2010 IEEE International Conference on Power and Energy (PECon2010), Nov 29 - Dec 1, 2010, Kuala Lumpur, Malaysia
Buck-Boost Interleaved Inverter for Grid Connected Photovoltaic System Omar Abdel-Rahim, IEEE Student Member, Mohamed Orabi, IEEE Senior Member and Mahrous E. Ahmed, IEEE Member APEARC, South Valley University, Aswan City 81542, Egypt
[email protected] voltage gain and low efficiency compared to two stage inverters. In [1], the proposed topology suffers from low gain and complex control. Flyback inverter proposed in [2] suffers from high stress on switches and also the efficiency decreases as the turns ratio increases. A single stage inverter which was proposed in [3] has the following advantages such as simple, compact, low cost and simple control, but suffers from low voltage gain and low efficiency. In [4], the proposed inverter could buck or boost input voltage, but suffer from complex control and the gain is not very high. In this paper a single stage inverter is proposed it has a very high gain, low switches stresses and simple control, as it will be explained in the next section. Figure 3 shows the schematic of the proposed inverter. The proposed inverter consists of two buck boost converters, one converter operates during positive half cycle and the other operates during the negative half cycle.
Abstract—in this paper, a single stage buck-boost inverter is proposed for grid connected PV system with a very high voltage gain. The proposed inverter not only boosts DC output voltage of the PV module but also converts it into AC voltage which is required for grid connection. Discontinuous conduction mode (DCM) is employed to achieve unity power factor with the grid voltage and a maximum power point tracking (MPPT) control. The proposed topology has several desirable features such as high gain, low cost, compact size and simple control. Only two switches operate at high switching frequency and so switching losses are minimized. Simulation and experimental results are given to prove the proposed system. Keywords— Single Stage, Buck-Boost Inverter, Low-Cost, Grid-Connected, PV system, Simple-Control, DCM, MPPT.
I. INTRODUCTION Renewable energy has become an important source of energy; Photovoltaic system (PV) is an example of renewable energy. PV modules convert sunlight into electrical power, so they provide an important source of energy. PV modules can't be connected to the grid directly, but this could be done by using power conditioning system, that is for example H-bridge inverter, to convert dc output power of PV modules into ac output power. Output voltage of PV modules is not very high, so we may be in need to connect more than one module in series to get the required dc voltage. H-Bridge Inverter, shown in Fig.1, is a buck inverter that has the requirement of input voltage greater than output voltage. In case of input voltage is smaller than output voltage, a boost converter is used before the inverter stage to provide the required voltage for the inverter as shown in Fig. 2. Inverters may be classified as single stage or two stages according to how is the input voltage will be boosted. In the two stage inverters, the first stage is boost converters which boost input voltage to become greater than output voltage and the second stage is a buck converter used to convert dc input voltage into an ac output voltage. In the single stage inverters, the inverter does two functions boosting the input voltage and converting DC power into AC power. Single stage inverters have some advantages over two stage inverters such as low cost and compact size but suffer from low
978-1-4244-8946-6/10/$26.00 ©2010 IEEE
Figure 1: Traditional voltage source inverter.
The proposed system is suitable for AC module technology, where each PV module can be attached to the grid directly. This application is good as electrical characteristics of the PV module are greatly affected by shading condition. Shadow causes the output power of the PV module to be reduced. The proposed inverter helps in solving this problem by reducing number of connected modules per inverter. A maximum power point tracking control is applied for better utilization of the PV module. As the module power is low and to simplify the control for unity power factor DCM mode is applied. The paper is organized in the following way.
63
(3)
(4)
Mode 3: when SW1 and all dioodes are OFF and inductor current becomes zero as shownn in Fig. 4 (d);
0
(5)
(6)
Fig. 5 shows inductor currentt and Fig. 6 shows diode current D4, from steady state analysis , it can be conclude that: Figure 2: Two stage inverter configuration.
0
(7)
(8)
means averagge of inductor voltage and
Where
means average of dioode current. Using (7) and (8) we could obtain the gain g of the converter as following:
√2
Where , D: the converter dusty cycle, Ts: switching period, L: converter inductor and R: load ws that the gain of the resistance. Equation (9) show converter is higher than traditiional buck boost converter by √2. Figure 7 shows a compparison between the gain of the switched inductor and the t traditional buck-boost converter. The figure shows the gain of the switched inductor is approximately onee and half higher than the conventional converter.
Figure 3: Schematic of the prooposed inverter.
Section II presents switched inducctor buck boost converter. Section III presents analysiss and operation of the proposed single stage inverter. Secction IV describes the MPPT control techniques used for tracking t maximum power from PV. Section V summarizess simulation result of the proposed system. Section VI summarizes experimental results of the proposed sysstem. II.
