2nd Int'l Conf. on Electrical Engineering and Information & Communication Technology (ICEEICT) 2015 Jahangirnagar University, Dhaka-I 342, Bangladesh, 21-23 May 2015
Maximum Power Point Tracking Using Very High Frequency Resonant DC/DC Converter for Photovoltaic Systems Md, SaifIftekhar, Md. Rabiul Hasan, Rana Banik, Rubaeat Umar, Chinmoy Barua Department of Electrical & Electronic Engineering Chittagong University of Engineering & Technology Chittagong, Bangladesh. E-Mail:
[email protected]@gmail.com Abstract- A modified Very-high-frequency (VHF, 30-
300 MHz) resonant converter has been introduced in this
proposed system of maximum power point tracking
(MPPT)
to
extract solar power out of PV arrays with greater stability. For the purpose of assessing the duty cycles of resonant, MPPT controller is used. The proposed modified design of this type of converter features reduced conduction loss at the load side, reduced core loss, low device voltage stress, high efficiency over a wide
load range,
and
excellent
transient
performance.
The
conduction losses have been reduced based on soft switching technique by replacing the diode with MOSFET. For all kinds of simulation and development, MATLAB Simulink is used. The overall system efficiency, amount of ripple current on both PV side and load portion, power on both sides and investigation on system stability under several conditions have been considered. From the simulation result, it can be seen that the overall system efficiency has been improved. The significant advantage of this proposed system is that there are low ripple content in both the current of PV side and load side.
KeywordsObservation
PV
System,
MPPT,
Perturbation
Algorithm,
Percent
Ripple,
VHF
Solar
&
Resonant
Converter.
I.
INTRODUCTION
The energy generation system from renewable sources (solar, wind etc.) is popular nowadays because of its simplicity of maintenance and for being environment friendly. The major concern is focused on solar energy since the dramatic improvement in semiconductor physics which made it easier to extract energy from the solar cell [1]. The main hindrance, behind the mass use of it, is low efficiency comparing to high installment cost. Nevertheless, solar energy is the main source of green energy whose pollution level is minimum comparing with other known energy sources [2] [3]. However, power generation from solar varies based on the variation of solar irradiance, temperature, dust, shadow etc. [4]. For variation of this parameter, the output voltage of the PV cell fluctuates and become inconsistent. So, it is now the main concern of scientists and researchers to increase the efficiency of energy extraction by using some techniques such as mechanical tracking, electrical tracking. The main problem of mechanical tracking is the bulky system. On the other hand, it is much convenient to use electrical tracking system by using MPPT algorithm which is operated by DC-DC converter. The MPPT
978-1-4673-6676-2/15/$31.00 102015 IEEE
tracking efficiency depends on the chosen MPPT algorithm and the converter topology [5]. In this research, the VHF resonant DC-DC converter has been modified to increase the converter efficiency and also the parameter of the whole has chosen in a way so that the MPPT efficiency has also been increased. II.
PROPOSED METHODOLOGY
In fig. 1, the basic form of the designed system has been shown. Resonant converter has been used in this particular system with a view to extracting solar power out of PV arrays with the greatest stability. MPPT algorithm is used for the purpose of assessing the duty cycles of resonant. In this system, Perturb and Observe (P&O) algorithm have been used as it is comparably practical and easy to implement [6]. For this particular algorithm, voltage and current are taken as reference and these are used to calculate extracted power from PV side continuously. If the power is greater than the previous calculated power, the duty cycle of the converter will climb up; otherwise, the duty cycle will be decreased according to this algorithm. Thus it changes the pulse width as well as the output voltage to track the maximum power point. In this research work, the resonant DC-DC converter topology [7] has been chosen to ameliorate its performance comparing to previous practice in MPPT system [8]. Soft switching technique, such as: zero voltage switching and zero current switching, is used in building the L-C network based resonant DC-DC converter [9]. It can run at higher switching frequencies in comparison with other PWM based converters. This paper shows that the overall performance can be upgraded just by replacing the diode of the load end, i.e. the previous design [7], with a switch. As an example, the conduction losses will be reduced due to the use of MOSFET
switch at the load side instead of diode.
Fig. 1. PV system with maximum power point tracking circuit.
Fig. 2. Equivalent circuit of a solar cell consisting of discrete electrical components.
