IRJET - Study of Technical Parameters in Grid-Connected PV System

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 07 Issue: 02 | Feb 2020 www.irjet.net p-ISSN: 2395-0072
© 2020, IRJET | Impact Factor value: 7.34 | ISO 9001:2008 Certified Journal | Page 2960
Study of Technical Parameters in Grid-Connected PV System
Mahesh Chapai1, Braj Kishor Shah2, Ishwor Kafle3, Ujjwal Adhikari4,
Dr. Basanta Kumar Gautam5
1Student, Dept. of Electrical Engineering, Western Region Campus, Gandaki, Nepal
4Associate Professor, Dept. of Electrical Engineering, Western Region Campus, Gandaki, Nepal
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Abstract - With the rise of the power electronics technology
and the decline in the price of photovoltaic(PV)panels, theuse
of the PV system as distributed generation has been
increasing. During the grid connection of the PV system,
parameters like voltage magnitude and frequency should be
maintained within tolerable limitssothatthepowerreliability
and quality are assured. These parameters obtained from the
PV source should be matched with grid parameters before
synchronism and shouldn’t be disturbed during the operation
as well. However, there are several conditionsthataffectthese
parameters, one being an islanding condition. During this
situation, the voltage and frequency show unhealthy nature
and hence the operation of the PV system in the islanded
condition under this scenario is forbidden. In this paper, the
overall modelling of the grid-connected PV system isdonefirst
and the output nature of voltage and frequency waveforms
from the inverter are studied. Later, the islanding scenario of
the overall system is modelled and hence the nature of voltage
and frequency waveforms are simulated and studied at
different load conditions. All the modelling and simulations
are done in MATLAB/Simulink environment.
Key Words: Mathematical modelling, Inverter controller,
Grid-connected PV system, Maximum Power Point Tracking
(MPPT), Islanding, MATLAB/Simulink.
1. INTRODUCTION
Due to the advantages of environment friendly energy
source with free availability, cheap operation and
maintenance cost, PV energy has become the promising
source of distributed generation [1]. To sum up the
simulation of overall grid connected three phase PV system,
several modelling stages have to be preceded, which are
modelling of PV array, maximum power point tracking
(MPPT), DC-DC converter, DC-AC converter and inverter
controller. Several works regarding the modelling of such
various stages have been presented in numerous research
papers. [2]-[4] are some collection of papers which
presented the modelling of PV modules with the use of
MATLAB/Simulink. The literature [5]-[7] reports several
techniques for implementing MPPT. Similarly,themodelling
of DC-DC converter which performs regulation of the DC
voltage as required is referred from [8],[9]. For grid
integration, DC should thus be converted to three phase AC
which is equivalent to grid alternating parameters. So, the
proper design of DC-AC inverter and its control unit is thus
required. The collection of literature [10]-[12] presents the
design of inverter along with the inverter controller which
ensures the parallel operation of the PV system and power
grid. Based on the study of this referred literature, the
former section of this research paper performs the
modelling of the aforementioned stages in
MATLAB/Simulink.
During the grid connection, it is required to ensure that the
parameters of the PV system should match the grid
parameters. These parameters include threephasevoltages,
its phase sequence and the frequency. Before the grid
connection, these parameters are studied in this paper so
that the reliability and quality of the power are ensured.
Also, an algorithm is modelled in this paper where the
inverter output parameters are compared with that of grid
parameters which decides the time of PV system and power
grid synchronization. There are several conditions that
might affect the nature of the aforementioned parameter.
One of those conditions is known as islanding. Islanding is
the state where the distribution generation (DG) like PV
energy source carries on with supplying local loads despite
the disconnection of grid [13]. The frequency and voltage
waveforms at point of common coupling (PCC) during the
islanding conditions show different nature according to the
availability of the local load capacity [14]. This research has
restricted its scope to studying andanalysingthevoltage and
frequency waveforms under several local loads condition.
Moreover, several research papers present islanding
detection and its mitigation techniques, some of them are
[15]-[17].
2. MODELLING
2.1 PV Array with Maximum Power Point Tracking
(MPPT) technique
The model representing the practical single diode PV
module is presented in figure 1. For many applications of
power system planning, sucha modelispreferredsinceithas
a smaller number of computation and yields less
computational error in comparison with doublediodemodel
[18]. Based on the following equations [19], the PV module’s
mathematical modelling is illustrated in the

