International Journal of Engineering Trends and Technology (IJETT) – Volume 14 Number 5 – Aug 2014
Photovoltaic Power Generation System with
Extended Boost Quasi-Z-Source Inverter
R.Revathy#1, R.Ravi#2
#1
#1,#2
Assistant Professor, #2Professor
Department of Electronics and Communication Engineering,
Jayalakshmi institute of technology, Dharmapuri, India
Abstract— This paper proposes a extended boost quasi-Z-source
inverter (qZSI) which is used as power conditioning unit for
residential solar powered system .The proposed topology used is
extended boost quasi Z source inverter and implemented using
three phase induction motor. The cascaded qZSI is derived by
the adding of one diode, one inductor and two capacitors to the
conventional qZSI. The proposed two-stage qZSI has all the
advantages of conventional qZSI such as voltage boost and buck
functions, continuous input current, improved reliability and a
new feature such that it reduces the shoot-through duty cycle by
over the 30% for the same voltage boost factor qZSI. The main
focus of this paper is to generate residential power using low
power input of renewable energy like solar along with two stage
qZSI (cascaded qZSI), single phase isolation transformer, and a
voltage doubler rectifier and three phase induction motor. Twostage qZSI in shoot-through and non-shoot-through operating
modes is analysed theoretically. The simulation and experimental
results
are
also
discussed
and
presented
using
MATLAB/simulink.
Index
Terms—
Quasi-Z-source
inverter(qZSI),Power
conditioning
unit(PCU),Voltage
doubler
rectifier(VDR),
photovoltaic
system
(PV),Maximum
power
point
tracking(MPPT).
I. INTRODUCTION
Power converter topologies employed in the PV power
generation systems are mainly characterized by two- or singlestage inverters. The single-stage inverter is an gives good
solution because of its low cost and reliability. However, its
conventional structure must be oversized to cope with the
wide PV voltage variation derived from changes of irradiation
and temperature. The Z-source inverter (ZSI) presents a new
single-stage structure to achieve the voltage boost/buck
character in a single power conversion stage, which has been
reported for various applications [1-2] like photovoltaic
systems. This type of converter can handle the PV dc voltage
variations over a wide range without overrating the inverter.
As a result, the component count and system cost is reduced
with improved reliability due to the allowed shoot through
state.
ZSI can boost the input voltage by a shoot-through
switching state in which both switches of the same phase leg
of the inverter conduct simultaneously and hence three phase
ZSI has nine switching states unlike VSI has eight states. The
ninth switching state is forbidden for the traditional voltage
source converters (VSI) because it causes the short circuit of
the dc link capacitors. In the Z-source inverter, the shoot-
ISSN: 2231-5381
through states are used to boost the magnetic energy stored in
the inductors L1and L2 without short-circuiting the dc
capacitors C1 and C2 which boost the inverter output voltage
during traditional operating states. If the input voltage is high
then shoot-through states are eliminated and ZSI begins to
operate as traditional VSI. The voltage-fed ZSI has a
significant drawback such as discontinuous input current
during the shoot-through mode. This problem is overcome
by introducing quasi-Z-source inverter (qZSI) with continuous
input current [3].The voltage-fed qZSI has all the advantages
of the ZSI and also provides the continuous input current[4].
This paper discusses the improvement method for the voltagefed qZSI with continuous input current and improved
reliability by the introduction of the extended quasi-Zsource(qZS) network, moreover the boost range is increased
by cascading another stage at front end without increasing the
number of switches for photovoltaic system. The extended
qZS-network is derived by the adding of one diode (D2), one
inductor (L3) and two capacitors (C3 and C4) to the
conventional qZSI, as shown in Fig.1(b). By the
implementation of the proposed two-stage qZS-network the
duty cycle of the shoot-through state could be sufficiently
decreased to 30% for the same boost factor of voltage and
component stresses when compared to conventional qZSI.
