International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
Modeling and Control of Grid Connected Photovoltaic SystemA Review
Zameer Ahmad1, S.N. Singh2
1
M.Tech Student, 2Senior Scientific Officer, Alternate Hydro Energy Centre,Indian Institute of Technology Roorkee
Roorkee, Uttrakhand (India) -247667
The power-electronic technology plays an important role
in distributed generation and in integration of renewable
energy sources into the electrical grid, and it is widely used
and rapidly expanding as these applications become more
integrated with the grid-based
Systems. During the last few years, power electronics
has undergone a fast evolution, which is mainly due to two
factors. The first one is the development of fast
semiconductor switches that are capable of switching
quickly and handling high powers. The second factor is the
introduction of real-time computer controllers that can
implement advanced and complex control algorithms [2].
Photovoltaic (PV) power supplied to the utility grid is
gaining more and more visibility, while the world‟s power
demand is increasing [3]. Not many PV systems have so far
been placed into the grid due to the relatively high cost,
compared with more traditional energy sources such as oil,
gas, coal, nuclear, hydro, and wind. Solid-state inverters
have been shown to be the enabling technology for putting
PV systems into the grid [4].
The photovoltaic (PV) field has given rise to a global
industry capable of producing many gigawatts (GW) of
additional installed capacity per year [5]. In 2010, the
photovoltaic industry production more than doubled and
reached a world-wide production volume of 23.5 GWp of
photovoltaic modules. Yearly growth rates over the last
decade were in average more than 40%, which makes
photovoltaic one of the fastest growing industries at
present. Business analysts predict that investments in PV
technology could double from € 35-40 billion in 2010 to
over € 70 billion in 2015, while prices for consumers are
continuously decreasing at the same time [6].
This review paper is organised as follows. In section II,
we described Evolution of grid-connected photovoltaic
system, in section III, we presented Modeling of
photovoltaic module with using of Matlab/simulink, in
section IV, discussed control techniques (power
conditioning system, MPPT) used in grid-connected
photovoltaic (PV) generation plants.
Abstract– This review-paper focuses on the latest
development of modelling and control of grid connected
photovoltaic energy conversion system. Modelling of
photovoltaic systems include modelling of SPV array, power
electronics
inverter/converter
based
on
MATLAB/SIMULINK. This present control algorithm of a
three-phase and single phase grid-connected photovoltaic
(PV) system including the PV array and the electronic power
conditioning (PCS) system, based on the MATLAB/Simulink
software. It also discussed advances in MPP tracking
technologies, the synchronization of the inverter and the
connection to the grid.
Keywords-- photovoltaic; converter/inverter, MPPT,
detailed full modelling; MATLAB/simulink; grid connected
PV system.
I. INTRODUCTION
The world constraint of fossil fuels reserves and the ever
rising environmental pollution have impelled strongly
during last decades the development of renewable energy
sources (RES). The need of having available sustainable
energy systems for replacing gradually conventional ones
demands the improvement of structures of energy supply
based mostly on clean and renewable resources. At present,
photovoltaic (PV) generation is assuming increased
importance as a RES application because of distinctive
advantages such as simplicity of allocation, high
dependability, absence of fuel cost, low maintenance and
lack of noise and wear due to the absence of moving parts.
Furthermore, the solar energy characterizes a clean,
pollution free and inexhaustible energy source. In addition
to these factors are the declining cost and prices of solar
modules, an increasing efficiency of solar cells,
manufacturing technology improvements and economies of
scale [1].
The increasing number of renewable energy sources and
distributed generators requires new strategies for the
operation and management of the electricity grid in order to
maintain or even to improve the power-supply reliability
and quality. In addition, liberalization of the grids leads to
new management structures, in which trading of energy and
power is becoming increasingly important.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
The continuously decreasing prices for the PV modules
lead to the increasing importance of cost reduction of the
specific PV converters, in section V, we have discussed
inverters circuit which are being used in grid-connected
photovoltaic systems and in section VI, we discussed
discussion and conclusion.
