International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
Investigation of Different Multilevel Current Source
Invertertopologies for Grid Connected PV System
Swapnil V. Narkhede1, N. S. Mahajan2, M. M. Ansari3
PG Student, Dept. of Electrical Engineering, SSBT’scollege of Engineering & Technology, Bambhori, Jalgaon,
Maharashtra, India 1
Assistant Professor, Dept. of Electrical Engineering, SSBT’scollege of Engineering & Technology, Bambhori,
Jalgaon, Maharashtra, India 2
Assistant Professor, Dept. of Electrical Engineering, SSBT’s college of Engineering & Technology, Bambhori,
Jalgaon, Maharashtra, India 3
Abstract—A multilevel inverter technology has much
more importance in the sector of renewable energy.
Mostly multilevel inverter creates the output by
combining of DC voltages means by rising of DC
voltages, the levels of the output increases. Multilevel
inverter system consist of inverter module with itsPV
source, maximum power point tracking (MPPT) unit
and DC link current controller. The moreapplicable
control and modulation methods established for such
type of inverters are multilevel sinusoidal pulse width
modulation. This review paper intends a gridconnected Photovoltaic (PV) system based on a
multilevel
Current
Source
Inverter
(CSI)
configurations as well as new development in the
sector of multilevel current source inverter and also
focuses on the comparative analysis of different
inverter topologies.
Keywords —Multilevel Current Source Inverter, PV
system, MPPT, Current Source Inverter (CSI),
Voltage Source Inverter (VSI), Z source Inverter
(ZSI).
I. INTRODUCTION
As we know that a solar is the most
important renewable source of energy which is widely
used in the sector of power generation. There are
many more development going in solar in order to
improve the PV system efficiency and decrease the
system losses as well as cost of the system. For this
reasonmultilevel inverters are proposed. A multilevel
inverter is more beneficial for higher power
application such as decrement in harmonics.It has
higher power rating because of reduced switching
device voltage and current stress as compared to two
level inverters. A voltage source inverter has been
commonly used multilevel inverter topology up to
now. But because of more advantages CSI are used in
its place of VSI. The reason behind this is that CSI has
in-built boosting and short circuit protection abilities,
direct output current controllability and AC side
simple filter structure. [1,2,15]
Generally there are basically two types of
grid connected PV systems which exist centralized
connected configuration as well as string connected
configuration. This configuration exist series and
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parallel connected PV modules in order to increase
power levels. But a string connected configuration has
more advantages for grid connected system. When
string type of configuration is used in which PV
modules are connected in parallel then because of
cloudy environment i.e. partial shading there is
divergence of maximum power point will reduce the
power output which intern affect the system efficiency.
By using multilevel inverters PV array can feed
improved quality of power even under partial shading
conditions also. As there is more number of levels in
output of multilevel inverters, they can generate high
quality output power .Incase of voltage source
multilevel inverter as the number of voltage levels are
incremented it leads to raise in number of
capacitor.But because of unequal charging and
discharging of input capacitors may generate offset in
output voltage waveforms. To overcome such problem
extra corresponding circuits are needed which intern
affect the efficiency of the system and increase the
cost as well as complication[2,5,8,12].
In current source multilevel inverter as the
number of levels in output increases which results in
increase in a number of sharing inductor but which are
lossy and bulky. In present days super conducting
magnetic materials (SCMM) are being used for an
inductor design, which reduce the losses in the
conducting material below certain threshold
temperature. Hence because of this the sharing
inductors are not only lossy but also exhibit the
problem of balanced current sharing. A certain current
source multilevel inverter topology is proposed in this
review paper. These topologies contain more number
of power switches to generate the multilevel in the
output current waveform which increases losses in the
system.This paper proposes a novel current source
multilevel
inverter
configuration,
which
is
implemented with less number of power switches
compared to previous topologies. The proposed
current source multilevel inverter configuration has
multiple inputs and can generate multi levels at the
output. A different PV module can be assumed to be
an input for proposed system. [3, 4,7, 14]
A CSI topology are preferred for interfacing
PV system to AC load for the following reason i) It
provide smooth DC side current. ii) CSI’s energy
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
storage element has longer lie time than VSI. iii) CSI
has an inherent voltage boosting capability that allows
integration of PV panels of lower output voltages. As
compared to two level CSI, multilevel CSI has huge
benefits for higher power application .These include
more stable operating conditions, direct control of the
output current, faster dynamic response in some
situations, easier fault control, etc. However the
growth in the area of power electronics and
microelectronics made it possible to reduce
theharmonics content with multilevel PWM inverters,
where the number of levels of the converters is
increased rather than increasing the size of the filters.
