A single stage photo voltaic grid-connected inverter using spwm

GJTE-Vol(1)-Issue(3) Oct 2014 ISSN: 2393-9923
Global Journal of Trends in Engineering
www.gjte.in 22
A Single-stage Photovoltaic Grid-Connected Inverter using SPWM
Shaik.Amanulla
P.G.Student, Dept. of EEE, G. Pulla Reddy Engineering College, Kurnool, Andhra Pradesh, India
ABSTRACT: Now a day’s solar energy gain its importance due to its vast availability in environment and Eco-friendly.
Solar power is gaining its trend for the use of solar cells in industry and domestic applications, because solar energy is
expected to play important role in future smart grids as a distributed power generation. Grid connected PV Inverters are
basically implemented with two-stage and single-stage conversion. Among this Single-stage conversion has more
advantageous than two-stage with improved efficiency, less weight and reduced losses. In this paper, a generalized solar
photo Voltaic (SPV) system for Matlab/Simulink model with constant and variable irradiation has been developed. Solar
PV cell is modelled using Matlab/Simulink. Solar PV cell behaviour under environmental changes is also considered.
Single-stage Grid integrated Solar PV system with VSI has been explained with Matlab/Simulink model. Sinusoidal
Pulse Width Modulation (SPWM) is used to generate the pulses for Voltage Source Inverter (VSI).
Keywords: Solar Photo Voltaic (SPV), Constant Irradiation, Variable Irradiation Voltage Source Inverter (VSI), SPWM,
Grid-Tie Inverter.
I. INTRODUCTION
Energy harvesting from Solar Photo Voltaic (SPV) system is the most essential and sustainable way because of its vast
availability and eco-friendly. The fundamental power generation units of solar power generation are PV modules. The P-V
and I-V characteristics of SPV cells are depend on the solar irradiation and cell temperature. The Matlab/Simulink provides a
user-friendly environment for the analysis of Power Electronic converters with PV modules. Generally, Grid connected PV
inverters are modeled with two-stage and single-stage conversion. In two-stage conversion, the first Boost converter is used to
boost up the PV output voltage; second allows the conversion of this power into high-quality ac voltage. The presence of
several power stages undermines the overall efficiency, reliability and increased cost. The single-stage has numerous
advantages, such as simple topology, high efficiency, etc. The single-stage conversion of Grid connected PV inverter is
shown in Fig. 1.Typically, simple inductor L is used to get the reduced ripples in the Inverter output.
Fig. 1 Typical configuration of a single-stage grid-connected PV system.
In Fig.1, It has PV panel, capacitor, VSI, inductor and finally Grid. Here Capacitor(C) is used to maintain the constant
voltage which is coming from PV panel. Generally the output of VSI will be stepped waveform with the presence of ripples.
To get the reduced ripples at the Inverter output side L is used as shown in Fig.1. Sinusoidal Pulse Width Modulation
(SPWM) is used to give the pulses to VSI. This study aims to develop a general purpose Simulink SPV module with Grid-Tie
Inverter system. This module can be easily reconfigured for the electrical response of PV panels in a wide range. In this the
behavior of SPV module with constant and variable irradiation are discussed. In solar based Grid-Tie Inverter system, PV
module itself is modeled to give desired open circuit voltage (Voc). Solar based Grid-Tie Inverters are useful for large-scale
solar power generation. Solar array modeling is first explained with mathematical equations and then, simulation results are
provided for constant and variable irradiation. VSI with SPWM is also explained. Grid-Tie Inverters are explained with some
basic theory. Finally Matlab Simulation of Grid connected PV inverter system for two different irradiation of PV has been
explained.
II. SOLAR PV ARRAY MODELING
PV Array is a combination of solar cells, connections, protective parts, supports, etc. In the present modeling, the focus is
only on cells. Solar cells consist of a p-n junction; various modeling of solar cells have been proposed in the literature.
Thus, the simplest equivalent circuit of a solar cell is a current source in parallel with a diode. The output of the current