The present technology, which is a hot research topic in
Germany, is the „string-inverter‟ [17], [18]. String-inverters
use a single string of modules, to obtain a high input
voltage to the inverter. However, the high DC voltage
requires an examined electrician to perform the
interconnections between the modules and the inverter.
On the other hand, there are no losses generated by the
string diodes and an individual MPPT can be applied for
each string. Yet, the risk of a hot-spot inside the string still
remains.
The AC-Module, where the inverter is an integrated part
of the PV-module, is also an interesting solution. It
removes the losses due to mismatch between modules and
inverter, as well as it supports optimal adjustment between
the module and the inverter. Moreover, the hot-spot risk is
removed. All this together; a better efficiency may be
achieved. It also includes the possibility of an easy
enlarging of the system, due to the modular structure. The
opportunity to become a „plug and play‟ device, which can
be used by persons without any education in electrical
installations, is also an inherent feature [13].
II. EVOLUTION O F GRID-CONNECTED P HOTOVOLTAIC
SYSTEM
A. The Past: The past technology of grid connected
photovoltaic system was based on centralized inverters,
which was interfaced to a number of modules. The modules
were normally connected in both series, called a string, and
parallel in order to reach a high voltage and power level.
This results in some limitation; such as the necessity of
high voltage DC cables between the modules and the
inverter, power losses due to a centralized MPP Tracking
(MPPT), mismatch between the modules and at last the
string diodes. If one of the modules in a string becomes
shadowed, then it will operate as a load with lower power
generation as a consequence. On the other hand, if the
modules are connected in parallel, the shadowed module is
still generating power, but the input voltage to the inverter
is inevitable lower due to the parallel connection [13]. A
third scheme is given in [7] – [11], where each module is
interfaced by a Generation Control Circuit (GCC). Hence,
an individual MPPT is assured for every single module,
which also lower the possibilities of hot spots.
According to [12], full shadowing of one PV-cell (in a
string of 160 cells) causes a temperature raise, inside the
cell, of more than 70 ºC above the ambient temperature,
whereas the non-shadowed cells only reach 22 ºC above the
ambient temperature (for an ambient temperature equal to
12 ºC). This is of great importance, because an overheated
cell rapidly decreases the modules lifetime.
B. The Present: The grid integration of RES applications
based on photovoltaic systems is becoming today the most
important application of PV systems, gaining interest over
traditional stand-alone systems. This trend is being
increased because of the many benefits of using RES in
distributed (aka dispersed, embedded or decentralized)
generation (DG) power systems. These advantages include
the favorable incentives in many countries that impact
straightforwardly on the commercial acceptance of gridconnected PV systems [14], [15]. This condition imposes
the necessity of having good quality designing tools and a
way to accurately predict the dynamic performance of
three-phase grid-connected PV systems under different
operating conditions in order to make a sound decision on
whether or not to incorporate this technology into the
electric utility grid [16].
III. MODELLINGOF P HOTOVOLTAIC SYSTEM
A. Modelling Of Photovoltaic Module/Array
The photovoltaic module is the result of associating a
group of photovoltaic cells in series and parallel, with their
protection devices, and it represents the conversion unit in
this generation system. The manufacturer supply PV cells
in modules, consisting of NPM parallel branches, each with
NSM solar cells in series shown in Fig. 1.
Fig. 1. Equivalent circuit of a PV array.
Although the mathematical and simulation photovoltaic
modules development began time ago, improvements of
these models are analyzed and presented continually. One
of the objectives of this study is a review of those existing
methods and models.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
M. G. Molina and P. E. Mercado presented the detailed
mathematical model that predicted the power production of
the PV generator becomes an algebraically simply model,
being the current–voltage relationship as given in equation
(1).
