[ieee 2013 annual international conference on emerging research areas (aicera) - 2013 international...

International Conference on Microelectronics, Communication and Renewable Energy (ICMiCR-2013)

An Improved Current Feedback Based Maximum Power Point Tracking Controller for Solar Photo-

Voltaic System Dnyaneshwar S. Karanjkar, Dr. S. Chatterji

and Shimi S. L.NITTTR, Chandigarh, India

Dr. Amod Kumar CSIO, Chandigarh, India

Abstract— Maximum power point tracking (MPPT) technique with improved current feedback has been proposed for solar photo-voltaic system in this work. A proportional integral derivative (PID) type controller with online set-point adjustment has been designed. The parameters of PID controller have been tuned using relay tuning method. Performance of this technique has been compared with perturb and observe (P & O) method of maximum power point tracking (MPPT). Matlab/Simulink model of solar photo-voltaic system and boost converter with proposed maximum power point tracking (MPPT) technique and perturb and observe maximum power point tracking (MPPT) technique has been designed. Simulation results showed the superiority of proposed method for tracking maximum power point (MPP) under rapidly varying solar radiations as compared to P & O maximum power point tracking (MPPT) technique.

Keywords— Maximum power point tracking; current feedback control; duty cycle control; PID control

I. INTRODUCTION

Solar energy is abundant and renewable in nature and can be considered as the future energy source. The need for renewable energy sources is on the rise because of the severe energy crisis in the world today. The major obstacle for the penetration and reach of solar photo-voltaic (PV) systems is their low efficiency and high capital cost. The output voltage of the solar PV panel is highly dependent on solar radiation and panel temperature. The power-voltage and current-voltage relationship of PV system are non-linear in nature. Switched mode DC-DC power converter is the major component in PV system responsible for ensuring maximum power generation.

The maximum power point tracking (MPPT) system controls the voltage and the current output of the PV system to deliver maximum power. Perturb and observe (P & O) technique, incremental conductance technique, fractional short circuit current technique, fractional open circuit voltage technique, ripple correlation control technique, parasitic capacitance technique, intelligent technique and current or voltage feedback technique are few of many available maximum power point tracking (MPPT) methods. A comprehensive review and analysis of around twenty four maximum power point tracking (MPPT) techniques for photo-voltaic system available until January 2012 can be found in [1].

Out of most widely used maximum power point tracking (MPPT) techniques, incremental conductance and intelligent techniques have higher accuracy but are complex in design. The P & O method is most commonly used in commercial applications. This method has moderate accuracy and ease of implementation. In this method the instantaneous PV voltage is periodically perturbed and output power is compared with previous perturbation cycle. Limitation of this method is development of oscillations near maximum power point. Accuracy and the tracking time of this method depends on size of perturbation. As compared to P&O technique, voltage or current feedback technique is a simple, inexpensive technique but mostly used in stand-alone PV system which has no battery. This method is based on comparison of panel voltage or current with a pre-determined reference value. The duty cycle of DC-DC converter is continuously adjusted to maintain the maximum power [2]. Due to pre-calculated reference voltage or this method is useful only for PV systems with constant radiation and temperature conditions; it cannot track the maximum power point during varying atmospheric condition. This method sometimes used along with conventional techniques. In contrast to other true maximum power point tracking (MPPT) controllers, like P&O method, no additional circuit is necessary for determining the power. Hence the power consumption is lower than the system using conventional techniques.

In the proposed work an attempt has been done to overcome the limitation of current feedback technique. An improved current feedback based technique has been proposed which can track maximum power point even under rapidly changing solar radiations. A proportional-integral-derivative (PID) type controller with online set-point adjustment has been designed. Parameters of PID controller have been tuned using relay tuning method.

The desired objective of the controller is to overcome inherent drawback of the conventional current feedback controller. The paper has been organized as follows: Section two gives brief introduction of solar photo-voltaic system modeling. The proposed improved current feedback based maximum power point tracking (MPPT) technique has been described in section three. Development of Matlab/Simulink

978-1-4673-5149-213$31.00 ©2013 IEEE


model of PV system and boost converter with proposed technique and perturb & observe technique and simulation results have been given in section IV. In the last section the conclusions have been discussed.

II. MODELING OF PV SYSTEM

A photo-voltaic module consists of several photo-voltaic cells connected in series and/or parallel. Series connections are responsible for increasing the voltage of the module whereas; the parallel connection is responsible for increasing the current in the array. Typically a solar cell can be modeled by connecting an inverted diode in parallel with a current source as shown in figure 1.

