Switched inductor boost converter for PV applications - IEEE Xplore

8 downloads 11 Views 743KB Size Report
introduced by replacing the inductor of the boost converter with a switched ... Power Point Tracking (MPPT) control is applied to extract the maximum power of the ..... 1, FEBRUARY 1998. [24] www.solarcellsales.com/techinfo/docs/bp-485.pdf.

Switched Inductor Boost Converter for PV Applications Omar Abdel-Rahim*, Mohamed Orabi*, Emad Abdelkarim*, Mahrous Ahmed* and Mohamed Z. Youssef** *APEARC, South Valley University, Aswan City 81542, Egypt, ** Bombardier Transportation, Kingston, Canada [email protected] Abstract— this paper introduces a boost converter with high dc gain as a solution for partial shading of photovoltaic (PV) module. Switched inductor boost converter (SIBC) is introduced by replacing the inductor of the boost converter with a switched inductor branch. As a result, the conversion gain ratio can be increased. The proposed converter is used as an interface between the PV system and the load. A Maximum Power Point Tracking (MPPT) control is applied to extract the maximum power of the PV module. Analyses, simulation, and experimental results are provided to validate the operation of the converter. Keywords— high gain, photovoltaic, partial shading, SIBC and MPPT.

I. INTRODUCTION Recently, renewable energy is the targeted solution for energy crises. In addition, it is a friendly solution which is free from any pollution. Photovoltaic (PV) module is considered as a very important source of renewable energy, because it is safe, robust and has long life time. The low voltage of the PV module is one of the challenges in case of grid connection applications or high voltage dc applications requirements. As a result, many PV modules should be connected in series to meet these application requirements. Partial shading has a serious bad effect on PV modules connected in series. It causes a big reduction in PV output power, and sometimes the controller is forced to disconnect the whole string if one or more PV modules are under partial shaded. The ac modules are the solution for partial shading problems. But the ac module requires high voltage gain conversion ratio because of the low voltage of the PV modules [1]-[2]. Theoretically, a dc–dc boost converter can achieve a high step-up voltage gain with an extremely high duty ratio near to 100% [3]. However, in practice, the step-up voltage gain is limited due to the effect of power switches, rectifier diodes, the equivalent series resistance (ESR) of inductors and capacitors, and the saturation effects of the inductors and capacitors. Many topologies have been presented to provide a high step-up voltage gain without an extremely high duty ratio [4]–[7]. However, these types are all complex and have a higher cost. The coupled inductor techniques provide solutions to achieve a high voltage gain, a low voltage stress on the active switch, and a high efficiency without the penalty of high duty ratio. Thus Switched Inductor Boost (SIB) type provides high gain and

978-1-4577-1216-6/12/$26.00 ©2012 IEEE

high efficiency. This paper introduces switched inductor boost type with PV module. SIBC is used to extract maximum power from PV module and provides output voltage with a level suitable for grid connection applications. The organization of this paper is as follows. Section II discusses the principle of operation of the switched inductor boost converter (SIBC). Small signal analysis of the SIBC is introduces in section III. Subsequently section IV provides the MPPT techniques. Then simulation results and experimental results are provided in section IV and V respectively. Finally, section VI summarizes the conclusion of the paper. IIPARTIAL SHADOW PROBLEM Electrical characteristics of the PV module are affected by environmental conditions such as the temperature, the solar irradiation, dust accumulation and the shadow caused by birds, clouds, and dust. Shading of solar cells not only reduces the cell power, but it also changes the open circuit voltage V , the short circuit current I , and the efficiency. The Partial shading is a common situation due to the shadow of buildings, trees, clouds, and dust, etc. Under partial shading condition, some of the series strings of PV modules are less illuminated which dissipate some of the power generated by the rest of the modules. It means that the current available in a series connection of PV modules is limited by the current of the less illuminated PV module [8]-[13]. This can be avoided by the use of bypass diodes which can be placed across a PV module. This is to allow the array current to flow in the right direction even if one of the strings is completely shadowed. The effect of shade on the performance of a PV depends on influences of multi parameters such as [13]: -Reduction of insulation. -Distribution of the shade on the PV generator (geometry of shade). -Modules with or without by-pass diodes. -Circuit design of PV array (series connection, or strings in parallel). Figure 1 shows a PV system consists of two strings connected in parallel, each string consists of four PV modules of 85W BP485 type. This system will be considered as an example to study the effect of partial shading on PV modules. First the system is under normal

2100

condition, maximum output power is found to be about 700W. Figure 2 shows the system when two PV modules is partially shaded, maximum power is reduced to about 350W and multiple peaks appear. Using dc-dc converters to boost output voltage of PV modules enables reduction in the number of PV modules connected in series, and hence reduces the problem of partial shading.

two modes of operations. Mode 1 occurs when switch SW1 is ON, this causes diodes D1 and D3 to be ON and diodes D2 and D4 to be OFF. Thus the two branches of inductors are charging in parallel. Figure 6 (a) shows the proposed converter circuit of mode 1. Inductor voltage and capacitor current in this mode are given in equations (1) and (2).