Fig.2. Model of a photovoltaic cell with two diodes
The current generated by the module is given by
equations (3)
(1)
Where: IA: PV array output current
VA: PV array output voltage
IPh: Solar cell photocurrent
IRS: Solar cell reverse saturation current (aka dark current)
q: Electron charge, 1.60217733e–19 Cb
A: P–N junction ideality factor, between 1 and 5
k: Boltzmann's constant, 1.380658e–23 J/K
TC: Solar cell absolute operating temperature, K
RS: Cell intrinsic series resistance
RP: Cell intrinsic shunt or parallel resistance
(3)
Where V and I represent the output voltage and current
of the PV; q is the electronic charge; Iph corresponds to the
light-generated current of the solar array. Is1, 2 represent
the current saturation of the two diodes; A1, 2 is ideality
factor of the junction of D1 and D2, K the Boltzmann‟s
constant, T the cell temperature.
Altas, and Sharaf, [21] presented a model as shown in
Fig. 3. The solar cell is modelled as a current source. Iph,
the photovoltaic current is proportional to the ambient
irradiance level and to the temperature of the panel. To
allow for losses, a series (Rs) and parallel resistance (Rp)
are commonly included in the circuit. In this model, the
parallel resistance was neglected in order to simplify the
model.
The photocurrent IPh for any operating conditions of the
PV array is assumed to be related to the photocurrent at
standard test conditions (STC) as given in equation (2).
(2)
The proposed model used theoretical and empirical
equations together with data provided by the manufacturer,
and meteorological data (solar radiation and cell
temperature among others) in order to accurately predict
the I–V curve. The three-phase grid-connected PV system
was simulated, under changing solar radiation conditions
while maintaining the cell temperature constant at 25 ºC in
MATLAB/simulink and validated the obtained result
experimentally [16].
S. Rustemli, F. Dincer, [19] Presented an accurate
photovoltaic module electrical model and demonstrated in
Matlab/Simulink for a typical Lorentz LA30-12S
photovoltaic panel. Such a generalized PV model was easy
to be used for the implementation on Matlab/Simulink
modeling and simulation platform. Especially, in the
context of the Sim Power System tool.
Simulation results showed that a photovoltaic panel
output power reduces as module temperature increases.
This situation was shown with Matlab/Simulink graphics.
There are lots of variety cooling systems for photovoltaic
panels. These systems may increase efficiency of panel
depend on the weather conditions.
F.Bouchafaa et al. [20] presented their work in the
model with two diodes as shown in figure 2.
Fig. 3. Model without parallel resistance
Output voltage is given by equation (4)
(4)
The curve fitting factor, A, was adjusted so that at rated
input values of temperature and irradiance the datasheet
values of output were obtained as model outputs. The same
approach was taken to obtain the temperature and
irradiance correction coefficients. Model had a correct
varying input temperature response, but irradiation
dependences could not be correctly set.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
Huan-Liang Tsai et al. [22] presented a generalized PV
model which was representative of the all PV cell, module,
and array had been developed with Matlab/Simulink and
been verified with a PV cell and a Commercial module
shown in Fig.4.
The intermediate dc-dc converter is built with an
Insulated Gate Bipolar Transistor (IGBT) as main power
switch Tb in a standard unidirectional boost topology that
employs an energy-storage reactor Lb, a rectifier diode Db
and a voltage smoothing capacitor C. The converter is
linked to the PV system with a filter capacitor CA for
reducing the high frequency ripple generated by the
transistor switching. The dc-dc converter output is
connected to the dc bus of the VSI.For modelling of
voltage source inverter they used IGBT. The VSI structure
is designed to make use of a three-level pole structure, also
called neutral point clamped (NPC), instead of a standard
two-level six-pulse inverter structure This three-level
inverter topology generates a more sinusoidal output
voltage waveform than conventional structures without increasing the switching frequency. The additional flexibility
of a level in the output voltage is used to assist in the output
waveform construction.
F.Bouchafaa et al. [20] the three-level NPC VSI,
presented, was one of the most commonly applied
multilevel topologies.