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source is directly proportional to the light falling on the cell (photocurrent). Thus, the diode determines the I–V
characteristics of the cell. The electrical equivalent circuit of a solar cell is shown in Fig. 2
Fig. 2 Solar cell electrically equivalent circuit
The output current I and the output voltage of a solar cell are given by
I =Iph− Ido – Vdo/Rsh (1)
=Iph−Io*(exp (q.Vdo/n.k.T) −1) −Vdo/Rsh (2)
V=Vdo−Rs*I (3)
Here, Ipv is the photocurrent, I0 is the reverse saturation current, Id0 is the average current through the diode, n is the
diode factor, q is the electron charge (q = 1.6˟10−19), k is the Boltzmann’s constant (k = 1.38∗10−23), and T is the solar
array panel temperature. Rs, the intrinsic series resistance of the solar cell; this value is normally very small. Rsh is the
equivalent shunt resistance of the solar array, and its value is very large. In general, the output current of a solar cell is
expressed by
I= Iph−Io*(exp (q*(V+Rs*I)/n.k.T) −1) –(V+Rs*I)Rsh (4)
In the above equation, the resistances can be generally neglected, and thus, it can be simplified to
I=ph−Io*(exp (q*V/n.k.T)−1) (5)
If the circuit is shorted, the output voltage V = 0, the average current through the diode is generally neglected, and the
short circuit current Isc is expressed by using
Isc=I= Iph−Ido− (Vdo/Rsh) (6)
Finally, the output power P is expressed as below,
P=V*I=V*(Iph−Ido−Vdo/Rsh) (7)
The below table is having the parameters and its values, which are used in the equivalent circuit of PV. To maximize the
short circuit current to 9.4A, 2PV panels are connected in parallel. The following parameters are useful to design Matlab
code with above equations, which is shown in Table1.
Table1. PV panel parameters used in simulation
Parameter Value
Short circuit Current(A) Isc=4.7
Open circuit Voltage(V) Voc=230
Shunt Resistance(Ω) Rsh=2000
Series Resistance(Ω) Rse=0.25
Amps/Kelvin(A/k) Ki=78*10-6
Volts/Kelvin(V/k) Kv=2.3*10-6
Temperature(°C) T=25
Diode factor n=1.3
Series connected cells Ns=384
The National Renewable Energy Laboratory (NREL) in their technical report used Matlab/Simulink since it presents
unique capabilities for developing control algorithms and power electronics modeling. In the research for a platform to
model a PV array or cell, many different programs were proposed but at the end the choice was clear in that
Matlab/Simulink and Simpowersystems would be the ideal modeling system since it offered accurate computations and
eased up on the power system block diagram design. Solar cell is acts as current source, so current source is taken from
Simpowersystems. Irradiation (insolation) and Temperature are taken as the input parameters for the panel. Generally
irradiation can be varied from 100W/m² to 1000W/m². The standard temperature condition (STC) for the panel is taken
as 25°c (298kelvins). For an ideal PV cell Rs is negligible and Rsh will be infinite but in a practical solar cell series
resistance (Rs) is taken as small and shunt resistance (Rsh) is taken as large.
A. PV cell characteristics under Constant Irradiation(constant insolation)
As shown in Fig.3 signal1 is having constant irradiation profile with 1000W/m² Irradiation. Here temperature is also
constant with 25°c.

