INTERNATIONAL JOURNAL OF CONTROL, AUTOMATION AND SYSTEMS
VOL.1 NO.2
ISSN 2165-8277 (Print) ISSN 2165-8285 (Online)
http://www.researchpub.org/journal/jac/jac.html
APRIL 2013
Design and Control of Large Scale Photovoltaic System
for High Power Applications

1
Hammad Abu-Zied, 1,2Ahmed G. Abo-Khalil
Abstract- Grid-connected PV application is currently driving
the global PV industry. This trend is being increased because
of the many benefits of using renewable energy sources in
modern distributed generation systems. The connection of
such distributed generator to the grid requires special
conditions for high quality electric power system. This paper
presents
a
three-phase
grid-connected
high-power
photovoltaic generation system. The main purpose of a gridconnected PV system is to transfer the maximum power
obtained from the sun into the electric grid. Therefore the use
of a PWM inverter with maximum power point tracking
(MPPT) capabilities and the ability of effectively connecting to
the AC power grid is required. The model of the PWM
inverter and a control strategy using dq0 transformation are
proposed. The maximum power point is controlled by
controlling the q-axis current component of the grid side,
while the power factor is controlled to unity by controlling the
grid d-axis current component. Simulation results and
analyses are presented to validate the proposed controller for
high-power, grid connected photovoltaic generation system.
simplicity, overall efficiency and a cost reduction.
The past technology, illustrated in Fig. 1(a), was based on
centralized inverters that interfaced a large number of PV
modules to the grid. The PV modules were divided into
series connections (called a string), each generating a
sufficiently high voltage to avoid further amplification.
These series connections were then connected in parallel,
through string diodes, in order to reach high power levels.
This centralized inverter includes some severe limitations,
such as high-voltage dc cables between the PV modules and
the inverter, power losses due to a centralized MPPT,
mismatch losses between the PV modules, losses in the
string diodes, and a nonflexible design where the benefits of
mass production could not be reached [3], [4].
The present technology consists of the string inverters and
the ac module [4]. The string inverter, shown in Fig. 1(b), is
a reduced version of the centralized inverter, where a single
string of PV modules is connected to the inverter. The input
voltage may be high enough to avoid voltage amplification.
The normal operation voltage is as low as 450 - 510 V. The
possibility of using fewer PV modules in series also exists,
if a dc–dc converter or line-frequency transformer is used
for voltage amplification. There are no losses associated
with string diodes and separate MPPTs can be applied to
each string. This increases the overall efficiency compared
to the centralized inverter, and reduces the price, due to
mass production [3]-[6].
The multi-string inverter depicted in Fig. 1(c) is the
further development of the string inverter, where several
strings are interfaced with their own dc–dc converter to a
common dc–ac inverter [4]. This is beneficial, compared
with the centralized system, since every string can be
controlled individually. Thus, the operator may start his/her
own PV power plant with a few modules. Further
enlargements are easily achieved since a new string with
dc–dc converter can be plugged into the existing platform.
A flexible design with high efficiency is hereby achieved
[7], [8].
The AC-module in Figure 1(d) is a reduction of the string
inverter, where each PV module has its own integrated
power electronic interface to the utility. The power loss of
the system is reduced due to removing the mismatch
between the modules, but the constant losses in the inverter
may be the same as for the string inverter. Also the AC-
Keywords: Photovoltaic, Maximum Power point tracking,
Boost Converter, Grid-utility.
I. INTRODUCTION
As people are much concerned with the environmental
problems and the fossil fuel exhaustion caused by the
conventional power generation, renewable energy sources
are becoming increasingly important. The photovoltaic
(PV) panel is widely used in residences and buildings since
it has no noise, no installation area limitation, and no
additional maintenance cost [1]. Recently, the photovoltaic
(PV) has developed into a mature technology and has
become acceptable worldwide. Therefore, the PV power
supplied to the utility grid is gaining more and more
visibility, while the world’s power demand is increasing.