1(a)
1(b)
Fig .1(a)Quasi-Z-Source inveter,1(b) Cascaded Qzsi
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International Journal of Engineering Trends and Technology (IJETT) – Volume 14 Number 5 – Aug 2014
II.SOLAR CELL AND MPPT
A solar panel is a packaged interconnected assembly of
solar cells which is also known as photovoltaic cells. Solar
panels converts light energy from the sun to electrical energy
through the photovoltaic effect. A solar cell is the building
block of a panel. A single solar cell can be modelled with a
current source, diode and two resistors. This model is called as
single diode model of solar cell and its equivalent circuit is
shown in Fig 2. Electrical connections are made in series to
achieve a desired output voltage and/or in parallel to provide
a desired current capability. I-V characteristic and P-V
characteristic is shown in Fig.3.
Different algorithms are proposed to track the maximum
power that can be obtained from a solar panel .The main type
includes hill climbing method like Perturb and observe
method, incremental conductance method and other methods
like constant reference voltage, current methods. In perturb
and observe method (P&O) a voltage perturbation is provided
with respect to the change in the power of the panel. In this
project this method is simulated. The flow chart of used
algorithm is given below in Figure 4
The conditions are,
dP/dV=0, MPP is reached
dP/dV<0, reduce the voltage by a value
dP/dV>0, increase the voltage by a value
Fig 2. Equivalent circuit of solar cell
The cell output current, I is given by the equation
(2.1)
Open-circuit voltage Voc of the cell is
(2.2)
Short circuit current is Isc = I L
(2.3)
Fig.4 Flow chart of P&O algorithm
Fig.3 P-V, I-V curve of solar cell
In solar panels, Maximum Power Point Tracking (MPPT) is
the automatic adjustment of electrical load to achieve the
maximum possible power extraction during moment to
moment variations of light level from sun, shading a n d
temperature. Solar cells have more complex relationship
between solar irradiation, resistance and temperature that
produces a non-linear output characteristic known as the IV curve. It is the purpose of the MPPT system to sample the
output of the cells and apply a resistance (load) to obtain
maximum power for any given environmental conditions as
the voltage of solar cell depends on irradiation or isolation.
ISSN: 2231-5381
II.CASCADED QUASI-Z-SOURCE NETWORK,
EQUIVALENT CIRCUITS AND ITS OPERATION
This paper shows an implementation of cascaded qZSI in the
power conditioning units (PCUs) for residential solar power
systems (Fig.5). PCUs are used to interconnect distributed
energy sources producing low dc voltage solar panels
(typically 40–80 V dc), to the residential loads (typically 230
V ac single-phase or 3 × 400 V ac). For safety and dynamic
performance requirements, the power conditioning unit should
be realized within the dc/dc/ac concept of conversion. This
means that low voltage from the solar panel first passes
through the front-end step-up dc/dc converter with galvanic
isolation and the output dc voltage is inverted in the threephase inverter and filtered (second dc/ac stage).The topology
proposed with a cascaded qZS-network, a high-frequency
step-up isolation transformer, and a voltage-doubler
rectifier(VDR) performs the concept of dc/dc step-up
converter suitable for PCUs.
A direct step-up dc/dc converter without input
voltage pre-regulation is simpler in control and protection and
provides high voltage gain when compared to traditional
isolated full bridge converters. The varying voltage from the
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International Journal of Engineering Trends and Technology (IJETT) – Volume 14 Number 5 – Aug 2014
solar panel passes through the high-frequency inverter to the
step-up isolation transformer. Due to preboosted voltage the
isolation transformer has moderate turns ratio (1:7-1:8),which
exerts appositive impact in terms of leakage inductance and
efficiency. This paper is gives a new power circuit converter
topology to be implemented in the photovoltaic power
generation such that the converter has advantages like high
reliability, continuous input current and isolation transformer
with reduced turns ratio.
Fig.8(a)
Fig.8(b)
Fig.8 Equivalent circuits of the cascaded qZS-network during two modes (a)
During the nonshoot- through (active) state and (b) during the shoot-through
state.
Fig.5 Power Conditioning Unit (PCU) for residential power systems using
proposed qZSI.