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Fig. 3 Constant irradiation profile with 1000 W/m² Irradiation.
When the irradiation profile with 1000W/m² is applied to the PV, the following Power-Voltage and Current-Voltage
characteristics are obtained. Here Voc=230V, Isc=9.4A and T=25°c (Standard Temperature condition). P-V and I-V
characteristics of PV are shown in Fig. 4.
Fig. 4 P-V and I-V characteristics of PV module with 1000W/m² irradiation.
From the Simulation results, it can observe that Vmax= 200V and Imax=8.5A and Pmax is around 1700W. These values
are useful to determine the Maximum Power Point (MPP). Pmax= Vmax*Imax. These values can be changed by solar
irradiation and cell temperature. In this paper, we can see how above parameters are changed during Variable irradiation.
Irradiation (insolation) will not be constant in practical world. Irradiation will be moderate in morning and evening times
and maximum during mid-day. If the irradiation is maximum, then we have maximum open circuit voltage and if the
irradiation is minimum, then we will have less open circuit voltage compare to previous.
B. PV cell characteristics under Variable Irradiation(Variable insolation)
As shown in Fig.5 signal2 is having variable irradiation profile with 600W/m² to1000W/m² Irradiation with Trapezoidal
signal. Here temperature is constant with 25°c (STC).
Fig. 5 Variable Irradiation profile with 600 W/m² to 1000 W/m².
The corresponding P-V and I-V characteristic of above irradiation profile is given below Fig. 6.
Fig. 6.P-V and I-V characteristics of PV module with 600W/m² to 1000W/m² irradiation.
From the Simulation results, it can observer that Vmax=210V, Imax= 7A and Pmax is around 1480W. Hence we can say
that PV module can be changed by the Irradiation changes. Vmax, Imax and Pmax are changed from previous one. PV
cell can also be changed by temperature but it is not considered here.

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III. VOLTAGE SOURCE INVERTER MODEL
Fig. 7 Three phase-VSI
Three-phase VSI is shown in Fig. 7. Normally 180° mode of operation is preferred over 120° mode of operation due to it
better utilization of switches. In 180° mode of operation, there are six modes of operation in a cycle and the duration of
each mode is 60°. To avoid the short circuit in any mode switches are on without conducting in same leg. If S1 is ON,
then S4 will be OFF and if S3 is OFF, and then S6 will be ON. Here logic 1 is taken when switch is ON and logic 0 is
taken when switch is OFF. In mode1 and mode8, all upper leg switches are OFF and ON respectively. So there is no
output Voltage. From mode2 to mode6, we will have +Vdc and –Vdc which is explained in Table2.
Table2.Switching operations of VSI with 180° mode
mode S
1
S
3
S
5
S
4
S
6
S
2
Van Vbn Vcn Va
b
V
bc
Vca
mode1 0 0 0 1 1 1 0 0 0 0 0 0
mode2 1 0 0 0 1 1 2/3 -1/3 -1/3 1 0 -1
mode3 1 1 0 0 0 1 1/3 1/3 -2/3 0 1 -1
mode4 0 1 0 1 0 1 -1/3 2/3 -1/3 -1 1 0
mode5 0 1 1 1 0 0 -2/3 1/3 1/3 -1 0 1
mode6 0 0 1 1 1 0 -1/3 -1/3 2/3 0 -1 1
miode7 1 0 1 0 1 0 1/3 -2/3 1/3 1 -1 0
mode8 1 1 1 0 0 0 0 0 0 0 0 0
In above table, Van, Vbn and Vcn are pole voltages. Vab, Vbc and Vca are line voltages. When upper leg switch is ON,
then Pole Voltage will be 2/3Vdc. If lower leg switch is ON, then Pole Voltage will be -1/3Vdc. Line Voltages are
calibrated as given below,
Vab= Van-Vbn
Vbc=Vbn-Vcn
Vca=Vcn-Van
They are various techniques to vary the inverter gain. The most effective method of controlling gain and output voltage is
to incorporate Pulse Width Modulation (PWM) control within the inverters. Sinusoidal Pulse Width Modulation
(SPWM) is one of the PWM techniques. In this method, there are three sinusoidal reference waves each shifted by 120°.
A carrier wave is compared with the reference signal corresponding to a phase to generate the gating signals to that
phase. Comparing the carrier signal Vcr with the reference phases Vra, Vrb and Vrc produces the gating pulses g1, g3
and g5 for the switches S1, S3 and S5 respectively, as shown in Fig. 8. The instantaneous line-to-line voltage is Vab=
Vs*(g1-g3). The output voltage Vab is generated by eliminating the condition that two switches in the same arm cannot
operate at the same time.