III. SOLAR PHOTOVOLTAlC SYSTEM A solar cell (basic unit of PV module) converts energy that lies in the photons of sunlight into electricity by means of particular semiconductor materials like silicon and selenium. The PV module generates a low output DC voltage. The equivalent circuit of a solar cell is shown in fig. 2. Solar cell's characteristic is exponential and there lies a non-linear relationship between the output current and voltage of a PV module [10] [II] [12] [13]. Solar cells are joined together generally in series and parallel to form PV modules. Here, the value of the energy storage components are computed by taking into account 10% ripple voltage and ripple current. The current equation [14] of a module is given below-
10
np1ph - np1rs[exp
=
e�:) - 1]
(1)
Here, 10 output current of the PV array; V output voltage of the PV array; ns the number of PV cells connected in series; np the number of such strings connected in parallel; Iph photo current; Irs reverse saturation current; ko Boltzman Constant. =
=
=
=
=
=
=
The photocurrent of the cell can be gained by
1ph
=
[lscr
+
ki (T - Tr )(--�-) 100
]
(2)
Here, Iscr short circuit current at reference temperature and radiation; ki temperature co-efficient of the short circuit current; Tr cell reference temperature; 2 S solar radiation presented in mW/cm =
=
=
=
IV.
MPPT ALGORITHM
Maximum Power Point moves throughout the day depending on the various factors of the environment. I-V and P-V curves at different irradiance and temperature have shown that PV panel terminal voltage and current are affected by the factor irradiance and temperature [13]. As the significant voltage shifts occur at MPP, so it should be continuously tracked to get maximum power. This is done by changing the duty cycle of DC-DC converter which perturbs the voltage continuously. A control algorithm is developed to ensure that the operating point is at its maximum value. There are various types of algorithm such as perturbation and observation (P&O), Incremental Conductance (IncCond) method, fuzzy logic etc.
Considering the simplicity of implementation, tracking speed with high efficiency, P&O method is chosen as an MPPT algorithm [11] to perturb the voltage to operate the PV array at MPP. To find out the MPP, operating voltage is perturbed by a small amount dv and then measured the change of power dp. If power increases, direction of the next perturbation will be same. And if power decreases then the direction of next perturbation will be in opposite. This process is repeated again and again until reaching at MPP. At this time operating point oscillates around the MPP. The oscillation around the MPP can be reduced by using small step size of dv. Tracking speed is reduced at this time. So, there is a trade-off between this oscillation and the tacking speed. This problem can be solved by using a larger step size at first and then that size is reduced to a small amount. The flow chart of the MPPT algorithm is shown in fig. 3. At first, voltage and current are sensed. Then, by using those values, available power has been measured. After that, this algorithm compares this value with the previous iteration value [14]. v. SIMULINK MODEL OF PROPOSED SYSTEM In this research work, resonant DC-DC converter is operated at very high frequency in the range of Megahertz (MHz). This type of resonant DC-DC converter is a classical class- inverter which is actually a new version and it is coupled to a resonant rectifier. The complete PV system (fig. 6) including MPPT using VHF Resonant DC-DC converter has been modeled in MATLAB/Simulink and SimPowerSystem environment. The electrical characteristic of the PV module under standard temperature, standard irradiance and 1.5 illumination (STC) that has been used for the purpose of simulation [14] has been listed in TABLE I. The switching frequency of this converter is setup at 100 MHz. All the selected and optimized component parameters (TABLE II) of the converter have been transferred in MATLAB. The schematic arrangement of the resonant DC-DC topology is shown in fig. 4 which operates at very high frequency [12]. It is basically an interconnection of class- inverter and resonant rectifier as mentioned earlier. The class- inverter is a multi-resonant network comprises LF, L2F, CF and C2F• This low ordered network is designed to approximate the symmetrizing properties of a quarter-wave transmission line. To reduce peak voltage stress across the switch, the component LF, Lm CF and C2F are tuned to resonate near the second harmonic of fs (Switching Frequency) to prevent a low drain to source impedance at the second harmonic. The converter uses air-core inductors required for the low energy storage, which eliminates magnetic core loss and introduces the possibility of easy integration. This kind of design is used for low device stress and very high frequency (VHF) operation for a fixed frequency and duty ratio, D. AC-DC power conversion is controlled by switching frequency. The resonant elements Lrect and Crect are selected to deliver the desired power at the specified output voltage. Here, the diode which exists in the original design (fig. 4) [12] has been replaced by a MOSFET switch (fig. 5) which has been placed at the load side of the resonant converter with a view to reduce
conduction losses caused by the use of diode in the previous topology. The PV module is designed using electrical characteristics to get the PV module's output voltage and current. By using this, we get the output power of PV module. This power then works as an input for converter and controller. At fixed temperature (25°C), the irradiance has been changed to test the operation. The illumination level 2 2 starts from zero W/m and reaches at 1000 W/m within less than one second. It stays there until 5.00 second. Sseconds 2 later the illumination level drops to 900 W/m and stays there for rest of the time period. It is clear that the extracted power 2 has negligible delay before reaching the MPP for 1000W/m 2 irradiation but has delay during dropping down to 900W/m 2 before reaching the MPP. At 1000W/m irradiance, the extracted average power by the MPPT is 100.09921W and the maximum power at that condition is 100. I04SW. MPPT Efficiency is the ratio of the extracted maximum power from the PV array to the power at the MPP. The MPPT efficiency has been found approximately 99.995%.