Figure 3: Traditional boost converter.

Figure 4: Switched inductor branch [14]. Figure 1: PV system at normal condition.

Figure 5: Switched inductor boost converter.

Figure 2: PV system under partial shading condition. Figure 6: Operation modes of SIBC (a) mode 1 (b) mode 2.

III-

ANALYSES OF THE PROPOSED SWITCHED INDUCTOR BOOST CONVERTER Figure 3 shows the conventional dc-dc boost converter. Figure 4 shows the switched inductor which has been introduced in [14]. It consists of two parts of inductors and three diodes. By replacing the inductor of the traditional dcdc converter with the switched inductor, the emerged circuit is called switched inductor dc-dc boost converter (SIBC). Figure 5 shows the boost converter after adding switched inductor branch and Fig. 6 shows the operation modes of the converter. The proposed converter has also

vL t i t

v t R

(1) (2)

Mode 2 occurs when switch SW1 is OFF, this causes diodes D1 and D3 to be OFF and diodes D2 and D4 are ON, and hence the two branches of inductors discharge in series. Figure 6 (b) shows the proposed converter circuit of mode 2. Inductor voltage and capacitor current in this mode are given in equations (3) and (4).

2101

vL t

.5

v t

i t

i t

v t

R

v t

.5

.5 d t

i t

1

d t

(6)

R

L

i

C

0

iL t v t L

L

1

C

1

ı̂L t v t

d t

d t

d t

iL t v t

RC

v t

0 v t

1

1

L

L

d t

1

d t

iL t

L

RC

1

d t v t

d t

D

v t IL C

v t

L

v t

RC

D L

(12) (13)

ı̂L t v t

A

B

v t d t

(14)

Where; A

D

0

L

D

B

(15)

D C

(7)

Substitute vL t and t in (5) andd (6) into (7), the following equations can be obtained;

D C

L

The model is (12) and (13) can be b rewritten as:

.5 v t

The differential equation of inductor volltage and capacitor current is as follow vL

L

ı̂L t

(5)

t

V

V

d t

(4)

Applying inductor volt second balance annd capacitor charge balance [15-16], equations (1), (2), (3) and (4) yield the following equations for the proposed connverter model. vL t 1 d t

̂L

(3)

L

0

RC V

V L

L IL

(16)

C

The control to PV output voltage transfer function is obtained from (15) and a step response r is drawn as shown in Fig. 8 to enable a propeer choice of the sampling frequency for the MPPT conntrol. From step response (8) drawing and choosing control criteria ε=5%, the suitable sampling time may be choosingg to be more than 4ms [17]. (9)

v t

(10)

From the dc analyses, the dc operaating point of the proposed converter with a constant duty ratio d D can be determined by letting the right-hand siides of differential equations (9) and (10) equal to zero annd solving the two resulting algebraic equations for iL and d v . This resulting in, G

V

D

V

D

IL

V R

D

Figure 7: SIBC and Booost converter gain.

The gain of the SIBC is higher thann traditional boost converter by a factor of (1+D). Fiigure 6 shows a comparison between the gain of the SIB BC and traditional boost converter as shown in the figure, thhe gain of the SIBC is higher than that of boost converter. Applying standard linearization techniques, a small-signnal model of the proposed converter can be derived from m (9) and (10) with the following steps: The state variables are illustrated as: iL IL ı̂L v V v d d d

(11)

Then, using (11), the small signal model can be obtained as: Figure 8: Step response of the control to input voltage transfer function.

