MATLAB/SIMULINK Modeling of photovoltaic water pumping systems without energy storage....
CHARACTERIZATION OF PHOTOVOLTAIC PUMPING SYSTEM MODEL WITHOUT ENERGY STORAGE BY MATLAB/SIMULINK 1
A. BOUSSAIBO
1,2,3
1
M. KAMTA
J. KAYEM
2
University of Ngaoundere/ Cameroon, IUT, Department of Electrical Engineering University of Ngaoundere/ Cameroon, ENSAI, Department of Process Engineering
[email protected]
2
3
3
D. TOADER
S. HARAGUS
3
4
N. MUNTEAN
Polytechnic University of Timisoara/ Romania, Fundamental of Physics for Engineers Department 4 Polytechnic University of Timisoara/Romania, Electrical Engineering Department
[email protected]
Abstract: In this paper, we study photovoltaic power
source without energy storage, but applying an almost stable voltage across a single phase induction motor which drives a centrifugal pump. The centrifugal pump supplies water at a flow rate and pressure set by the operator. The simulated model is a system PV Boost/MPPT/MLI-Inverter/MLI-Motor-Pump. The whole system is simulated in MATLAB / SIMULINK software with ode23t solver and variable step size. An algorithm allows the extraction of a maximum power of the PV generator and to maintain the Boost-converter output voltage almost constant through systematic lock duty ratio in a given interval. The PWM control of the inverter is operated, and with an input frequency, the operator can fix the rotational speed of the pump. Due to the control law V/f = C, the torque is kept constant, thereby allowing the pump to deliver water at the frequencies less than the rate value. The limits values of the speed and pressure below which the flow of water pumped is zero are defined. Keywords: Photovoltaic, Centrifugal pump, MPPT.
INTRODUCTION
In this work, we chose to explore the possibility of transferring the maximum power of the PV generator to a centrifugal pump in charge of feed a filtration unit with fixed water flow and pressure. And secondly, it is a matter of studying the behavior of the entire system without battery batter y storage. The use of electric pumps in rural areas in subsaharan Africa remains hampered by the lack of connected grid power supply. In this part of the continent, 92 % of the rural populations have no access to electricity [1]. This large deficit of electricity supply in the rural area is due to the prohibitive cost of extending the conventional network [2].
In the interest of improving the living conditions of the populations, the research for alternative energy resources has become a priority [3]. For this purpose, the Sahel has a very important potential solar energy, estimated at 7 kWh / m² / day [4]. The use of this solar resource for the production of electricity is well adapted to rural areas and especially isolated [4-5]. However one of the major drawbacks of photovoltaic generator, is the dependence on weather conditions which have a very random behavior and sometimes very severe for photovoltaic modules. Added to this, the use of batteries in the system becomes costly in the short or long term because of their life very closely linked to the conditions of use. Several studies are being conducted to improve the efficiency of photovoltaic systems. The maximum power transfer is the most studied by various authors [6]. MPPT algorithms are well developed for the control of static converters, which couples the PV generator and the load. Among these algorithms, the most common are: perturb and observe, incremental conductance, fuzzy logic, etc. [7-8]. Otherwise, the automatic control of the switches of the inverter by the Pulse Width Modulation is implemented to allow the pump to run at low speed but with high torque through the constant V/f ratio when the generator power decreases. However, it is important to use this law in the filtration application which required fixed water flow and pressure in a specific interval. In this paper, the desired is, in the one hand to allow the operator to set the flow rate and pressure required, and to maintain a nearly constant voltage at the input of the inverter without using the battery on the other hand. In the paragraphs
that follow, using the algorithm "disturbed and observe (P & O)," a model in MATLAB / Simulink of the entire system through its various sub-parts is proposed. Model of the PV-generator
The photovoltaic generator has a current / voltage characteristic, strongly nonlinear direct result of the behavior of semiconductor junctions which are the basis of its realization [9]. Considering the electrical model of a diode PV cell shown schematically in fig. 1, the current generated by the cell is given by Kirchhoff's law:
= − −
(1)
Table 1. Features of PV Module Photovoltaic H750 Helios module parameters
Short-circuit current at STC Open circuit voltage at STC Current at MPP Voltage at MPP Maximum power at STC Number of series-connected cells cells
4.01 A 21.6 V 3.47 A 17.3 V 60 Wp 36
The diagram in fig. 2 shows the Simulink model of a single module compiled from expressions (2) and (4). and are resolved in Matfile from their following analytical expressions:
− = . . −
(5)
expression is obtained by solving a system of
Fig. 1. PV cell model
The current of the cell is linked to the sunlight and temperature by the following expression [10]: = + ( − ) (2) Where is the temperature coefficient, , the cell temperature and , the temperature in standard operating conditions; , the radiation /2 received by the cell and 0 radiation in standard conditions. The diode current is given by the formula [11]: = +∗ − 1 (3)
equations, derived from (4), where on one hand, the current and voltage are replaced by their values at the maximum power point, and on the other hand, the current is replaced by the short-circuit value and the voltage by the open-circuit value. is the saturation current of the diode, and strongly related to temperature [10]. It is given by equation (6): 3
1 1 = . − −
(6)
1.38 10−23 / is the With = . Where = 1.38 Boltzmann constant, the cell temperature in Fig. 2. Model Simulink of PV module Kelvin, n is the ideality factor of the diode and 1.602 2 10−19 is the electron charge. = 1.60 Model of the Boost-Converter The magnitude of the current and voltage produced Boost-converter makes it possible to efficiently by a single cell is very low. l ow. It is important i mportant to involve convert a DC voltage from a lower level to a higher them in series and in parallel in order to have level [12]. The relationship between the duty ratio sufficient value for our application. In our case, the and the electrical parameters of the circuit are given number of cells in series per module is 36 according by: to the manufacturer's specifications as presented in = 1− (7) Table 1 below. The series combination of three rows of six modules in parallel is the PV generator needed = (1 − ) (8) for our application. If we denote by , the number of cells in series per module, the formula (1) takes The switch is an IGBT. Diode D is chose to offer the following form: a very small recovery period with a minimum voltage + = − ∗ − 1 (4) drop across it during ON period. The expressions (9)
and (10) are respectively inductor and capacitance [13-14]. They are calculated at the boundary operation conditions of the circuit.
= .∆ = .∆
Initialization
(9)
Read P
(10)
ΔP=P-P_old
We opted to use the SimPowerSystem blocks to model the Boost-converter as shown in fig.3. A Controlled Voltage Source is used to convert the Simulink signal from PV generator to electrical signal applied to the Boost-converter. The command signal of the Insulated Gate Bipolar Transistor (IGBT) is obtained through a comparator with the input of which a triangular signal is injected at a frequency of 100 kHz and a duty ratio. This duty ratio is supplied by a MPPT control algorithm described later. Iin
reads the instantaneous power delivered by the generator.
Iou
Vin
Vout
Fig. 3. Model Simulink of Boost-
N
N
N
N
Y
D_old>D
D_new=D-ΔD
Y
ΔP>0
D_new=D+ΔD
D
