Ali Jasim, National Aerospace University “Kharkiv Aviation Institute”, Postgraduate Student,

Yuri Shepetov, National Aerospace University “Kharkiv Aviation Institute”, Associate Professor, Candidate of Science

An Intelligent Technique By Using The Method of Constant Coefficient of Short Circuit Current Under Pulse Width Modulation Control of The Photovoltaic Power System

Abstract: Photovoltaic (PV) system is extensively increasing since it is clean, pollution free, and inexhaustible and by consider available resource as a future energy supply. The PV array output power is used to directly control the Pulse-widthmodulation (PWM), dc/dc boost converter, thereby reducing the complexity of the system. The resulting system has high efficiency with lower cost. This paper presents an improved Constant Coefficient of Short Circuit Current (CCSCC) Maximum Power Point Tracking (MPPT) technique under PWM control of photovoltaic (PV) power generation systems to obtain the maximum output power. The solar panel is modelled and analyzed in MATLAB/SIMULINK.

Keywords: photovoltaic system, modeling of PV panels, Constant Coefficientof Short Circuit Current, Boost converter and Simulation Results.

1. IntroductionThe growing demand for electrical energy all over the world has caused a

great need to consider renewable energy sources as a technological option for sustainable energy supply. Among the renewable energy sources photovoltaic (PV) energy is now becoming one of the fastest growing renewable energy technologies due to the continuous cost reduction and technological progress. PV is the field of technology related to the application of solar cells by converting sunlight directly into electricity.

Photovoltaic (PV) generation is becoming increasingly important as a renewable source since it exhibits a great many merits such as cleanness, little maintenance and no noise. Due to the nonlinear relationship between the current and the voltage of the PV cell, it can be observed that there is a unique maximum power

point (MPP) at a particular environment, and this peak power point keeps changing with solar illumination and ambient temperature.

An important consideration in achieving high efficiency in the PV power generation system is to match the PV source and load impedance properly for any weather conditions, thus obtaining maximum power generation. Therefore, the system needs a maximum power point tracking (MPPT) which sets the system working point to the optimum and increases the system’s output power. It is common that the efficiency of a solar cell is very low. Some methods are used so as to match the source and load properly, thereby increasing the efficiency of solar cell. This is done by utilizing a boost converter whose duty cycle is varied by using an MPPT algorithm.

Maximum power point tracker is an electronic DC to DC converter that optimizes the match between the solar array (PV panels), and the load. The Powerpoint tracker is a high frequency DC to DC converter.

This paper introduces a new index, which designs an intelligent technique by using the method of constant coefficient of short circuit current under pulse width modulation control of the photovoltaic Power system [1-6]. The paper is organized in the following way. Section two presents Nomenclature about everything related to study. In section three presents the entire proposed PV system configuration which components are used and also discuss about the mathematical modeling of the PV array, Maximum Power Point Tracking, analyzing the boost converter. In section four simulation results of numerical experiments under considerations are discussed. Finally, conclusions are made in section five.

2. Nomenclature PV – Photovoltaic. Ipv=I – Output Panel current, (A). Vpv=V – Output Panel voltage, (V). Iph – Photovoltaic Current, (A). Io – Reverse saturation current of the diode, (A). Q – Electron charge (1.602 10-19), (C). Rs – Series resistance of the cell, ( ).Rsh – Shunt Resistance of the cell, ( ).KB – Boltzmann constant (1.38 10-23), (J/K). N – The diode factor.

ISC – Nominal Short-Circuit Current, (A). VOC – Nominal Open Circuit Voltage, (A). VT – Thermal Voltage (V). IMP – Current at the maximum power point, (A). VMP – Voltage at the maximum power point, (V). KI – Constant coefficient of short circuit current. G – Illumination, (W/m2).T – PV cell temperature, (K). toff – On/off duty cycle of the switching controls. PW – Relative pulse width (pulse ratio). P – power generated by the PV array, (W). Pout – Output power, (W). PPV – Input power, (W). D – Duty cycle. Fcn1 – Represent the equation between ISC & G. Fcn2 – Represent the equation between VOC & G.D1,D2 – Diode1, Diode 2. C – Capacter, (F). VO – Output voltage of the DC-DC Boost Converter.

