 Open Access
 Total Downloads : 256
 Authors : Harsh Kumar Jain, Monika Vardia
 Paper ID : IJERTV4IS050274
 Volume & Issue : Volume 04, Issue 05 (May 2015)
 DOI : http://dx.doi.org/10.17577/IJERTV4IS050274
 Published (First Online): 11052015
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Comparison of ASS and INC Method of Maximum Power Point Tracking for Photovoltaic Array
Harsh Kumar Jain, Monika Vardia Department of Electrical Engineering Geetanjali Institute of Technical Studies Udaipur, India
Abstract – An efficient maximum power point tracking (MPPT) technique is need to improve the efficiency of a solar photovoltaic (PV) array. An asymmetrical step size (ASS) method of maximum power point tracking (MPPT) for photovoltaic generation is proposed in this paper in which present and past slope of power (P) with respect to voltage (V) are compared and based on this step size of the voltage is changed. The main advantage of proposed method is fast tracking of maximum power point of PV array. Also it works successfully in dynamic as well as steady state condition and track maximum power point when irradiation level changes. The obtained simulation results are compared with conventional incremental conductance method (INC). The results show that the ASS method is better than the conventional INC method for tracking MPPs of PV array.
Keywords Maximum Power Point Tracking, Photovoltaic System, Asymmerical Step Size, Incremental Conductance.

INTRODUCTION
Effective renewable energy sources are those sources in which energy supplies that are refilled by natural processes as fast as we use them. All renewable energy comes, ultimately, from the sun. Solar energy comes directly from the sun and is used to produce electricity, heat, and light. However, there are two main disadvantages of photovoltaic (PV) system, the high installation cost and the low conversion efficiency of PV modules which is only in the range of 917% [1]. Besides that, PV characteristics are nonlinear and it is very much weather dependent. Fig. 1 and Fig. 2 show the IV and PV characteristics of a typical PV module for a series of solar irradiance levels with constant temperature [2]. It can be noticed that PV output voltage greatly governed by temperature while PV output current has approximate linear relationship with solar irradiances. It can be seen from the PV characteristic curve that there is only one peak operating point which is named as the maximum power point (MPP). Due to the high capital cost of PV array, maximum power point tracking (MPPT) control techniques are essential in order to extract the maximum available power from PV array in order to maximize the efficiency of PV array. Therefore, a DCDC converter is inserted between PV generator and load or battery storage. MPPT algorithms are used to control the switching of DCDC converter by applying pulsewidth modulation (PWM) technique [2].The basic block diagram of solar system is shown in Fig. 3.
Fig. 1. CurrentVoltage characteristic curves of PV array with change in irradiation level at constant temperature
Fig. 2. PowerVoltage characteristic curves of PV array with change in irradiation level at constant temperature
PV
Module
3
1 1
LOAD
DCDC
Converter
= ()
{ [ ( )]} (5)
MPPT
Control Procedure
Digital Controller
Fig. 3. Basic Block Diagram of photovoltaic system

MODEL OF PV ARRAY
The PV cell is the key unit of PV power system, mainly made up of silicon and compound semiconductor material, which has nonlinear characteristic between power and voltage. A PV array is made up of series or parallelconnected combinations of solar cells. The output current and voltage generated by a PV array depend on solar irradiation level, array temperature and load resistance. The model of a PV array is usually described by its currentvoltage (IV) characteristic and by the equivalent circuit as shown in Fig. 4.
Fig. 4. Equivalent circuit of solar cell
Where Ios is the diode reverse saturation current which depends on temperature, not solar radiation. Ior is the diode reverse saturation current at reference temperature Tr (298K), Iscr is the normal short circuit current of the cell under the temperature of 298K and the irradiation (S) of 1kW/m2, Eg is the band gap energy of the cell semiconductor.

MPPT METHOD
The conventional incremental conductance method is reviewed in the part A of this section followed by an asymmetrical step size method.

