CC BY
9NAUKI INZYNIERYJNE I TECHNICZNE / ТЕХНИЧЕСКИЕ НАУКИ
POWER ELECTRONIC FOR SINGLE-PHASE GRID-CONNECTED PV SYSTEM
Jasim Ali
Postgraduate Student, National Aerospace University "Kharkiv Aviation Institute",
Yuri Shepetov
Associate Professor, Candidate of Science, National Aerospace University "Kharkiv Aviation Institute",
An intelligent technique a high performance of a power electronic interface for single-phase grid connected (SPG) PV system. The functions of the power converter of a PV system consists of Maximum Power Point Tracking (MPPT), a combination of different methods (hybrid methods) is promising, including short circuit current method with of one of the direct methods incremental inductance, DC/DC boost converter, DC/AC universal bridge, which convert the solar dc power into high quality ac power for feeding into the grid power. The proposed system components models are implemented in Matlab/Simulink environment and interfaced with SimPowerSystem toolbox. The suggested topology has several desirable features such as better utilization of the PV array, higher efficiency. Further, due to the very nature of the suggested topology, the PV array appears as a floating source to the grid. All the analytical, simulation and experimental results are presented.
Key words: Photovoltaic array, maximum power point tracking (MPPT), Power control, SPG, Matlab/Simulink.
1. Introduction
Green technology is promising future of the world. it aims at finding ways of producing energy that does not deplete natural sources of energy. it refers to the alternative technology which reduces fuel use and expects less damage to living things and environment. Supposedly, it is the use of a clean technology to reduce pollution caused by the consumption of fossil fuel in the production of energy. one of the most important renewable energy sources is solar radiation [3-6].
PV array is a semiconductor device that generates DC electricity out of sunlight. it is a combination of PV modules connected in series and parallel.
Energy produced by the PV array depends on some parameters such as temperature and solar irradiance. The PV array is connected to a Maximum Power Point Tracker (MPPT) in order to optimize the Dc output power of the PV array by varying the operating voltage of the PV array. The Dc power then is converted to AC power using an inverter before transported to the utility grid.
The output of PV array varies with irradiation, and hence the duty cycle of the controller is adjusted automatically to supply a constant DC voltage to the inverter circuit, the output of which is
directly connected to the grid.
The MPPT, a maximum power point tracking control to extract maximum power from the PV arrays at real time is considered necessary in PV generation system and feeds it to the single-phase utility grid.
Converters interfacing PV module with the grid involves two major tasks. one is to ensure that the PV module is operated at the maximum power point. The other is to inject a sinusoidal current to the grid. Later, these tasks shall be reviewed in this paper. Simulating results are obtained for the normal regime [10 -13].
2. global configuration
The system formation of the power converter is shown in Figure 1.The grid connected PV system transfers power generation from the PV into utility grid.
The first stage is the DC-DC boost converter, which will raise the relatively low solar voltage to a level suitable voltage for the DC link directly connected to the inverter. The second stage is the DC to AC converter that operates in a voltage controlled mode, which will inject to the single phase grid [10-13].
GRID or
STAND ALONE AC LOAD
MPPT Algorithm
hybrid MPPT methods
Fig. 1: Block diagram of configuration of grid connected photovoltaic system.
2.1. PV GENERATOR MODEL
Photovoltaic cell is the most basic generation part in PV system, which are basically a p-n semiconductor junction that directly convert solar radiation into DC current using the photovoltaic effect. It means the photocurrent generated, when the sunlight hits the solar cell can be represented with a current source Iph. The single-diode mathematic model is applicable
to simulate silicon PV cells. This model was represented at and used at this work. PV, usually considered to have the same characteristics, are arranged together in series and parallel to form arrays. Cells connected in parallel increase the current and cells connected in series provide greater output voltages [1-4, 6-10]. The equivalent circuit of PV array as Eq. (1) can be described as illustrated in Fig. 2.
#
i
N,
X
Jï g
N
S
^r v
Fig. 2: Single-diode mathematical model of a photovoltaic array.
Ish
N
N,
Np Rs I
S
-VW
Rp
+
V
Where the output panel current and voltage, are represented as I, (A) and V, (V). The current source Iph, (A) represents the cell photocurrent and IS is the saturation current (A). The series and shunt resistances of the cell, (O) are represented as RS and RP. NS and NP number of the series and parallel cell. PN junction ideality factor is represented as A, Boltzmann constant KB (1.38x10-23), (J/K), q stands for electron charge and PV cell temperature T in Kelvin. Tref PV cell temperature worked in Standard test condition, Tref =298, (K).