3. Proposed PV systemFigure 1 shows the proposed PV system which is single stage power conditioning

system, used for feeding the DC loads. The PV system consists of different elements like solar PV array, short circuit current MPPT method, boost converter, energy storage element and net load. Here the PV array is a combination of series and parallel solar cells. This array develops the power from the solar energy directly and it will be changed by depending upon the temperature and solar irradiances.

The DC-DC boost converter is controlled so as to track the maximum power point of the PV array and to transfer the energy to the net load [3-5].

Figure 1. Block Diagram of Power Conditioning PV System

3.1. Mathematical Modeling of PV Array The output obtained from the panel is variable DC voltage, this voltage

depends upon the solar radiation intensity and temperature. The simple equivalent circuit of PV cells is shown in figure 2 [2-8].

From the circuit in figure (2) the output panel current can be expressed as Eq. (1).

( ) ( )1S

Bq V I R

NK Tp sh o shI RI eI R

VI.

(1)

Figure 2. Equivalent Circuit of Photovoltaic Cell

Three operation points on the I-V curve are of special interest for understanding solar cell operation and for designing photovoltaic systems:

Short Circuit Current MPPT Method

BoostConverte

rNet

Load

Id

Rs

Vpv

+Ipv

-Isc(Iph )

Rsh

Ish

The short circuit point (I=ISC, V=0); the open circuit point (I=0, V=VOC) and the maximum power point (I=IMP, V=VMP).

At the short circuit operation point (I=ISC, V=0) the solar cell model Eq. (1) Can be rewritten as in Eq. (4), which allows to approximate the photo-generatingcapabilities (Iph) of the solar cell with the short circuit current (ISC) under certain conditions Eq. (2).

,SC ph s sh o phI for R R Iand II . (2)From here we can infer the main factors influencing the voltage of the solar

cell are: the temperature through the thermal voltage (VT) as in Eq. (3), followed to a lesser extent by the irradiance through (Iph).

T BTV = K q ; (3)

(4)

The solar cell operation can be described at the open circuit (I=0, V=VOC) as in Eq. (5), and assuming a high shunt resistance (Rsh), the solar cell open circuit voltage (VOC) can be approximated as in Eq. (6).

(5)

(6)

The resistances Rs and Rsh are usually neglected in order to simplify the model and under certain conditions Eq. (2) (ISC Iph) Therefore, Eq. (1) Can be simplified to Eq. (7) [4-8].

(7)

Under an open circuit condition at the PV array can be expressed as Eq. (8).

(8)

The maximum power point (MPP) is may be the most important parameter relating to PV system performance and operation.

At the maximum power point, where IMPP and VMPP are the Current and Voltage at maximum-power point. From (7) and (8), regarding that exp(V/NVT)>1 under normal operation of the diode, the following expressions can be approximated as Eq. (9).

(9)

The output voltage of the PV generator can be expressed as a function of the output current, in terms of parameters such as VOC and ISC.

(10)

The maximum power point generated by the PV array can be expressed as Eq. (11) and Eq. (12).

(11)

(12)

As initial data for Model there were used experimental data from solar panel educational bench in school laboratory in National Airspace University «KhAI», the Department of space technology and alternative energy sources with Si PV cell manufactured by Siemens Corp [2-3]. The common structure of PV Panel Simulation Model is represented in Figure 3.

Simulation of the I-V curve Fig.4a) & P-V curve Fig.4b) of PV module under changing illumination are represented in Figure 4.

Figure 3. PV Panel Simulation Model

a) b) Figure 4. Simulation of I-V curve a) & P-V curve b) of PV module under

changing illumination

3.2. MPPT Technique There are different methods through which the maximum power point in the P-

V curve can be obtained. The I-V and P-V characteristics of a PV cell depend upon the solar radiation intensity and temperature [2-5].