2102

IV- RESULTS AND DISCUSSION power was found to be about 122 W. experimental setup is PV generation efficiency and power quality are the shown in Fig. 20. fundamental issues. PV power sources are usually TABLE 1 POWER STAGE COMPONENTS integrated with control algorithms that have the task of ensuring maximum power point (MPP) operation. Many Components description algorithms have been developed for tracking the maximum Cp 100µF power point of a solar array [18]–[22]. Most commonly Cf 220µF used are the perturb-and-observe (P&O) algorithm and the L1=L2 3.3 mH incremental conductance algorithm. In this paper the P&O SW1 IXFT36N50P of MPPT will be implemented [23]. Schematic of the D1=D2=D3 RURG30015 complete system is depicted in Fig. 9. It consists of the PV model, the SIBC, and the load. The PV current and voltage D4 1n50 are received from sensors by the controller where the MPPT control can adjust the duty cycle using PWM control. The proposed converter has been simulated using PSIM software to verify the performance of the proposed configuration. Circuit parameters of the proposed converters are C 220 μF, L L 3.3 mH , and switching frequency of 40 kHz. For single PV module, Figure 10 shows the PV output voltage, current, and power. The maximum power point is achieved and found to be approximately 85 W. Figure 10 shows the PV model current and voltage. It can be noted that at normal environmental conditions, the MPP of the considered PV model: the PV voltage equals approximately 17 V and the PV current equals approximately 5 A. Figure 11 shows the output voltage which approximately equals 125 V. As a result the gain of the SIBC is about 6. Figure 12 shows the MPPT control Figure 9: Schematic of the experimental system. output signal (duty cycle) which far away with safe value from the unity. This reflects the idea of the proposed converter which shows high dc-dc gain ratio at low duty cycle far away from unity. To validate the proposed system, a prototype was built and experimentally was tested in the Lab. The components used to build power stage are same as in simulation given in table 1. Two PV modules of the BP485 85W PV module are used [24]. The control was implemented using Spartan 3 FPGA xc3s200 kit to produce the PWM modulation signal for the SIBC switch. The data are fed to the FPGA controller from sensors and ADC circuits. Input and output voltage of the SIBC is depicted in Fig. 13 at 0.8 duty cycle, the figure prove the high gain of the SIBC. Diodes D1 and D3 are on in synchronization with the switch SW1, currents diode D1 and switch SW are depicted in Fig. 14 and 15, while current of diode D2 is shown in Fig. 16.

Figure 10: one PV module output (a) power (b) voltage and current.

First only one PV module was used and maximum power point was achieved at different time of a day. The test has been done in Aswan city, Egypt around 2 and 3 PM during the summer time where the temperature is around 44oC. As a result the extracted power was less from the rated maximum of the cell. Figures 17 and 18 show the PV current, PV voltage, and PV maximum power at 2 and 3 PM PV modules; respectively. The maximum power was found to be about 52 W and 56 W respectively. Then two PV modules were used at the same conditions. Figure 19 shows the PV current, PV voltage and PV maximum power at 2 PM PV modules. The maximum

2103

Figure 11: SIBC output voltage.

Figure 12: Maximum power point tracking control output.

Figure 15: Switch SW current.

Figure 13: Ch 1 SIBC input voltage and Ch2 output voltage at D =.8. Figure 16: Diode D2 current.

Figure 14: Diode D1 current

Figure 17: PV maximum powers at 2PM, Ch4 PV current, Ch2 PV voltage and Math is the total power.

2104

Maximum power point tracking is applied for the proposed converter to extract maximum power point from the PV module. A prototype has been built in the Lab to test and validate the operation of the converter; the test was done on one and two PV modules. Simulation and laboratory results have been provided. ACKNOWLEDGMENT The authors gratefully thank the ministry of Science, Egyptian Science and Technology Development Fund (STDF), for supporting this project. REFERENCES [1] Figure 18: PV maximum power at 1 PM, Ch4 PV current, Ch2 PV voltage and Math is the total power. [2] [3]

[4] [5] [6] [7]

Figure 19: PV maximum power at 2 PM, Ch4 PV current, Ch2 PV voltage and Math is the total power.

[8]

[9]

[10] [11]

[12] Figure 20: Experimental setup.

V- CONCLUSIONS A switched inductor boost converter (SIBC) is presented in this paper which has very large dc-dc conversion ratio at low duty cycle far away from unity. It is suitable for ac modules besides it can offer a solution for the problem of shading and grid connection applications for low voltage dc voltage of the available PV modules.

[13] [14]

[15] [16]

