Design, modeling and performance investigation of gc

Journal of Energy Technologies and Policy www.iiste.org
ISSN 2224-3232 (Paper) ISSN 2225-0573 (Online)
Vol.2, No.1, 2011

Design, Modeling and Performance Investigation of GC

PVGS
K.Rajambal, G.Renukadevi, N.K.Sakthivel
Department of Electrical Electronics Engg , Pondicherry Engineering College , Pondicherry ,India
* E-mail of the corresponding author: renunila_1977@yahoo.com

Abstract
This paper proposes a new topology for the power injection system that is based on the parallel association
of two voltage source inverters: one is operated using a quasi-square voltage waveform strategy and the
other operates with a PWM based strategy. The aim of this topology is that the quasi-square inverter injects
the power from the photovoltaic generation system and the PWM inverter controls the current quality. The
proposal optimizes the system design, permitting reduction of system losses and an increase of the energy
injected into the grid.
Keywords: quasi square wave inverter, pulse width Modulation inverter, photo voltaic power system,
modulation Index (M).

1. Introduction
Since the start of the industrial age more than 150Years ago, the world economy was running on fossil fuels,
which were cheap as there was no cost associated with their Production, but only with their extraction and
transportation. The negative effects on the environment became visible only in the last 30 years. Renewable
energy resources will be increasingly important part of power generation in the new Millennium. Besides
assisting in the reduction of the emission of greenhouse gases, they add the much needed flexibility to the
energy resource mix by decreasing the dependence on Fossil fuels. Due to their modular characteristics,
ease of the Installation and because they can be located closer to the user photovoltaic (PV) systems have
great potential as distributed power source to the utilities. PV systems are installed on the roof of the
residential buildings and connected directly to the grid.(called grid-tie or grid-connected ). A grid-tied PV
system consists of two main stages a PV module and power injection System as shown in Figure. 1. In
these PV systems , power Conditioning system (PCS) should have high efficiency and Low cost The
power generated from the renewable energy systems are tied to the utility grid (Doumbia 2004) and
(Jih-heng Lai 2003). In remote places where there is less/no feasibility of utility grids, renewable energy
Systems provides electricity to the isolated region. These isolated renewable energy systems can be
employed to power residential applications. For renewable energy sources, the output voltage and power
typically depends on a variety of uncontrollable factors. for example: radiation intensity determines the
obtainable voltage and power output of a solar panel, wind speed determines voltage and power of a wind
electrical generator; and the output Voltage and power of a fuel cell changes with the operating
temperature, fuel and air flow rates (Detrick 2005) to (DeSouza 2006). To obtain the required
voltage output for varying input conditions, power conditioning Systems (PCS) are introduced
as interfacing scheme between the PV panels and grid . The existing power conversion topologies
used in power conditioning system consist of boost converter and a pulse width
modulation inverter or a multi level inverter. The boost converter enhances the low
voltage output from the renewable energy sources during low input conditions And the
inverter converts the dc power into ac of required voltage and frequency. The two stage
conversion system also increases the system cost and decreases the efficiency of the
system. In this project, an inverter topology without the intermediate dc-dc converter has
been presented. This scheme reduces the system losses and increases the efficiency and increase of the
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Vol.2, No.1, 2011
energy injected into the grid. The proposed inverter is designed for a kW photovoltaic generation system.
Photovoltaic generation scheme along with the inverter is simulated in MATLAB/Simulink. Simulation is
carried out to study the power flow characteristics for varying solar intensities and modulation indices and
the results are presented.
2. Description of the Pvgs with QSWI-PWM Inverter
Fig.1 shows the inverter topology. This Photovoltaic Generation system presents a topology of the
power injection system (PIS) that has the function of injecting the power produced by the photovoltaic cell
groups, converting the energy from the original DC form to the final AC form with the desired electrical
characteristics. The part of the PIS that carries out this conversion is the inverter (Carrasco 2006) .Usually,
Pulse width modulation (PWM) based inverters (Kwon 2006) or multilevel topology inverters are used
(Gupta 2006). In this paper a new topology for the PIS is presented based on the parallel association of two
voltage source inverters. (VSI): one is operated using a quasi-square voltage waveform strategy
(quasi-square waveform inverter, QSWI).It can be operated with the operating frequency of grid frequency
(50Hz) and the other operates with a PWM based strategy (high-switching-frequency inverter, HSFI).and
this inverter can be operated as high switching frequency of 20kHz.The general purpose of the QSWI is to
inject the power generated by the PVGS into the grid. In order to achieve this, controlling the fundamental
component of the inverter and that of the HSFI is to be responsible for controlling the quality of the current
injected into the grid. The mathematical modeling of the various system components are discussed below:
(a) Modeling Of PV Cell