By controlling the parameters like current or voltage or both the pinnacle can be obtained. Short Circuit Current (SCC). This method represents an indirect approach, The short circuit current (ISC) technique is based on the measurement of the PV module SCC when its output voltage is equal to zero, and the PV module maximum output current at MPP (IMP), is linearly proportional to (ISC) [1-4]. In order to match the two currents, the error current is used to regulate the duty ratio of DC-DC

Rsh

0.001736*u-0.057Fcn1

Voc

s +-

-s

0.0105*u+13.675Fcn 2

D2Rs

C

D1

IscE

E

+VIN

Illumination1

1

2-VIN

converter and the relationship between the PV module output current and SCC at MPP (IMP), which can be described by the following Eq. (13).

MPP I SCI k I . (13)

Where KI is a constant in the range 0.78-0.92, (KI <1) that can be calculated from the PV curve [4-8]. The SCC flowchart is shown in Figure 5.

Figure 5. Flow chart of the (SCC) Method

This method has a disadvantage an undeveloped but a rapid technique of tracking the MPP. To track the power, this MPPT technique requires the value of SCC by isolating the PV array. The power output is not only reduced when finding ISC but also because the MPP is never perfectly matched. A way of compensating KIis proposed such that the MPP is better tracked while atmospheric conditions change. The performance stages of the suggested technique are as follows. It measures the Isc during the start of the MPP tracking. The value of short circuit current is then converted numerically to maximum Power Point current using Eq. (13) [1-4]. After calculating the duty cycle D, the controller reduces the error.

The duty cycle D is used to drive the DC-DC converter and is adopted as the initial point of performance for the constant coefficient of short circuit current. The constant coefficient current method starts tracking the real MPP with very small steps after operating the DC-DC converter at approximated MPP. However, under varying environmental conditions the limit helps the system to Fastly track the MPP [3-10].

Decrease Duty Cycle

Increase Duty Cycle

UpdateReference ?

PV short circuit condition

Measurementof SC

PV workcondition

IMP=KI SC

Measurement of IMP

IMP=Iref

IMP>Iref

No Yes

No

Yes No

Yes

3.3. Pulse Width Modulation (PWM)Pulse width modulation (PWM) is a powerful technique for controlling analog

circuits with a processor's digital outputs. The PWM technique is used to control the closing and opening switches. The switching scheme applied is unipolar. The PWM signal is used to control ON/OFF switching state of the IGBTs (insulated-gate bipolar transistor) will function in driver model that created to control the switching scheme. The duty cycle of a square wave is modulated to encode a specific analog signal level by using a higher resolution counter. The benefit of choosing the PWM over analog control increases noise immunity, which the PWM is sometimes used for communication [2, 4, 5, 10]. The system formation of the PWM is shown in figure 6a. And the systematic formation of the Pulse generation is shown in figure 6b.

a) b) Figure 6. Block Diagram of PWM a) and Pulse Generation b)

3.4. DC-DC Boost Converter In DC-DC Boost converter output voltage (VO) is greater than the input voltage

(VPV) of boost Converter. Consider a boost type converter connected to a PV module with a resistive load as illustrated in Figure 7.

This Figure shows a step up or the PWM boost converter. It consists of a DC input voltage source Vpv; boost inductor L, controlled switch T, diode D, filter capacitor C, and the load resistance RL.

Here at the circuit, we can be observed that when the switch S is in the on state (close the switch S), the current in the boost inductor increases linearly and energy is stored in inductor L and the diode D was off at that time of the output RC circuit [3, 8, 9, 10].

When the switch S is in the off state (opens the switch S), the diode D was on at that time the energy which was stored in the inductor is transferred to a resistive

Pulse GeneratorDDuty Cycle Gate

Switch (T)

Pulse Width Modulation

EIGBT

+VIN 1

-VIN 2 Pulse generator

1period

2*pi

1Duty Cycle

+-

integretor signRem(u(1),u(2))/2/pi

u1

u2

1- (Rem(u(1),u(2))/2/pi)

((Rem(u(1),u(2))/2/pi) +1)/2 1Gate

1s

load through diode D and at the same time inductor current will fall. The role of capacitor in the circuit is for producing a continuous output voltage VO [10].

Figure 7. Circuit diagram of DC-DC boost converter

The power switch is responsible for modulating the energy transfer from the input source to the load by varying the duty cycle D [2, 3, 6, 10].

