Improvement of Maximum Power Point Tracking Perturb and Observe Algorithm for a Standalone Solar Photovoltaic System

Extraction of maximum power from PV (Photovoltaic) cell is necessary to make the PV system efficient. Maximum power can be achieved by operating the system at MPP (Maximum Power Point) (taking the operating point of PV panel to MPP) and for this purpose MPPT (Maximum Power Point Trackers) are used. There are many tracking algorithms/methods used by these trackers which includes incremental conductance, constant voltage method, constant current method, short circuit current method, PAO (Perturb and Observe) method, and open circuit voltage method but PAO is the mostly used algorithm because it is simple and easy to implement. PAO algorithm has some drawbacks, one is low tracking speed under rapid changing weather conditions and second is oscillations of PV systems operating point around MPP. Little improvement is achieved in past papers regarding these issues. In this paper, a new method named “Decrease and Fix” method is successfully introduced as improvement in PAO algorithm to overcome these issues of tracking speed and oscillations. Decrease and fix method is the first successful attempt with PAO algorithm for stability achievement and speeding up of tracking process in photovoltaic system. Complete standalone photovoltaic system’s model with improved perturb and observe algorithm is simulated in MATLAB Simulink.


INTRODUCTION
P V is the fastest growing renewable technology of producing energy in the current world because of its priceless benefits like its abundance, environment friendly, cheap maintenance, pollution free (noise and toxic gases), simplicity and easy implementation [1,2]. But this growing technology has some serious problems of efficiency from manufacturing materials, change of illumination and temperature. The problem of manufacturing material has been overcome to some extent by using different materials, and 46% efficiency has been achieved in December 2014 by Japanese AIST (National Institute of Advance Industrial Science and Technology) [3]. Solar PV cell are made up of semiconductor material, mostly with silicon.
Semiconductor materials used other than silicon are CdTe

Improvement of Maximum Power Point Tracking Perturb and Observe Algorithm for a Standalone Solar Photovoltaic System
photoelectric effect, when exposed to sun light it generates or releases electrons from its conduction band by absorbing the photons of sunlight. These free electrons of PV cell are then captured results an electric current. So, in short "PV cell use solar above bandgap photons as a power source and converts these above bandgap photons directly into electrical energy when exposed to sun light" [4]. These PV cells are connected in the desired order to form modules and arrays. Several PV cells are connected to each other in series to form a PV module and several PV modules are connected to form a PV array. PV modules and arrays are connected in a specific order to get the required output voltage and output current. PV cell generate a DC voltage and can feed dc-loads directly and ac-loads by using inverter before the load, and can also be connected to grid by using conversion devices. To understand the concept of PV cell it can be drawn electronically as shown in Fig. 1 [5].
Where I ph is Photon Current, I d is Diode Current, I s is Shunt Current, and I s is Output Current.
PV cell presents nonlinear V-I characteristics with one/ single point where power production from PV cell/panel is maximum and that single point is known as MPP [6][7][8][9].
A typical V-I (Voltage-Current) and P-V (Power-Voltage) curves of PV cell with MPP are in shown in Fig. 2. V-I characteristics curve represents with brown color in Fig.   2. The blue line in Fig. 2 represents a power curve.
The maximum power is calculated using Equation (2). P max = U mpp x I mpp (2) Where I sc is current when circuit is short, I mpp is maximum possible current, U mpp is maximum possible voltage, and  Here the algorithm selected is PAO algorithm but this algorithm has two main problems. One is slow tracking speed during rapidly changing weather conditions and the other is its oscillations around MPP. Several methods have been used to overcome these two problems like using variable step size which improves the performance but only to a limited extent. Slow tracking speed problem is removed to some extent but problem of oscillations around MPP is still there but with less margin.
In this paper, "decrease and fix" method is introduced to overcome these two problems completely and to achieve higher efficiency. Decrease and fix method is

PERTURB AND OBSERVE ALGORITHM
In PAO algorithm, operating voltage of PV cell/array is perturbed with increment, and resulting change in power is measured and saved as comparative value in algorithm. impedance of solar PV system to transfer maximum power from source to load [15]. Conventional flow of PAO and observe algorithm is shown in Fig. 3.

FIG. 3. CONVENTIONAL PERTURB AND OBSERVE ALGORITHM [5]
This PAO algorithm is a popular method but also have some drawbacks. Oscillations around the MPP and slow tracking speed during rapidly changing weather conditions are the two drawbacks [16]. These drawbacks results in high energy losses. Several methods have been used to overcome these problems [16][17] like using variable step size which improves the performance but not significantly [18]. Both slow tracking speed problem and problem of oscillations around MPP were overcome to some extent by using variable step size PAO algorithm.   In Table 1

Improvement of Maximum Power Point Tracking Perturb and Observe Algorithm for a Standalone Solar Photovoltaic System
It is observed in Tables 1-2that     Temperature has different effect at the generated voltage and current. Temperature is inversely proportional to PV panel's generated voltage and directly proportional to current generated by PV panel. As the temperature increases PV panel's current will increase and voltage will decrease. If temperature decreases PV panel's current will decrease and voltage will increase. 3D graphical representation of PV cell/panel power with changing illumination and temperature at different loads is shown in Fig. 8.  Fig. 9.
It can be clearly observed in Fig. 9

CONCLUSION
In this paper, three main problems of PV array (oscillations,