SSRG International Journal of Communication and Media Science (SSRG-IJCMS) – volume 2 Issue 5 November to December 2015
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(PG Scholar, Communication Systems, PRIST University, Thanjavur, INDIA)
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(Assistant Professor, Electronics & Communication Engineering, PRIST University, Thanjavur, INDIA)
ABSTRACT : Power electronic converters, especially
DC/AC Sinusoidal Pulse Width Modulation inverters have been extending their range of use in industry because of their numerous advantages. They typically synthesize the stair –case voltage waveform (from several dc sources) which has reduced harmonic content. This paper describes the development of design, modelling and simulation of the proposed seven level quasi-Z-source cascaded multilevel inverter (QZS-CMI) based PV systems which inherits the advantages of traditional CMI while overcoming issues with imbalance DC-link voltages. The modified inverter can produce a better sinusoidal waveform by increasing the number of output voltage levels. This paper also aims to analyse and compare total harmonic distortion in conventional cascaded three level, five level and modified seven level H-bridge inverters and evaluates THD in the respective outputs.
Simulations of the circuit configuration have been performed in MATLAB/Simulink and the results were verified.
Keywords - Cascaded Multilevel Inverter (CMLI), quasi-Z-source multilevel inverter (QZS-CMI), Total
Harmonic Distortion (THD), photovoltaic (PV) power system.
I.
INTRODUCTION
In recent years, applying various multilevel inverter topologies to PV systems is getting more and more attention due to the large power-scale and high voltage demands. Among various topologies, Cascaded H-
Bridge (CHB) inverter has unique advantages and has been identified as a suitable topology for PV systems.
However, the DC-link voltage in each inverter module is not constant, because PV array voltage varies due to the changes of environmental conditions such as temperature and solar irradiation or partial shadows. These cases will cause an imbalance DClink voltage among different H-bridge modules.
Furthermore, in the conventional cascaded multilevel inverter (CMI) based PV system, each module is a buck inverter because the first component of the output AC voltage, always is lower than the input DC voltage.
The quasi-Z-source inverter (QZSI) has been employed for PV power generation system due to some unique advantages and features. Unlike quasi Zsource inverter, ZSI has a discontinuous input current during the shoot-through state due to the blocking diode. Quasi-Z-source cascaded multilevel inverter
(QZS-CMI) based PV systems is proposed which inherits the advantages of traditional CMI.
Modern power electronics based devices have put a great effect on the development of new powerful applications and industrial solutions. But at the same time, these advances have increased the harmonic problems in line currents, which make distortion in the voltage waveforms. Diode power rectifiers, thyristor converters and static VAR compensators (SVCs) are examples of power electronics applications. The series connection of several bridges allows working with much higher voltages and the stepped voltage waveforms to eliminate the voltage stress in associated equipment, such as transformers. Moreover, the bridges of each converter work at a very low switching frequency which allows working with low speed semiconductors and Low switching frequency losses. Filters are used for compensation of contaminating load with small power factor and to feed the load during voltage dips. The multilevel inverters perform power conversion in multilevel voltage steps to obtain improved power quality, lower switching losses, better electromagnetic compatibility and higher voltage capability. Considering these advantages, multilevel inverters have been gaining considerable popularity in recent years.
Comparing conventional two level inverter systems with multilevel inverter systems has the advantages that the lower harmonic components on the output voltages can be eliminated and EMI problem could be decreased. Due to these merits, many researches on multilevel inverters have been performed at simulations and an idea of using multilevel inverter instead of conventional inverter is developed.
II. TOPOLOGY DESCRIPTION
The topology of the seven- level Quasi Z-Source inverter consists of a series of single phase H bridge inverter units, Quasi Z Source impedance networks
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SSRG International Journal of Communication and Media Science (SSRG-IJCMS) – volume 2 Issue 5 November to December 2015 and DC voltage sources. DC sources can be obtained from batteries, fuel cells, solar cells.
III. QUASI Z-SOURCE INVERTER
The QZSI extends several advantages over the ZSI such as continuous current from the input DC source, cut down component ratings, and enhanced reliability.
Figure 1 shows the basic topology of QZSI.
Fig. 1 Basic topology of QZSI
IV. MODES OF OPERATION AND
CIRCUITANALYSIS
A. Active or Non-Shoot through state
In this state the load is directly connected to the supply through the switches, so the equivalent circuit of Quasi Source Inverter Bridge becomes a current source.