Sampling Time Chosen in Simulink Design
50µs
TABLE!. ELECTRICAL CHARACTERISTICS OF PV MODULE AT STC Maximum power (Pmax) Voltage at MPP (Vmpp) Current at MPP (Impp) Open circuit voltage (Voe) Short circuit current (Ise) Diode Constant (a) Array with NssxNpp Modules Fill Factor, FF Short Circuit Current Temp. Coefficient Open Circuit Voltage Temp. Coefficient Module Size Maximum Power Temp. Coefficient NOCT
100W±5% 17.50V 5.72A 21.60V 6.35A 1.55 Nss:Npp 1:1 0.729805 6. 928mAfC =
The output power of proposed resonant converter is shown in TABLE III for different irradiance levels which are very much satisfactory. For example, the result shows that at irradiance, G = 1000 W/m2 and 600 W/m2, the output power is 99.76W
-0.068VfC 36 Cells (4x9) (0.5±0.05)%fC 2YC
TABLE II. Optimized parameters of resonant DC-DC converter (New Topology). Parameters
Fig. 3 Flow chart for Perturbation & Observation method.
Fig. 4. Electrical circuit of very high frequency (VHF) resonant DC-DC converter.
Specification
Inductor, Lj
10llH
Inductor, L2 Inductor, L3
2SIlH 6S0llH
Capacitor, C 1
140llF
Capacitor, C2 Capacitor, C3
470llF
Capacitor, C4
470llF
Load Resistor, R
3.0700
470llF
P&O Cycle
100Hz
Switching Frequency
100 MHz
Fig. 5. Electrical circuit (Proposed) of very high frequency (VHF) resonant DC-DC converter.
Fig. 6. Simulink model of proposed methodology.
and 61.16W respectively. The input and output power of this resonant converter for variable irradiance levels; i.e. for 2 2 1000W/m and 900 W/m are represented graphically in fig. 7 and fig. 8 respectively.
Converter Efficiency is the ratio of the output power Po to the input power Pin.
Converter Efficiency [T]m(%)]
�
=
(5)
Pin
Overall Efficiency [T] (%)] MPPT Efficiency [T]m(%)] x Converter EfficiencY[T] c(%)] (6) 2 Here, at 1000W/m , for previous DC-DC resonant converter, Overall efficiency [T] (%)] [T]m (%)] x [T] c (%)] =
VI. RIPPLE CONTENT
The percent ripple exists in current in both PV side and load side is computed by the equation belowf.
· Currentpeak(Upper)-Currentpeak(Lower) O X 10 Rlpp I e = CurrentAvg.
