Scholarly article on topic 'Grid Connected Photovoltaic System, for a 800W'

Grid Connected Photovoltaic System, for a 800W Academic research paper on "Electrical engineering, electronic engineering, information engineering"

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Abstract of research paper on Electrical engineering, electronic engineering, information engineering, author of scientific article — Ali Bouhafs, Bendaas Mohamed Lokmane, Djarallah Mohamed

Abstract The purpose of this work is on analyzing large, grid connected with three-phase inverter system, which presents the three-phases grid-connected inverter designed for a 800W photovoltaic system. These PV systems are interfaced to the grid invariably by a power electronic DC-DC converter and inverter. Many of the important characteristics of the PV generation are influenced by the design and performance of those power electronic converters. The power plant that features a maximum power point tracking (MPPT) scheme shows accurate and fast response, and it is integrated in DC-DC converter. The inverter has a role to generate a current sinusoidal and to inject it into the networks electrical supply. The whole system which presented is simulated in Matlab.

Academic research paper on topic "Grid Connected Photovoltaic System, for a 800W"

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Energy Procedia 74 (2015) 414 - 422

International Conference on Technologies and Materials for Renewable Energy, Environment and

Sustainability, TMREES15

Grid connected photovoltaic system, for a 800 W

Ali BOUHAFS, BENDAAS Mohamed Lokmane, DJARALLAH Mohamed.

Université Kasdi Merbah Ouargla, Laboratoire Génie Electrique, Faculté des Sciences et Appliquées,

Ouargla 30000 AJgérie.

Abstract

The purpose of this work is on analyzing large, grid connected with three-phase inverter system, which presents the three-phases grid-connected inverter designed for a 800W photovoltaic system. These PV systems are interfaced to the grid invariably by a power electronic DC-DC converter and inverter. Many of the important characteristics of the PV generation are influenced by the design and performance of those power electronic converters. The power plant that features a maximum power point tracking (MPPT) scheme shows accurate and fast response, and it is integrated in DC-DC converter. The inverter has a role to generate a current sinusoidal and to inject it into the networks electrical supply. The whole system which presented is simulated in Matlab. © 2015TheAuthors. Publishedby ElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) Keywords: inverter, boost converter, PV system

I.INTRODUCTION

The Earth is known as the blue planet because of the color of the oceans that cover two thirds of its surface. This planet, the third planet from the Sun, is the only one where the right conditions exist to sustain life, something that makes the Earth special. It has liquid water in abundance, a mild temperature, and an atmosphere that protects it from objects that fall from outer space. The atmosphere also filters solar radiation thanks to its ozone layer. Slightly flattened at its poles and wider at its equator, the Earth takes 24 hours to revolve once on its axis. This plant is so rich by many things which made the man thought how to use them as sort of energy. Nowadays, the world changes to become more developed, therefore the demand of the energy has known increasing recently. The massy usage of the fossil fuels, such as the oil, the coal and the gas, result in serious greenhouse effect and pollute the atmosphere, which has great effect on the world. Meanwhile, there is a big contradiction between the fossil fuels supply and the global energy demand, which leads to a high oil price in the international market recently. The energy shortage and the atmosphere pollution have been the major limitations for the human development. How to

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4 .0/).

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) doi:10.1016/j.egypro.2015.07.639

find renewable energy is becoming more and more exigent. All of these reasons urge the scientists to look for new power source; one of these sources is the solar energy. The abundance of solar energy in the most of the 2world make the design of effective and profitable facilities based on solar modules is especially important [1, 11].

Nodaway there are software, which intended to simulate of photovoltaic systems such as Pvsyst, Hybrid, Ilse, Pvsol, Ashling, and PvWatts, etc. The traditional tools of design before mentioned can carry out extensive and precise analyzes, but generally, they do not allow the user to modify the algorithms. This degree of freedom can be only given in an open architecture: MATLAB allows this process of disposition to modify any existing routine or to include new ones. The use of photovoltaic systems is common now. The solar energy has been used in systems of illumination, electrification, signaling, and communication means of reception for education via satellite in far located communities, heating, pumping and purification of water etc., but as it happens with every new technology, there is not sufficient investigation regarding the conditions of work and operation of these systems. When they are evaluated, designed or when economic analyses of the system for benefiting from solar energy are made a rigorous, detailed information is required.

Converters interfacing PV module with the grid involves two major tasks. One is to ensure that the PV module is operated at the maximum power point. The other is to inject a sinusoidal current to the grid. Later, these tasks shall be reviewed in this paper. Simulating results are obtained for the normal regime.

2. SYSTEM TOPOLOGY

Fig 1 shows the power converter structure used to interface the photovoltaic array with the power grid. The first stage is the boost converter, which will raise the relatively low solar voltage to a level suitable (720 V) for the DC link directly connected to the inverter. The second stage is the DC to AC converter that operates in a current controlled mode, which will inject unity power factor current to the grid.

