Copyright © 2015, American-Eurasian Network for Scientific Information publisher
JOURNAL OF APPLIED SCIENCES RESEARCH
ISSN: 1819-544X
EISSN: 1816-157X
JOURNAL home page: http://www.aensiweb.com/JASR
2015 September; 11(14): pages 59-66.
Published Online 10 September
Research Article
Total Power Control for Quasi Z-Source Inverter –Based Grid Tie Single Phase
Photovoltaic System
1D.Umarani
1Assistant
2Associate
and 2Dr.R.Seyezhai
Professor, Department of EEE, SSN College of Engineering, Chennai, India
Professor, Department of EEE, SSN College of Engineering, Kalavakkam, Chennai, India
Received: 12 May 2015; Accepted: 18 July 2015
© 2015
AENSI PUBLISHER All rights reserved
ABSTRACT
A total power control scheme including DC link voltage and PWM control for Quasi Z-source inverter based grid-tie single phase
photovoltaic power system is proposed. The DC link voltage control is employed to control the voltage of the inverter H-Bridge and
provides a solution to the voltage imbalance problem. Independent MPPT is employed to control the shoot through states of the Quasi Zsource inverter and also the phase shifted PWM is employed for conventional non shoot through pulses to the inverter switches. For
MPPT, the Perturb and Observe algorithm has been used. The Phase shift sinusoidal PWM technique has been implemented for generating
the pulses to which the shoot-through from the independent MPPT will be added. The PV module, the impedance network and the output
filter has been designed for the inverter with output power of 0.6 kW. The Phase shift triangular carrier PWM has been implemented to
add with shoot through states from the independent MPPT. The detailed design of the total power controlling action is disclosed in this
paper. Simulations were carried out using MATLAB/SIMULINK and the simulation results are discussed.
Key words: Quasi Z-Source inverter, DC link voltage, MPPT, Double carrier PWM & Multilevel inverter, Photovoltaic system.
INTRODUCTION
As the renewable energy usage has been
increasing day by day we need to efficiently trap it.
So at every step of designing the stand-alone PV
systems care must be taken to reduce the system size
and cost without compromising its performance. As
so many researches have been focused on the source
and load side of the PV systems, we have taken our
investigation towards the Quasi Z source inverter
(QZSI) topology. The QZSI is a sub topology of ZSource inverters. These inverters are gaining
attention in the PV systems as they can boost the
output voltage in a single stage and draws continuous
current from the source. The QZSI topology allows
independent control of power delivery with high
reliability and distributed MPPT can be employed for
this topology [4]-[7]. It has so many advantages
compared with traditional voltage source inverters
and are very suitable to PV systems. As the solar
power fluctuates stochastically, it will be difficult to
control the output voltage and power that is fed to
grid. In this paper, a total power control scheme has
been discussed for controlling the power of the PV
based single phase grid tied QZSI. To effectively
control the QZSI, independent control of DC link
voltages and the pulse width modulation (PWM) are
necessary [9]. The maximum power point tracking
(MPPT) of the PV source has been employed with
perturb and observe (P & O) algorithm for generating
shoot through states and PWM has been employed
with Phase shift triangular carrier PWM. The DC
link voltage control and the grid injected power
control is studied.
Topology Of Quasi Z-Source Inverter:
Figure 1 shows the circuit configuration of
Quasi Z-Source inverter (QZSI). As like any other
Corresponding Author: D.Umarani, Assistant Professor, Department of EEE, SSN College of Engineering, Chennai, India
E-mail: umaranid@ssn.edu.in
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D.Umarani and Dr.R.Seyezhai, 2015 /Journal Of Applied Sciences Research 11(14), September, pages 59-66
inverter, the QZSI has a DC or PV source. Instead of
connecting the voltage source directly to the inverter
H-bridge, an impedance network is connected in
between. It has certain significance over
conventional voltage source inverters. By employing
the unique impedance network between the voltage
source and the inverter bridge, the following
advantages can be achieved.
1. The AC output voltage will be more than the
input DC rail voltage.