Therefore, the THD for multilevel CSIs will be lower
than that of a usualCSI .[3,4,5,9]
II. CLASSIFICATION OF PV CELL INVERTER
A. Centralized Inverter:
In this type, the strings of PV modules are connected
in parallel with each other through string diodes for
reaching high power levels and this arrangement is
connected to the multilevel inverter. Grid connected
inverters are basically line commutated using
thyristor, which involves many harmonics in
current and has poor power quality.[3] Centralized
inverter has some limitations which are as follows:
a. High voltage DC links between the PV
module andthe multilevel inverter.
b. Power deficiency because of incorporated
MPPT.
c. Mismatch between PV modules.
Fig. 2. String Inverter [4]
C. Multistring Inverter:
It's the additional development in string
inverter. In this technology some strings are
linked to their particular DC converter and at last
to the AC inverter. Each string might be
structured separately. This configuration has
more efficiency as compared to other
configuration. [3]
Fig. 3. Multi-String Inverter [4]
III. PROPOSED MULTI LEVEL CURRENT SOURCE
INVERTER.
Fig.1. Centralized Inverter[4]
B. String Inverter:
It is modified technology of centralized
inverter technology. In this technology single
string is connected to inverter.It has several
advantages i.e.
a. No losses associated with string diodes
and separate MPPT can be applied to
each string.
b. Overall efficiency of system increases.
[3]
Fig.4. Multilevel Current Source Inverter [2]
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
A. Operation principle:
The proposed five level current source
inverter’s circuit diagram is shown in Fig. 4. The
proposed system consist of two PV sources and single
phase five level current source inverter. Low
frequency current unfolding circuit and CL filter. In
this topology, power switches S11, S12, S22 and S21 are
operated at high frequency and Sp1, Sp2, Snl and Sn2
are operated at low (grid) frequency. By using this
configuration, five levels +i1, +i2,0,-i2,-i1can be
generated in a wave form of currentwhich are given as
follow:
(a) Level +i1 or –i1: All high frequency switches are
operatedwith sinusoidal pulse width modulation
(SPWM) and the two low frequency switches are ON
according to the polarity of grid voltage.
(b) Level +i2 or -i2: one top and one bottom high
frequency power switch is operated with SPWM and
other two high frequency power switches are ON and
two low frequency switches are in ON position
according to the polarity of grid voltage.
(c) Level 0: all high frequency power switches are in
ON position. For each current level, there is a
contribution of two PV sources. For (b), Lll and L12
(L22 and L21) are assumed to be equal in value. Thus
current sharing is half of Ipvl.The proposed topology
can be operated with balanced andunbalanced PV
sources without affecting the quality of grid current by
proposed control strategy and switching pattern of
devices as shown in Table I. [2,5]
Table-I: Switching states of proposed CSI
S11
S12
S21
S22
SP1
SP2
Sn1
Sn2
iA
0
0
0
1
1
1
0
0
+i2
0
1
1
1
1
1
0
0
+i1
1
1
1
0
0
0
0
0
0
1
0
0
0
0
0
1
1
-i2
0
0
0
0
0
0
1
1
-i1
+i1= Ipv1+ Ipv2
(1)
Fig. 5. Equivalent circuit in Mode- I[2]
Mode-2 (+i2 level):
Fig. 6. Equivalent circuit in Mode-2[2]
Fig.6 shows the equivalent circuit inthis
mode-2. During this mode, the five level CSI is
operated in positive half cycle of grid voltage to
maintain the +i2level by turning ON the power
switches (Spl and Sp2), turning OFF the power
switches ((SI2, and S21) or (Sll, S22) and modulating the
switches ((S11, S22) or (SI2, and S21) at high frequency
according to sinusoidal grid current reference
waveform. In this mode, the current magnitude is
given by the summation of half of two PV module
currents (Ipvl and Ipv2).
(2)
B. Modes of Operation:
The operation of the proposed topology is
divided into five modes in positive and negative half
cycles of the grid voltage.These are explained as:
Mode-3 (0 level):
Mode-1 (+il level):
Fig. 5 shows the equivalent circuit to this
mode. During this mode, the CSI is operated in
positive half cycle of grid voltage to maintain the +il
level by turning ON the power switches (Spland Sp2)
and modulating the switches (S11, S12, S22 and S21) at
high frequency according to sinusoidal grid current
reference waveform. In this mode, the current
magnitude is thesummation of two PV module
currents (Ipvl and Ipv2). [2,6]
Fig.7. Mode-3 (Zero state)[2]
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
Fig.7 shows the equivalent circuit duringthis mode.