Conventional Incremental Conductance(INC) Method
The IC MPPT method has a simple structure and its implementation is easy. The IC MPPT method uses PV characteristics curve. Fig. 3 shows the IC MPPT method. The maximum power point in IC method is when a slope of PV is zero. A left side of MPP has the positive slope and right side has the negative slope.
Fig. 5. Control theory of INC MPPT
The slope of PV curve can be expressed as follow.
Where IPV is the PV array output current, VPV is the PV
= () =
+
(6)
array output voltage, IPH is the generated photo current under a given solar irradiation, ID is the PN junction dark current of the
cell unit, q is the charge of an electron (1.6Ã—1019 C), n is the P The A, B, C point in fig. 5 is as follows.
N junction curve constant, when PV cell output high voltage, n=1, otherwise n=2, K is the Boltzmanns constant (1.38Ã—1023 J/K), T is the cell absolute temperature (K), RS and RP are the
= +
< 0 ( ) (7)
intrinsic resistances associated with the silicon PV array, RS is
=
+
= 0 ( ) (8)
the equivalent series resistance of the PV array, and RP is the equivalent shunt resistance of the PV array. The value of RP is
thousands of ohms.
=
+
> 0 ( ) (9)
According to Fig. 4, when shunt resistance is neglected in
the equivalent circuit, the relation of the output current of PV array can be expressed as the following equations [3]:
= (1)
= {exp [( + ) 1]} (2)
= [ + ( )] (3)
= { exp [( + ) 1]} (4)
Fig.6 shows flow chart of conventional INC method in which fixed step size is uses to change the PV output voltage to track the maximum power point according to slope of power with respect to voltage.
When operating point is in between 0.6pu and 0.75pu (Near the MPP), slope of power with respect to voltage decreases. When operating point is in the right side of the MPP, the magnitude of slope of power with respect to voltage increases and at the MPP the slope of power with respect to voltage is zero. The ASS MPPT method proposed in this paper uses a maximum step size when operating point of the photovoltaic system is far from the MPP i.e. when present and past slope of power is equal whereas minimum step size is used when an operating point is near to the MPP i.e. present and past slope of power is not equal. MPP is reached when change in present power and past power is zero.
START
Initial V (k1) =0; P (k1) =0
Measure V (k); Calculate I (k) & P (k) =V (k)*I (k) Calculate V (k+1) =V (k) +ISV; I (k+1) &
P (k+1) =V (k+1)*I (k+1)
Measure V (k1); Calculate I (k1) & P (k1) Measure V (k); Calculate I (k) & P (k) Measure V (k+1); Calculate I (k+1) & P (k+1)
Fig. 6. Flow chart of INC MPPT method

Assymetrical Step Size(ASS) Method
The MPPT performance of a convenional MPPT method is depend on the step size of voltage. The large step size is improved the tracking speed, but the accuracy of tracking is decreased. The small step size is improved the tracking
 P (k+1) P (k) 
NO
( + 1) ()
 
( + 1) ()
NO
YES
YES
accuracy, but the tracking speed is slowed [4]. Fig. 7 shows the
V (k+2) = V (K+1) + MSV
PV characteristic curves (P1 & P2) and 
curves with change in irradiation condition.
V characteristic
( + 1) > ()

NO
( + 1)

()
YES
V (k+2) = V (K+1) + SUV V (k+2) = V (K+1) – SDV
Update: V (k+1) = V (k+2)
V (k) = V (k+1)
V (k1) = V (k)
Fig. 8. Flow Chart of ASS MPPT Algorithm
Fig. 7. Slope of power versus voltage under different irradiation conditions
The V characteristics of P1 shows that when operating
point is in between 0pu and 0.6pu (far from the MPP and in left
side of MPP), slope of power with respect to voltage is constant.
Present slope of power with respect to voltage is defined as:
( + 1) ( + 1) ()
( + 1) = ( + 1) ()
Past slope of power with respect to voltage is defined as:
() () ( 1)
() = () ( 1)
Fig. 8 shows flow chart of ASS method proposed in this paper in which ISV, MSV, SUV, SDV & represent initial step voltage, maximum step voltage, step up voltage, step down voltage and tolerance of change in power or change in slope of power with respect to voltage respectively.


