Abstract-- In recent years, renewable energy sources have become most popular day by day to generate electrical energy. Among all other types of renewable energy sources, solar is one of the best alternatives for conventional sources as it offers more advantages. This paper is mainly concentrated on the area of charge controller in solar PV power conditioning system. In this paper, a P&O method MPPT charge controller is designed and it is converted into digital domain to use in FPGA. Compared to analog controllers, digital controller offers more advantages like high accuracy, less effected by noise etc., MPPT charge controller is designed to perform 1) to convert the variable DC electrical energy which is coming from solar into fixed DC and 2) to track the maximum power from the solar PV module under all operating conditions. This fixed DC can be used to feed the DC loads or charge the batteries. Keywords- Solar, PV, solar PV module, MPPT, DC-DC Boost converter, P&O method, FPGA.

I. INTRODUCTION One of the major concerns in power sector, the power

demand is increasing day-by-day but the unavailability of enough resources to meet the power demand using the conventional primary energy sources. The generation of energy from the use of the non-renewable energy sources causes pollution problems and also these sources are not available for long time. Our researchers and engineers are focused on these problems and they have found that there are many alternative energy sources can be used instead of fossil fuels.

After the search for the alternative ways, the solar photovoltaic (PV) system and wind power generation systems are found to be more advantageous compared to all different types of renewable energy sources for the desirable environmental and safety aspects. Among these two renewable sources, solar power systems attract more attention [1]. Solar energy sources has its own advantages like long life with little maintenance, high power capability per unit of weight and Tripti Saini is with the Department of Electrical & Electronics Engineering, Graphic Era University, Dehradun-248002, India (e-mail: tripti.saini@gmail.com).

Dogga. Raveendhra (Assistant Professor) is with the Department of Electrical & Electronics Engineering, Graphic Era University, Dehradun-248002, India (e-mail: doggaravi09@gmail.com)

Padmanabh Thakur (Associate Professor) is with the Department of Electrical & Electronics Engineering, Graphic Era University, Dehradun-248002, (e-mail: tonu_arth@rediffmail.com).

978-1-4673-5630-5//13/$31.00 2013 IEEE

highly mobile. In the solar photovoltaic, solar cell is the basic element which converts the solar energy into the electrical energy. To form solar PV module there are combination of several solar cells need to be used. The output of solar cell depends on the weather conditions (temperature and irradiation), which are fluctuating in nature. Under normal operating conditions, each solar PV module offers unique maximum operating point. If solar PV module operates at that optimum operating point, can get maximum efficiency at the load. To achieve good efficiency from the solar PV system there is a need to extract maximum power from module. This can be achieved by placing MPPT charge controller as a part of power conditioning system between solar PV module and DC load.

The solar energy comes from the sunlight in the form of solar irradiation and temperature [2]. It converted variable DC by using solar PV module. The output of solar cell depends on the temperature, insulation, spectral characteristics of sunlight and so on. The energy conversion efficiency of solar PV array is very low and thus, the maximum power point tracking (MPPT) control technique is required to extract the maximum power from the solar PV array in order to increase the overall efficiency of the solar PV system [3].

Every photovoltaic solar cell has optimum operating point which is called maximum power point (MPP). This maximum power point can varies if there will be any variation in the cell temperature and irradiation level. MPPT tries to match the impedance of the module with the load/battery by changing the duty ratio of the DC-DC converter. In this paper 24V solar PV module is modeled with the help of mathematical equations in the MATLAB. For this designed solar PV module, DC-DC boost converter has to be designed properly to attain high DC bus voltages. In this paper 24V to 100V DC-DC boost converter is designed and it is controlled with P&O method MPPT controller. This P&O method MPPT controller is implemented in digital domain to use in FPGA. This FPGA based MPPT controller is converts variable DC into fixed DC of 100V magnitude.

II. PROPOSED SYSTEM Solar PV system is combination of several components like

solar PV module, power conditioning system and battery or load. Solar PV module is formed by connecting group of solar PV cells in series or parallel depends upon voltage or current requirements whenever there is need of high voltages, there is a need of series connected solar PV modules similarly for the requirement of high currents there is a need of parallel

Stability Analysis of FPGA Based Perturb and Observe Method MPPT Charge Controller for

Solar PV System

Tripti Saini, Dogga Raveendhra and Padmanabh Thakur

connected solar PV modules. These arrangements can provide variable DC supply which is function of both temperature and irradiation. This variable DC should be converted into fixed DC by using MPPT based boost converter. Generally boost converter is preferred in solar PV System rather than buck converter because of availability of voltage from solar module is low in solar PV system. This boost converter is a part of power conditioning system (PCS).

Usually Power Conditioning Systems (PCS) plays major role in solar PV systems. These are designed with the help of power electronic devices. PCS is used to manage the electrical energy effectively and to provide high quality maximum power from solar PV module. There is used a boost DC-DC converter to get the desired high level DC output voltage from variable low level DC power supply like solar PV module. This proposed solar PV system provides constant DC Supply by using the FPGA based MPPT and the DC-DC boost converter.