This type of VSI has several advantages over the
standard two-level VSI, such as a greater number of levels
in the output voltage waveforms, lower dV/dt, less
harmonic distortion and lower switching frequencies. The
main draw- back of this type of converter is the voltage
imbalance produced in the capacitors of the DC-link when
one of the phases is connected to the middle point or
Neutral Point (NP).
R. Mechouma et al. [23] focused on different
technologies for connecting photovoltaic (PV) modules to a
three-phase- grid. Some of three-phase topologies are
presented, compared according to the type of control (i.e.
the PWM method; the bang-bang method or the fuzzy logic
method or numerical control); and a comparison with
single-phase inverters was given.
Fig. 4. Equivalent circuit models of generalized PV.
The proposed model takes sunlight irradiance and cell
temperature as input parameters and outputs the I-V and PV characteristics under various conditions. This model had
also been designed in the form of Simulink block libraries.
The masked icon makes the block model more userfriendly and a dialog box lets the users easily configure the
PV model. Such a generalized PV model is easy to be used
for the implementation on Matlab/Simulink modeling and
simulation platform. Especially, in the context of the
SimPowerSystem tool, there is now a generalized PV
model which can be used for the model and analysis in the
field of solar PV power conversion system. A model of PV
module with moderate complexity which includes the
temperature independence of the photocurrent source, the
saturation current of the diode, and a series resistance was
considered based on the Shockley diode equation.
IV. CONTROL O F GRID -C ONNECTED PV SYSTEM
The control structure of the grid-connected PV system is
composed of two structures Control:
1. The MPPT Control, which the main property is to extract
the maximum power from the PV generator.
2. The inverter control, which have the main goal:
- Control the active and regulate the reactive power injected
into the grid;
- Control the DC bus voltage;
- Ensure high quality of the injected power.
B. Modeling Of Converter/Inverter
For grid-connected PV applications, two hardware
topologies for MPPT have been mostly studied worldwide,
known as one-stage and two-stage PV systems.
M. G. Molina, and P. E. Mercado, [16] selected to model
the two-stage PV energy conversion system. They
including a dc-dc converter (or chopper) between the PV
array and the inverter connected to the electric grid various
control objectives are possible to pursue concurrently with
the PV system operation at the cost of slightly decreasing
the global efficiency of the combined system because of the
connection of two cascade stages.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
A. MPPT Control: The maximum power that can be
delivered by a PV panel depends greatly on the insulation
level and the operating temperature. Therefore, it is
necessary to track the maximum power point all the time.
Many researchers have been focused on various MPP
control algorithm to lead the operating point of the PV
panel to optimum point [24].
F.Bouchafaa et al. [20] propose an intelligent control
method for the maximum power point tracking (MPPT) of
a photovoltaic system under variable temperature and
insulation conditions. This method uses a fuzzy logic
controller. It can be deduced that the fuzzy controller is fast
controller in the transitional state and presents also a much
smoother signal with less fluctuations in steady state. It was
able to find the point of maximum power in a shorter time
runs.
[16] proposed a The proposed multi-level control
scheme for the three phase grid-connected photovoltaic
system consisting of external, middle and internal level, is
based on concepts of instantaneous power on the
synchronous-rotating dq reference frame. They used
“perturb and observe”, for MPP
F. Huang et al. [25] a microcontroller based automatic
sun Tracker was designed and implemented. The automatic
sun tracker is implemented with a dc motor and a dc motor
controller.
The novelty of this unit is that the switching device of
the chopper is not only used for power conversion but also
for Maximum Power Point (MPP) detection. MPP is
determined by simple embedded software with a current
sweep approach.
Amrouche, et al. [26] proposed artificial neural network,
(ANN) based modified P&O method to predict the power
value during the next perturbation cycle so that the value of
perturbation step can be adjusted for next perturbation
cycle.
Zhang. L, et al. [27] built a Genetic Algorithm trained
Radial Basis Function Neural Network (GA-RBFNN)
model to predict the reference DC bus voltage of the
control system to maximize the output power.