Figure 1 Ideal and practical model of a typical PV cell It has its own series and parallel resistances. Series

resistance Rs is due to the hindrance in the path of flow of electrons from n to p junction and parallel resistance is due to the leakage current. Several PV modules can be connected in series-parallel configuration to form PV arrays as shown in figure 2. The basic mathematical expression of the PV array is given by equation 1. 1 (1)

In equation 1., I represents the output current of the PV array, V is the output of the PV array, Iph is the photo-current (approximately equal to short circuit current, Isc), I0 represents the reverse saturation current, np is the number of modules connected in parallel, ns is the number of modules connected in series, q is the charge of the electron, k is the Boltzmann’s constant, a represents the p-n junction ideality factor (1<a<5, a =1 being the ideal value) and T is panel temperature.

Practical PV arrays composed of several inter-connected photovoltaic cells and can be described by equation [2]. 1 (2)

In equation 2, Iph and I0 represents photo-voltaic current and saturation currents respectively, Vt =NskT/q is the thermal voltage of array, Rs and Rp are equivalent series and parallel array resistances.

Figure 2 Equivalent circuit of a PV module with series and parallel connected cells

The output current of solar cell Ipv depends on series and parallel resistances. Manufacturers datasheet only provides the nominal short circuit current Isc,n . The assumption Isc ≈ Ipv has mostly been used in photovoltaic models the value of series resistance is low as compared to parallel resistance. The light generated current of solar cell is given by equation 3.

, (3)

where Ipv,n is Ipv at nominal condition, T and Tn are the actual and nominal temperatures, S and Sn are the actual and nominal radiation and the temperature coefficient of short circuit current represented by KI. The diode saturation current also depend on radiation and temperature as: , (4)

In equation 4, Eg is the semiconductor bandgap energy and I0,n is the nominal saturation current given as:

, , , , (5)

where Vt,n is the thermal voltage of ns series connected cells at the nominal temperature Tn. The equation for short circuit current is given as: , (6)

where Isc,n is the short circuit current at nominal condition and α is exponent given by [5]:

, / ,/ (7)

where Isc,n and Isc,1 are the short circuit currents under radiation intensities Sn and S1 respectively.

Figure 3 P-V and I-V characteristics of a solar cell

I-V and P-V characteristics of a typical solar cell at a constant panel temperature and solar radiation are shown in figure 3 [4].

III. PROPOSED WORK

The current feedback or constant technique is the simplest MPPT technique, but it fails to track the maximum power point during changing environmental conditions. Proposed Improved Current Feedback Control technique includes dynamic set-point tracking based on actual solar radiation. Dynamic set-point tracking assures the dynamic operating point of the controller at Imp. The block schematic diagrams of conventional and the proposed current feedback maximum

This work has been sponsored by AICTE, Government of India under QIP scheme

Rs

Vo

+

-

i

Practical model

Ideal model

Rp

id iph

np1 2

-

+

Load V

I

ns

1

2

3

Cur

rent

I-V curve

P-V curve

PmaxIsc

Imp

VocVmp Voltage

Power


power point tracking schemes have been shown in figure 4 and figure 5 respectively.

Figure 4 Current feedback control scheme

Figure 5 Block diagram of proposed scheme for MPPT Improved current feedback controller for MPP tracking is

designed based on following relation between Isc and Imp: Imp= k * Isc (8)

where, the factor k is always <1 and the value of k is considered to be in between 0,78 and 0,92 [6]. The common value of k is 0.9 that means when Pv output current considered to be 90% of the short circuit current, the PV module operates at maximum power point. In the present work Isc has been determined based on equation (6) & (7) and values of Isc,1 and S1 from the I-V curves provided in the manufacturer. Based on equations (6), (7) and (8) set-point tracker system has been designed.

Main building block of the current feedback controller is the PID (proportional plus integral plus derivative) controller. It is widely accepted control strategy in industrial applications. It generates output control signal based on error between measured and desired level of the parameter under control. The output of the controller can be represented in Laplace domain as (equation 9): + (9) where, Kp is proportional constant, Ti is integration time and Td is derivative time. For precise and accurate control and stable operation proper tuning of these control parameters is essential. The tuning procedure becomes complex and difficult when process to be controlled is highly nonlinear.