100% (3)
The average value of the current ripple in the PV side for the previous topology is about 5.55853A and current ripple is approximately 5.13% of the average value. On the contrary, for resonant converter based MPPT system, the average value of the peak current is around 5.7022415A and the current ripple is almost 0.74585% of the average value. The average value of the current ripple in the load side for the previous topology is about 5.51695A and current ripple is approximately 3.3987% of the average value. On the other hand, for resonant converter based MPPT system, the average value of the peak current is roughly 5.7009105A and the current ripple is almost 0.0021225% of the average value. This can be considered as ripple free system. The ripple current characteristic is shown in the fig. 9 and fig. 10 for 2 irradiance level 1000W/m . By this way, this resonant converter based MPPT system is much better than conventional topology of resonant converter based MPPT system in terms of ripple current characteristics. VII. EFFICIENCY
MPPT Efficiency is the ratio of the extracted maximum power from the PV array to the power at the MPP. MPPT Efficiency
[T]m(%)]
=
p. -E!:. PM
Here, Pin Extracted maximum power from the PV array; PM the power at the MPP. =
=
(4)
=
=
99.99%
x
97.68%
=
97.67%
But for proposed resonant converter, Overall efficiency [T] (%)]
=
[T]m (%)]
x
[T] c (%)]
99.99% x 98.68% 98.67% 2 Again, for 650W/m , for previous DC-DC resonant converter, Overall efficiency [T] (%)] [T]m (%)] x [T] c (%)] =
=
=
=
97.01%
x
93.58%
=
90.782%
But for proposed resonant converter, Overall efficiency [T] (%)] =
=
[T]m (%)]
98.74%
x
x
[T] c (%)]
99.6722%
=
98.4163%
Therefore, overall system efficiency increases almost 1% (at 2 2 1000W/m ) and around 7.6343% (at 650W/m ) with the proposed MPPT scheme. Similar results are obtained for other irradiance values which are summarized in TABLE III and IV. This result clearly signifies that the proposed resonant converter is much better for tracking down MPP as well as for delivering the load than the previous one. The dynamic MPPT performance with this new resonant converter is also satisfactory and is much better compared to the dynamic MPPT performance with the previous resonant topology. Besides, the result shows that resonant converter based MPPT controller's efficiency in this research is more than the existing MPPT controller system proposed recently [13]. Furthermore, after comparing the proposed system in this paper and the system mentioned by K. Kavitha et al [15]; the proposed one has been
found to be more preferable in practical field considering both the efficiency and the non-inverting output. TABLE III. COMPARISON OF POWER UNDER VARIABLE IRRADIANCE Irradiance G(W/m')
PM(W)
Resonant Converter(New) P'n(W) Po(W)
Resonant Converter(Old) P'n(W) Po(W)
550
51.5028
50.612
50.45
50.02
46.52
650
62.1420
61.359
6l.l6
60.28
56.41
700
67.5034
66.652
66.42
65.86
60.98
750
72.8888
71.496
71.26
7l.l2
66.40
850
83.7227
82.768
82.22
81.38
76.00
900
89.1673
89.499
88.21
86.58
81.74
1000
100.1045
100.097
99.76
100.100
97.78
TABLE IV. COMPARISON OF EFFECIENCIES UNDER VARIABLE IRRADIANCE Irradiance G(W/m')
MPPT Efficiency Resonant Converter (New) 11m (%)
Resonant Converter( Old) 11m (%)
Converter Efliciency
550
98.27
97.13
Resonant Converter (New) 11c (%) 99.672
Resonant Converter(Old) 11c (%)
650
98.74
97.01
99.6722
93.58
700
98.74
97.57
99.6721
92.59
750
98.09
97.71
99.6723
93.34
850
98.86
97.20
99.6723
93.39
900
99.25
97.10
99.6723
94.41
1000
99.99
99.99
98.68
97.68
93.00
Fig. 9. Current Ripple characteristics in PV side for irradiance 1000W/m2.
Fig. 10. Current ripple characteristics at load side for irradiance 1000W/m2.
VITI.
�.
ACKNOWLEDGEMENT
The authors would like to express gratitude to the faculty members of the Department of Electrical & Electronic Engineering, Chittagong University of Engineering & Technology, Bangladesh for giving continuous support, encourage and showing genuine interest in this research work. X.
[I] [2] Fig. 7. Extracted power from PV array vs. Time for variable irradiance.
[3] [4]
[5]
Fig. 8. Output Power of DC-DC Converter vs. Time for variable irradiance.
CONCLUSION
The proposed VHF resonant converter along with the P&O MPPT algorithm improves the performance of PV system in a way so that the performance of converter is also improved. In this system, the conduction loss has been reduced by replacing the diode with MOSFET. The results from the simulation show that the proposed converter with MPPT algorithm gives better efficiency and improvised ripple characteristics compared to the old topology and other recent work.
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