The characteristics of a solar cell relating the cells voltage to current are expressed
by the equations, that are given below ,The PV cell output current is given as,

q(Vpv + I pvRs
I pv = I ph − I0[exp( ) −1] ......... ( 1 )
AKT
The PV cell output voltage is given as,

AKT I ph − I pv + I0
Vpv = ln( ) − I pvRs ……..( 2 )
q AKT
The light generated current is given as,

I ph = [ I scr + K i (T − 298)]λ / 100 …… ( 3 )

The saturation current is given as,

T 3 qEgo 1 1
I 0 = I or ( ) exp[ ( − )] ..…… (4 )
Tr BK Tr T
(b) Power Injection System
A direct connection to the inverters without a previous DC/DC converter has been chosen. The upper
inverter in the figure.1 (inverter 1) is the Quasi Square Wave Inverter and the lower inverter (inverter 2), is
the High Switching Frequency Inverter. Both inverters share the same DC bus, which is connected to the
PVGS. The capacitor between the PVGS and the PIS must absorb the active power fluctuations (that
always exist in a single-phase system). Therefore it achieves constant power extracted from the PVGS, by
keeping the DC voltage at the output terminal of the PVGS constant under these power fluctuations. The
QSWI is responsible for injecting the energy produced by the PVGS. This inverter deals with high currents
but at low switching frequency (50-60 Hz), resulting in lower losses than if the inverter is operated with a
PWM technique. This inverter injects a current with a high total harmonic distortion (THD) into the grid.
The PWMI is connected in parallel with the QSWI and working as a part of the PIS and its corrects the
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Vol.2, No.1, 2011
current produced by the QSWI improving its THD.The PWMI (working as APF) not only can correct the
QSWI current, but also correct the current demanded by a non-linear load connected to the same point of
common coupling (PCC) than the PVGS. (Usually between 10 to 20 kHz).

(C) Design of Inductance
The QSWI inductance value (L1) must be selected to permit the injection of
the maximum power That the PVGS can generate (which depends on the irradiance
and temperature conditions).The power injected into the grid from the PVGS (neglecting
the PIS losses),

PS = V S I S ,1 = PPVGS …….………….. (5)

As the current is in phase with the grid voltage. The RMS fundamental current
component injected by the QSWI (I1) can be determined by ,

V1 − VS
2 2

I1 = …..……….…… (6)
L1ω
From that two equations we get,

VS
L1 = V1 − VS
2 2
…….………….. (7)
ωPPVGS
The inductance with proposed DC voltage is given by,

VS π
L1 = (0.9Vdc cos )2 −VS .
2
.………………… (8) and,
ωPPVGS 6

Vdc − 2VS
L2 = ..….……………… (9)
i2,slope
The coupled inductors enable the advantage of, output ripple current reduction due to AC
magnetic field cancellation within the inductor core. Improved efficiency due to lower peak
currents. Reduction in required output capacitance. Faster transient response due to the ability
to use lower effective inductance values. Reduced overshoot or undershoot during load transients.
Frequency range up to 2 MHZ.