The simplest and accurate technique of PID parameter tuning for non-linear systems is the relay based tuning method. It avoids trial and error, and ensures stability of the plant [7]. Figure 6 shows use of boost converter for elevating the output voltage Vo respect to the input voltage Vin by fixing the duty cycle D (with a pulse width modulator) according to the following relation [8]:

(10)

Figure 6 Configuration of boost converter in PV system Boost converter is supposed to be one of the traditional

benchmark problems of non-linear control. From boost converter point of view the control scheme shown in figure 5 is of feed-forward type. In such a case input to output steady state relation needed for feed-forward controller design. Hence in order to design the controller duty ratio to inductor current small signal transfer function given in equation (11) [8] has been considered. The small signal transfer function (GiLd) that relate the inductor current iL(s) with the variation of the duty cycle d(s) around the operation point under zero initial conditions is given by: / (11)

Modified relay tuning method proposed in [9] has been adapted to tune PID parameters in this work. The procedure of The procedure of relay tuning is as follows: step i. replace a relay with amplitude d in place of PID controller as shown in figure 7. Step ii. ii. record the plant output a and period P, and step iii. Calculate the ultimate period Pu equal to observed period P, given by equation 12.

Pu = P (12) and the ultimate gain is inversely proportional to the observed amplitude, and given by:

(13) After calculation of the ultimate gain and period, Ziegler -Nichols tuning rules (Table 1) have been used to find PID parameters.

Figure 7. Relay tuning method

TABLE 1

Kp Ti Td 0.6 Ku Pu/2 Pu/8

According to modified relay tuning method [9], correction

factor k has been added to equation (13) of ultimate gain as follows: . (14)

IV. SIMULATION RESULTS A model of solar PV module (BP MSX 120) with boost

converter of specifications [4] mentioned in Table 2 has been developed in Matlab/ Simulink/ SimPower system platform. Boost converter has been designed based on design

- Ipv

+

PV Panel

Boost Converter

PWM

Load

Current Feedback Controller

Iref

S

- Ipv

+

PV Panel

Boost Converter

PWM

Load

Current Feedback Controller

Setpoint Tracker

+ Vo

IO

S

+

C

L i

Load PV Array

ProcessPID

-Step


specifications mentioned in Table 3 as per design guidelines given in [8]. Boost parameters used for simulation purpose have been provided in Table 4.

TABLE 2 PV MODULE (BP MSX120) PARAMETERS AT STC

Short circuit current Isc 3.87A

Open circuit voltage Voc 42.1 V

Current at maximum power point I MPP 3.56 A

Voltage at maximum power point V MPP 33.7 V

Number of cells in series Ns 72

Temp, coeff. of short circuit current (0.065 0.015)%/ oC

Temp. coeff. of open circuit voltage -(80 10) mV/ oC

Pmax 120 W

Ideality factor A 1.3977

Module series resistance Rs 0.473 Ω

Module parallel resistance Rsh 1367 Ω

TABLE 3 BOOST CONVERTER DESIGN SPECIFICATIONS Vin 33.7 V Vout 54.0 V Pmax 120 W Switching frequency 10 KHz Max. inductor current ripple 10 %

TABLE 4. BOOST CONVERTER PARAMETERS Inductor 444 µH

Rloadmax 24.3 Ohm Cin 78.6 µF

Cout 154.6 µF D 0.3759

Simulink models of PV system with P & O MPPT and proposed method of MPPT have been shown in figure 8 and 9 respectively. Figure 10 shows the solar irradiation pattern used for the comparative analysis of performance of the perturb & observe MPPT and proposed method of MPPT scheme for rapid variation s in solar radiation. Simulations have been carried out under discrete power GUI in ODE45.

Figure 8. P & O MPPT simulation model

Figure 9. Improved current feedback MPPT simulation model

Figure 10. Solar irradiation pattern for simulation study

Figure 11 (a)

Figure 11 (b)

Figure 11 (c) (continued)

V-

V+

P and O MPPT

Discrete,Ts = 2.5e-06 s.

pow ergui

v

+

-

VM1

Vpv

IpvD

T

S

+

Rload

+

L

Ir

Irradiance(W/m2)

g CE

25+273.15

Temperature (K)

i+ -

+

Cin

+

Cout

V-

V+

Improved current feedback MPPT

Discrete,Ts = 2.5e-06 s.

pow ergui

Iref

Ipv

D

T

S

Iref

+

Rload

+

L

Ir

Irradiance(W/m2)

g CE

25+273.15

Temperature (K)

i+ -

+

Cin

+

Cout

120

PV

Pow

er (W

)

Time (sec)

35

PV V

olta

ge (V

)

Time (sec)

3.5

PV c

urre

nt (A

)

