EJSR_53_2_09

European Journal of Scientific Research ISSN 1450-216X Vol.53 No.2 (2011), pp.231-238 © EuroJournals Publishing, Inc. 2011 http://www.eurojournals.com/ejsr.htm

Modeling, Analysis and Simulation of Pumping System

Fed by PV Generator

N. Chandrasekaran Asst.Prof., PSNA College of Engg.&Tech, Dindigul

K. Thyagarajah

Principal, KSR College of Tech, Tiruchengode

Abstract

The limited resources and high cost of conventional energy sources like oil, gas, coal, as well as the continued environmental pollution are some of the main reasons for their widespread introduction in electrical power systems. The development and usage of these alternative energy technologies are still restricted. Solar energy has the greatest potential of all the sources of renewable energy. In order to make solar energy competitive with the other forms of renewable energies, a better exploitation of its advantages especially environmental side, this will be possible only with the development of the less expensive and high output efficiency systems. The objective of this work is to bring a contribution to the study of the behaviors of the photovoltaic generators and converters used to feed a well defined load. In this case an asynchronous machine actuating a centrifugal pump is considered. This can be achieved through modeling and simulation of the various stages that constitute the overall system.

1. Introduction Energy has become an important and one of the basic infrastructures required for the economic development of a country. Energy security is imperative for sustained growth of economy.

The concern for environment, due to excessive use of fossil fuels, has led to a remarkable global effort to harness alternative energy resources. The renewable energy resources, such as, solar, wind, biomass, geothermal, etc. are environment friendly and perennial in nature [1].

Solar energy can be a major source of power. Its potential is 178 billion MW which is about 20,000 times the world’s demand. So far it could not be developed on a large scale because of large space requirement, uncertainty of availability of energy at constant rate, due to clouds, winds, haze, etc.

Utilization of solar energy is of great importance to India since it lies in a temperature climate of the region of the world where sun light is abundant for a major part of the year.

The applications of solar energy which are enjoying most success today are solar water heating, solar cookers, food refrigeration, solar furnaces and solar photovoltaic (PV) cells. In this work, solar photovoltaic cells, which can be used for conversion of solar energy directly into electricity for water pumping in rural agricultural purposes is concentrated.

Modeling, Analysis and Simulation of Pumping System Fed by PV Generator 232

Figure 1: Block diagram of water pumping system

The energy conversion in photovoltaic system for the application in a water pump system is shown in fig 1.It permits solarly generated electrical power to be delivered to the load in our case an electric pump. It consists of solar array, DC-DC converter, battery storage, inverter and load.

Solar array may be tracking arrays or fixed arrays. A tracking array is defined as one which is always kept mechanically perpendicular to the sun-array line so that all times it intercepts the maximum isolation. A fixed array is usually oriented east west and tilted up at an angle approximately equal to the latitude of the site. Fixed arrays are mechanically simpler than tracking arrays. A DC – DC Converter can be considered as DC equivalent to an AC transformer with a continuously variable turns ratio. Since the output of the solar array is very less, a cuk regulator is used to increase the output voltage level. The cuk regulator provides an output voltage which is less than or greater than the input voltage[7]. The input current of this regulator is continuous. It has low switching losses and has high efficiency. The solarly generated electric energy may be stored in the battery storage. It provides continuous power supply to the load without interruption. Inverter is a solid state circuit which converts the battery bus voltage into AC of required frequency and phase to match that needed to integrate with the utility grid or to a frequency sensitive load like AC motors[10]. The configurations of inverters are 120 degree conduction mode and 180 degree conduction mode. Among these two configurations, the inverter with 180 degree conduction mode is used to get its own technical advantages. 2. Modelling of Photovoltaic Array The direct conversion of sunlight into electric power, without any intermediate action, is defined as being the photovoltaic effect. The solar cell is the basic element in charge of the solar energy transformation into electric power[9]. Fig. 2 represents the simplified equivalent electric circuit of a photovoltaic cell.

In order to get a precise modeled output, instead of using circuit model directly Mathematical model of PV array is suggested in most of the literature. And this work has used the following mathematical model of a solar cell.

Figure 2: Equivalent electric circuit of a photovoltaic cell.

233 N. Chandrasekaran and K. Thyagarajah

The classical equation of a PV cell describes the relationship between current and voltage of the cell (neglecting the current in the shunt resistance of the equivalent circuit of the cell) as,

⎥⎥⎦

⎤

⎢⎢⎣

⎡−⎟⎟

⎠

⎞⎜⎜⎝

⎛ −−= 1exp0 A

IRVIII pvsepv

Lpv

Assuming,

1exp ⟩⟩⟩⎟⎟⎠

⎞⎜⎜⎝

⎛ +

AIRV pvsepv

90 10−≈scI

I

IL = Isc Therefore, Ipv = Isc – Id

Where, ( )⎥⎦

⎤⎢⎣

⎡+= −

pvsepvoc

scd IRVV

II 7.20exp10 9

Vpv =Voc The PV generator consists of solar cells connected in series and parallel fashion to provide the

desired voltage and current required by the load.