3. Simulation Results

The individual models of the PV array and inverters are simulated and their characteristics are studied.
Then the models of PV array and inverters are integrated along with grid and simulation is carried out to
study the power flow characteristics for varying solar intensities. The effect of modulation index is studied
and the optimum modulation is identified for different solar intensities for maximizing the power output.
The simulation results are discussed below,
(a) Characteristics of the PV module.
The model equations (1) - (4) of the PV cell detailed in section II (a) are used to obtain the I-V
14

Vol.2, No.1, 2011
characteristics of the PV module given in appendix. Figure.2 shows the I-V characteristics for varying solar
intensities at constant temperature of 25°C.It observed that the current increases with increase in intensity
thereby increasing the power output of the solar cell. It is observed that the PV array generates 550V
Voltage and 8.8A current at the rated intensity of 100mW/cm² with 34 modules in series and 2 modules in
parallel. Figure.3 shows that I-V characteristics for varying temperature at rated intensity of 100mW/cm².It
is observed that the current variation for marginal temperature variation from 25°C to 65°C.The output
power versus power characteristics of the PV array for different intensity is shown in Figure.4 It is observed
that the power increases with increasing voltage and reaches a maximum value and starts decreasing for
any further increase in voltage. It is also seen that the output power increases with increasing intensity.
(b) PIS results
The quasi square wave inverter and pulse width modulation inverter are modeled in MATLAB-SIMULINK.
The performance of the grid connected PIS which includes PWM and QSW Inverters are studied for
different modulation indices. Figure.5 shows the firing pulses for Quasi Square Wave Inverter. A reference
sine wave of 50Hz is compared with constant dc voltage to generate the firing pulse. The output voltage is
varied by controlling the width of the pulse. In this switching scheme, the positive and negative half cycles
are present for an interval less than the half the period of the output frequency. The QSWI is responsible for
injecting the energy produced by the PVGS. The steady state voltage and current waveforms of the inverter
are presented in Figures.6&7. It is seen that peak current of the QSWI is about 17 A without the controller.
A reference sine wave of 50Hz is compared with triangular wave of 20 kHz to generate the firing pulse.
The width of each pulse varies as a sine fashion. The frequency of the reference determines the output
frequency of the inverter. The number of pulses in each half cycle depends on the carrier frequency. The
output voltage is controlled by adjusting the modulation index. The PWMI is connected in parallel with the
QSWI and working as a part of the PIS and its corrects the current produced by the QSWI improving its
THD are seen from Figure.11. The main advantage of the proposed system is the inverter loss decrease,
because the QSWI has low losses which are due principally to conduction. The switching losses are small
because the switching frequency matches the grid frequency, near 50Hz. The conduction losses could be
even further reduced if a low ON- voltage semiconductor is selected, since no high speed switching
semiconductors are needed for this inverter. But also losses in the HSFI (principally due to the switching
losses) decrease notably, because the current levels for this inverter are lower than those produced if it was
working alone (without the QSWI cooperation). This can be observed in Figure.10. Where the maximum
instantaneous current value is lower than 3 A, a value that is significantly lower than the maximum value of
15 A for the total injected current, Figure.11. If one assumes that the switching losses are proportional to the
maximum instantaneous current value, these losses will be reduced to approximately 20%. Therefore the
high switching semiconductors used in this inverter will have a current ratio of about five times lower than
the semiconductor used in a conventional PWM inverter for the same task. The reduction in losses allows
one to use a smaller aluminium radiator. Furthermore, the QSWI inductor can be built with a conventional
core coil designed for a working frequency of 50 Hz (ferromagnetic core and conventional cupper wire),
and the HSFI inductor can be built with an air core coil with smaller section cupper wire (usually special
cupper wires must be used due to the high switching frequency), because the RMS current value is smaller
than if it operates alone.

4. Control system
The PIS control can be divided into the blocks shown in the schematic diagram of Figure. 12.
(a) Maximum Power Point Tracking (MPPT) block:
The objective of this block is to set and to maintain the PVGS at its maximum power point (MPP). When
the PVGS is working at this point, one hast that,

dPPVGS
=0 ..….……. (10)
dVPV
The MPPT scheme is shown in Figure. 13. In the proposed system, this condition is achieved by
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Vol.2, No.1, 2011
considering the power derivative as the error input of a proportional integral (PI) controller. In this way if
the PI controller is well-designed, the power derivative will become zero in the steady state and the MPP
will be tracked. A saturation function has been included to prevent the improper operation of the integral
part of the controller during start-up transients. A low-pass filter is included to eliminate the components in
the power derivative due to the switching frequency.
(b) Reference Supply Current Generation Block
This is the principal block of the proposed control system, because it guarantees that the current extracted
from the PVGS is the desired one and so the MPP is tracked. The RMS value of the fundamental current
component that must be injected into the grid is determined by neglecting the PIS losses:

VPV I PV ,ref
I S ,ref = ..….……. ( 11 )
VS
This reference current value is tuned by the output of an additional PI whose input is the error between the
reference current that must be extracted from the PVGS(determined by the MPPT block) and the actual
one(measured from the system). The aim of this tuning is to compensate the system losses that have not
been considered in (11).The RSCG block contains a synchronization module in fig.14. that generates two
sinusoidal signals with unity RMS value, which are in phase (the first one) and in quadrature (the second
one) with the fundamental grid voltage component, and an RMS grid voltage fundamental component
calculation module .As a result the reference supply current is a sinusoidal wave in phase with the grid
voltage, and its RMS value is equal to that given by (11).
(c) Signal Generation for Inverter 1
This block implements the collaboration between the two inverters and their principle of operation was
described in above. Figures 15 and 16 shows the schematic diagram for each inverter. In Figure.15 the
function block “Fcn” implements equation (11) , and the function block “Fcn1” is used to avoid values
greater than 1 for the arc cosine function of this equation.
(d) Switching Signal Generation for Inverter 2
The QSWI is operated in a quasi-open-loop obtaining the value of β from the reference supply current
(determined in part B), not its measured value. The HSFI switching signals are generated based on the error
existing between the reference supply current and the measured one, by using a hysteresis band that
compares the error with zero for a fixed sample period (Figure.16).
5. Controller results:
The closed loop control systems are simulated with the parameters given in Appendix. The simulation
results of the PVGS and Inverter are studied individually and the overall system performance for varying
solar intensities and modulation indices are observed. From Figure.17 to 21 shows that the performance of
the PVGS and the grid connected system results of the both inverter currents are simulated at rated intensity
of 100mW/cm². The main advantage of the proposed system is the inverter loss decrease, because the
QSWI has low losses which are due principally to conduction. The switching losses are small because the
switching frequency matches the grid frequency, near 50Hz. The conduction losses could be even further
reduced if a low ON- voltage semiconductor is selected, since no high speed switching semiconductors are
needed for this inverter. But also losses in the HSFI (principally due to the switching losses) decrease
notably, because the current levels for this inverter are lower than those produced if it was working alone
(without the QSWI cooperation). This can be observed in Figure. 21. Where the maximum instantaneous
current value is lower than 3 A, a value that is significantly lower than the maximum value of 15 A for the
total injected current shown in Figure.22. If one assumes that the switching losses are proportional to the
maximum instantaneous current value, these losses will be reduced to approximately 20%. Therefore the
high switching semiconductors used in this inverter will have a current ratio of about five times lower than
the semiconductor used in a conventional PWM inverter for the same task. The reduction in losses allows
one to use a smaller aluminium radiator. Furthermore, the QSWI inductor can be built with a conventional
core coil designed for a working frequency of 50 Hz (ferromagnetic core and conventional cupper wire),
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Vol.2, No.1, 2011
and the HSFI inductor can be built with an air core coil with smaller section cupper wire (usually special
cupper wires must be used due to the high switching frequency), because the RMS current value is smaller
than if it operates alone. The total inductor losses will be lower: (a) the electric losses in inductor of inverter
1 decrease because it has a lower resistance (R1) then if it had been built with air core coils
(because far fewer turns of copper wire are needed). Ferromagnetic losses will be small since the
operation frequency is 50 Hz.(b) the electric losses in inductor of inverter 2 decrease too, because the RMS
of the current that flows through it(I 2) is reduced by five times Although the cost could seem to increase
due to the more complex topology and control and to the higher number of semiconductors needed, others
factor related to cost should be taken into account: decrease in losses and so decrease in the size of the
aluminium radiator, possibility of using ferromagnetic core inductors for filtering the greater fraction of the
current (this inductors are about 30% less expensive than air core inductors), possibility of using slower
semiconductors (so cheaper devices) for the greater current fraction, and using lower rating high frequency
semiconductors (because they operate with a lower current fraction).