As we mention earlier that solar PV system is the combination of several elements like PV module, power condition elements and DC load. As shown in figure-1, the circuit diagram there is two systems one is PV Module and another one is MPPT based DC-DC boost converter. This MPPT algorithm is implemented in FPGA to control the switch of MOSFET in DC-DC converter.

Fig.1 Proposed FPGA based MPPT charge controller

A. Model for solar PV system In the figure-2 there is show a simple equivalent

circuit diagram for solar PV system. In the circuit, it includes a photo current source, a diode, a load, a series resistor and a parallel resistor.

Fig.2 Solar PV system

The PV cell output current can be defined as follow in equation 1.

I = Iph Id (eVd/nVt - 1) (Vd/Rsh) (1)

Iph = Short-circuit current due to sunlight Id = Diode current R = Resistance load Rs = Series resistance

Rsh= Shunt resistance q = Electron charge (1.6 e-19C) k = Boltzmanns constant (1.38 e-23J/K)

B. Maximum power point tracking In the solar PV system, the efficiency is very low because of

solar PV module provides very low efficiency. The method which is used to increase the efficiency of the solar PV system by tracing the maximum power point, such a method is called maximum power point tracking (MPPT). By using this technique, one can get the maximum possible power from the solar PV system. In other words MPPT is a device which tries to match the impedance of the module with that load or battery to extract the maximum power. Resultantly, various MPPT techniques with different characteristics performance have been proposed in past years [4]. Some methods of them are listed below [5].

Perturb & observe method Incremental conductance Fraction open circuit voltage Fractional short circuit current Fuzzy logic control Neural network

Fig.3 MPPT charge controller

Among all methods, P&O method is chosen to draw maximum power from the solar PV module and it is implemented in digital domain.

C. Perturb and observe method Perturb and observe method is most common method of

MPPT. In P&O method, the MPPT algorithm is based on the calculation of the PV output power and the power change by sampling both the PV current and voltage [6]. The most basic purpose of the perturb & observe algorithm, it adjust the operating voltage of photovoltaic solar system and observe whether the output power of the system is increase or decrease. If it observes that the output voltage is increase, the algorithm will continues to adjust the output voltage in the same direction. If it observes that the output voltage is decrease, the algorithm adjusts the output voltage in the opposite direction.

Fig.4 Flowchart of the P&O algorithm

D. DC-DC Boost converter The main purpose of DC-DC boost converter, it converts

low level DC voltage into high level DC voltage [7]. Generally DC-DC boost converter operates in two modes either in charging mode or discharging mode, which are based on the closing and opening of the switch respectively. Inductor, a diode, and a high frequency switch are the main elements of the DC-DC boost converter [8]. In the figure-5 there is shown a basic circuit of the boost converter.

Fig. 5 DC-DC Boost converter circuit

Charging mode-: In the charging mode of operation, the switch is closed and inductor gets start charging by the source through the switch. In this mode diode is reverse biased. The diode restricts the flow of the current from source to load [9].

Discharging mode-:In the discharging mode of operation, the switch is open and inductor start discharges and together the source charges the capacitor and meets the load demands. The variation of the load current is very small and sometime in many cases is assumed constant throughout the operation of the DC-DC boost converter.

E. FPGA (Field Programmable Gate Array) A Field Programmable Gate Array (FPGA) is pre-fabricated

device that can be electrically programmed to become any kind of digital circuit. It is an integrated circuit which is an array of logic cells. It can be made or designed as requirement of users.

In this paper the proposed system is controlled with the help of FPGA i.e., the MPPT Perturb & Observe (P&O) method is programmed in VHDL by using Xilinx space system generator to control the DC-DC boost converter.

III. SIMULATION RESULTS In this paper, 100V MPPT charge controller is designed with

the help of small signal analysis and presented simulation results. 100V DC Solar PV system MATLAB Simulink model is shown in figure-6. In this model, solar PV module is modeled in MATLAB 2008a with help of mathematical equations. The left portion of the MATLAB Simulink represent the solar PV system, right portion represents DC-DC boost converter and down portion represents the MPPT Perturb & Observe (P&O) algorithm. Irradiation, Insulation and Short Circuit currents are taken as inputs to this solar PV module model. From this model variable DC is obtained. By this variable DC we get the maximum power by using the maximum power point tracking (MPPT) [10].

Fig.6 MATLAB Simulink

To design DC-DC boost converter properly with good load and line regulations small signal analysis is used. From this analysis, obtained small signal plant transfer function and controller transfer functions are shown in below for 24V to 100V conversion.