During this mode, the power switches, Spl and Sp2are
turned OFF, while the remaining switches are in ON
position.
magnitude is the summation of two PV module
currents (Ipvland Ipv2). [2,8,10]
Mode-4 (-i2 level):
The complete synthesis of CSI output current for one
grid frequency cycle is shown in Fig. 10. [2]
-i1 = - Ipv1 – Ipv2
(4)
Fig.10. Synthesis of CSI output current waveform [2]
Fig. 8. Equivalent circuit in Mode-4[2]
Fig. 8 shows the equivalent circuitduring this
mode. During this mode, the CSI is operated in
negative half cycle of grid voltage to maintain the i2level by turning ON the power switches (Snl and
Sn2), turning OFF the power switches ((S12, S21) or
(Sl1, S22) and modulating the switches ((Sl1, S22) or
(S12, and S21)) in high frequency according to
sinusoidal grid current reference waveform. In this
mode, the current magnitude is the summation of half
of two PV module currents (Ipv1 and Ipv2) as given
below:
(3)
Mode-5 (-i1 level):
C. Control strategy and switching pulse generation:
Modulation strategy and control strategy of
the proposed five level current source inverter is as
shown in Figs.11,12.
Fig. 11.Modulation strategy used for the proposed CSI.[2]
Available maximum power is fed into the
grid from the two PV sources based on the Impp1 and
Impp2 references, which are generated by the
Incremental Conductance MPPT algorithm. Five level
current output of CSI is generated by using the two
carriers
and
Sinusoidal
Pulse
Width
Modulation(SPWM) technique as per switching states
shown in Table 1. This PWM technique compares the
reference current wave form with two high frequency
carrier waveforms. Reference current waveform is
obtained by the multiplication of PLL output and
addition of Impp1, Impp2. [2,7,11]
Fig.9. Equivalent circuit in Mode-5[2]
Fig. 9 shows the equivalent circuitduring this
mode. During this mode, the CSI is operated in
negative half cycle of grid voltage to maintain the –
i1level by turning ON the power switches (Snl and Sn2)
and modulating the switches (Sl1, S12, S22 and S21) in
high frequency according to sinusoidal grid current
reference waveform. In this mode, the current
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Fig. 12. Control strategy used in the multilevelCSI. [2]
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D. Advancement In Multilevel Inverter
Z-Source Inverter:
likely done in VSI which will enhances the circuit
reliability. [17]
Improvement in multilevel inverter is an
impedance source inverter which is generally known
as Z- source inverter. This inverter is employed with
L,C impedance network to connect the main converter
circuit to the power source which indirectly provides
enhancement in input voltage which is not possible in
VSI as well as CSI. Z source inverter has more
number of applications in DC-DC, AC-DC, and ACAC, DC-DC power conversion. Traditional VSI, CSI
are shown in fig. 13,14
Fig. 15: General Structure of the Z-Source Inverter [17]
E. Comparative Analysis of Different Inverter
Topologies
Table 2.Comparative Analysis of Different Inverter
Topologies
Topics
Centralized
Inverter
String
Inverter
Multistring
Inverter
Low
Moderate
High
2 Maintenance
Simple
Robust
Robust
3 Power Loss
More
Less as
compared to
Centralized
inverter.
Less as
compared to
both
configuration
4 Efficiency
Moderate
More
More
5 Upgradability
Low
Moderate
High
6 Continuity of
Service
Very Low
Lower as
compared to
multistring
inverter
Higher than
all topologies
1 Cost
Fig.13: Traditional V-Source Inverter[17]
Fig.14: Traditional I-Source Inverter [17]
Fig. 15 shows a two port network which
consist of split inductor and capacitors and which are
connected in X shape format to give an impedance
source (Z source) coupling to inverter or converter to
a DC source or load or another converter. A DC
source or load may be voltage or current source or it
may be load. A DC source can be battery, fuel cell,
diode rectifier, thyristor converter, inductor, capacitor
etc. Switches employed in converter/inverter can be a
combination of switching devices and diodes which is
shown in fig. 13, 14
ZSI is widely employed to decrease and increase the
output AC voltage which is not done by VSI and CSI.
ZSI has compatibility to short DC link which not
ISSN: 2231-5381
In previous days one of the easy way to
connect large number of PV module to network was
the use of centralized inverters.Though this
configuration is broadly used, particularly for high
power values of PV fields, it shows many drawbacks
which are summarized in the following table. A string
inverter and multistring inverter configurations are
advanced technology than centralized inverter
configuration and in these configuration photovoltaic
fields arranged in strings rather than arrays.
V. CONCLUSION
The multilevel current source inverter is able
to extract maximum available power from PV source.
Even during the partial shading conditions five level
current source inverter can extract and feed maximum
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
available power in to the grid with both balanced and
unbalanced PV sources. As less number of power
electronics component are used this CSI topology both
cost as well as size get reduced.
Centralized inverter topology were used in
past but now string inverter and multistring topologies
are used which provides overall higher efficiency of
system and reduces cost of the system. Multistring
inverter topology is the latest development in the field
of inverter topology which provides individual control
of each string. So that anyone can start his own PV
plant with few modules which is main benefit of
multistring inverter topology.
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