The relation between output voltage and the input voltage (solar cell) is given as the equation (14):

(14)

4. Simulation results Output power of PV unit is strongly depended from the value of the Relative

Duty Cycle (D), and for each value of net voltage there is corresponded certain value of PW which provides maximum output power (Fig 7a). With increasing of net voltage the values of Duty Cycle (D) increase too. In the same manner for each value of sunshine illumination there is corresponded certain value of Duty Cycle (D) which provides maximum output power (Fig 7b). With increasing of illumination the optimal Cycle (D) decreases.

Thus the task of control of studying the PV unit consists in finding for each moment of optimal Cycle (D) which correspond to changeable external parameters (illumination and net voltage) for providing maximum output power. The values of optimal Cycle (D) are collected in Table 1.

There are also exist the losses of power due to dissipate energy under transformation (Fig. 8a). They are more with Cycle (D) increasing. But all the same, this loss is repaid through increasing of output power.

IL D

S C Vo

VL IDL

Ic IR

a) b) Fig. 7 Output power as function from PW for different net Voltage (a)

and different Illumination (b)

Voltage, Illumination, W/m2

V 600 700 800 900 100014 0.0001 0.0001 0.0001 0.0001 0.000115 0.0001 0.0001 0.0001 0.0001 0.000116 0.0001 0.0001 0.0001 0.0001 0.000117 0.0001 0.0001 0.0001 0.0001 0.000118 0.001 0.0001 0.0001 0.0001 0.000119 0.01 0.0001 0.0001 0.0001 0.000120 0.11 0.04 0.0001 0.0001 0.000121 0.18 0.09 0.001 0.0001 0.000122 0.19 0.15 0.05 0.001 0.000123 0.22 0.17 0.09 0.05 0.0124 0.26 0.18 0.12 0.09 0.0625 0.28 0.19 0.15 0.12 0.0926 0.29 0.24 0.19 0.16 0.1227 0.3 0.25 0.21 0.2 0.1728 0.32 0.28 0.24 0.22 0.1929 0.34 0.3 0.28 0.24 0.2130 0.37 0.33 0.3 0.27 0.24

Table 1. Values of optimal Duty Cycle (D) provided maximum output power

0 0,1 0,2 0,3 0,4 0,5Pulse Width

14 V16 V18 V20 V22 V24 V

Illumination – 800 W/m2

02468

101214161820

0 0,1 0,2 0,3 0,4 0,5Pulse Width

Output Voltage – 17 V

600 650 700750 800 850900 950

Illumination, W/m2

10000

5

10

15

20

25

Regulator transformation efficiency is shown in (Fig. 8b). It was calculated as the ratio between output and input power for certain external conditions.

a) b) Fig. 8 Input/Output power of regulator (a) and relationship between them (b)

The integrated 3D relationship of optimal Duty Cycle from illumination and net voltage is shown in (Fig. 9a).

The integrated 3D relationship of maximum output power (under optimal D) from illumination and net voltage is shown in (Fig. 9b).

a) b) Figure 9. 3D relationship of MP Duty Cycle a) and MP from illumination

and net voltage b)

0

5

10

15

20

25

30

35

0 0, 05 0,1 0, 15 0,2 0, 25 0,3 0, 35 0,4 0, 45

PpvPout

Illumination – 1000 W/ 2

Net voltage – 21 Vm

Duty Cycle

0,820,840,860,880,9

0,920,940,960,98

0 0,1 0,2 0,3 0,4 0,5 0,60

Illumination – 1000 W/ 2

Net voltage – 21 Vm

Duty Cycle

051015202530354045

Conclusion This study presents an energy-efficient, fast-tracking MPPT circuit PV energy

reaper. Firstly, it presents the characteristics of the PV system and mathematical model. The maximum power point tracking (MPPT) strategy based on the constant coefficient of the short circuit current method is proposed. The results gained from simulation employing short circuit current approach display the effectiveness of the proposed power tracking and control strategies with quick power tracking response and well direct current output.

However, by using this MPPT method we have increased efficiency. This method computes the maximum power and controls directly the extracted power from the PV. The proposed method offers different advantages which are: good tracking efficiency, response well high and controls for the extracted power.

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