MATLAB/Simulink environment, which is shown in figure 2.
The desired power output can be achieved with the
introduction of the number of PV modules in series and
parallel configuration.
Iph = [Isc + Ki*(Tc - Tref)] * (G/Gref) (1)
Io = (2)
I=Iph-Io - (3)
Iph : Photocurrent
Isc : Current during short-circuit
Ki : Coefficient of the temperature of current during
short-circuit condition
Tc : Temperature of the module (K)
Tref : Module temperature at STC
G : Irradiance (W/m2)
Gref : Irradiance at STC (W/m2)
VOC : Open circuit voltage
Kv : Coefficient of the temperature of voltageduringthe
open-circuit condition
k : Boltzmann’s constant
Ns : No. of cells
Q : Charge of electron
Io : Saturation photocurrent
A : Ideality factor
Rs : Series Resistance
Rp : Parallel resistance
I : PV current
V : PV voltage
MPPT techniqueplaysasignificantpartinmaximizingthe
efficiency of the PV model. Several techniques of MPPT like
perturb, and observation (P&O), fractional open-circuit
voltage, fuzzy logic control, incremental conductance, and so
on have been researched [20]. Among which, the P&O
technique is widely used because of its effortless
implementation. In thisresearch,weuseperturbandobserve
method with a variable current parameter which is referred
to from [21]. Figure 3 represents the flowchart of the
implemented P&O algorithm.
Figure 2: PV module mathematical model
Figure 3: P&O algorithm
2.2 DC-DC converter
The DC-DC converter is such designed that the required
utilization voltage output can be achieved either through
increasing or decreasing the DC voltage obtained from the
PV array system. Several converters like a buck, boost or
buck-boost converterare employedtoachievetheregulation
of voltage through the modes of switching [22]. In this
research, the modelling of boost converter based on the
circuit diagram is shown in figure 4 and the related
equations [23]. Based on the following equations, the
mathematical model of the system is representedinfigure5.
Figure 1: Single diode PV module

Figure 4: Boost converter
When the switch is in ‘ON’ state,
(4)
(5)
When the switch is in ‘OFF’ state,
(6)
(7)
And, D=1- (8)
Figure 5: Mathematical model of the boost converter
2.3 Three Phase Grid Tied Inverter
A three-phase distortion-free, alternatingcurrent(AC)is
required during the grid connection of the PV system. Fig 6
shows a circuit diagram of the two-level inverter with a
three-phase voltage source(VSI)and withsixswitches,anda
dc source ‘V’. The mathematical expression from the above
circuit is referred from [24] and is given below. The
mathematical model based on theseexpressionsisfiguredin
figure 7.
Vrn = Vro- Vn (9)
Vbn = Vbo-Vn (10)
Figure 6: Circuit diagram of inverter
Vyn =Vyo-Vn (11)
So, Vrn + Vbn + Vyn = Vro +Vbo + Vyo -3Vn (12)
And, at balanced condition, Vrn + Vbn + Vyn = 0
Vn= (Vro +Vbo + Vyo)/3 (13)
Vrn = (2Vro-Vbo-Vyo)/3 (14)
Vbn=(2Vbo-Vro-Vyo)/3 (15)
Vyn=(2Vyo-Vro-Vbo)/3 (16)
Figure 7: Mathematical model of inverter
The inverter must beabletosynchronizeparameterslike
voltage, phase and frequency between the PV system and
grid. Moreover, a unity power factor must also be
maintained on the grid side[23].Tomaintainsuchcondition,
the current controller is modelled in this research which is
shown in figure 9. In the model, the current obtained from
the MPPT technique is compared to the dq componentof the
grid current. Discrete PID controller is used to obtaining the
reference dq voltage which is fed to a pulse generator to
obtain the necessary switching pulses to the inverter.
Further, the LCL filter is designed to obtain the ripple-free
three phase waveform. In order to damp the switching
harmonics, small rated inductors can be employed for LCL
filter which is advantageous in term of cost and dynamic
operation [25].