6. Experimental results:
The prototype model of this scheme has been implemented in hardware. The prototype model has been
tested with a laboratory Prototype (Figure.23) solar module (supplying 12 VDC) is connected to a four
branches inverter (MOSFET-IRF840) and it can be connected into the load. The results are shown in Figure.
24 to 29.The proposed PIS behaves as in the simulation setup. The output voltage of the 15W PV module is
found to vary from 15.07V to 18.09Volt over a day. The corresponding output voltages of the inverter
voltage are varying from 11.8V to 13.2Volt.The output voltage and current waveform are observed and
presented. The hardware results are compared with the simulation results. It is found that the hardware a
result closely matches with the simulation results, thus validating the simulation model of the grid
connected PVGS.

7. Conclusion
The power characteristics of the photovoltaic generation system is investigated for varying solar
intensisties.The power output of the photovoltaic module, the voltage and currents of the inverters and the
power export to the grid are observed for different intensities. The effect of modulation index is studied and
the optimum modulation is identified for every solar intensities for maximizing the power output. A closed
loop control is designed to maximize the power output for varying intensities. The proposed system
optimizes the system design, permitting the reduction of the system losses (conduction and switching losses,
and Joule effect losses in inductors) and so increases the energy effectively injected into the grid. There will
consequently be an increase in profit when selling this energy.
APPENDIX
PV Generation System

Parameter Value
34
Number of series connected cells

2
Number of parallel connected cells

SHELL BP150-P
Photovoltaic cell reference

8.8A
MPP current (25°C, 100mW/cm²)

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Vol.2, No.1, 2011
550V
MPP voltage (25°C, 100mW/cm²)

Power rating at rated intensity of 100(mW/cm²) 3kW

Power Injection System
50mh ×2
Filter inductance L1

25mH ×2
Filter inductance L2

20 kHz
Switching frequency

REFERENCES:
Doumbia.M.l.,Agbossou, K.;Bose, T.K.(2004), “Islanding protection evaluation of inverter-based
grid-connected hybrid renewable energy system, Electrical and Computer Engineering",1081-1084
Jih-heng Lai (2003), “Power Electronics applications in renewable energy system”, Industrial Electronics
Society, IECON.
Detrick, A. Kimber, and L.Mitchell (2005), “Perfomance Evaluation Standards for Photovoltaic Modules
and Systems,” proceedings of the 31th IEEE Photovoltaic Specialists Conference, Lake Buena Vista, FL.
Armenta-Deu.C (.2003) “Prediction of battery behaviour in SAPV applications Renewable Energy”
28:1671-1684.
K.C.A.deSouza,O.H.Goncalves,D.C.martins(2006). “study and optimization of two dc-dc power structures
used in a grid- connected photovoltaic system”,power electronics specialists conference,PESC06,pp.1-5
Carrasco. J.M, Franquelo, L.G, Bialasiewicz. J.T, Galvan, E, Portillo-Guisado. R.C, Prats.M.A.M, Leon.,
J.I, Moreno-Alfonso.N (2006), “Power-Electronic Systems for the Grid Integration of Renewable Energy
Sources: A Survey”. IEEE Transactions on Industrial Electronics, vol. 53, issue 4, pages:1002 - 1016.
Weidong Xiao, Ozog, N.; Dunford.W.G (2007) “Topology Study of Photovoltaic Interface for Maximum
Power Point Tracking”. IEEE Transactions on Industrial Electronics, vol 54, issue 3, pages 1696 – 1704.
Kwon.J.-M, Nam.K.-H, Kwon.B.H (2006), “Photovoltaic Power Conditioning System with Line
Connection”. IEEE Transactions on Industrial Electronics, vol. 53, issue 4, pages 1048- 1054.
Gupta.A.K, Khambadkone.A.M (2006). “A Space Vector PWM Scheme for Multilevel Inverters Based on
Two-Level Space Vector PWM”. IEEE Transactions on Industrial Electronics, vol. 53, issue 5, pages
1631 – 1639.
Rico, Amparo,Cadaval, Enrique Romero; Montero, Maria Isabel Milanes (2007) “Power Injection Control
System and Experimental Model based on Manufacturer Characteristic Curves for a Photovoltaic
Generation System”. Compatibility in Power Electronics,CPE '07, pages 1 – 7. May 29 2007.