Fig. 2 Equivalent circuit of QZSI in active state
By writing mesh equations for this circuit we get,
(1)
(2)
(3)
Since the diode is turned on at this switching state
(4)
B. Zero state
During one of the traditional zero state the load is shorted by the turning ON of upper devices or lower devices alone, so the inverter bridge is represented by a current source with zero value
Fig. 3 Equivalent circuit of QZSI in zero state
C. Shoot-through state
The special feature of QZSI is the shoot through state, during this state the devices in the same limb are triggered ON. Since there are inductors and capacitors available in the topology this switching that lasts for a very short duration does not damage the switches as in the case of VSI. The equivalent circuit of the inverter bridge is shown below.
Fig. 4 Equivalent circuit of QZSI in shoot through state
This state is switched either at the beginning or at the end of the zero state when the voltage across the load is zero to eliminate the damage of the switches. The analysis of the circuit is shown below
………………….(6)
…………………. (7)
…………………. (8)
………………… (9)
V. CASCADED H-BRIDGE MULTILEVEL (CHB)
INVERTERS
In the field of research of power electronics multilevel inverters are producing more interest. Multilevel inverters are the most attractive technology for the medium to high voltage range, which includes power distribution, power quality. The general structure of the multilevel converter is to synthesize a near sinusoidal voltage from several levels of DC voltages.
As more steps are added to the waveform, the harmonic distortion of the output wave decrease, approaching zero as the number of levels increases. A cascaded multilevel inverter consists of a series of H bridge inverter units. Series H bridge inverter appeared in 1975.
Among the different structures of multilevel inverter cascaded multilevel inverter is more preferable and popular. A cascade multilevel inverter consists of a series of H-bridge (single-phase full bridge) inverter units in each of its three phases. Each H bridge unit has its own dc source. Each SDCS (separate D.C. source) is associated with a single-phase full-bridge inverter. In this circuit, the number of voltage levels is defined by m= (2H+1), where H is the number of Hbridge cells per phase leg. The Neutral Point Clamped converters requires neutral point control and capacitor voltage balance. With the increase in the output voltage level, the requirement of the clamping diodes
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SSRG International Journal of Communication and Media Science (SSRG-IJCMS) – volume 2 Issue 5 November to December 2015 increases, the neutral point control and the power circuit become complex. The Flying capacitor converter balances voltage naturally but the capacitors has to be pre-charged. Although this converter is modular in structure, but requires large number of flying capacitors as the output voltage level increases and thus adds to the cost. In contrast to this, CHB converters have no voltage balancing problems due to separate DC sources. Its power rating can be increased by the series connection of the cells.
In CHB converter, each switching devices always conduct for half cycle, hence distributing the current stress equally among the switching devices. No clamping diodes present as in NPC and no voltage balancing capacitors present as in flying capacitor circuit. Separate DC sources eliminate the need of the voltage balancing circuits. It can work at reduced power level when one of its cell or SDCS is damaged.
Soft switching techniques can be applied to CHB. The converter circuit is based on the series connection of single-phase inverters with separate dc sources. The resulting phase voltage is synthesized by the addition of the voltages generated by the different cells. In a 3level cascaded inverter each single-phase full-bridge inverter generates three voltages at the output: +V dc
,
0,- V dc
(positive dc voltage, zero and negative dc voltage).
This is made possible by connecting the capacitors sequentially to the ac side via the power switches.
Fig. 5 Single Phase 3 Level H bridge
The resulting output ac voltage swings from -V dc
to
+V dc
with three levels, -2V dc to+2V dc
with five-level and -3V dc
to +3V dc
with seven-level inverter. The staircase waveform is nearly sinusoidal, even without filtering. The number of voltage levels in a CHB inverter can be found by following equation.
In equation (10) m is the number of level and H is the number of H-Bridge. m=(2H+1)
…………………… (10)
The voltage level m is always an odd number for the
CHB inverter. As shown in fig. 6 is the five level structure of cascaded H bridge where two H bridge structure.
Fig. 6 Single Phase 5 Level H bridge.
