2014 Annual IEEE India Conference (INDICON)
1
Modeling and Simulation of Hybrid MPPT Based Standalone PV System with Upgraded Multilevel Inverter A. Rout, S. Samantara, G. K. Dash, S. Choudhury, R Sharma, B. Dash
Abstract— This paper presents modeling and simulation of a standalone PV (Photo-Voltaic) system with upgraded multilevel Inverter using MATLAB/SIMULINK. The modeling includes a 25 level inverter which claims that the total harmonic distortion (THD) reduced to approximately 3%. The proposed 25 level inverter uses only 12 numbers of switches which is very less than that of diode clamped, capacitor clamped and cascaded H-Bridge multilevel inverter (MLI). In this paper a hybrid MPPT is used to extract maximum power from PV string/Cell at different irradiance. It also gives a comparative analysis of THD with and without Boost converter based PV system. Index Terms— Boost Converter, MPP, MPPT, MLI, PV System, THD
I. INTRODUCTION
I
st
N the 21 Century, it is a big challenge to meet the power demand with the existing conventional resources. To meet the power demand the PV system is rapidly penetrating to the energy market because of its favorable nature like clean, safe , abundant and could serve the society for longer period. The solar panel is the intermediate which converts the solar energy into electrical energy and the power generated by the panel generally depends on the panel temperature and irradiance. To get the required voltage/power level, the panels are connected in series which is facing towards south and sun is moving from east to west, so there is a variation in the panel temperature and in the irradiance. As a result power variation occurs. In order to get a constant level of output, maximum power point tracking (MPPT) technology is introduced. There are different types of MPPT algorithm like true seeking A. Rout is with the Department of Electrical & Electronics Engineering, ITER, S‘O’A University, Odisha, India (e-mail:
[email protected]). S. Samantara is with the Department of Electrical & Electronics Engineering, ITER, S‘O’A University, Odisha, India (e-mail:
[email protected]). G.K. Dash is with the Department of Electrical & Electronics Engineering, ITER, S‘O’A University, Odisha, India (e-mail: dash.gopalkrishna07 @gmail.com). S. Choudhury is with the Department of Electrical Instrumentation & Control Engineering, ITER, S‘O’A University, Odisha, India, (e-mail:
[email protected]) R. Sharma is with the Department of Electrical Instrumentation & Control Engineering, ITER, S‘O’A University, Odisha, India, (e-mail:
[email protected]) B. Dash is with the Department of Electrical Instrumentation & Control Engineering, ITER, S‘O’A University, Odisha, India, (e-mail:
[email protected])
978-1-4799-5364-6/14/$31.00 ©2014 IEEE
methods and quasi seeking methods [1] are used in the PV system. The P&O (hill-climbing method) method is used in many cases in which small perturbations are introduced in the system in order to vary the operating point such that the maximum power point is achieved [8]. The drawback of this method is described by [9], that they can easily lose track of Maximum Power Point (MPP) if irradiation changes rapidly and an alternative to the P & O method that is Incremental Conductance (IC) is proposed in [9]. The method is more adaptive to sudden change in atmospheric conditions. Many people uses parasitic capacitance method to get Maximum Power Point (MPP) but this method leads to complexity in control circuit [10]. In [2] a feed forward fuzzy based algorithm is used to change the duty ratio of the converter so as to match the output voltage of PV cell with the corresponding voltage at MPP. This paper proposes a hybrid MPPT which uses advanced P&O with IC and it is found that the tracking algorithm is suitable for any insolation/irradiance. The PV system generally employs boost converter along with an inverter [3] to supply AC power to the utility. The converter/inverter uses power semiconductor switches for their required operation. The tracking algorithm changes the duty cycle of the converter in such way that the PV cell output voltage equals the voltage corresponding to maximum point at any irradiance/insolation. Use of converter/inverter in a PV system introduces harmonics to the utility which deteriorate the power quality. Many topologies are proposed in the literature to reduce harmonic contained in the utility voltage/current. A digital PI current control algorithm is used in a five level neutral clamped inverter to achieve high dynamic performance with low THD [4]. A new type of multilevel inverter with less number of switches is proposed in [11] to reduce THD. To produce multilevel ac output, different level of dc input is provided by means of several PV strings [5]. In this paper an upgraded multistring 25-level inverter is proposed, that uses only 12 numbers of switches which is very less than that of conventional multilevel inverter. It is found that the harmonic contain in the utility voltage is about 3% and also meets the IEEE-519 standard. In this paper a comparison has been made on THD with and without using boost converter along with the PV cell/string. The model of PV system with and without boost converter is shown in Fig.1 and Fig.2 respectively.