Figure 8: Inverter Controller model
2.4 Islanding Scenario
Figure 9 shows the layout of the PV system integrated
with the grid according to the model that is done in
MATLAB/Simulink. The arrangement of two breakers is
made where breaker 1 is supposed to imitate the condition
of islanding and the breaker 2 is supposed to isolate the PV
system with the grid which can be termed as islanding
scenario. According to the scope of the work, thestudyofthe
parameters like voltage, and frequency is done in this
research during the islandingcondition.Duringtheislanding
situation, the nature of the mentioned parametersaccording
to the power demand of the local load [26]. So, the
waveforms at the following different situations are studied
in this paper.
• When PV generation is less than load power
• When PV generation is greater than load power
• When PV generation is equal to load power.
Figure 9: PV grid integration layout
3. SIMULATION RESULTS
After the completion of overall modelling, the model is
simulated and analyzed in several situations. Before
synchronizing the PV source with the grid, it is necessary to
observe the voltage magnitude, its phase sequence and the
frequency. So, the waveform of the inverter’s output is
simulated first. During the analysis, a scenario is created
where the reference parameters for the inverter controlleris
provided through the grid. The grid provides reference
parameters of line voltage and line current. According to this
maintained scenario, the voltage and frequency waveforms
are obtained which are shown in figure 10 and figure 11
respectively. In these figures, it is observed that the
magnitude and phase sequence of voltage and the frequency
from the inverter becomes equivalent to that ofthereference
grid parameters after a certain period of time. So, the
synchronization breakers are programmed to be operated
after the achievement of tolerable limits of the voltage and
frequency. The flowchart representing such algorithm is
shown in figure 12 below.
Figure 10: Voltage waveform from an inverter
Figure 11: Frequency waveform of the inverter
Figure 12: Flowchart to program synchronizing breaker
The modelling of 100 kW of the PV system and the power
grid of 1 MVA of power with 5 km of distribution feeder line
is done in our research. After the successful synchronization
of the PV system with a power grid, the scenario of islanding
is imitated according to the layout presented in figure 9.
During the islanding scenario, the voltage and frequency

show distinct characteristics according to the connection of
load power. In the model, a disconnecting breaker is used in
the grid side which is programmed to disconnect grid from
PV system at t = 1 sec. A single-phase voltage and the
frequency are simulated and analyzed in several scenarios
which are described below.
• When PV generation is less than load power
Figures 13 and 14 show the nature of voltage and
frequency waveform under the condition of higher PV
generation. In the figure, we can observe that the phase
voltage is sagging along with the increasing frequency after
the encounter of islanding condition. The frequency after
islanding is found to increase in linear nature.
Figure 13: Phase voltage waveform when Ppv > Pload
Figure 14: Frequency waveform when Ppv > Pload
• When PV generation is greater than load power
frequency waveform under the condition of higher load
power. In the figure, we can observe thatthephasevoltage is
swelling along with the increasing frequency after the
encounter of islanding condition. Similar to the above-
mentioned condition, the rate of increase in frequency after
islanding is linear in nature.
Figure 15: Phase voltage waveform when Ppv < Pload
Figure 16: Frequency waveform when Ppv < Pload
• When PV generation is equal to load power
frequency waveform under the condition of equivalent PV
power and load power. In the figure, we can observethatthe
phase voltage and frequency are almost equivalent to the
original values after the encounter of islanding condition.
Figure 17: Phase voltage waveform when Ppv = Pload
Figure 18: Frequency waveform when Ppv = Pload
4. CONCLUSIONS AND RECOMMENDATION
During the study, it is found that the characteristics of
several technical parameters at PCC change with the change
in characteristics of the grid. The paper presents the
modelling of the overall PV energy system with grid
connection along with the study of the contributing
parameters. According to the studyoftheoutputparameters
from an inverter, an algorithm is also included which
ensures proper synchronizationofPVenergysystemandthe
power grid. Moreover, the research can be furtherexpanded
to study the stability behaviour of the system with the PV
energy sources included in the conventional grid. Similarly,
the behavioural study of these technical parameters due to
several faults in the grid is also recommended.

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