T DC-AC INVERTER III. PROPOSED SINGLE STAGE The proposed system connsists of two buck boost converters; each converter operates for a half cycle as shown in Fig.8. Operation of the t proposed inverter is as follow, Switches SW1 and SW4 S operate at switching frequency equal to grid fundam mental frequency, each one operates for one half cycle. SW W4 operates in the positive half cycle while SW1 operates in the negative half cycle. Switches SW3 and SW2 opperate at high switching frequency. To provide a highh quality grid current and reduce the size of the output fillter each converter operates for only one half cycle. SW3 operates o in the positive half cycle while SW2 operates inn the negative half cycle. Figure 9 shows the operation modes m of the inverter during positive half cycle. During thhe positive half cycle the inverter has three modes of operation: Mode1: when switch SW3 iss ON as shown in Fig. 9 (a). Mode2: when diode D4 is ON N Fig. 9 (b). Mode3: when both SW3 andd D4 are OFF as shown in Fig.9(c). In order to inject a sinusoidal ac current into the grid at unity power factor, the operatioon of converters must be in
O CONVERTER SWITCHED INDUCTOR BUCK-BOOST
Traditional buck-boost converter coould operate as a buck or boost converter according to the adjusted duty cycle. To increase the gain of the convverter, its inductor has replaced by the switched inductor proposed in [5] with this replacement the gain of the converter is improved. Figure 4 shows the propoosed buck- boost converter. The converter operates in DC CM when , that's inductor current ripple is higherr than its average value, When the converter operates in DCM, it has three modes of operation as shown in Fig.4. Mode 1 as shown in Fig. 4 (b) takes place when SW1 annd diodes D1 and D3 are ON and the steady state equatioon of the converter is given by: (1)
(9)
(2)
Mode 2 occurs when diodes D2 and D44 are ON and SW1 is OFF as shown in Fig.4 (c);
64
the DCM mode. The proposed inverrter has only four switches so that it has lower cost andd lower switching losses as compared to two stage invertter. The high gain buck boost converter helps in reducinng number of PV modules connected in series which helpps in reducing the effect of environmental condition, such as shadow, on the performance of the PV module annd MPPT control operation. IV.
V.
The proposed system wass simulated using PSIM software. Two PV modules of the BP485 85W PV module were used [8]. Electrrical characteristics of the used PV module are shown in Table 1. Two PV modules were connected in series to give output power equal to 170 W. Circuit parameters arre as follow fs = 10 kHz, input capacitor Cp =10 mF, filter capacitor Cf = 1uF, switched inductors L1=L2=1220 µH for each converter and output filter inductor Lf =3.5 = mH and grid voltage and frequency are 311V and 500Hz, respectively.
MPPT CONTROL ALGORITHM
PV module which is connected to grid must always deliver its maximum power to the grid. Any change in the environmental conditions will shift the operating point of the PV module to a lower power point, a maximum power point tracking (MPPT) controlleer is used with PV modules to ensure that the PV moduless always operate at its maximum power. Maximum pow wer point tracking algorithm proposed in [7] will be ussed in this paper. Figure 10 is the flowchart of the used MPPT control technique. Where , and are the momentary m voltage and current of the PV array. And are the previous voltage and current, respectively. The
P
SIMULATTION RESULT
term can be Figure 5: inductor current.
I
V, making the calcuulation easier. The replaced by I V major check of this algorithm is achieveed by detecting I
Figure 6: diode D4 current.
I
V, and then D (duty) will be adj djusted in order to move the operating point into the direcction of maximum power point of the PV array. The algoorithm begins with checking if dV 0 or not. If dV 0 , then dI is checked. For dI 0 , D is held unchannged. For I 0 , D must be incremented, while if dI 0 , D must be decremented. On the other hand, if dV d 0 , then I V
I V
V should be checked. For I
held unchanged. But if I I
I V
V
I V
V
0, D is
0, then D must be
decreased and if I V 0, D must m be increased. V Then, the algorithm continues until thee power reaches to its maximum value.
Figure 7: Comparison between switched inductor buck boost converter and traditional buck b boost converter.
Figure 4: (a) schematic of the proposed Switcheed inductor buck-boost converter; (b) mode 1; (c) mode2; (d) modee 3.
Figure 8: Proposed single sttage inverter.
65
operate at grid fundamental frequency, while Fig. 14 (a) and Fig. 15 (a) show that Switches SW3 and SW2 operate at high switching frequency for only one half cycles. Inverter Efficiency was found 87% at full load. This is a good efficiency for DCM operation with such high voltage gain.