A. Modes of operation
To regulate the varying input voltage, the front-end
qZSI has two different operating modes: shoot-through and
non-shoot through. In the non-shoot-through mode, the qZSI
performs only the voltage buck function. This operation mode
is typically used during light-load conditions, when the output
voltage of an FC or a solar panel reaches its maximum. The
inverter is controlled in the same manner as with the
traditional VSI, utilizing only the active states when one and
only one switch in each phase leg conducts. The transistors in
the full-bridge configuration are controlled alternately in pairs
by T1 and T4 or T2 and T3(Fig. 3) with 180◦-phase-shifted
control signals. In this operating mode, the duty cycle of
inverter switches should not exceed 0.5. The equivalent circuit
of the extended boost qZSI during non-shoot-through i.e
active states is shown in Fig.6
When the input voltage drops below the predefined
value, the qZS inveter starts to operate in shoot-through mode.
In order to boost the input voltage during this mode, a special
switching state called shoot-through state is implemented in
the PWM inverter control. During the shoot through states, the
primary winding of the isolation transformer is shorted by all
the four switches (Fig 7). Moreover, the shoot through states
are used to boost the magnetic energy stored in the dc-side
inductors L1, L2, and L3 without short circuiting the dc
capacitors C1 to C4. This increase in the magnetic energy and
boost the voltage on the inverter output during the active
states. The equivalent circuit of the Extended qZSI during the
shoot-through states is shown in Fig.7.There are seven
different shoot-through states namely three shoot-through
states via any one of the phase leg, three shoot-through states
from combinations of any two phase legs and one shoot
through state by all the three phase legs conducting at same
time.
Fig.6 Equivalent schemes of the inverter during the non-shoot-through
(active) states: (a) positive and (b) negative half-cycles.
Fig.7 Equivalent scheme of the inverter during the shoot-through states.
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Fig.9 Generation of PWM signals with shoot-through states during zero states.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 14 Number 5 – Aug 2014
III.CIRCUIT ANALYSIS OF THE CASCADED
QZS-NETWORK
In the non-shoot-through mode, the inverter bridge viewed
from the dc side is equivalent to a current source [Fig.6].
From Fig.8(a), for the active states, the inductor voltages are
represented as,
vLI = VIN-VC1
(1)
vL2 = VC4-VC2 = VC1 -VC3
(2)
vL3 = -VC4
(3)
From the equivalent circuit of the extended qZSI during the
shoot-through state [Fig. 8(b)], the voltages of the inductors
can be represented as,
vL1 =VIN +VC2
(4)
vL2 =VC4 + VC1
(5)
vL3=VC3
(6)
Let us consider that the duty cycles of the shoot-through and
non-shoot-through states are DS and (1 − DS),
correspondingly. At steady state, the average inductor
voltages over one switching period are zero
VL1 = t ʃt+T vL1 dt = 0
(7)
VL2 = t ʃt+T vL2 dt =0
(8)
VL3 = t ʃt+T vL3 dt =0
(9)
Higher stage qZS-networks can be designed by just multiple
repeating of the parts D2−C3−L3−C4 [Fig. 1(b)]. For the nth
stage qZS-network the boost factor of the input voltage is
given by,
Bn = 1/ [1‒DS.(1+n)]
(21)
where n is the number of stages (n = 1 for traditional qZSIs, n
= 2 for two-stage or extended qZSIs,etc).The shoot-through
duty cycle of the qZSI with two-stage qZS network and
positive input voltage should never exceed the value defined
by (21)
Ds,max<1/3
(22)
IV.COMPARISON OF CASCADED qZSI WITH THE
TRADITIONAL qZSI
A. Boost Performance Comparison
In contrast to the proposed two-stage qZS-network, the boost
factor of the traditional topology based on two capacitors, two
inductors, and one diode [single-stage qZS-network, Fig. 1(a)]
is
B = 1/(1-2DS)
(23)
From (1)–(9), we obtain
VL1 = ͞vLI = DS (VIN + VC2) + (1-DS) ( VIN - VC1) = 0
(10)
VL2 = ͞vL2 = DS (VC4 + VC1) + (1-DS) ( VC4 - VC2) = 0
(11)
VL2 = ͞vL2 = DS (VC4 + VC1) + (1-DS) ( VC1 - VC3) = 0
(12)
VL3 = ͞vL3 = DS (VC3 ) - (1-DS) ( VC4) = 0
(13)
Solving (10)–(13),
be found as
VC1 = VIN
VC2 = VIN
VC3 = VIN
VC4 = VIN
the voltages of capacitors C1 to C4 could
* [(1-2DS) /(1-3DS)]
* [2DS /(1-3DS)]
* [(1-DS) /(1-3DS)]
* [DS /(1-3DS)]
(14)
(15)
(16)
(17)
The peak dc-link voltage across the inverter bridge is given as,
̂vdc =VC1 +VC2 + VC3 + VC4 = V IN * 1/(1-3DS)
(18)
The boost factor B of the input voltage is,
B = ̂vdc / V IN =1 /(1-3DS)
(19)
For the desired input voltage boost factor B, the duty cycle of
the shoot-through state is calculated as
DS = (1-Bˉ1 ) /3
(20)
ISSN: 2231-5381
Fig.10 Shoot-through duty cycle as a function of the voltage boost
factor for different types of qZS-networks.