The PV grid-connected systems have become one of the
most important applications of solar energy [2]. The
increase of average PV system size may lead to new
strategies like eliminating the DC-DC converter that is
usually placed between the PV array and the inverter, and
moving the MPPT to the inverter, resulting in increased
2
1
Electrical Engineering Department, Assiut University, Assiut, Egypt
On leave to Majmaah University, Majmaah, Saudi Arabia
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INTERNATIONAL JOURNAL OF CONTROL, AUTOMATION AND SYSTEMS
VOL.1 NO.2
ISSN 2165-8277 (Print) ISSN 2165-8285 (Online)
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Centralized
technology
APRIL 2013
Multi-String technology
String technology
String diodes
PV modules
AC-Module technology
DC
DC
DC
DC
AC
DC
AC
AC
DC
1-phase
Connection
3-phase
Connection
(a)
DC
DC
DC
1 or 3-phase
Connection
(b)
DC
AC
AC
(c)
AC
1-phase
Connection
(d)
Fig.1 Photovoltaic system technologies. A) Past centralized technology, b) Present string technology, c) Present multi-string technology, d) Latest ACModule technology.
module concept supports optimal operation of each module,
which leads to an overall optimal performance [9].
The voltage-power characteristic of photovoltaic array is
a non-linear because of the variation that caused by solar
irradiation and temperature. Therefore, it is very important
for a PV to operate at the maximum power point to reduce
the cost of the generated power to the system installation
cost. Many maximum power point tracking (MPPT)
techniques for photovoltaic (PV) systems are well
established in the literature. The most commonly known are
hill-climbing [10], fractional open-circuit voltage control
[11], perturb and observe (P&O) [12], and incremental
conductance [13], the parasitic capacitance [14], the
constant voltage [15]. There are lesser known, but
sometimes very appropriate, methods such as maximizing
load current or voltage [16], fractional short-circuit current
control [17], array reconfiguration [18], linear current
control [19], fuzzy control [20], neural network dc link
capacitor droop control, pilot cells, current sweep, limitcycle control, and several others [21], [22].
In this paper, a design and simulation of a high
performance three-level inverter of a three-phase gridconnected PV system and its controller. A 500 kW PV grid
connected systems is used to investigate the performance of
high power 3-phase PV generation systems. The control
scheme of a three phase current-controlled SVPWM
inverter in synchronous- rotating d-q frame able to control
the output active and reactive power is presented.
Validation of models and control algorithms is carried out
through computer simulations PSIM software.
II. SOLAR ARRAY CHARACTERISTICS
Solar cells are devices that convert photons into
electrical potential in a PN junction, of which equivalent
circuit is shown in Fig. 2(a). PV module is composed of n
of these cells in series, as shown in Fig. 2(b), in order to
reach a high voltage at the terminals. The connection of PV
cells in series is named a string. Due to the complex
physical phenomena inside the solar cell, manufacturers
usually present a family of operating curves (V-I) as shown
in Fig. 3. These characteristics are obtained by measuring
the array volt-ampere for a different illumination values.
From these characteristics, the optimum voltage or current,
corresponding to the maximum power point, can be
determined. It is clearly seen in Fig. 3 that the current
increases as the irradiance levels increase. The maximum
power point increases with a steep positive slope
proportional to the illumination.
The main parameters which influence the illumination
levels on a surface at a fixed tilt on earth are the daily and
seasonal solar path, the presence of clouds, mist, smog and
dust between the surface and the sunlight, and the shade of
the object positioned such that the illumination level is
reduced, etc.