The inverter should be able to supply a continuous power from the DC link bus to a three phase utility line (300 V / 50 Hz). The output L filter is employed to reduce the ripple components due to PWM switching operation.

A. PV GENERATOR

The PV generator is a set of several photovoltaic cells put in series and in parallel to produce the wishing power.

The density of power radiated from the sun at the outer atmosphere is 1.373 kW/m2. Final incident sun light on the earth surface has the peak density of 1 kW/m2 at noon in the tropics. Solar cell can convert the energy of sunlight directly in to electricity. A simplified equivalent circuit of a solar cell consists of a current source in parallel with a diode variable resistor is connected to the solar cell generator as a load. The relationship between the current and voltage may be determined from the diode characteristics equation:

* = IPH- 'o - l) = v - h...........(1)

Where q is the electron charge, k is the Boltzmann constant, ph I is the photocurrent, 0 I is the reverse saturation current, d I is the diode current and T is the solar cell operating temperature (K).

B. BOOST CONVERTER

Figure 2.(a) shows a typical boost converter. A boost converter is also called a step-up converter because the output voltage is higher than the input voltage. As a result, the output current is lower than the input current because of the power balance. Similarly, there are also two operation modes when the switch Q is turned ON and OFF. The equivalent circuits in these two modes are shown in Figures 2.(b) and 2.(c), respectively. During Mode 1, the inductor current increases linearly because

....................(2)

Moreover, the inductor stores energy from the power source while the capacitor discharges to supply the load. During Mode 2, both the energy stored in the inductor and from the power source are transferred to the load and the capacitor. The inductor current decreases linearly because

= ...............(3)

Similarly, the net energy changed in the inductor should be zero during one period in the steady state, which means the current increased in Mode 1 should be equal to the current decreased in Mode 2. That is,

kT (l-Jflr { . r4,

-[vs=—^(va-vs>............(4)

From which the output voltage can be derived as

(a) Topology

C _L Load

C _L Load

(b) Mode 1 ((Q: ON)

(C) Mode 2 (Q: OFF)

Fig. 2: boost converter

Indeed, this is a boost converter because

k= — > i for k e (0,1).................(6)

The waveforms of the inductor current and the capacitor voltage are shown in Figures 1.15(d) and 1.15(e), respectively [13].

C. THE MAXIMUM POWER POINT TRACKING

MPP tracking is the technique that ensures that the PV cell under any change in irradiance level and the cell temperature conditions gives the maximum available power. In other words, there is a need to track the MPP in order to maximize the power delivered to the load from the PV cell under any circumstance that causes the output voltage of the power system to lose regulation. By controlling the boost converter which is connected to the PV cell is possible to track the MPP. There are different control methods for the MPP tracking. In this paper, the perturb and observe method is implemented due to its ease of implementation. The perturb and observe method is based on the constant measuring of the PV current and voltage and calculation of its power output while the working point is moving in order to reach the maximum power. This algorithm has two parameters: 1) the time interval between the time when measuring is done and the time when the working point moves from its optimal value and 2) the increment of the movement of the working point itself.

The MPP tracking method is depicted in Fig.4. It can be seen that at the MPP it is dP/dU = 0 There are 5 possible movements of the working point. For positions 1 and 2, the derivative dP/dU

has a positive value and there is a need to increase the voltage to reach the MPP. In other hand, positions 3 and 4 have negative derivatives and there is a need to decrease the voltage.

Since the boost converter is used for the MPP tracking, voltage decrease is done by incrementing the duty ratio a, and vice versa, when there is a need to decrease voltage, the boost duty ratio a is decremented. There are four possible situations for controlling the boost converter in order to ensure the MPP tracking:

1. dU(k) > 0 and dP(k) > 0 ) a(k + 1) = a(k) - c,

2. du(k) < 0 and dP(k) < 0 ) a(k + 1) = a(k) - c,

3. du(k) > 0 and dP(k) < 0 ) a(k + 1) = a(k) + c,

4. du(k) < 0 and dP(k) > 0 ) a(k + 1) = a(k) + c,

Where c is small positive number, which is defined as a constant value for which the duty ratio is changed depending on the controller needs either for increasing or for decreasing the value of a [18].

Fig. 3: Maximum Power Point

D. INVERTER

Power Inverter is a device that converts a DC source into an AC source. Inverters are used in a wide range of applications, from small switched power supplies for a computer to large electric utility applications to transport bulk power.

Power inverters produce one of three different types of wave output. In this case we are choosing the Square Wave [16].

The three different wave signals represent three different qualities of output power. Square wave inverters were the first type of inverters made and are now obsolete.

E.PULSE WIDTH MODULATION

Pulse width modulation (PWM) is a method, which the switched voltage pulses are produced for different output frequencies and voltages. A typical modulator produces an average voltage value equal to the reference voltage within each PWM period.

PWM provides a way to decrease the Total Harmonic Distortion (THD) of load current. The THD requirement can be met when the output of a PWM inverter is filtered since the unfiltered output of a PWM based inverted will have a relatively high distortion.