Figure 3: Voltage- power Characteristics of PV generator

This PV generator exhibits a nonlinear voltage-current characteristic that depends on the insulation (solar radiation). The PV generator Voltage-Current and Voltage-Power characteristics at five different values of G are shown in Fig. 3. (G is the solar insolation in per unit)From which, at any particular value of G, there is only one point at which the PV generator power is maximum[8]. This point is called the Maximum Power Point (MPP) [2]. To locate its position, the corresponding voltage (Vmg) and current (Img) must be determined first. If the externally applied voltage is in the reverse direction, say during a system fault transient, the current remains flat and the power is absorbed by the cell. However, beyond a certain negative voltage, the junction of PV cell breaks down as in a diode and the current rises to a high value. In the dark, the current is zero for voltage up to the breakdown voltage which is the same as in the illuminated condition.


3. Modeling of DC/DC Converter DC converters can be used as switching-mode regulators to convert a DC voltage normally unregulated to a regulated DC output voltage. The regulation is normally achieved by PWM at affixed frequency and the switching device is normally BJT, MOSFET or IGBT. The output of DC converters with resistive load is discontinuous and contains harmonics. The ripple content is normally reduced by an LC filter [3]. Switching regulators are commercially available as integrated circuits. The designer can select the switching frequency by choosing the values of R and C of frequency oscillator. The transistor switching loss increases with the switching frequency and as a result the efficiency decreases [4]. In addition, the core loss of inductors limits the high frequency operation. There are four basic topologies of switching regulators namely, Buck regulators, Boost regulators, Buck-Boost regulators, Cuk regulators. For the application taken up here used the Cuk regulator as a voltage regulator. It is similar to the buck-boost regulator i.e., it provides an output voltage that is less than or greater than the input voltage, but the output voltage polarity is opposite to that of the input voltage.

The capacitive accumulation chopper shown in allows the supply of a current load starting from a current source, where the capacitor C1 plays the role of an auxiliary source of voltage. During the charging of C1, there is a current generator outputting in voltage receiver, as in a parallel chopper. When C1 discharges, it constitutes a voltage generator outputting in a current receiver, as in a series chopper. The circuit is equivalent to a cascade of parallel and series choppers, from where its name Cuk Regulator converter [3]. For the modeling of the Cuk Regulator chopper, a model has been selected. It has the input voltage from the voltage of the photovoltaic generator, with the output voltage of the chopper used to supply the voltage inverter [2]. The circuit diagram of cuk regulator is as shown in Fig.4.

Figure 4: Cuk regulator

It is supposed that the losses on the level of the chopper are negligible and Power input = Power output

Therefore, the equation which governs the chopper is given by

⎟⎠⎞

⎜⎝⎛−

=d

dVV sa 1

Where, Va, - Output voltage Vs- Voltage of the photovoltaic generator d - Cyclic ratio When the switch ‘S’ is off, the inductor currents iL1 and iL2 flow through the diode. The

capacitor C1 is charged through the diode by energy received from both the input and L1. Load current decreases, because Vc1 is larger than Vd. Energy stored in L2 feeds the output. Therefore, iL2 also decreases [3]. When the switch is on, VC1 reverse biases the diode. The inductor currents iL1 and iL2 flow through the switch. Since VC1 > Vo, C1, discharges through the switch, transferring energy to the


output and L2 and therefore, iL2 increases. The input feeds energy to L1 causing iL1 to increase. The inductor currents iL1 and iL2 are assumed to be continuous. The main aspects in the design of cuk regulator are to design the various components in the circuit for the required output voltage in accordance to the given input voltage [7].

In this paper, the input voltage to the cuk regulator is the output voltage derived from the PV array model. In the following equations, by changing the duty ratio of the DC – DC converter, the net value of all the components and the corresponding output voltage also been changed. The governing equations of cuk regulator are as follows [6].

( )2

1

12- k R

L =kf

( )2

12

k RL

f−

=

1 2kC =fR

21

8C =

fR

Where, R - Resistance of the load in Ω. f - Switching frequency in Hz; k - Duty ratio. With above all equations, the cuk regulator is modeled and the same model is simulated using

MATLAB SIMULINK. The output of the regulator is matching with the modeled cuk regulator output. In this regulator, both the input current and the current feeding the output stage are highly ripple free unlike the buck – boost converter where both these currents are highly discontinuous. It is possible to simultaneously eliminate the ripples in IL1 and iL2 completely, leading to lower external filtering requirements [4]. 4. Modeling of Inverter DC to AC converter is known as inverter. The function of an inverter is to change a dc input voltage to a symmetric ac output voltage of desired magnitude and frequency. The output voltage could be fixed or variable at affixed or variable frequency. A variable output voltage can be obtained by varying the input dc voltage and maintaining the gain of the inverter constant. On the other hand, if the dc input voltage is fixed and it is not controllable, a variable output voltage can be obtained by varying the gain of the inverter, which is normally accomplished by pulse-width-modulation control within the inverter. The inverter gain may be defined as the ratio of the ac output voltage to dc input voltage [5]. Three phase inverters are normally used for high power applications. Three single phase half bridge inverters can be connected in parallel to form the configuration of a three phase inverter. The gating signals of single phase inverters should be advanced or delayed by 120 degrees with respect to each other to obtain three-phase balanced voltages. A three phase output can be obtained from a configuration of six transistors and six diodes as shown in Fig.5. Two types of control signals can be applied to the transistors: 180° conduction or 120° conduction. The 180° conduction has better utilization of the switches and is the preferred method.[6] In 180o mode each transistor conducts for 180°. Three transistors remain on at any instant of time.