Veerachary. M, et al. [28] implemented a feed-forward
MPPT scheme for coupled inductor interleaved boost
converter fed PV system by using fuzzy logic controller,
while ANN is trained offline to estimate the voltage
reference.
Joe-Air Jiang, et al. [29] designed a three-point weight
comparison method to avoid rapidly moving of the
operating points of PV when it is under varying atmosphere
conditions which could overcome the drawback of P&O
method.
References [30, 31] proposed neural fuzzy network for
MPPT control scheme. The neural network used to train
sets of data off-line for inputs of fuzzy logic controller,
while the fuzzy logic controller used to control the duty
cycle effectively and hence the MPP can be tracked
effectively.
B. The Inverter Control: Inverter interfacing PV
module(s) with the grid involves two major tasks. One is to
ensure that the PV module(s) is operated at the maximum
power point (MPP). The other is to inject sinusoidal current
into the grid. In grid-connected PV system, different
inverter topologies and controllers are usually used for
interfacing the PVG and the utility grid [32]-[33]. Two
inverter configurations and three inverter topologies can be
identified in such applications, namely: central inverter,
string inverter and integrated inverter for configuration; and
topologies with or without transformer [34]. These
interfaces use various PWM single-phase inverters
configurations and topologies, governed with different
suitable control strategies to transfer powers and to shape
the utility line current, making it follow a reference
sinusoidal waveform.
M. G. Molina et al. [16] proposed multi-level control
scheme for the three phase grid-connected photovoltaic
system consisting of external, middle and internal level, is
based on concepts of instantaneous power on the
synchronous-rotating dq reference frame as given in Fig. 5.
Fig. 5. Multi-level control scheme for the three-phase grid-connected
PV system.
Ismail et al. [35] presented the development of single
phase sinusoidal pulse width modulation (SPWM)
microcontroller-based inverter. The attractiveness of this
configuration is the use of a microcontroller to generate
sinusoidal pulse width modulation (SPWM) pulses. The
power circuit topology chosen is full bridge inverter. Fig. 6.
Shows the full bridge inverter topology. It consists of DC
voltage source or photovoltaic module, four switching
elements (MOSFETs), LC filter, transformer and load.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
The full bridge topology is chosen with considerations
that it must be capable of delivering high current at low
voltage. This property is important if the inverter is
designed for photovoltaic applications.
In terms of topologies, a large interest has been shown
by the scientific community in multilevel inverters [43],
[44], [45], [46], [47], and [48]. At the megawatt range, the
two-level voltage source converter, if compared with
multilevel converters, is not be able to ensure the power
quality required by the standards, the maximum allowed
switching frequency, the higher voltage operation and the
reduction in filter size. The multilevel converters are useful
in achieving an interconnection of the photovoltaic strings
in a better way to reach higher voltages that are closer to
that one at the point of common coupling. The cascaded
half bridge solution is based on the series connection of the
H-bridge, so that a natural voltage increase is obtained and
the adoption of a boost stage or step-up transformer is not
needed.
Resonant harmonic compensators [49] allow performing
selective harmonic compensation in grid-connected
photovoltaic inverters in order to guarantee high
performances in terms of frequency content of the current
injected into the grid. Such a technique is based on the use
of a bank of generalized integrators, namely second-order
band pass filters tuned to resonate at a predefined
frequency.
The control of reactive power has been proposed in
Cagnano et al. [50]: in this paper a decentralized controller
that is able to minimize grid losses by actively managing
the reactive power supplied by PV inverters is proposed. It
allows reducing the burden due to the centralization of the
controller.
In J. Vasquez et al. [51] a converter, controlled by means
of the droop control technique, which is able to provide
active power to local loads and to inject reactive power into
the grid providing voltage support at fundamental
frequency.
Soeren Baekhoej et al. [4] presented a grid-connected
photovoltaic (PV) system with direct coupled power quality
controller (PQC), which uses an inner current control loop
(polarized ramp time (PRT)) and outer feedback control
loops to improve grid power quality and maximum power
point tracking (MPPT) of PV arrays.