As seen from Fig.10, the proposed two-stage qZS-network
features a 33.3% smaller shoot-through duty cycle at the same
boost factor of the input voltage than the traditional qZS
network. It means that the time of the shoot-through states of
the two-stage qZSI could be decreased by 33.3% for one
switching period. The shoot-through duty cycle of the qZSI
with the two-stage qZS-network and positive input voltage
should never exceed one-third of the switching period, while
in the traditional (single-stage) topology, it is, in theory,
possible to use the shoot through duty cycle values up to onehalf. Practically, it is not advisable to operate at high shootthrough duty cycle values because it will cause high power
losses in the components, which could seriously reduce the
converter’s efficiency.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 14 Number 5 – Aug 2014
TABLE I
OPERATING VOLTAGES OF THE INDUCTORS AND CAPACITORS OF
THE TRADITIONAL AND TWO-STAGE QZS-NETWORKS
Parameters
V C1
V C2
V C3
V C4
Traditional qZSnetwork
VIN * [(1-D S) /(1-2D S)
VIN * [(D S) /(1-2DS)]
-
Extended qZSnetwork
VIN * [(1-2DS) /(1-3DS)
VIN * [2DS /(1-3D S)]
VIN * [(1-DS) /(1-3DS)]
VIN * [(DS) /(1-3DS)]
VL1
VIN + VC2
VIN + VC2
VL2
VC1
V C4 + VC1
VL3
-
V C3
For the same operating conditions and target value of the dclink voltage, the capacitors in the two-stage configuration
should have capacitance values that are 33.3% lower than
those of a traditional topology. Neglecting transients and
voltage drops, the maximum voltage across the switches (T1
to T4) and the diodes (D1 and D2) will be equal to the peak
dc-link voltage of the converter for both of the compared
topologies. The average current through the diodes of the
qZS-network equals the average current through the inductors
in both of the compared topologies.
Fig.13 Subsystem Block Of Proposed Circuit
TABLE II
SIMULATION PARAMETER
Serial
No
1
2
3
4
5
Paramerters
Values
L1,L2
C2,C4
C1=C3
C5=C6
Switching Frequency
10µH
10µF
220µF
20µF
1 khz
6
7
8
9
10
11
12
Input PV voltage (V1)
Cascaded qzsi output (Vdc)
Transformer ratings
Transformer ratio
Voltagedoubler output(Vout)
Induction motor rating
Motor speed
60.85v
108v
100 VA,10kHz
1:1.5
364v
1000VA 500Vrms 50Hz
1484 rpm
V.SIMULATION RESULTS
Fig.11. Proposed Circuit(Cascaded Qzsi) Simulation
The PCU with cascaded qZSI is simulated using
Matlab/Simulink R2009b and simpower system in matlab
library shown in Fig.11and Fig.13 and the parameters are
shown in above Table II. Here solar panel subsystem has five
12v solar panel in series with irradiation 1000 for each panel
and gives the output of 60v with 4.85A for normal sunny day
as shown in Fig.12.perturb and observe method of MPPT
algorithm is used to maintain the constant voltage around
60v.this voltage is fed as input to the power conditioning
circuit.
Fig.14 solar panel output voltage
Fig 12.subsystem of solar panel
ISSN: 2231-5381
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International Journal of Engineering Trends and Technology (IJETT) – Volume 14 Number 5 – Aug 2014
Fig.15 solar panel output current
Fig.20 Three phase line current Iab ,Ibc,Ica.