There are six modes of operation in a cycle and the duration of each mode is 60 °. The transistors are named in the sequence of gating the transistors.


The gating signals are shifted from each other by 60° to obtain three phase balanced voltages. The load may be connected in star or delta. The switches of any leg of the inverter cannot be switched on simultaneously; this would result in a short circuit across the dc link voltage supply

Figure 5: Three phase inverter

Similarly, to avoid undefined states and thus undefined ac output line voltages, the switches of any leg of the inverter cannot be switched off simultaneously; this can result in voltages that depend on the respective line current polarity. 5. Simulation and Results The mathematical model of PV cell is simulated using MATLAB simulation as shown in fig.6.

Figure 6: PV Cell Model

From the above simulink model of solar cell, the following open circuit voltage and the corresponding current are obtained. It can be observed from the simulation result that 19v dc is developed by the solar cell.

Figure 7: Simulation output of PV Cell


The simulink block set arrangement of pumping system fed by the combination of cuk regulator and inverter is shown below.

Figure 8: Simulation model of pumping system

The above shown simulink configuration consist of Cuk regulator, three phase inverter and induction motor. For simplifying the power stage with minimum number of components and for achieving maximum performance, the cuk regulator is preferred over other regulators. The cuk regulator is here used in boost operation model. The dc voltage received from solar cell is boosted up to the required level and then fed into three phase inverter.

The parameters of three phase induction motor are shown in the following table.

Parameters Type / Value Rotor Squirrel cage Reference frame Rotor Nominal Power 3 HP Line – Line Voltage 415 Frequency 50 Stator Resistance 0.435 Stator Inductance 2mH Rotor Resistance 0.816 Rotor Inductance 2mH Mutual Inductance 69.31mH Number of poles 4 Inertia 0.089 Friction factor 0

The waveform in fig.9 shows the simulated output of the combination of cuk regulator, inverter

and induction motor. The voltage received from solar cell is 19v dc which is fed to cuk regulator. The output voltage of cuk regulator is 240v dc which will be the input to the inverter. The induction motor receives 415v ac from inverter for its operation to which the pump is connected. The first two simulation outputs show the line voltage and currents. The next two wave forms are the variation of torque and the speed with respect to time. The system reaches to the speed of 1500 rpm at 0.15 secs.


Figure 9: Simulation outputs of pumping system

6. Conclusion This work paved a path on the modeling and simulation of the photovoltaic pumping systems using Simulink under MATLAB programming environment. The various components like PV array, cuk regulator and inverter of Photovoltaic pumping systems have been modeled with practical data and it is validated using MATLAB. The results obtained from the simulation of the system are satisfactory. This work will be a contribution to the analysis of the photovoltaic pumping system with regards to the results of simulation of the model. A reliable and simple scheme of PV array has been successfully developed for the first time to supply a three-phase remote load of induction motor with constant frequency balanced voltages. References [1] Benlarbi K , Mokrani L , Nait-Said MS. A fuzzy global efficiency optimization of a

photovoltaic water pumping system . Solar Energy 77 (2004); 203–216 . [2] Betka A, Moussi A.Performance optimization of a photovoltaic induction motor pumping

system .Renewable Energy 29 (2004); 2167–2181. [3] Arrouf M,Ghabrour S.Modelling and simulation of a pumping system fed by photovoltaic

generator within the Matlab/Simulink programming environment. Desalination 209 (2007) ;23–30.

[4] Muhammad H.Rashid.Power Electronics circuits,devices,and Applications.3rd edition.Pearson Education,2004.

[5] Nejib Hamrouni, Moncef Jraidi ,Adnene Cherif .Theoretical and experimental analysis of the behaviour of a photovoltaic pumping system. Solar Energy 83 (2009) ;1335–1344.

[6] Bhim Singh, Putta Swamy CL,Singh BP.Analysis and development of a low-cost permanent magnet brushless DC motor drive for PV-array fed water pumping system.Solar Energy Materials and Solar Cells .51 (1998); 55-67.

[7] Singh MD,Khanchandani KB.Power Electronics.2ne edition. Tata McGraw-Hill Publishing Company Limited,2007.

[8] Odeh I , Yohanis YG, Norton B.Influence of pumping head, insolation and PV array size on PV water pumping system performance .Solar Energy 80 (2006) ;51–64.

[9] Betka A, Attali A.Optimization of a photovoltaic pumping system based on the optimal control theory. Solar Energy 84 (2010); 1273–1283.

[10] Mezghanni D, Andoulsi R, Mami A, Dauphin-Tanguy G.Bond graph modelling of a photovoltaic system feeding an induction motor-pump.Simulation Modelling Practice and Theory 15 (2007); 1224–1238.

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