Photocurrent mainly depends on the solar insolation and cell' working temperature, which is described as Eq. (2) [2-4, 6-10].
Where ISC is the nominal short circuit current, (A) , KI is the constant coefficient of short circuit current, G , and T , PV
ret ref
cell illumination and temperature worked in STC, Gref =1000 (W/m2), Tref =298,(K).
On the other hand, the cell's saturation current varies with the cell temperature, which is described as Eq. (3) [2-4].
T
rs
ref j
■ exp
qE,
3
AK
B
{Tref T)
(3)
Where Eg is the band-gap energy of the Si solar cell, = 1.16 (eV) depending on the cell material and Irs is the reverse saturation current, (A).
Photovoltaic cells, usually considered to have the same characteristics, are arranged together in series and parallel to form arrays. Cells connected in parallel increase the current and
cells connected in series provide greater output voltages. Three operation points on the I-V curve are of special interest for understanding solar cell operation and for designing photovoltaic systems:
The short circuit point (I=ISC, V=0); the open circuit point (1=0, V=VoC) and the maximum power point (I=IMP> V=VMP) [14, 6-9]. The common structure of PV Array Simulation Model is represented in Figure 3.
Illumination
(G)
Figure 3: PV array model implemented in Matlab/Simulink.
Where functions Fcnl, Fcn2 are the linear equations between short circuit current (I ) & illumination (G) and between open circuit voltage (V ) & illumination (G) respectively. The diodes
CD
Ipv
Œ)
Vpv
D1 and D2 .Simulation of the I-V curve Fig. 4(a) & P-V curve Fig. 4(b) of PV module under changing illumination are represented in Figure 4.
2 1,6
« 1,2
:= O)
<3 0,8j
0,4
it—4—«-
-
--±_
II—■— m—■— —
— -♦— Modeling, 1000 W/m2 -■— Modeling, 800 W/m2 -A— Modeling, 600 W/m2 -■- Modeling, 400 W/m2
50
(a)
100 150
Voltage, V
200
350 300 250 200
ai
I 150 100 50 0
-♦— Modling, 1000 W/m2 ---■— Modeling, 800 W/m2 -A— Modeling, 600 W/m2 -B- Modeling, 400 W/m2
250
50
(b)
100 150
Voltage, V
200
250
Figure 4: Simulation of I-V curve a) & P-V curve b) of PV module under changing illumination.
2.2. HYBRID METHODS MPPT (PROPOSED METHOD) As the output power supplied by the photovoltaic system depends very much upon the weather conditions, i.e. solar irradiation, temperature and shading effects due to clouds, an important consideration in the design of efficient solar array systems is to track maximum power point correctly.
The MPPT methods can be classified into three broad categories: indirect methods, direct methods and hybrid methods. The conclusion is formulated that using of a combination of different methods is promising, including short-circuit current method with of one of the direct methods.
Short Circuit Current (Isc) is one of the simplest indirect methods. There is an approximately linear relationship between the short circuit current (Isc) of the solar panel and the MPP current (Imp), which can be described by the following equation [1-4, 6-13].
Imp ~ -1 SC
(4)
Where, KI is proportionality constant, has to be determined according to the PV array in use. The constant KI [1-6] is generally found to be between 0.78 and 0.92. The SCC flow chart is shown in Fig. 5.
0
0
0
Figure 5: Flow chart of the (SCC) Method.
An additional switch usually has to be added to the power converter to periodically short the PV array so that ISC can be measured using a current sensor. This increases the number of components and cost. A boost converter can be used, where the switch in the converter itself can be used to short the PV array.
This method has a rapid technique of tracking the MPP. To track the power, this MPPT technique requires the value of SCC by isolating the PV array. A way of compensating KI is proposed such that the MPP is better tracked while atmospheric conditions change. The performance stages of the suggested technique are as follows. It measures the ISC during the start of the MPP tracking. The value of short circuit current is then converted numerically to maximum Power Point current using Eq. (4) [1-4, 6-9]. After calculating the duty cycle D, the controller reduces the error.
The duty cycle D is used to drive the DC-DC converter and is adopted as the initial point of performance for the constant coefficient of short circuit current. The constant coefficient of the short circuit current method starts tracking the real MPP with very small steps after operating the DC-DC converter at approximated MPP. However, under varying environmental conditions the limit helps the system to fast track the MPP [3-4, 6-13].
2.3. BOOST CONVERTER
A schematic circuit of the basic boost topology is shown in Fig. 6(a). A boost converter is also called a step-up converter because the output voltage is higher than the input voltage. As a result, the output current is lower than the input current because of the power balance. The operation of the boost is best understood by its two modes of operation, when the switch S is turned ON and OFF [2-4,6-13].