The equation of the PV output current I is expressed as
a function of the array voltage V
I  I sc - I o e
17
q (V  IRs )
KTk
- 1} - (V  IRs )/Rsh
(1)
INTERNATIONAL JOURNAL OF CONTROL, AUTOMATION AND SYSTEMS
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APRIL 2013
Impp is the current at maximum output power,
Rs
ID
D
Isc
voltage at maximum power, I sc is the short circuit current
Icell
Rsh
and Voc is the open circuit voltage of the array. Equation
Vcell
(2) is only applicable at one particular operating condition
of illumination G and cell temperature Tc .The parameter
variations can be calculated by measuring the variation of
the short-circuit current and the open-circuit voltage in
these conditions using the parameters at the normal
illumination and cell temperature. Equation (2) is used for
the I-V and P-V characteristics for various illumination and
fixed temperature ( 25[ oC ] ) in Fig. 3.
(a)
VPV
I PV
III. SYSTEM DESCRIPTION
Isc,n
Isc,2
Isc,1
(b)
A conventional two-stage energy conversion system is
connected between the PV array and the electrical power
system as shown in Fig. 4. A boost converter is used to
increase the PV voltage and it also plays a role of an
intermediate circuit for tracking the maximum power point.
The inverter circuit converts the direct current to the
alternating current which flows into the utility or local
loads. The connection to the utility grid is made by means
of a step-up  –Y transformer, and L-filters are included in
order to reduce the perturbation on the distribution system
from high-frequency switching harmonics generated by the
PWM control of the VSI.
The inverter controller has two main functions. One is to
synchronize the output current with the grid voltage, which
means the power factor is equal to unity. The other is that it
controls the DC-link voltage.
To achieve those two goals, the three-phase line current
is transformed from ABC reference frame to a dq rotating
reference frame. The q-axis current is used to control the
DC link voltage to the predetermined value. On the other
hand, the d-axis current is used to control the output power
factor to unity. As shown in Fig. 5. , two line currents ia
Fig. 2 Equivalent circuit of PV array.
Fig. 3 V-I and P-I characteristics at constant temperature.
where V and I represent the PV output voltage and
current, respectively; Rs and Rsh are the series and shunt
resistance of the cell (in Fig. 2); q is the electronic charge;
I sc is the light-generated current; Io is the reverse
Saturation current; K is the Boltzman constant, and Tk is
the temperature in K .
and ib are measured to be used in the inner-loop current
Equation (1) can be written in another form as [23]
I  I sc {1  K1[e K2V  1]} - (V  IRs )/Rsh
controller to control the MPPT and the power factor. Two
line voltages vab and vbc are also measured to calculate
m
Where the coefficient K1 ,
Vmpp is the
(2)
the angle of the rotating reference frame. The calculated
angle is then used to synchronize the output current with the
grid voltage. By adjusting the q-axis current to the optimum
operating value, the DC-link voltage would be controlled to
the reference value. The reference q-axis current is then
compared with the real q-axis current and the error value is
controlled using PI controller until the two values become
the same. In addition, the reference d-axis current is set to
zero for unity power factor control. In the same way, the
reference current is compared with the real d-axis current
and the error is controlled using another PI controller.
K 2 and m are defined as
K1  0.01175,
K 2  K 4 /(Voc )m ,
K4  ln(( K1  1) / K1 ) ,
K3  ln[( I sc (1  K1 )  I mpp ) / K1I sc ],
m  ln( K3 / K 4 ) / ln(Vmpp / Voc )
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Three-Phase
Voltage Source
Inverter
DC-DC Conveter
PV Array
Line
Filter
+
V
- dc
Cdc
Coupling
Transformer
ia
Lf
+
-
APRIL 2013
ea
i b eb
i c ec
Inverter
gating
Converter
gating
PWM
PWM
Vpv
System
Controller (1)
Vdc
ia, b
eab, bc
System
Controller (2)
IPV
Fig.4 Power circuit for PV system.