Time (sec)

1000

Irra

diat

ion

(W

/m2 )

Time (sec)


(d)

Figure 11 (d)

Time (sec)

Figure 11 (e)

Figure 11.(a) to (e) Response of P & O MPPT with 0.001 delta D Response of perturb and observe MPPT system with 0.001

step change in duty ratio (delta D) have been shown in figure 11 (a) to (d). Response of improved current feedback MPPT system with Kp=2.9, Kd=0.01, Ki=1 (Ki * Kp =Ti while Kd/Kp=Td is) have been shown in figure 12 (a) to (d). Figure 13 shows startup response of proportional controller with Kp ranging from 1 to 5. Value of settling time and overshoot has been found to be minimum at the value of Kp around three. As compared to the response of P and O MPPT system power point tracking is accurate and fast in case of response of the proposed system under dynamic conditions. During steady state condition power point tracking is almost similar in both the cases but proposed technique provides smooth voltage and current regulation as compared to the P and O technique. The proposed method provides additional flexibility in terms of PID tuning parameters as compared to the other technique.

Figure 12 (a)

Figure 12 (b)

Figure 12 (c) Figure 12 (a) to (d) Response of improved current feedback MPPT with

Kd=0.01, Kp=3, Ki=1 (continued)

Figure 12 (d)

Figure 12 (e) Figure 12. (a) to (e) Response of improved current feedback MPPT with

Kd=0.01, Kp=3, Ki=1

V. CONCLUSIONS An improved current feedback control based MPPT

technique has been proposed in this paper. Proposed controller has been designed using dynamic set-point tracking based on the current and radiation sensing. Simulations have been carried out to compare performance of proposed technique

2.5 B

oost

cur

rent

Time (sec)

50

Boo

st v

olta

ge

110

Boo

st p

ower

(W

)

120

PV

pow

er

(W)

Time (sec)

35

PV

volta

ge

(V

)

Time (sec)

3.5

PV

cu

rren

t (A

) Time (sec)

110

Boo

st p

ower

(W

)

Time (sec)

2.5

Boo

st c

urre

nt

Time (sec)

50

Boo

st v

olta

ge


with P & O technique under rapid variations of radiations. Simulation results show that proposed method has superior performance than P and O technique. Only drawback of this technique is need of additional radiation sensing system.

Figure 13 Startup response of Proportional controller for MPPT

REFERENCES [1] B.Subudhi, and R. Pradhan, “A comparative study on maximum power

point tracking t echniques for photovoltaic power systems”, IEEE Transactions on Sustainable Energy, vol. 4, no. 1, January 2013, pp. 89-98.

[2] Oscar L., Maria Teresa Penella, “A New MPPT Method for Low-Power Solar Energy Harvesting”, IEEE Transactions on Industrial Electronics, vol. 57, no. 9, September 2010, pp. 3129-3138.

[3] C. Carrero, J. Amador, and S. Arnaltes,”A single procedure for helping PV designers to select silicon PV module and evaluate the loss resistances”, Renewable Energy, 2007.

[4] M. G. Villalva, J. R. Gazoli and E. Ruppert F., “Comprhencsive Approach to Modeling and Simulation of Photovoltaic Arrays”, IEEE Transactions on Power Electronics, 2009 vol. 25, no. 5, pp. 1198-1208.

[5] Wei Zhou, Hongxing Yang and Zhaohong Fang, “A novel model for photovoltaic array performance prediction”, Science Direct J. of Applied Energy, vol. 84, 2007, pp. 1187–1198.

[6] J.Surya Kumari1, Ch. Sai Babu2 and J. Yugandhar, “Design and Investigation of Short Circuit Current Based Maximum Power Point Tracking for Photovoltaic System”, International Journal of Research and Reviews in Electrical and Computer Engineering (IJRRECE), Vol. 1, No. Science Academy Publisher, United Kingdom, vol. 2, June 2011 pp. 63-68.

[7] David I. Wilson, “Relay-based PID Tuning”, Automation & Control, Feb/March, 2005, pp.10-12.

[8] Muhammad H. Rashid, “Power Electronics Handbook”, Academic Press, 2001, pp. 435-436.

[9] S.A. Misal, Vivek S Sathe, R. W. Gaikwad, and Dhirendra, “Modified Relay Tuning Method for Level Control Model”, International Journal of Chemical Engineering and Applications, Vol. 2 , No. 5 , October 2011, pp.307-309.

Kp=1

Kp=2

Kp=3 Kp=4

Kp=5

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