Single-phase Inverter-interfaced Islanded Microgrids
with PV Systems
M. I. Azim
Engineering Student, Department of Electrical and Electronic Engineering, Rajshahi
University of Electrical and Electronics Engineering, Bangladesh
[email protected]
Abstract. This paper focuses on photovoltaic (PV) integrated single-phase inverter-based
islanded microgrids; in which solar energy is used to run a load, household bulb. The overall
system is constructed via MATLAB simulation, where inverter is included with a view to
converting dc solar energy into ac load. Filtering is also done to avoid the effects of the
harmonic components. Finally, the system is practically demonstrated by designing an
inverter and the experimental outcomes satisfy the performance criteria of a PV-based
microgrid system.
Keywords: PV system, inversion circuit, low pass filter, household lamp.
1 INTRODUCTION
Renewable energy sources (RESs) are usually installed as DG units and they are popular
nowadays to meet the electricity demand due to the economical, technical and environmental
developments [1]. DG units have the ability of providing more local services than the
traditional generators [2] and inverters connect them to the load-side [3], [4]. Microgrids can
be termed as networks integrated with DG units and loads [5]. In general, microgrids provide
plenty of advantages including renewable energy utilization, low system losses and capital
investments and system reliability [6]-[13]. Islanded microgrids, having an autonomous
operating feature, are commonly utilized in order to maintain adequate power flow and
desired voltage profile [14], [15].
PV is one of the most renowned renewable energy sources in today’s world. This is
because some advantages are provided by them such as small system size, noise-free
operation and feed in tariff [16]-[18]. PV gives dc output but the grid voltage is ac. Hence,
fast acting dc/ac converters (inverters) are used extensively in PV systems [19] - [22].
Single-phase PV inverters for islanded microgrids are discussed by the authors in [23],
[24]. It is shown that proper modulation should be implemented in the switching devices of
the inverters otherwise; harmonics components may exist at the output terminals of the
inverters. This may lead to the mal-operation of the system [25]-[27]. Consequently, in [28],
a low-pass filter is used so that Total Harmonic Distortion (THD) can be kept within the
defined range.
The practical implementation of PV-based microgrids is studied in [29]. The sound operation
of the system is dependent on the precise design of solar inverter. Therefore, the design of a
practical solar inverter is described in this paper and the performance of the designed circuit is
evaluated by operating a load (light lamp).
The rest of the paper is organized as: Section 2 provides the modeling of PV system
including the mathematical expression of PV arrays and solar energy integration into
microgrid system. The experimental set-up is given Section 3. This section contains the realtime performance of the designed system and the last section outlines the summary of the
work.
2 MODELING OF PV SYSTEM
One of the well-known renewable resources is solar energy, which is abundant in nature
especially in most summer predominant countries. PV modules are used by solar islanded
systems to supply total electric needs. Apparently, PV systems have a number of advantages
over conventional power generating technologies. PV systems can be designed for a variety of
applications and operational requirements, and can be used for either centralized or distributed
power generation. Moreover, PV systems have no moving parts, are modular, easily
expandable and even transportable in some cases. More importantly, energy independence
and friendly environmental operation are the two prominent features of PV systems. In
general, PV systems that are well designed and properly installed require free fuel such as
sunlight, minimal maintenance and have long service lifetimes [30]. For this reason, grid
connected PV systems are gaining popularity due to the feed in tariff and battery cost
reduction.
However, the intermittent PV generation varies with the changes of atmospheric conditions.
Maximum Power Point Tacking (MPPT) techniques are used to deliver maximum power into
the grid [31]. Configuration of a typical grid-tied PV system [32] is illustrated in Figure 1. A
grid-tied PV system is proposed in this paper which serves energy to the grid so that on-site
critical load can be backed up when grid power is not available. As is observed in “Fig. 1”, a
number of conversion processes are applied to get desired voltage that can be applied to the
grid system.
Fig. 1. Block diagram of PV-based microgrid system [32].
A photovoltaic cell is nothing but a p-n junction diode which converts solar irradiation into
electricity in a direct way. It is also known as solar cell. A single photovoltaic cell can be
modeled by utilizing a current source, diode, series and shunt resistors as seen in “Fig. 2”.
Fig. 2. Single diode model of a PV cell.
The characteristic equations of a PV cell can be expressed as [33]-[36]:
I pv  ( I sc  k i (T w  T r )) s
3
Tw [
I rs  I rr ( ) e
Tr
qE g(
1