Fig.16 Second Stage Output(Cascaded Qzsi)
Fig.17 Single Phase Inverter Output
Fig.21. Motor Speed
On simulating the PCU, we can observe that the
input voltage of 60V is been boosted to 150V as shown in
Fig.14.This shows the boost ratio is 1.8,consisting of small
value of inductors and capacitors. Switching pulse is given by
means of pulse generator with time period of 0.5T for each
transistor and pulse width of 50%. Single phase inverter used
here inverts the dc voltage of 150v from cascaded qZSI to ac
voltage which is a square wave (Fig.17).An isolation
transformer with small step up ratio 1:2 is used here ,due to
losses in the transformer the output of transformer is not
exactly twice. The VDR connected doubles the output from
transformer exactly(Fig.18) from 200V to 400V.The dc output
obtained from VDR is again fed into a three phase inverter
and induction motor, which can run the motor at a speed of
1484 rpm (Fig.21). The Fig.19 and Fig.20 shows the line
voltage and current of three phase inverter which is around
347V. Thus the proposed step up dc-dc converter,which
included cascaded qZSI, transformer and VDR can boost a
low dc voltage to run a three phase induction motor. Thus the
proposed circuit can be connected to residential loads.
Fig.18 Voltage Doubler Output
VI.EXPERIMENTAL VERIFICATIONS
Fig.19 Output three phase inverter( line voltages) Vab, V bc , V ca
ISSN: 2231-5381
The following calculations shows the theoretical output to
be obtained at each stage of the whole system , Table III shows
the comparision of theoretical value and simulation results
obtained using Matlab/simulink.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 14 Number 5 – Aug 2014
Bmax =Vdc/ Vin,
Hence Output of cascaded qZSI(Vdc)=160V
Ds =[1- (Bmax )̄1 ]/3 =0.167
Transformer ratio=1:1.33.
Therefore,Transformer o/p (Vtr) =212.8V
Voltage doubler rectifier o/p(Vout)= Vc1+ Vc2 (or) 2*
Transformer o/p =425.6V
Vph (rms) =√2/3* Vd =200.63V,
Vl (rms) =√2/√3 * V d =347.5V
Given:
P=115w Vin =80v f=10khz
(23)
qZS network inherits all the advantages of traditional
solutions and reduced the shoot-through duty cycle by over
30% at the same voltage boost factor and component stresses
as the conventional qZSI.By cascading or increasing the
number of stages of qZS network the shoot through duty cycle
can be further decreased.Voltage stress is lower when
compared to ZSI.The turns number of the secondary winding
of the isolation transformer could be reduced by turns ratio of
1 : 2 in the case of VDR instead of 1 : 10 of traditional fullbridge rectifiers due to the voltage doubling effect available
with the VDR. The high-frequency step-up isolation
transformer(1:2) provides the required voltage gain as well as
input output galvanic isolation..The proposed cascaded qZSI
can be applied to almost all dc/ac, ac/dc, ac/ac, and dc/dc
power conversion schemes.It can be used in demanding
applications as power conditioners for renewable energy
sources like FCs and solar panels.Thus the proposed circuit
is implemented using three phase induction motor and
simulated using Matlab/simulink.
By Sustituting The Values, C1 =C2= C3 = C4=9.8µF
REFERENCES
[1]
(24)
By Sustituting The Values, L1 = L2 = L3 =7.7mH.
TABLE III
COMPARISION OF THEORITICAL AND SIMULATION PARAMETERS.
Parameters
V dc
Theoretical
value
120V
Simulation
results
108V
B max
2
1.825
V tr
212.8V
199.5V
V out
425.6V
363.8V
V ph (rms)
200.63V
171.5V
V l (rms)
347.5V
360V
C 1 ,C2,C 3 , C4
9.8µF
20 µF
L1 , L2 , L3 ,
7.7mH
10 µF
VII.CONCLUSION
The paper focuses on PCUs using cascaded qZSI as a major
part for residential power system which is fed by a low voltage
renewable energy source, solar panel.The proposed cascaded
ISSN: 2231-5381
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