To reduce the complexity, cost and number of power
conversions, which results in higher efficiency, a single
stage CCVSI is used. The system operation has been
divided into two modes (sunny and night). In night mode,
the current controlled inverter (CCVSI) operates to
compensate the reactive power demanded by nonlinear or
variation in loads. In sunny mode, the proposed system
performs PQC to reduce harmonic current and improve
power factor as well as MPPT to supply active power from
the PV arrays simultaneously.
Fig. 6. Full bridge inverter topology.
All commercial inverters in this review (Soladin120
[36], OK4 [37], OK5 [37] and Sunmaster 130 [36] (a three
stage inverter)) are based on the resonant principle. In the
case of the OK4 inverter the DC-DC converter are used to
amplify the voltage but also to modulate the rectified
sinusoidal current, which is „unfolded‟ in the secondary
stage.
The next inverter [38] is based on the series resonant
DCDC converter and a modified full bridge grid-connected
inverter, cf. Fig. 7. The inverter is modified in such a way
that it cannot operate as a rectifier; hence problems with
standby losses are solved. Two additional diodes do this.
The DC-DC converter is, as stated before, based on the
series resonant converter, where the leakage inductance in
the transformer together with the capacitor inserted in the
main path forms a resonant-tank. The resonant tank
together with the output capacitances of the switches makes
the inverter zero-voltage switching.
The DC-DC converter is operated at 100 kHz with a
duty-cycle slightly smaller than 50% in order to avoid
shoot-through.
Fig. 7. The inverter proposed in [36, 42]. The series resonant DC-DC
converter amplifies the voltage from the PV-Module and the grid
connected inverter generates the sinusoidal grid-current.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
V. T HREE-P HASE GRID CONNECTED INVERTER FOR
P HOTOVOLTAIC S YSTEMS
The inverters are categorized into some classifications:
the number of power processing stages; the use of
decoupling capacitors and their locations; the use or no of
the transformers; the type of three phase inverter; whether
they are preceded by a DC/DC converter or not .Some of
three-phase topologies are presented. Fig.8. shows typical
circuit diagram of a MOS-equipped VSI with BoostConverter for a PV module generation system. This
inverter is a viable alternative to a VSI+BC due to its
voltage step-up characteristic [2], [3]. The CSI directly
connected to the PV module features a single stage power
conversion system for feed-in and MPPT
Fig.10. Circuit diagram of a 3-phase BJT equipped inverter topology
[53]
The topology shown in fig.11. is a three phase four-wire
BJT-equipped inverter with split DC-link. It was a simple
topology and the advantage is that a three-phase split-link
inverter essentially becomes three single-phase half-bridge
inverters and permits each of the three legs to be controlled
independently, making its current tracking control simpler
than the four-leg inverter.
Fig.8. Typical circuit diagram of a MOS-equipped With BoostConverter for a PV module generation System
The inverter shown in Fig. 9. is a viable alternative to a
VSI+BC due to its voltage step-up characteristic [2], [3].
The CSI directly connected to the PV module features a
single stage power conversion system for feed-in and
MPPT.
Fig.11 Topology of the three phase four-wire inverter with split dclink [52]
In order to maximize the success of the PV systems a
high reliability, a reasonable cost, and a user-friendly
design must be achieved in the inverter topology. Fig.12
depicts the multi-string topology is commonly used in PV
applications. It permits the integration of PV strings of
different technologies and orientations (north, south, east
and west) [54].
Fig.9. Circuit diagram of a MOS-equipped CSI for a PV module
generation system.
The topology (depicted in fig.10) is less interesting for a
low-voltage distribution network which is typically a fourwire system.
Using the three phase three-wire topology, only two
parameters can be controlled, which is disadvantageous in
case active power filtering functions are desired.