Figure 6. (a): Circuit diagram of DC-DC boost converter
The equivalent circuits in these two modes are shown in Figures 6(b) and 6(c), respectively. During Mode 1, from Eq. (5) we can notice the inductor current increases linearly because
(5)
VL = Vpv
_m
(b)
V = Vpv - Vo
c =r= Vo
(c)
C4= Vo
Figure 6(b): Power switch turned ON, diode turned OFF of mode 1. Figure 6(c): Power Switch turned OFF, diode turned ON of mode 2.
Moreover, the inductor stores energy from the power source while the capacitor discharges to supply the load.
During Mode 2, both the energy stored in the inductor and from the power source are transferred to the load and the capacitor. From Eq. (6) we can notice the inductor current decreases linearly because dlj
L-jT-Vpv-K
(6)
From Eq.(7) we can notice, the net energy changed in the inductor should be zero during one period in the steady state, which means the current increased in Mode 1 should be equal to the current decreased in Mode 2. That is,
DT
I'pv
(1
£Vy,
PV
v„
L L " - (7)
The power switch is responsible for modulating the energy transfer from the input source to the load by varying the duty cycle D [2- 4, 6-13]. The relation between output voltage and the input voltage (solar cell) is given as the Eq. (8).
Vo 1
V,
PV
l -D
(8)
2.4. DC-AC INVERTER
The DC/AC inverter is connected photovoltaic array with boost converter to single phase grid, which used the DC output voltage of the PV array into AC voltage to be connected to the electric utility grid. The single phase full bridge voltage source inverter consist of four power switching blocks. C is used to filter the noise on the DC bus. After the inverter an LC harmonics filter is used to eliminate the high frequencies in the output inverter voltage. Each block of the switching blocks consists of a semiconductor switch (IGBT) and antiparallel diode. To create proper gating signals for switches, pulse with modulation is used [10-13].
3. RESULTS OF NUMERICAL EXPERIMENTS
Output power of PV unit is strongly depended from the value of the duty cycle, and for each value of voltage grid there is corresponded certain value of duty cycle which provides maximum output power Figure 7(a). With increasing of voltage grid the value of duty cycle does increase too.
In the same manner for each value of sunshine illumination there is corresponded certain value of PW which provides maximum output power Figure 7(b). With increasing of illumination the optimal PW decreases.
+
+
Figure 7: output power as function from duty cycle for different net Voltage (a) and Illumination (b).
Thus the problem of control of studying the PV unit consists in finding for each moment of optimal duty cycle which correspond to changeable external parameters (illumination and voltage grid) for providing maximum output power. The matrix of optimal duty cycle is shown on in Table. 1.
There are also exist the losses of power due to dissipate energy under transformation Figure 8(a). They are more with PW increasing. But all the same, this loss is repaid through increasing
of output power.
Regulator transformation efficiency Figure 8(b). Was calculated as the ratio between output and input power for certain external conditions.
The integrated 3D relationship of optimal duty cycle 9(a) and output power 9(b) from illumination and voltage grid is shown in Figures 11.
Table. 1
Matrix of values of optimal PW providing maximum output power
Voltage, Vgrid Illumination, W/m2
600 700 800 900 1000
210 0.12 0.15 0.17 0.18 0.19
220 0.13 0.16 0.18 0.19 0.2
230 0.15 0.18 0.19 0.21 0.22
Table. 2
Matrix of values of optimal maximum output power. Providing PW.
Voltage,Vgrid Illumination, W/m2
600 700 800 900 1000
210 86.3 115.8 148.8 185.6 224.1
220 85.2 114.4 147.2 182.6 221.2
230 83.2 112.1 144.9 180.4 219.1
(a) Duty Cycle (b) Duty Cycle
Figure 8: The relationship between duty cycle & I/O power (a) and & efficiency (b).
Figure 9: 3D relationship of MP duty cycle (a) and Max. output power (b) from illumination and net voltage
4. Conclusion
This study presents to suggest a control methodology for single phase grid connected to PV systems by power electronic. The DC-DC converter was used to boost the output voltage of the PV array and perform MPPT by using hybrid methods including constant coefficient of the short circuit current
method with generally of one of the direct methods. In order to inject a high quality AC current into the single phase grid, a single phase - second stage a voltage-source inverter VSI was conversion efficiency of the PV system. The dynamic behavior of each subsystem is investigated showing the interaction between different components of grid connected PV system.
NAUKI INZYNIERYJNE I TECHNICZNE
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