2. The upper and lower devices can be gated
simultaneously without creating short circuit with the
source.
3. Overdrive can be achieved without additional
converter stages to obtain the required output which
reduces the system cost and reduces efficiency.
4. They are less vulnerable to EMI noise.
Fig. 1: Quasi Z-source Inverter
Operating States Of The Quasi Z-Source Inverter:
There are two operating states in a QZSI. They
are non-shoot through state and shoot through state.
The equivalent circuits of the two states are shown in
fig.2 and 3. With the QZSI, the unique LC and diode
network connected to the inverter bridge modify the
operation of the circuit, allowing the shoot-through
state. This network will effectively protect the circuit
from damage when the shoot-through occurs and by
using the shoot-though state, the QZS network boosts
the dc-link voltage. The non-shoot through state
consists of usual active and zero states of any VSI.In
boost mode some or all of the zero states are replaced
by shoot-through states depending on the PWM
technique used.
Fig. 2: QZSI in non-shoot through state
Fig. 3: QZSI in shoot through state
Impedance Network Design Of Qzsi:
The inductance and capacitance values of the
QZSI are calculated as follows.
Inductance:
∆𝑇.𝑉 𝐿
𝑇 .𝑚 .𝑉
𝐿1 = 𝐿2 =
= 𝑜 𝑖𝑛
∆𝐼
Capacitance:
2.𝐼𝐿 .𝑅𝑐
(1)
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D.Umarani and Dr.R.Seyezhai, 2015 /Journal Of Applied Sciences Research 11(14), September, pages 59-66
𝐶1 = 𝐶2 =
2.∆𝑇.𝐼𝑐
∆(𝑉𝑐1 +𝑉𝑐2 )
=
𝑇𝑜 .𝑚 .𝐼𝐿
2.𝐵.𝑉 𝑖𝑛 .𝑅𝑣
(2)
Where,
To - Shoot-through Interval, M - Modulation
Index, Rc
- Peak current ripple in %,Rv - Peak
voltage ripple in %,
IL
- Rated Load current andfs
frequency
- Switching
Pv Module Design And Mpp Tracking Using P & O
Algorithm:
Photovoltaic cell equivalent electrical circuit is
shown in figure 4.
Fig. 4: Equivalent circuit of photovoltaic cell
The I-V and P-V characteristics of the solar cells
are non-linear in nature. The solar cell characteristics
mainly depend on the existing atmospheric
conditions such as temperature and irradiance. The
P-V characteristics of solar cell are shown in figure
5.The I-V curve shown in figure 6 is for the PV
module under sunlight and dark conditions.
Fig. 4: P-V characteristics of a PV cell
Fig. 5: P-V characteristics of a PV cell
The PV cell can be mathematically modelled
using the following equations.
The open circuit voltage given by Voc is obtained
when load current reaches zero.
𝑉𝑜𝑐 = 𝑉 + 𝐼𝑅𝑠ℎ
The diode current expression is given by
(3)
𝑄 𝑉 𝑜𝑐
𝐼𝑑 = 𝐼𝐷 𝑒 𝐴𝐾𝑇 − 1
(4)
The load current is given by the following
equation
𝑄 𝑉 𝑜𝑐
𝐼 = 𝐼𝐿 − 𝐼𝐷 𝑒 𝐴𝑘𝑇 − 1 −
𝑉𝑜𝑐
𝑅𝑠ℎ
(5)
Where,
ID – Diode saturation current,Q – Charge of an
electron = 1.6×10-19 C,A - Curve fitting constant,k -
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D.Umarani and Dr.R.Seyezhai, 2015 /Journal Of Applied Sciences Research 11(14), September, pages 59-66
Boltzmann constant = 1.38×10-23 J/K,T – Absolute
Temperature in K
Independent Mppt Control:
Perturb and Observe algorithm has been
implemented for tracking maximum power point. It
is a very simple algorithm which requires the
measurement of solar panel’s output voltage and
current. The perturbation at each step is used to find
out the maximum power. The flow chart of the P &
O algorithm is shown in figure 6.
