI FLC MLI CHB MLI
Fig. 15: Diode comparison for all types of Inverter
Weight(Kg)
25
20
15
10
5
0
Weight(Kg)
5level 5level 5level
NPC MLIFLC MLI CHB MLI
Fig. 16: Weight Comparison for all types of Inverter
Fig. 17: Cost Comparison for all types of Inverter
THD(%)

THD Analysis
THD (%)
40
20
0
5level 5level 5level
NPC FLC MLI CH MLI
Type of Inverter
THD (%)
Fig. 18: THD comparison
The total harmonic distortion in diode clamped inverter (NPC) is much more than Flying capacitor multilevel inverter (FLC) and Cascaded HBridge Multilevel Inverter (CHBMLI), so for further studies regarding modulation index only FLC and CHB multilevel inverter are considered.
THD calculations obtained from the SIMULINK file. All of the THDs are for stator current in the electrical motor. The THD comparison between the 5level Cascaded H Bridge Multilevel Inverter and Flying capacitor multilevel inverter has been done which shown in table.2 and by Fig.19. The output voltage of all the 5level topologies is the same, since the same switching pattern is used for all of them.
Table .2: THD Comparison of Flying Capacitor Multilevel Inverter and Cascaded HBridge Multilevel Inverter
Cascaded HBridge Multilevel Inverter
Flying capacitor multilevel inverter
MA1
MA2
THD (%)
MA1
MA2
THD (%)
.4
.4
76.77
.4
.4
92.92
.2
.8
52.43
.2
.8
63.80
.8
.2
51.81
.8
.2
63.33
.6
.6
44.15
.6
.6
50.44
.8
.6
41.79
.8
.6
44.57
.8
.8
38.31
.8
.8
42.40
.8
1.2
31.68
.8
1.2
37.67
1
1
26.89
1
1
36.13
1.8
1.8
25.3
1.8
1.8
24.28
1.6
1.6
23.6
1.6
1.6
25.46
1.2
1.2
21.71
1.2
1.2
29.94
1 3 5 7 9 11
Amplitude Modulation (Ma)
Flying capacitor
multilevel inverter THD (%)
100
80
60
40
20
0
THD (%)
Fig. 29: THD Comparison of Flying Capacitor Multilevel Inverter and Cascaded HBridge Multilevel Inverter


CONCLUSIONS


Each topology has some advantages and disadvantages. By increasing the number of levels, the THD will be decreased but on the other hand cost and weight will be increased as well. Compared to mlevel DCMLI, FCMLI uses m1 capacitors on the dc bus, the CMI uses only (m1)/2 capacitors for same mlevel. Clamping diodes are not required for FCMI and CMC. But balancing capacitors are must for FCMI. But for CMI such balancing capacitors are completely absent. The cascaded Hbridge has the lowest weight and cost between the multilevel inverters, but its power losses is more that all the other topologies. It is seen from the study of diode clamped, flying capacitor clamped multilevel inverter, Cascaded HBridge Multilevel inverters that cascaded inverters have better THD. Due to decrease in the price of switching devices Cascaded H Bridge MLI is more promising for Photovoltaic (PV) applications. A multilevel cascaded inverter is also gven preference over others because it utilizes two or more than two different strings of SPV array as a voltage source for each cell of a multilevel inverter.
The Flying capacitor clamped inverter has the lowest power losses between all of the other topologies, since there is no diode in its topology. It has two big problems.
First is that it is heavier than the other topologies. Also the
THD Comparison of Flying capacitor
multilevel inverter and Cascaded H
Bridge Multilevel Inverter
cost of this inverter is more than other inverters.
Cascaded HBridge
Multilevel Inverter THD (%)
The diode clamped multilevel inverters power losses are lower than cascaded Hbridge the cost will not be that much higher than the cascaded Hbridge. It seems that diode clamped inverter is a topology between all other topologies that THD, cost and power losses are between other types of inverters.
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About the Auorth
Pramod Kumar Verma was born in Lucknow, Uttar Pradesh, India, on 20 June 1986. He Received the Bachelors Degree from Uttar Pradesh Technical University (UPTU), Lucknow and M.Tech from Integral University. He is with the ITM Group of Insititution,Lucknow , as Assistant Professor in the Electrical and Electronics Engineering Department from August, 2012.
Ahmad Faiz Minai Received the Masters Degree from Aligarh Muslim University, Aligarh and pursuing PhD from Integral University, Lucknow, U.P. India. He is with the Integral University, as an Associate Professor in the Electrical and Electronics Engineering Department from July, 09.