(1)
1
Tr Tw
kn
)
]
(2)
q(
I  n p I pv  n p I rs [e{
np
V I Rs

)
V ( )  I Rs
ns n p }
ns
]
knT w
Rsh
(3)
P  VI
(4)
where, I, V and P are the output voltage, current and power of the photovoltaic cell
respectively. Tw and Tr are working and reference temperatures. S is solar irradiation, Isc is
short circuit current at reference temperature, ki is short circuit current temperature
coefficient, Irr is cell saturation current at reference temperature, k Boltzmann’s constant, q is
charge of electron, n is the ideality factor of model parameter of the cell, Eg is band energy of
the used semiconductor material, Iph is photon current, Irs is cell saturation current, ns and np
are number of series and parallel cells and Rs and Rsh are series and shunt reactors
respectively. To simulate the performance of the photovoltaic cell these parameters are
assigned deliberately like Tw=250 C, Tr=250 C, S=800W/m2, ki=0.00023 A/K, Isc=5.29 A,
Irr=10 nA, k=1.38065×10-23 J/K, q=1.6022×10-19 C, n=1, Eg=1.12 eV, ns=36, np=2, Rs=0 ohm
and Rsh=infinity ohm, each cell area=125mm×125mm=0.0156 m2 and input
power=800×0.0156×36×2=898.56 W. The characteristic curves of photovoltaic cell have
been shown in “Fig. 3”.
Fig. 3. PV: I-V and P-V Curves.
The efficiency of a photovoltaic cell is very low. In order to increase the efficiency, methods
are to be undertaken to match the source and load properly. One such method is the Maximum
Power Point Tracking (MPPT). The MPPT technique adjusts the PV array voltage in order to
extract available maximum power under all atmospheric conditions. MPPT uses V and I to
detect the slope and generates Pmp to track the maximum power [37]. In this paper,
incremental conductance method as presented in [38] is used to obtain the maximum power.
At maximum power point (MPP).
dP
0
(5)
dV
Where, P=VI, using this relation from (5)
I
I

(6)
V
V
∆I/∆V is the incremental conductance and I/V is the instantaneous conductance. MPP can be
obtained considering the following conditions:
1) At MPP, ∆I/∆V=-I/V
2) At the left of MPP, ∆I/∆V>-I/V
3) At the right of MPP, ∆I/∆V<-I/V
If a PV system satisfies condition 1, the voltage V is ascertained at MPP voltage and fixed at
this voltage until the MPPT encounters a change due to the change in atmospheric conditions.
If the atmospheric conditions change in such a way that the PV system holds condition 2, then
it is essential to increase the reference voltage to achieve MPPT and the opposite is true for
condition 3 [39]. From “Fig. 3”, it can be realized that condition 3 is applicable in this paper
and the voltage and current obtained in MPP, in which Pmp=VmpImp.
Boost regulator; in which the output voltage is greater than the input voltage is appended in
the system to step up the input voltage to a desired magnitude without a transformer and to
attain a high efficiency. As far as this strategy goes, a single-phase full bridge inverter
converts dc voltage into ac voltage. In order to reduce the harmonics, present in output
voltage, an LC low pass filter is used. The implementation of an LC filter could trigger a
parallel resonance which tends to amplify the harmonic voltages and currents in ac network
leading, in some cases, to potential harmonic instabilities owing to the fact that the filter
capacitance has a profound impact on the harmonic performance [40]. PV output voltage is
given in “Fig.4”.
Fig. 4. PV: output voltage.
3 EXPERIMENTAL ANALYSIS
The silicon type two modules-based PV panel (containing 36 cells) were set at
an azimuth angle of 230. “Figs. 5-7” highlight the experimental set-up
procedures and the obtained outputs are provided in Table 1 and illustrated in
“Fig. 8”.
Fig. 5. Single crystal siliconPV panel.
Fig. 6. Connection of PV panel to the measuring unit.
Fig. 7.
Data observed from measuring unit.
Table 1. Measured Values of Current, Voltage, and Power.
Current, I (A)
5.29
4.57
4.48
3.86
3.63
3.20
2.60
1.82
0
Voltage, V (V)
0
1.8
5.3
12
13.5
14.6
17.8
20.1
21.94
Power=V  I(W)
0
8.226
23.744
46.320
49.005
46.720
46.280
36.582
0
Fig. 8. Solar I-V and P-V curves (practical results).
A single-phase conversion circuit (includes inverter) was designed, as seen from “Fig. 9”, and
it was found from “Fig. 10” that the practical PV output voltage waveform contains certain
amount of harmonics though it is negligible.
Fig. 9. Single-phase conversion circuit.
The output of the conversion circuit was connected a 5W lamp and “Fig. 11” shows that when
there was not enough sunlight, the lamp shined dimply and the bulb shined brightly when
there was ample sunlight which can be noticed from “Fig. 12”.
Fig. 10. Practical voltage waveform of the designed circuit.
Fig. 11. Lamp response at low solar irradiation.
Fig. 12. Lamp response at high solar irradiation.
4 CONCLUSION
This paper presents not only the simulated approach but also the practical analysis to integrate
PV inverter-dominated islanded microgrid system. The experimental results validate the
simulated outcomes. A three-phase inverter-based islanded microgrid system can be designed
using the same strategy in the future.
Acknowledgments
I would like to thank Photovoltaic Lab of Rajshahi University of Engineering and
Technology, Bangladesh to carry out this project at undergraduate studies.
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