Fig. 6: Flow chart of P & O MPPT algorithm
Figure 7 shows the simulation circuit of designed PV module along with MPPT implemented using P and O
algorithm.
Fig. 7: Simulation of PV Module with MPPT
The simulation parameters for the PV module has been listed in table 1.
Table 1: Simulation parameters of PV array
PV Parameters
Open circuit voltage Voc
Short circuit current I sc
No of cells Ns
Insolation G
Ideality factor A
Operating temperature T
Rating
105.1V
2.55 A
36
1000 W/m2
1.5
298 K
Figure 8 represents the shoot through pulses generated from the independent MPPT control.
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D.Umarani and Dr.R.Seyezhai, 2015 /Journal Of Applied Sciences Research 11(14), September, pages 59-66
Fig. 8: Shoot through pulses generated from MPPT
Depending upon the variations in the PV output,
the Perturb and observe algorithm tries to always
operate the PV in its maximum power point by
changing the reference wave for shoot through pulse
generation.
Phase Shift Carrier Pwm:
The reference wave generated from the DC link
voltage control will be used to generate PWM pulses
using phase shifted triangular carriers. The resulting
output pulse will be added along with the shoot
through pulses produced from the independent
MPPT. The Phase shift technique has been chosen
out of the other carrier amplitude PWM techniques
such as PD, POD, APOD because it has balanced
switching action and there is no difficulty in
implementing shoot through states.Figure 9
represents the PS-PWM carriers.
Fig. 9: Phase shift triangular carrier PWM
Total Power Control And Dc Link Voltage Control:
The power injected to the grid is the PV string
output power. Let the power reference be P n*.
The peak value of the grid current is
I grid= 2Pn*/Vgrid peak
(6)
To ensure unity power factor operation, the
phase locked loop (PLL) is used to measure the
Fig. 10: Control scheme of the QZSI
phase of the grid voltage. The grid current is
measured and fed back to the current loop. The
output voltage Vtotalis given by the current loop to
produce the modulation index. With the modulation
index and the shoot-through duty ratio from the
MPPT, the required gating signals are produced for
the QZSI [9]-[10].Figure 10and 11 shows the control
scheme and power control of the QZSI.
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D.Umarani and Dr.R.Seyezhai, 2015 /Journal Of Applied Sciences Research 11(14), September, pages 59-66
Fig. 11: Power control of the whole system
Simulation Results of The Total Power Control
Scheme:
Simulation of the QZSI with total power control
and DC link voltage control has been carried out
using MATLAB/Simulink.
The simulation parameters of the QZSI has been
listed in table 2.The gating pulses for the QZSI are
shown in figure 13.
Table 2: Simulation parameters of QZSI
QZSI Parameters
Input Voltage
Inductors L1,L2
Capacitors C1, C2
Boost Factor B
Switching frequency fs
Fig. 12: Simulation of QZSI with Power control of the whole system
Fig. 13: Gating pulses for the QZSI
Rating
105 V
3 mH
2200 µF
1.66
5kHz
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D.Umarani and Dr.R.Seyezhai, 2015 /Journal Of Applied Sciences Research 11(14), September, pages 59-66
The output current , output voltage and output power fed to the grid from the PV sourced QZSI has been
shown in figure 14.
Fig. 14: Grid current, Grid Voltage and output power of the QZSI
The grid voltage and grid current was obtained
as 230 V and 2.55 A respectively.So the total output
power was found to be 600 W in the simulation
result.
References
Conclusion:
The total power control of QZSI grid tie single
phase photovoltaic system has been discussed
clearly. The PV system acts as source for the QZSI
and it was modeled mathematically using equations
in MATLAB/Simulink. Also independent MPPT is
employed with Perturb and Observe algorithm to
control the shoot through states of the QZSI when
there are drastic variations in PV source. With an
input voltage of 105 V obtained from PV, the QZSI
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and the shoot through of the QZSI. The controllers
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2.5 A and the power output was around 600 W.
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Acknowledgement
The authors would like to thank the ALL INDIA
COUNCIL FOR TECHNICAL EDUCATION for
funding the project and to carry out the research
work.
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