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Vol.2, No.1, 2011

Figure.1. Power Conditioning System for a Grid connected Photovoltaic System Figure.2 I-V characteristics of PV module for varying solar
intensities at rated cell temperature

Figure.3 I-V characteristics of PV module for different cell temperature Figure .4 Power characteristics for various solar
at rated intensity intensities at rated cell temperature

Figure.5. Firing pulse for QSWI Figure.6. Quasi Square Wave Inverter Voltage

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Vol.2, No.1, 2011

Figure.7. Quasi Square Wave Inverter Current Figure.8. Firing pulse for PWMI

Figure.9.Pulse Width Modulation Inverter Voltage Figure.10. High Switching Frequency Inverter
Current

Figure.11. Current Injected into the grid Figure.12 Maximum Power Point Tracking (MPPT)

Figure.13 MPPT scheme Figure.14.RSCG block contains a synchronization module

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Vol.2, No.1, 2011

Figure.15. Inverter Switching Figure.16. Inverter Switching

Figure.17 PVGS Current at 100mW/cm² Figure.18 Power Extracted from the PVGS

Figure.19 Supply reference current rms Figure.20.Quasi Square Wave Inverter Current

Figure.21. High Switching Frequency Inverter Current Figure.22.Current Injected into the grid

21

Vol.2, No.1, 2011

Figure.23. Hardware setup Figure.24. Firing pulse for QSWI

Figure.25. Firing pulse for PWMI Figure.26. Quasi Square Wave Inverter Voltage

Figure.27. Pulse Width Modulation Inverter Voltage Figure.28. High Switching Frequency Inverter Current

Table 1
DC AC
VOLTAGE VOLTAGE CURRENT POWER
15 12.2 0.51 6.22
16 12.8 0.65 8.32
17 12.9 0.67 8.64
18 13.5 0.69 9.31

Fig.29. Load Voltage

22

Vol.2, No.1, 2011
Table 2

DC AC VOLTAGE AC VOLTAGE
TIME VOLTAGE (Peak) (Rms) CURRENT POWER

9.30 am 15.07 16.68 11.8 0.49 5.78
10.30
15.19 17.53 12.4 0.54 6.69
am
11.30
16.16 18.10 12.8 0.62 7.93
am
12.30
16.27 18.24 12.9 0.64 8.25
pm
1.30 pm 18.09 18.66 13.2 0.66 8.71

2.30 pm 17.92 17.67 12.5 0.64 8.00

3.30 pm 15.08 16.68 11.8 0.60 7.08

4.30 pm 11.83 11.03 7.8 0.5 3.9

Bibliography of authors
K.Rajambal received her Bachelor of Engineering in Electrical &
Electronics, Master of Engineering in power electronics and Ph.D in Wind Energy
Systems in 1991, 1993 and 2005 respectively from Anna University, Chennai,
India. She is working as a Associate professor in the Department of Electrical and
Electronics in Pondicherry Engineering College,Pondicherry, India. Her area of
interest includes in the fields of Wind Energy systems and Photovoltaic Cell, Power
Converter such as DC-DC Converters, AC-AC Converters and Multilevel Inverters
with soft switching PWM schemes and power electronics application towards
power systems. She has published papers in national, international conferences and
journals in the field of non renewable energy sources and power electronics.
G.Renukadevi received her Undergraduate Degree in Electrical Engineering from
The Institution of Engineers, India in 2006 and Master of technology in Electrical
Drives and Control from Pondicherry Engineering College, Pondicherry, India in
2009. Now pursuing Ph.D in Pondicherry Engineering College, Pondicherry, India.
Her field of interest is power electronics, Drives and control, Modeling, AI
techniques and control systems.

N.K.Sakthivel received Master of technology in Electrical Drives and Control in
2010 from Pondicherry Engineering College, Pondicherry, India in 2009.His field of
interest is power electronics, renewable energy and control systems.

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Design, modeling and performance investigation of gc

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