2
The PV strings are directly connected to the inverter without boost converter. The proposed model is shown in Fig.4.
Fig. 1. PV System without Boost Converter MPPT Boost Converter 1
MPPT Boost Converter 2
Proposed 25-level Inverter
MPPT Boost Converter 3
MPPT
Load
Boost Converter 4
Fig. 2. PV System with Boost Converter Fig. 4. Matlab/Simulink diagram of PV system without Boost Converter
II. MODELING OF PV SYSTEM A. Modeling of PV Cell A PV cell is a device made of semiconductor materials which converts solar radiation into electricity. Equivalent circuit of the PV cell is shown in Fig.3.
LOAD
Fig. 3. Equivalent circuit of PV Cell
Applying Kirchhoff’s Current Law at the junction of Insolation current Iph, Diode current ID, shunt resistance Rsh and the series resistance Rse, we get
I = I ph − I D − I sh
(1)
C. Proposed Twenty five level Multilevel Inverter The 25-level proposed inverter uses only 12 switches compared to other type of conventional multilevel inverter which uses 48 switches and 12 separate dc sources. But in proposed inverter the requirement of separate dc source is only four. Separate dc supply to the MLI is provided using PV strings. To maximize the voltage level the number of dc voltage cell should be equal to the number of cascaded submultilevel cell. i.e. c1 = c 2 = ....c m (3) Where c1 is the number of dc voltage source in the 1st cell, c2 is the number of dc voltage source in the 2nd cell, m is the number of cascaded sub-multilevel cells. In this model c1 = c 2 = c = 2 and value of dc voltage in the 1st and 2nd sub multilevel cells are 19.16V and 95.83V respectively. In the proposed model the value of 2nd sub multilevel cell is five times greater than that of 1st sub multilevel cell according to (4).
Vdc 2 = (2c + 1) k −1 Vdc1
⎡ ⎛ V + IR se ⎞ ⎤ ⎡ V + IR se ⎤ ⎟⎟ − 1⎥ − ⎢ I = I ph − I 0 ⎢exp⎜⎜ ⎥ (2) V R T sh ⎣ ⎦ ⎝ ⎠ ⎣ ⎦ Where I is the Cell current, I0 is the reverse saturation current, V is the cell voltage,
VT =
KT is the thermal q
voltage, K is the Boltzmann constant, T is the temperature in Kelvin, q is the charge of an electron. B. PV system without Boost Converter In this paper the modeling and simulation of PV system is done using MATLAB/SIMULINK. Four PV strings are used for the modeling. Out of 4 strings, two strings are containing 34 PV cells each in a series parallel fashion to provide output voltage of 19.16V each, whereas other two strings contain 170 PV cells in series parallel fashion to provide output voltage of 95.83V each. The total rating of the PV array is 230V, 1kW.