2105

R.C. Campbell, “A circuit-based photovoltaic array model for power system studies,” Power Symposium, 2007. NAPS '07. 39th North American Issue Date: Sept. 30 2007-Oct. 2 2007 On page(s): 97 - 101. M. G.Villalva, J.R.Gazoli, and E.R.Filho, “Comprehensive approach to modeling and simulation of photovoltaic arrays, “IEEE Trans. Power Electronics, VOL. 24, NO. 5, MAY 2009 Amitava Das, Vinay Kumar Rajput, Amrita Chakraborty, Mainak Dhar, Sukanya Ray, Rakhi Dutta "A new transformerless dc – dc converter with high voltage gain" 2010 International Conference on Industrial Electronics, Control and Robotics B. R. Lin and F. Y. Hsieh, “Soft-switching zeta–flyback converter with a buck–boost type of active clamp,” IEEE Trans. Ind. Electron., vol. 54,no. 5, pp. 2813–2822, Oct. 2007. T. F. Wu, Y. S. Lai, J. C. Hung, and Y. M. Chen, “Boost converter with coupled inductors and buck–boost type of active clamp,” IEEE Trans. Ind. Electron., vol. 55, no. 1, pp. 154–162, Jan. 2008. K. C. Tseng and T. J. Liang, “Novel high-efficiency step-up converter,” Proc. Inst. Elect. Eng.—Elect. Power Appl., vol. 151, no. 2, pp. 182–190, Mar. 2004. R. J. Wai, C. Y. Lin, R. Y. Duan, and Y. R. Chang, “Highefficiency DC DC converter with high voltage gain and reduced switch stress,” IEEE Trans. Ind. Electron., vol. 54, no. 1, pp. 354– 364, Feb. 2007. V. Di Dio , D. La Cascia, R. Miceli, C. Rando,” A mathematical model to determine the electrical energy production in photovoltaic fields under mismatch effect,” Clean Electrical Power, 2009 International Conference on Digital Object Identifier: 10.1109/ICCEP.2009.5212083 Publication Year: 2009 , Page(s): 46 – 51. Y.Ji, J.Kim, S.Park, J.Kim, and C.Won” C-language based pv array simulation technique considering effects of partial shading,” Industrial Technology, 2009. ICIT 2009. IEEE International Conference on Digital Object Identifier: 10.1109/ICIT.2009.4939687 Publication Year: 2009 , Page(s): 1 – 6 H.Patel and V.Agarwal,” Matlab-based modeling to study the effects of partial shading on pv array characteristics,” IEEE Trans. Energy Conversion, VOL. 23, NO. 1, MARCH 2008. A.Kajihara, T.Harakawa, “Model of photovoltaic cell circuits under partial shading, “Industrial Technology, 2005. ICIT 2005. IEEE International Conference on Digital Object Identifier: 10.1109/ICIT.2005.1600757 Publication Year: 2005 , Page(s): 866 – 870. S. Silvestre and A. Chouder,” Shading effects in characteristic parameters of pv modules,” Electron Devices, 2007 Spanish Conference on Digital Object Identifier: 10.1109/SCED.2007.384007 Publication Year: 2007 , Page(s): 116 – 118. R. E. Hanitsch, Detlef Schulz and Udo Siegfried “Shading effects on output power of grid connected photovoltaic generator systems” Rev. Energ. Ren. : Power Engineering (2001) 93-99 B.Axelrod, Y.Berkovich and A.Ioinovici," Switchedcapacitor/switched-inductor structures for getting transformer less hybrid dc–dc pwm converters," IEEE Transactions On Circuits And Systems—I: REGULAR PAPERS, VOL. 55, NO. 2, MARCH 2008. "Fundamental of Power electronics" second Edition Robert W. Erickson J.Sun, D.M. Mitchell, M.F. Greuel, T. Krein and R.M. Bass, " Averaged modeling of pwm converters operating in discontinuous

[17]

[18] [19] [20] [21]

[22]

[23]

[24]

conduction mode," IEEE Trans. Power Electronics, VOL. 16, NO. 4, JULY 2001. N. Femia, G.Petrone, G.Spagnuolo, and M. Vitelli, “Optimization of Perturb and Observe Maximum Power Point Tracking Method” IEEE Transactions on Power Electronics, Vol. 20, No. 4, July 2005, pp: 963 – 973. W. Xiao and W. G. Dunford, “A modified adaptive hill climbing mppt method for photovoltaic power systems,” in Proc. 35th Annu. IEEE Power Electron. Spec. Conf., 2004, pp. 1957–1963. N. S. D’Souza, L. A. C. Lopes, and X. Liu, “An intelligent maximum power point tracker using peak current control,” in Proc. 36th Annu. IEEE Power Electron. Spec. Conf., 2005, pp. 172–177. Y.-C. Kuo, T.-J. Liang, and J.-F. Chen, “Novel maximum-powerpoint tracking controller for photovoltaic energy conversion system,” IEEE Trans. Ind. Electron., vol. 48, no. 3, pp. 594–601, Jun. 2001. G. J. Yu, Y. S. Jung, J. Y. Choi, I. Choy, J. H. Song, and G. S. Kim, “A novel two-mode MPPT control algorithm based on comparative study of existing algorithms,” in Conf. Record Twenty-Ninth IEEE Photovoltaic Spec. Conf., 2002, pp. 1531–1534. Safari. A and S. Mekhilef, “Simulation and Hardware Implementation of Incremental Conductance MPPT with Direct Control Method Using Cuk Converter,” IEEE Trans. on Ind. Elect., Vol. 58, Issue 4, pp: 1154 - 1161, 2011. ChihchiangHua, Member, IEEE, Jongrong Lin, and ChihmingShen” Implementation of a DSP-Controlled Photovoltaic System with Peak Power Tracking” IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 45, NO. 1, FEBRUARY 1998 www.solarcellsales.com/techinfo/docs/bp-485.pdf.

2106

Suggest Documents