VI. SEVEN LEVEL CASCADED QUASI-Z
SOURCE INVERTER
Fig.7 shows the topology of the proposed single phase
Quasi-Z-source seven level cascaded Inverter, consisting of a split inductors (L1 and L2) and two capacitors (C1 and C2) are connected with the input
DC sources and switches. The diode D will effectively protect the circuit from damage when the shootthrough occurs and by using the
Fig.7 Seven Level Cascaded Quasi-Z-Source
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SSRG International Journal of Communication and Media Science (SSRG-IJCMS) – volume 2 Issue 5 November to December 2015
Table. I
Switching scheme for seven level CMLI
S
1
S
1
2
S
3
0 0
S
4
S
5
1 1
S
6
S
1
0 0
’
S
2
’
S
3
1 1
’
S
4
0
’
S
5
S
6
’ ’
Vab
0 1
+3
Vdc
+2
1 0 0 1 1 0 0 1 1 0 1 0
Vdc
1 0 0 1 0 1 0 1 1 0 1 0
+1
Vdc
1 0 1 0 0 1 0 1 1 0 1 0 0
-
0 1 1 0 0 1 1 0 1 0 0 1 1V dc
-
0 1 1 0 0 1 1 0 0 1 0 1 2V dc
-
0 1 1 0 0 1 1 0 0 1 1 0 3V dc shoot though state, the (quasi-) Z-source network boosts the dc-link voltage. Comparing with the normal
Z-source inverter, the impedance are arranged so as to form the represented structure of qZSI.
The proposed Quasi- Z-source based seven level cascaded inverter is controlled with their AC outputs transiting between the seven distinct voltages. They are: +3V dc
, +2V dc
, +V dc
, 0, -V dc
, - 2V dc
and -3V dc
. To obtain the seven levels, the required switching scheme is given in Table.1.
The presented qZSI is expected to perform better, since performance limitations commonly associated with dead-time delay which was avoided. The qZSI network is responsible for the voltage boost up. The inversion is performed by supplying PWM signals to the switches of the circuit in a certain fashion so as to produce seven levels at the output. Inductors L1 and
L2 have the same values and Capacitors C1 and C2 have the same values. QZSI network is a symmetrical network. The operating states of the qZSI are shoot through zero state and non shoot through zero state. In this proposed inverter the number of bridges required is 3 hence it consists of 12 switches. where; m= Number of levels
VII. TOTAL HARMONIC DISTORTION
Fig. 8 Levels in Multilevel Inverter
Total harmonic distortion (THD) is a term used to describe the net deviation of a nonlinear waveform from ideal sine waveform characteristics. Total harmonic distortion is the ratio between the RMS value of the harmonics and the RMS value of the fundamental.
H (n) = 4/ [ cos 1 + cos 2 + cos
3 +…+ cos s] n =1, 3, 5, 7….
The angles α1 ,α2, α3,… ,αs, can be chosen such that the voltage total harmonic distortion is a minimum.
Generally, these angles are chosen so that predominant lower frequency harmonics, 5th, 7th,
11th, and 13th, harmonics are eliminated.
(11)
VIII. COMPARATIVE ANALYSIS OF SINGLE
PHASE H BRIDG MULTILEVEL INVERTERS
Fig. 9 Three Level H bridge model in MATLAB
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SSRG International Journal of Communication and Media Science (SSRG-IJCMS) – volume 2 Issue 5 November to December 2015
Fig. 10 Output voltage waveform of Three Level H bridge model in MATLAB.
Fig. 11 THD of Three Level H bridge model in MATLAB
Fig. 12 Five Level H bridge model in MATLAB
Fig. 13 Output voltage waveform of Five Level H bridge model in MATLAB
Fig. 14 THD of Five Level H bridge model in MATLAB
Fig. 15 Seven Level H bridge model in MATLAB
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SSRG International Journal of Communication and Media Science (SSRG-IJCMS) – volume 2 Issue 5 November to December 2015
Fig. 16 Output voltage waveform of Seven Level H bridge model in MATLAB
Fig. 17 THD of Seven Level H bridge model in MATLAB
In this paper the comparison of THD for single Phase cascaded H bridge multilevel inverter starting from 3- level to modified 7- level. Single phase cascaded H
Bridge inverters are implemented in
MATLAB/SIMULINK. A MOSFET is selected as a switch. The switches are triggered at regular intervals.
Table 2. Comparison of THD of single phase cascaded h bridge multilevel inverter
S.no
1
2
3
Level
3
5
7
THD
30.19%
18.82%
17.17%
IX. CONCLUSION
The proposed system is a combination of QZSI and
CHB multilevel topology and has both advantages of them. This paper presents the comparison of THD of 3 level, 5 levels, and modified 7 level single phase cascaded H bridge multilevel inverter. The results show that as the number of levels increases the THD reduced in single phase inverter. The simulation results show the effectiveness of the proposed QZS-
CMI based single-phase PV system.
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