Fig. 5. Circuit diagram of proposed twenty-five level inverter
(4)
3
Here k=2= number of cascaded cell, Vdc1 is the dc input voltage of the 1st cell in volt and Vdc2 is the dc input voltage of the 2nd cell in volt. The number of level is equal to N L = (2c + 1) = 25 . Circuit diagram of the proposed MLI is shown in the Fig.5. Table I provides the information about the logical switching sequence for making twenty five levels in the output voltage. m
TABLE I
Logical Switching Sequence for MLI S S S S T T T T T T T T 1 2 3 4 1 2 3 4 5 6 7 8
VLOAD
1 0 0 0 1 1 1 0 0 1 1 0 0 1 0 0 1 1 0 0 0 1 1 1 0
12Vdc 11Vdc 10Vdc 9Vdc 8Vdc 7Vdc 6Vdc 5Vdc 4Vdc 3Vdc 2Vdc Vdc 0 -Vdc -2Vdc -3Vdc -4Vdc -5Vdc -6Vdc -7Vdc -8Vdc -9Vdc -10Vdc -11Vdc -12Vdc
0 1 0 1 0 0 0 0 1 0 0 1 0 0 1 1 0 1 1 1 1 0 1 0 1
1 1 1 1 1 0 1 0 0 0 0 0 0 1 1 1 1 1 0 1 0 0 0 0 0
0 0 0 0 0 1 0 1 1 1 0 1 0 0 1 0 0 0 1 0 1 1 1 1 1
1 1 1 1 1 1 1 1 0 0 1 1 1 0 0 1 1 0 0 0 0 0 0 0 0
1 1 0 1 1 1 1 0 0 0 1 1 0 0 0 1 1 1 0 0 0 0 1 0 0
0 0 1 0 0 0 0 1 1 1 0 0 0 1 1 0 0 0 1 1 1 1 0 1 1
0 0 0 0 0 0 0 0 1 1 0 0 1 1 1 0 0 1 1 1 1 1 1 1 1
1 1 1 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 0 0 0
1 1 1 0 0 1 1 1 1 1 0 0 0 1 1 1 0 0 0 0 1 1 0 0 0
0 0 0 1 1 0 0 0 0 0 1 1 0 0 0 1 1 1 1 1 0 0 1 1 1
0 0 0 1 1 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 1 1 1
D. PV System with Boost Converter and MPPT PV strings are connected to the load through boost converter and MLI. Initially the PV module provides the required amount of voltages to the dc-dc converters using proposed MPPT techniques. Converter output is fed to MLI to supply power to the AC load. In this system seven PV strings of voltage rating 19.16V each are used for the purpose of modeling. The basic components of the given PV system are 1. Boost Converter 2. MPPT 3. MLI 1) Modeling of Boost Converter The state space model of boost converter [7] using Matlab/Simulink is shown in Fig.6.
Fig. 6. State space model of Boost Converter
The state equations are given in (5)-(6).
(1 − D ) V + 1 V di L −r iL − =− c dt L L L dVc (1 − D ) 1 = iL − Vc dt C CR L
(5) (6)
Where iL is the current through the inductor in A, Vc is the voltage across the capacitor in V, D is the duty ratio. The parameters involved in the design of boost converter are given in the Table II. TABLE II
Design parameters of Boost Converter S.N 1.
PARAMETER Inductance L (mH)
VALUES 1
2.
Capacitance C (µF)
62
3.
Inductance series resistance(r)
0.1
4.
Load resistance R(Ω)
10
5.
Switching frequency Fs (kHz)
50
The duty ratio D is given by (7)
D=
Vout − Vin Vout
(7)
Where Vout is the desired output voltage in Volt and Vin is the input voltage in Volt. 2) MPPT algorithm The hybrid MPPT used in the PV system uses both advanced P&O and IC algorithm simultaneously to track maximum power irrespective of any changes in irradiance. The algorithm is shown in Fig.7. The advanced P&O algorithm [12] estimates reference voltage between every two perturbs, which significantly increases the tracking speed without disturbing the tracking accuracy. In IC algorithm [6] step size is generally fixed. Larger is the step size larger is steady state oscillation, which leads to low efficiency. Such dilemma is solved by using variable step size instead of fixed step size.
4
Fig. 7. Hybrid MPPT Algorithm used for PV System
5
In this paper the accurate duty cycle to get MPP is provided to the dc-dc converter by comparing the updated pulses from both the algorithm. 3) Multi-level Inverter The proposed MLI is already discussed in sec. II.C. III. SIMULATION RESULTS A. PV System without Boost Converter The output voltage waveform is shown in Fig.8. It is observed that the proposed MLI has twenty five level output with twelve switches. The model makes switching loss less as compared to the conventional multi-level inverters because of less number of switches. Fig. 10. Output voltage waveform of MLI with boost converter
The harmonic content in the output voltage is found through FFT analysis. It is measured by THD and is found to be 12.14%. The FFT analysis is shown in Fig.11.