Fig.12 Topology of the three phase four-wire Multi-string inverter
[54]
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International Journal of Emerging Technology and Advanced Engineering
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Fig.13 depicts a three-phase inverter with stabilizer and
transformer [55]. This topology is very influenced by
variations in the load. When the Load is getting bigger the
variation follows up and the sinusoidal characteristics are
becoming worse off.
Thus, simplified models are suitable for system studies
that try to identify the impacts of PV systems on the
electric network.
In the past few years, developing new topologies for
power conditioning units and applying new control
techniques were the focus of many studies, almost
saturating this field of research. Also, the application of
new maximum power point tracking algorithms received a
lot of attention. However, most of these algorithms fail to
operate properly in the case of partial shadings, which is
the case where parts of the PV array are shaded by clouds
or nearby buildings. The conventional MPPT algorithms
are not capable of solving the problems of multiple peaks
that established in the P-V characteristic curves of the PV
systems due to partial shaded conditions. Therefore, further
research should be done to extract maximum power
effectively from the PV systems under non-uniform
insolation.
The use of storage devices with PV systems is currently
receiving a lot of attention. These devices can be used to
bridge fluctuations in the output power of PV systems, shift
the peak generation of the system to match the load peaks,
and provide reactive power support. One of the main
challenges that still face the use of storage devices is the
high cost associated with their installation. Thus, studying
the economical aspect of installing these devices is of great
importance.
Grid-connected PV systems can provide a number of
benefits to electric utilities, such as power loss reduction,
improvement in the voltage profile, and reduction in the
maintenance and operational costs of the electric network.
However, improper choice of the location and size of the
PV systems and unsuitability of the output power profile of
the PV system to the profile of the electric network can
impose operational problems on the network. Moreover,
the fluctuations in the output power of these systems add to
the complexity of the problem. Large, centralized PV
systems, installed in distribution networks, require more
attention at the time being.
Partial shading of PV arrays is considered one of the
main challenges that face MPPT techniques. In this case,
there might exist multiple local maxima, but only one
global maximum power point. The task of the PCU is to
identify and operate at the global MPP. The research in this
field is active.
Multilevel inverters posses the advantage of generating
output voltages with extremely low distortion, generating
smaller common-mode voltage and drawing current with
very low distortion but a complex topology, a large number
of components and a complicated control strategy have to
be overcome.
Fig.13 Topology of three-phase inverter with stabilizer and
Transformer [55]
VI. D ISCUSSION AND C ONCLUSION
The accuracy of any of these models is usually
dependant on the location where the PV system is being
installed, thus, it is important to choose a suitable model for
the case under consideration. One of the main activities in
this area is the development of irradiance models suitable
for specific locations. The fluctuations in irradiance due to
passage of clouds also received a lot of attention from
researchers, where most of the work done in this field
relied on the frequency domain analysis. One field that still
requires more attention is the prediction of irradiance,
which is a complicated task as compared to the prediction
of wind speed. This is mainly because of the variety of
factors that affect the accuracy of prediction including the
wind speed and direction and type, height and thickness of
clouds.
Modeling of the PV cells is one of the mature areas in
the field. There are a variety of models available in the
literature and can be divided into two main categories;
detailed and simplified models. Detailed models attempt to
represent the physics of the PV cell and are usually suitable
for studies that require the detailed cell information such as
implementation of maximum power techniques and
analysis of the effect of change in irradiance and
temperature on the performance of the PV cell. On the
other hand, simplified models usually provide a direct
estimate of the maximum power generated from the PV cell
at certain operating conditions.
47
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
[12 ] P. Rooij, M. Real, U. Moschella, T. Sample, M. Kardolus,
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Nevertheless, the multilevel inverter will play an
important role in the future. Advanced inverter, controller,
and interconnection technology development must produce
hardware that allows PV to operate safely with the utility
and act as a grid resource that provides benefits to both the
grid and the owner.
Advanced PV system technologies include inverters,
controllers, related balance-of-system, and energy
management hardware that are necessary to ensure safe and
optimized
integrations,
beginning
with
today‟s
unidirectional grid and progressing to the smart grid of the
future.
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