As switches SW2 and SW3 are modulated using sine wave this cause a large oscillation in the PV output voltage, this large oscillation make MPPT controller doesn't work well so that a large capacitor is used in the input, The value of the switched inductors is very small as the converter operates in the DCM mode and output filter is very small due to sine modulation of the converter switches SW2 and SW3. Figure 11 shows simulation results of the grid current which is in phase with a grid voltage. Unity power factor is achieved without any feedback from grid current this is due to the converter operating in DCM mode. Figure 12 shows output power of the PV module, the figure indicates that the MPPT control is operating well and able to extract maximum power from PV modules. Figure 13 shows one inductor current as shown each conductor carry current for only one half cycle, the current is discontinuous and the ripple is high due to DCM operation.
(a)
Figure 10: Flowchart of the MPPT control.
Table .1 Electrical characteristics of Bp 485 PV module
(b)
(c) Figure 9: operation modes of the inverter during positive half cycle (a) mode 1 when SW3 is on while D4 is off (b)mode 2 when SW3is off while D4 is on (c) mode 3 when SW3 and D4 are off
Figure 14 and 15 show the switches’ pulses. Figure 14 (b) and Fig. 15 (b) show that switches SW4 and SW1
66
Electrical Characteristics Maximum power (Pmax)
BP 485
Voltage at Pmax (Vmp)
17.8V
Current at Pmax (Imp)
4.9A
Short-circuit current (Isc)
5.4A
Open-circuit voltage (Voc)
22.0V
Temperature coefficient of Isc
(0.065±0.015)%/ °C
Temperature coefficient of Voc
-(80±10)mV/°C
Temperature coefficient of power
-(0.5±0.05)%/ °C
85W
Figure 11: grid current multiplied by 50 and grid voltage Figure 15: Switches SW1 and SW2 pulses (a)-switch SW2 pulses (b)switch SW1.
Figure 12: Output power of PV module.
Figure 16: Switches SW1 and SW4 Experimental pulses
VI.
EXPERIMENTAL RESULT
A prototype for the proposed PV system has been established in the lab. The prototype was built using five inductors four 120µH for the two converters and one 3.5 mH for the filter, three MBR40250G diodes and one RURG8060 diode, two capacitors 10 mF and 1µF, low switching frequency switches, SW1 and SW4, were constructed using two IRFP27N60KPBF switches while the two converters switches were built using IXFT36N50P switches. FPGA kite were used to generate switches pulses and to perform maximum power point tracking on the inverter. Figure 16 shows switches SW1 and SW4 pulses, as shown each switch operates for only one half cycles. Figure 17 shows switch SW3 pulses, the switch is operating at switching frequency equal to 10 kHz to obtain high quality output and reduce output filter. First, the PV output has been checked for MPPT. A resistive load of 880 ohm was used as a load. The obtained PV output which will be the input voltage of the inverter is 20V as shown in Fig. 18, this creates an output voltage of the inverter of 314 V as shown in Fig. 19. The shape of the voltage is sinusoidal and with low ripple.
Figure 13: Inductor current.
Figure 14: switches SW3 and SW4 pulses (a)-switch SW3 pulses (b)switch SW4
67
cycles and operates in DCM to provide unity power factor control. Due to the high voltage gain of the converter, a reduction in the number of modules connected in series is used and this helps in overcoming the environmental variations such as shadows which reduce output power of the PV modules. Simulation and experimental results assure the proposed idea of the single stage interleaved inverter for high voltage gain applications.
ACKNOWLEDGEMENT The authors gratefully thank the ministry of Science, Egyptian science and technology development funds (STDF project No 346), for supporting this project. Figure 17: Switch SW3 Pulses.
REFERENCES [1]
[2]
[3]
R. C´acere and I. Barbi, " A boost dc–ac converter: analysis, design, and experimentation," IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 14, NO. 1, JANUARY 1999. N. Kasa and T. Iida, “Fly back type inverter for small scale photovoltaic power system,” in Proc. IEEE IECON, 2002, vol. 2, pp. 1089–1094. S. Jain and V. Agarwal, " A Single-Stage grid connected inverter topology for solar PV systems with maximum power point tracking," TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 5, SEPTEMBER 2007
[4]
[5] Figure 18: PV output voltage.
C. Wang, “A Novel single-stage full-bridge buck-boost inverter,” IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol.19, No. 1, January 2004. B. Axelrod, Y. Berkovich and A. Ioinovici, "switched-capacitor/ 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 [6] [7]
Figure 19: Output voltage of the inverter.
VII. CONCLUSION A single stage buck boost inverter was proposed in this paper, the inverter has desirable features such as simple control, low switching losses, lower number of switches and low cost. The inverter consists of two interleaved buck boost converter each one operates for only one half
68
T. Liang, Y. Kuo and J. Chen, ‘‘Single-stage photovoltaic energy conversion system,’’ Electric Power Applications, IEE ProceedingsVolume: 148.2001. www.solarcellsales.com/techinfo/docs/bp-485.pdf.