e
ebc
eab
VPV
IPV
Cdc
+
V
- dc
ia
MPPT
SVPWM
VPV
v qe*
Vdc
Voltage
controller
q-axis
current
controller
iqe
-
L L
iqe
ide
d-axis
current
controller
*
+ ide
+
-
e
v de*
+
i*qe
ib
abc/dq
va* vb* vc*
IPV
Vdc*
+
In this paper, P&O algorithm is used to extract the
maximum power point tracker. P&O involves a
perturbation in the q-axis component of the grid-side
current. From Fig. 6, it can be seen that incrementing
(decrementing) the PV current – or line q-axis currentincreases (decreases) the power when operating on the left
of the MPP and decreases (increases) the power when on
the right of the MPP. Therefore, if there is an increase in
power, the subsequent perturbation should be kept the same
to reach the MPP and if there is a decrease in power, the
perturbation should be reversed. The process is repeated
periodically until the MPP is reached. The system then
oscillates about the MPP. The oscillation can be minimized
by reducing the perturbation step size. However, a smaller
perturbation size slows down the MPPT. The flowchart of
P&O method is shown in Fig.7.
abc/dq
-
ide
Fig.5 Control circuit for the PV system.
Fig.6 Operation in photovoltaic V-I curve using P&O Method.
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APRIL 2013
IV. SIMULATION RESULTS
The full detailed model of the proposed three-phase gridconnected PV energy conversion system is implemented in
the PSIM software environment as depicted in Fig. 8. The
PV array consists of 60 strings of 50 modules (NS=50,
NP=200) of 50 Wp model with voltage and current at
maximum power 17.4[V] and 2.87[A]; respectively. The
PV array provides a peak installed power of 500KW at
irradiation level 1000 W/m2.
Fig. 9 shows that the PV current, voltage and power at
full load. Fig. 10 shows that the DC link voltage dynamics
at continuous variable irradiation level. Fig. 11 shows the
grid voltage and current instantaneous waveforms. From
this figure, it is clear that, the power factor operation is
unity.
Fig.7 Flow chart for P&O method.
Fig.8 Detailed model and control scheme of the grid-connected PV solar system in the PSIM environment.
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APRIL 2013
V. CONCLUSIONS
This paper is focused on the modeling and simulation of a
Grid Connected Large Photovoltaic System controlled by
the Voltage Oriented Control for both steady state and
transient analysis. It does not require an intermediate stage
of DC/DC control, as the optimum DC voltage is set by the
inverter itself by means of a Perturb & Observe MPPT
algorithm which is used for tracking the operating point for
achieving maximum power output from the PV array, with
change in irradiation level. The future work focuses on
developing anti-islanding algorithm for large-scale gridconnected PV systems.
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APRIL 2013
of electrical engineering, Faculty of Engineering, Assiut
university. His main field of interest are Advanced power
system analysis, dynamics, stability, and control, Smartgrid and micro-grid systems, Application of power
electronics- Application of intelligent control methods to
power systems, Renewable energy systems, especially wind
power& photovoltaic systems.
First A. Hammad abu-Zied was born
in Assiut, Egypt, in 1969. He received
the B.S. and M.S. degrees in Electrical
Engineering from Assiut University in
1993 and 1998 respectively. The Ph.D.
degree in Electrical engineering from
Assiut University, and Darmstadt
university, Germany,
in 2004
(Cotutelle).
Since 2004, he has been a staff
member at the department of electrical engineering,
Faculty of Engineering, Assiut university. His main fields
of interest are Electric motors drives, power electronics
applications, and renewable energy systems. He was
lecturer in Omar El-Moktar university in 2005 to 2010. He
is the author of one book, and more than 40 articles. Assiut
university, Electrical Engineering Department, faculty of
Engineering, Assiut, Egypt.
Second B. Ahmed G. Abo-Khalil was
born in Egypt, in 1970. He received the
B.S. and M.S. degrees in Electrical
engineering from the Assiut University,
in 1992 and 1997 respectively. the
Ph.D. degree in Electrical engineering
from Yeungnam University, South
Korea, in 2007.
Since 2007, he has
been a staff member at the department
22