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Fig. 8 SPWM for VSI.
The phase-voltages (Van, Vbn, and Vcn) are identical, but 120° out of phase without even harmonics; moreover,
harmonics at frequencies multiple of three are identical in amplitude and phase in all phases.
IV. GRID-TIE INVERTER
Generally, Grid-Tie Inverter (GTI) is a power converter that converts direct current (DC) to alternating current (AC) with
ability to synchronize to interface with a utility line. The GTI must synchronize its frequency with that of the grid (e.g.
50 Hz) using a local oscillator and limit the voltage to no higher than the grid voltage. A high-quality modern GTI has a
fixed unity power factor, which means its output voltage and current are perfectly lined up. As we already discussed in
Introduction, Grid connected PV Inverters are implemented with single-stage than two-stage due to its reliability and
simple topology. In developed countries like United States and Germany are implementing this type of Grids with PV. In
this paper, simple inductor L is used to reduce the ripples in Inverter output. Capacitor is placed across the inverter input
side to maintain PV voltage as constant. The overall Matlab simulation setup for Grid connected PV Inverters are
explained as follows.
V.SIMULATION WORK
The Matlab simulation set up is shown in Fig. 9. It is consist of PV and Capacitor to the input side of Inverter.
PV with equivalent circuit is designed with Irradiation and Temperature as input parameters. The gate pulses are
generated by SPWM technique. An Inductor with parasitic resistance was added to give approximate sinusoidal output of
inverter. Three-phase load is connected across the inverter and then, it is fed to the infinite bus bar (GRID). Pi-section is
also added between load and Grid. Here Active and Reactive power blocks are also added to observe the Active and
Reactive powers before and after load. Here phase to phase Rms values of load and grid are selected as same as inverter
to observe the accurate results.
Fig. 9 Grid connected PV Inverter with simulation set up in Matlab.

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A. The following simulation results are related to Grid connected PV with Irradiation 1000W/m².
Fig. 10.PV and Inverter output waveforms.
.
Fig. 11 Inverter phase currents and voltages waveforms before and after Load (GRID).
Fig. 12 Active and Reactive power waveforms before and after load.
In practical environment irradiation is not going to be constant throughout the day. In morning and evening sessions, it
will be moderate. Irradiation will be high during afternoon session. The average irradiation throughout the day will be
600W/m², but not 1000W/m².
B. The following simulation results are related to PV with grid-inverter with irradiation of 600W/m².
Fig. 13.PV and Inverter output waveforms.

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Fig. 14.Inverter phase voltage and current waveforms before and after load (GRID).
Fig. 15.Active and Reactive power waveforms before and after load.
PV Panel output voltage can be varied by varying the external load connected to the PV. PV panel current and
power can be varied by varying the Irradiation. Hence there is a change in PV output current but not in PV output
voltage. The simulation results shows, there is a change in Active and Reactive powers before and after the load. If we
change the load then there will be change in PV output voltage. Here Irradiation is varied, so there is change in PV
current and power. There by active and reactive powers are changed with reduced manner which are shown Fig. 15 (after
load) when compared with Fig. 12 (before load). This Simulink model not uses transformer so it can be called as
Transformer less Grid-connected PV inverter. This is advantageous than Grid-connected PV inverters with transformers.
VI. CONCLUSION
This paper studies about the PV simulation under constant and variable irradiation. PV with grid-tie inverter system is
simulated for different irradiation. This PV system has been studied about the PV output voltages and currents with
different irradiation profiles. Also this system will be useful to simulate the high rated grids with PV. This system is
closely related to single-stage conversion of solar power without tracking Maximum Power Point from PV panel. Single-
stage conversion has several advantages than two-stage conversion without including Boost-converter. This can be
further extended to improved Grid connected PV Inverters with single-stage topology.
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Biography
Shaik Amanulla was completed his B.Tech in electrical and electronics engineering from KORM engineering college,
kadapa, A.P. in the year 2012. He is doing his post graduation study in G Pulla Reddy Engineering College, Kurnool
with specialization in Power Electronics. His area of research includes renewable energy systems and Power electronic
converters. Email id- aman.gprec@gmail.com.

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