PERFORMANCE AND RESULTS
The BPSX150[5] PV module of BP Solar Power Company (Madrid, Spain) is chosen for modeling, which is made up of 72 seriesconnected multicrystalline silicon photovoltaic cells and the electrical characteristics at standard test condition (STC) of irradiance (1kW/m2) and module temperature (250C) are shown in Table I.
TABLE I. PARAMETERS OF BPSX150 SOLAR MODULE
Parameter
Value
Maximum Power (Pmax)
150W
Voltage at Pmax (Vmp)
34.5V
Current at Pmax (Imp)
4.35A
Warranted minimum (Pmax)
140W
Short circuit current (Isc)
4.75A
Open circuit voltage (Voc)
43.5V
Maximum system voltage
600V
Temperature coefficient of Isc
(0.065Â±0.015)%/0C
Temperature coefficient of Voc
(160Â±20)mV/0C
Temperature coefficient of power
(0.5Â±0.05)%/0C
NOCT
47Â±20C
To track MPP the value of step size for ASS method are taken MSV = 4V, ISV = 0.5V, SUV = 0.1V, SDV = 1V and
= 0.01. Fig. 9 shows PV characteristic curves with change in irradiation level and tracking of MPP from an operating point voltage (8V) for irradiation (S) of 0.8kW/m2.When S is changed to 1kW/m2 and 1.2kW/m2 the MPP automatically shift from MPP of 0.8kW/m2 to the MPP of 1kW/m2 and 1.2kW/m2 respectively. The value of maximum power obtained and time to track MPP using ASS method and INC method are compared in Table II.
Fig. 9. PV characterstic curves with tracking of maximum power and change in irradiation level using ASS method
TABLE II. MAXIMUM POWER OBTAINED WITH CHANGE IN IRRADIATION LEVEL
Irradiation level (S)
Maximum Power (Pmax) obtained with ASS method
Maximum Power (Pmax) obtained with INC method
Time to track MPP
with ASS
Time to track MPP
with INC
0.8 kW/m2
114.451W
113.998W
0.03s
0.34s
1 kW/m2
146.725W
146.474W
0.02s
0.37s
1.2 kW/m2
179.658W
179.621W
0.02s
0.37s
Fig. 10. PV characterstic curves with tracking of maximum power using INC method at irradiation level of 1kW/m2
Fig. 11(a) shows that after every 0.2 sec the irradiation level changes randomly and Fig. 11(b) shows that when irradiation level changes, the maximum power tracked corresponding to irradiation condition using ASS method.

CONCLUSION
(a)
(b)
Fig. 11. Response characterstics of MPPT control with radiation changing using ASS method
A novel asymmetrical step size algorithm for MPPT of photovoltaic system was developed in this study and it is compared with INC method. The ASS algorithm considers the asymmetrical step size based on the slope of power with respect to voltage curve of a PV array. The characteristics of a BPSX150type PV module are modeled and analyzed. Its output voltage and current has nonlinear characteristics depending on the atmospheric conditions such as the irradiance and temperature. Because of the nonlinear characteristics, a control circuit must drive the PV module for the maximum power. A solar MPPT controller based on an asymmetrical step size algorithm is designed for a PV energy system. The MPPT controller based on microprocessor, including a DC/DC converter circuit, has a better response under rapid atmospheric conditions, can fast track the maximum power point by using the ASS method. This method can improve the dynamic and steady state performance of the PV system. MATLAB simulation results verify the feasibility and effectiveness of the ASS method over INC method.
REFERENCES

Roberto Faranda, S.L., Energy Comparison of MPPT Techniques for PV Systems, WSEAS Trans. on POWER SYSTEMS, vol. 3, No.6.

Mei Shan Ngan, Chee Wei Tan, A Study of Maximum Power Point Tracking Algorithms for Standalone Photovoltaic Systems, IEEE Applied power electronics colloquium (IAPEC), 2011.

Yao Chuanan, Yu Yongchang, An improved hillclimbing method for the maximum power point tracking in photovoltaic system, International Conference on Machine Vision and Humanmachine Interface, 9780769540092/10 IEEE, DOI 10.1109/MVHI.2010.158, 2010.

Yong Tian, Bizhong Xia, Zhihui Xu, Wei Sun, Modified Asymmetrical Variable Step Size Incremental Conductance Maximum Power Point Tracking Method for Photovoltaic Systems, Journal of Power Electronics, Vol. 14, No. 1, pp. 156164, January2014.

http://www.comel.gr/pdf/bpsolar/BPSX150.pdf