A. Specifications of PV System Temperature variations range of PV System (T) =

(293K-307K) Irradiance (G) =1000W/m2

DC-DC Boost converter is designed for 24 to 100V with the following specifications-:

Drain to source resistance rDS = 0.1800 Inductor L = 7.0000e-004H

Inductor Internal resistance rL = 0.1900 Capacitance C = 4.7000e-005F Capacitor ESR rC = 0.1110 Diode Forward resistance RF = 0.0720 Input Voltage Vi = 24V Output Voltage Vo = 100V Output Power Po = 150W Switching Frequency Fs = 10kHz Ts = 1.0000e-004Sec Duty Ratio D=0.5 Output Current Io = 1.0417A

B. 24 to 100v, 150w Boost Converter Small Signal Transfer Function analysis

From the specification of DC-DC boost converter, Plant Transfer function (Tp) is given by

..... (2)

The Plant Transfer function of DC-DC boost converter is having negative coefficients in numerator. To make it stable, design a charge controller in z-domain which is given by

.... (3)

Equation 3, represent the controller for boost converter in z-domain. After designing of charge controller, bode plot has been drawn for the loop gain of closed loop DC-DC boost converter is shown in figure-11. From this bode plot, observed that the gain margin of the charge controller is 3.92o at the phase crossover frequency 2.12e5 Hz. And Phase margin of the charge controller is 57.5o at the gain crossover frequency 1.26e4 Hz. From these frequency domain specifications i.e., PM and GM are observed that positive. Therefore, the designed closed loop DC-DC converter is this PV system is fully stable.

Fig. 7Changes in Irradiation

From the MATLAB Simulink, get the waveform of changes in irradiation and changes in temperature are shown in fig.7 and Fig. 8 respectively.

Fig.8 Changes in Temperature

The simulation result of photovoltaic module current and voltage is shown in figure-9 and 10 respectively. Gate pulses from FPGA are obtained to control DC-DC boost converter shown in figure-12. And the simulation result DC-DC boost converter current and voltage is shown in fig.13 and 14.

Fig. 9 Photovoltaic Current

Fig.10 Photovoltaic Module Voltage

Fig. 11 Loop Gain Bode Plot of closed loop DC-DC Boost converter

Fig. 12 Gate Pulses from FPGA to control DC-DC Boost Converter

Fig. 13 Current of DC-DC Boost Converter

Fig. 14 Current of DC-DC Boost Converter

IV. Conclusion This paper presents the complete analysis of small signal

mode DC-DC Boost Converter and MATLAB Simulations of

FPGA Based Perturb and Observe Method MPPT Charge Controller Implementation for Solar PV System. Designed FPGA Based Perturb and Observe Method MPPT Charge Controllers Loop gain is found to be 57.5o Phase margin, which is fully controlled and stable for line and load variations.

REFERENCES [1] Azadeh Safari and Saad Mekhilef, Simulation and Hardware

Implementation of Incremental Conductance MPPT With Direct Control Method Using Cuk Converter, IEEE Trans. Ind. Electron., Vol. 58, no. 4, April 2011.

[2] Kinal Kachhiya, Mukesh Patel, Makarand Lokhande, MATLAB/Simulink Model of Solar PV Module and MPPT Algorithm, National Conference on Recent Trends in Engineering & Technology, 13-14 May 2011.

[3] Hanja Cha and Sanghoey Lee, Design and Implementation of Photovoltaic power conditioning system using a current based maximum power point tracking, Industry Applications Society Annual Meeting, 2008. IAS 08. IEEE, vol., no., pp,.1-5,5-9 Oct. 2008.

[4] Caston Urayai and Gehan A.J.Amaratunga, Single Sensor boost converter-based maximum power point tracking algorithms, Applied Power Electronics conference and Exposition(APEC), 2011 Twenty Sixth Annual IEEE vol., no., pp.1238-1243 6-11 March 2011.

[5] Subudhi, B., Pradhan,R., A comparative study on maximum power point tracking techniques for photovoltaic power system, Sustainable Energy, IEEE Transactions on, vol.4, no.1, pp.89-98, Jan.2012.

[6] Mohamed Azab, A New Maximum Power Point Tracking for Photovoltaic Systems, International Journal of Electrical & Electronics Engineering, 3:11 2009.

[7] Raveendhra, Dogga.; Pathak, M.K.; Panda, A.; "Power conditioning system for solar power applications: Closed loop DC-DC convertor fed FPGA controlled diode clamped multilevel inverter," Electrical, Electronics and Computer Science (SCEECS), 2012 IEEE Students' Conference on , vol., no., pp.1-4, 1-2 March 2012.

[8] Grzesiak,W., MPPT Solar Charge Controller for High Voltage Thin Film PV Modules, Photovoltaic Energy Conversion Conference Record of the 2006 IEEE 4th World Conference on,vol.2,no.,pp.2264,2267, May 2006.

[9] Daohang Wang, A Method for Instantaneous Measurement of PV V-I Characteristics and its Applications for MPPT Control, Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE, vol.,no.,pp.002904,002907,20-25 June 2010.

[10] Yuncong Jiag, Abu-Qahoug, J.A, Multiple Solar Panels Maximum Power Point Tracking Using the output Current, Telecommunication Energy Conference (INTELEC), 2011 IEEE 33rd International, vol.,no.,pp.1,5, 9-13 Oct, 2011.

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