A MATLAB-Simulink-Based Solar Photovoltaic Array (SPVA) Module with MPPT Ravi Nath Tripathi , Alka Singh, Manoj Badoni Department of Electrical Engineering Delhi Technological University, Delhi, India
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Abstract: The performance of a solar photovoltaic array (SPVA) is dependent upon the temperature and irradiance level and it is necessary to study the characteristics of photovoltaic (PV) array. To utilize PV power or extract maximum power from PV array, the maximum power point tracking (MPPT) technique is essential to study and implement. In this paper, an equivalent electrical circuit of PV system has been modelled and its characteristics are studied. Response of the PV array with different irradiance level is also obtained. Incremental conductance algorithm of MPPT is modelled and developed to extract maximum power from PV array. The design of dc-dc boost converter has been carried out. It is observed that maximum power from PV array is achieved at input and output side of the dc-dc converter. The simulation work is done in MATLAB/SIMULINK environment. Keywords: Solar photovoltaic array (SPVA), incremental conductance (IncCond), MPPT, DC-DC boost converter.
I. INTRODUCTION The demand for quality power is increasing rapidly due to increased use of electrical and electronic gadgets and appliances. However, the sources of conventional (nonrenewable) energy are limited and will last for a fixed number of years. Therefore, we are approaching and searching newer techniques to use renewable energy in a very efficient and economic manner. Solar Energy is a good choice for electric power generation. The solar energy is directly converted into electrical energy by solar photovoltaic module. The photovoltaic array is formed using number of series and parallel modules of solar photovoltaic cells [1]. Photovoltaic array is modeled on the basis of its equivalent electrical model. There are different types of electrical equivalent model of solar cells which are present like single diode model, double diode model etc. The model taken and simulated in this paper is single diode electrical equivalent model of photovoltaic cell. The single diode model of photovoltaic cell is shown in Fig.1 Different types of photovoltaic cells will yield different energy output. Power output of a Solar PV module changes with change in the direction of sun, changes in solar irradiance level and variations in temperatures. It is known that the efficiency of the solar PV module is low and it is in the range
of 13%. Since, the module efficiency is low it is desirable to operate the module at the peak power point so that the maximum power can be delivered to the load under varying temperature and irradiance conditions. Hence maximization of power, improves the utilization of the solar PV module. A maximum power point tracker (MPPT) is used for extracting the maximum power from the solar PV module and transferring that power to the load. Considering the investment cost of the PV system, it is always a prerequisite to operate PV at its Maximum Power Point (MPP). Different MPPT methods are implemented and developed and they vary in their implementation techniques, complexity, speed and cost etc. [2]. To implement the maximum power point techniques (MPPT), there is a need to have DC-DC converter connected to the solar photovoltaic system (SPV). The maximum power point tracking is done by changing the duty cycle of the DC-DC converter using different MPPT techniques to operate the system at maximum power point (MPP). If a PV array is connected to the converter then by changing the duty cycle of the converter the PV array current will change. Due to change in PV array current PV array voltage and consequently power also change [2] - [7]. II. MODELLING OF PHOTOVOLTAIC ARRAY (PVA) Single diode model of solar cell as shown in Fig.1 is used for the modelling of photovoltaic array. A. Equations related to Solar Cell A solar cell can be modelled as an electrical equivalent model in the form of single diode model, double diode model, model with and without shunt resistance and with or without series resistance. The most commonly used model for modelling and simulation purpose is single diode model with series and shunt resistance and it has been used for modelling purpose in this paper also. The current-voltage relationship equation of Fig. 1 is given I
I
V R
e
I
NV
I
V R
I
R
where V, I
Solar Cell output voltage and current
(1)
B.
Diode ideality factor
e or q
Electronic charge (1.6 x 10-19 C)
k
Boltzman's constant (1.38 x 10-38 J/0K)
Iph
Photocurrent, function of irradiation level
Is
Reverse saturation current of diode
Vt
Thermal voltage (kT/q)
Rse
Series resistance of cell
Rsh
Shunt resistance of cell
Photovoltaic array (PVA) model
When we are modelling Photovoltaic array (PVA) there are two important points on V-I characteristics that need to be noted. The first one is open circuit voltage (VOC) and the second is the short circuit current (ISC). On both these points the power generated by the PV array is zero. Therefore the equation related to these two conditions is being changed and given as (2)
And the equation (1) become I
ISC
V IRse
IS e
1
NVt
(3)
If PV module is having open circuit situation then I=0 and the equation of open circuit voltage is shown as [1][4] N T
VOC
ln
ISC IS
1
Tref
Normal cell operating temperature
Sref
Normal cell operating solar irradiation
The equation for the photovoltaic array (PVA) will also be modified depending upon the number of series (Ns)and parallel (Np) modules (cells) and subsequently the series and parallel resistance get altered also. Series module (cells) increases the output voltage of PVA and the parallel module increases the output current of the PVA [5]. The equation incorporating series and parallel modules (cells) is given in equation (6) I
ISC ≈ Iph
Operating temperature and solar irradiation
The increase in temperature causes a reduction in the output voltage of the PV array with the p-n junction voltage dependency and the increase in irradiance parameter causes an increase in short circuit current and the open circuit voltage also increases little bit but it mainly affects short circuit current.
Fig.1. Equivalent circuit of single diode solar cell.
N
T, S
(4)
where,
N I
N I e
V IR
NV N
1
V IR R
(6)
Here the series and shunt resistance in equation (6) are taken as Rse
C.
Rse
Ns Np
, Rsh
Rsh
Ns Np
(7)
Maximum Power Point Tracking (MPPT)
The tracking of maximum power point (MPP) of a solar photovoltaic array (SPVA) is an important aspect of the SPVA system. There are several MPPT methods that have been studied and researched [1]-[4]. The methods vary in complexity, response, cost, sensors required etc. The most commonly employed method is Hill climbing and Perturb and Observe (P & O). Both methods follow the same fundamental method but the way of implementation is different from each other. In Hill climbing method, the perturbation is done in the duty ratio of DC-DC converter but in case of P & O a perturbation in the operating voltage of photovoltaic array (PVA).
ISC
Short circuit current
Principle and Algorithm of Incremental Conductance:
VOC
Open circuit voltage
In this paper, the maximum power point tracking (MPPT) algotithm used is incremental conductance (IncCond) shown in Fig.2. This technique is based on the principle that the slope of the PV array power curve is zero at the MPP and at the left of the maximum power point (MPP) it is positive and negative at right of the maximum power point (MPP), given as
It is clear from Equation (3) and (4) and also well known that the outputs of solar photovoltaic (SPV) array is dependent upon the parameters such as solar irradiance and temperature. The effect of SPV parameters should be incorporated in the equations so that we should get the desired output results related to practical photovoltaic array (PVA).Therefore the equations of PVA should be incorporated with irradiance and temperature parameters and equation of current is ISC where,
ISC
S S
1
T
T
K
(5)
dP/dV = 0 dP/dV > 0 dP/dV < 0
at MPP left of MPP right of MPP
(8)
But P V
I
V
I V
≈I
V
∆I
(9)
∆V
and therefore by differentiating power with respect to voltage we will get the new condition given ΔI/ΔV = (10)
I/V
ΔI/ΔV > ΔI/ΔV