There are many PWM schemes, the most popular ones being sinusoidal PWM (SPWM). It can be implemented using analog techniques, the remaining PWM require the use of a microprocessor.

For the three-phase inverter case, three-phase reference voltage signals of variable amplitude and frequency are compared in three different comparators with a common triangular carrier wave of fixed amplitude and frequency. Each comparator output forms the switching-state of the corresponding inverter leg.

Since there are, six switches and only three control signals the transistors are opposite paired which means the switching control for two transistors in the same leg are opposite.

In Figure .1 we see the typical topology of a three-phase inverter. Switch Q1 has the opposite reference signal of Q2, Q3 has the opposite of Q4 and finally Q5 has the opposite of Q6.

Since the PWM carrier or switching frequency is much higher (the recommended min value is at least 10x higher) than the frequency of the reference voltage leading to the approximation that the reference voltage is nearly constant during a PWM period TPWM .

The mean value of the output voltage resulting from a reference voltage being constant

There are two types of carrier signals: triangular and saw-tooth. Saw tooth shaped always sets the output to a high level at the beginning of each PWM period resulting in asymmetrical pulses that have an edge-aligned PWM with a high-level pulse at the beginning. On the other hand, pulses of a symmetrical PWM signal, like triangular, and are always aligned with respect to the center of each PWM period causes less current and voltage harmonics. Figure 4.3 is the resulting PWM.

The inverter is used to convert the DC power to AC power at the required output voltage and frequency. The solar PV cell, fuel cell, which produces the DC is supplied as the input to the current source inverter. The large value of inductance is placed in series

Fig. 4: Sinusoidal pulse with modulation.

With the input source for providing the constant current to the inverter. Therefore, the inverter is an adjustable-frequency voltage source. The configuration of AC to DC converter and DC to AC inverter is called a DC-link converter. Inverters are broadly classified into two types, voltage source inverter and current source inverters. A voltage-fed inverter (VFI) or more generally a voltage-source inverter (VSI) is one in which the dc source has small or negligible impedance. The voltage at the input terminals is constant. A current-source inverter (CSI) is fed with adjustable current from the dc source of high impedance that is from a constant dc source. i.e...The current across the input terminals is maintained constant.

F. SIMULATION RESULTS

The grid connected PV system in fig. 1 is simulated by using MATLAB/Simulink software environment in order to validate the control methodology discussed earlier. For all simulations, the temperature is assumed constant at all AC voltages and currents peak value are in per-unit on 300V and 30A at feeder circuits.

Under steady-state condition, some simulation results have been chosen to illustrate the system response at standard test condition (STC). The three phase grid voltage and grid current waveforms at PCC are shown in fig. 8. As we can see, the system response under STC is very good when the steady-state condition is reached in few AC periods. The injected current components are shown in fig. 4(c) where both components track their references closely. Simulation results about decoupled active power control have also been provided in fig. 6. The total harmonic distortion of the current (THDC) at PCC in time domain and frequency domain are shown in fig. 8.

In order to investigate the effectiveness of the control algorithms, a dynamic simulation is done with solar irradiance is suddenly dropped from 400 to 600 w/m2 at 0.2 sec as it is shown in fig. 9. Simulation results under transient condition are shown in fig. 5 through fig. 9.

Fig. 5 show the behavior of the PV array output power during simulation run. As can be seen, the effect of solar irradiance on the array terminal voltage is small compared to its effect on output current. However, this was expected where the I-V characteristics of the PV array in fig. 3 confirm this result.

0 0.05 0.1 0.15 0.2 0.25 0.3

Time (S)

Fig 5. : PV power change in time

The dynamic behaviors of three phase grid voltage and grid current at PCC are shown in fig.7-8. As also expected, the grid voltage waveform and the current amplitude do not change during the transient since the voltage it is set by the AC network.

-35* 30 V 25 I 20 0 15 1 10-< | i i i i i i i i i

r ............■.......; i i i i i i i i i

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Time (s)

Fig.6: injected active power (W)

FFT analysis

0.4 0.3 0.2 0.1

ntal (50Hz) = 37.27 , THD= 0.60%

T-1-1-1-1-r~

20 40 60

80 100 120 140 160 180 200 Frequency (Hz)

Fig.8 :injected current waveform with its THD

Fig. 9: change in Irradiation in the time

G. CONCLUSION

This paper attempt to suggest a control methodology for three-phase grid connected PV systems. The DC-DC converter was used to boost the output voltage of the PV array and perform MPPT by using IC technique. In order to inject a high quality AC current into the grid, a three phase - two levels VSI was conversion efficiency of the PV system, which is greatly affected by the choice of transformer primary voltage and DC link voltage. As the transformer, primary voltage increased the total conversion efficiency increased. On the other hand, there is no general rule about the dependency of total conversion efficiency on DC link voltage. Thus, PV planners and simulation programmers should consider these factors to design more efficient PV systems.

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