Fig. 8. Output voltage waveform of twenty-five level inverter
The harmonic content in the output voltage is found through FFT analysis. It is measured by THD and is found to be 3.21%. The FFT analysis is shown in Fig.9.
Fig. 11. THD of 25- level inverter with boost converter
IV. CONCLUSION
Fig. 9. THD of 25-level inverter without boost converter
B. PV System with Boost Converter The output voltage across the MLI has 25 levels output with twelve switches. The MLI is fed from the output of the boost converter, which has already been discussed in the earlier section. The output voltage waveform is shown Fig.10
This paper proposes a standalone PV system with and without boost converter using 25-level inverter. The filtration of lower order harmonics is a challenging issue. So as to reduce the harmonic contents the level of the output voltage is increased up to 25 levels by proper arrangement of multilevel inverter with reduced number of switches. The comparative analysis of THD between the two system discussed in this paper says that the THD is increased by introducing the DCDC converter which is approximately found to be 12% but it is less as compared to the other conventional multilevel inverters used for PV system . It also reduces the rating of the PV cell. The rating of the PV cell is higher in the system when the dcdc converter is not included.
6
V. REFERENCES Periodicals: [1]
[2]
[3] [4] [5] [6]
V. Salas , E. Olias , A. Barrado and A. Lazaro "Review of the maximum power point tracking algorithms for stand-alone photovoltaic systems", Sol. Energy Mater. Sol. Cells, vol. 90, no. 11, pp.1555 -1578, 2006. M. Veerachary, T. Senjyu, K. Uezato, "Feedforward maximum power point tracking of PV systems using fuzzy controller", IEEE Trans. Aerospace and Electronic Systems, Vol. 38, No. 3, pp. 969-981, July 2002. S. Jain and V. Agarwal “A single-stage grid connected inverter topology for solar PV systems with maximum power point tracking", IEEE Trans. Power Electron., vol. 22, no. 5, pp.1928 -1940 2007. A. Ravi, P. Manoharan,, J. V. Anand, “Modeling and simulation of three phase multilevel inverter for grid connected photovoltaic systems”. Solar Energy , 85, 2811–2818. N. A. Rahim and J. Selvaraj "Multistring five-level inverter with novel PWM control scheme for PV application", IEEE Trans. Ind. Electron., vol. 57, no. 6, pp.2111 -2123 2010. M. D. Goudar, B.P. Patil and V. Kumar “A review of Maximum peak power tracking algorithm for photovoltaic systems” Int. Jour. EET, vol 1, no.1,pp.85-107, 2010.
Books: [7]
M.H. Rashid, Power electronics: Circuits, Devices and Application, 3rd ed. Pearson Education, 2004, pp.217-221.
Papers from Conference Proceedings (Published): [8]
H. Al-Atrash, I. Batarseh, K. Rustom, “Statistical modeling of DSPbased hill-climbing MPPT algorithms in noisy environments Applied Power Electronics Conference and Exposition”, IEEE APEC 2005, vol. 3, 6–10 March 2005, pp. 1773–1777. [9] K. Hussein, I. Muta, T. Hoshino, and M. Osakada, “Maximum photovoltaic power tracking: An algorithm for rapidly changing atmosphere conditions,” in Proc. Inst. Elect. Eng., vol. 142, Jan. 1995, pp. 59–64. [10] A. Branbrilla,M. Gambarara, A. Garutti, F. Ronchi, “New approach to photovoltaic arrays maximum power point tracking” , in Proc. 30th IEEE Power Electronics Conference, 1999, pp. 632–637. [11] V.S. Prasadrao k, P. Sudharani and G Tabita, “A new multi level inverter topology for grid interconnection of PV systems" in Proc. PESTSE, Mar.2014, pp.1-5. [12] Liu C., Wu B., and Cheung R., “Advanced Algorithm for MPPT Control of Photovoltaic Systems,” 1st Canadian Solar Buildings Research Network Conference, Aug. 2006.
