abstract key words: introduction · also, pulse width modulation technique is used in grid...
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Investigation on Voltage stability of the DC bus in a Single phase
Grid connected PV system C. Benin Pratap
1, Geevin S R
1, G. Shine Let
2
1EEE-Department of Electrical Sciences,
2ECE-Department of Electrical Sciences,
Karunya Institute of Technology and Sciences
Coimbatore, India
Abstract
This paper presents an investigation on the voltage stability of the DC bus
when subjected to continuous change in irradiance and temperature. It is
important to have a stable voltage at the input of the Voltage source inverter for
grid integration. A single phase Grid connected PV system is used for this
analysis. Perturb & Observe technique is used for Maximum power point tracking.
The output voltage and power at the grid is also taken for analysis .The bus
voltage is maintained by an MPPT algorithm which controls the switching pulses
of a boot converter which is connected to the dc bus. The voltage output of the
DC bus is maintained by minimum response time and complexity.
Keywords: MPPT, Voltage stability, Grid, Boost Converter
Introduction
The most popular and abundant available renewable energy is solar
energy. This is used as an alternative energy source in most of the commercial and
industrial power generation. Solar energy is an economical, alternate energy
resource. Using solar energy, electricity is generated in a clean, reliable and quiet
manner. Advancement in electronics and various control techniques made it use
for many applications. Solar energy is converted into electric energy by using
photovoltaic (PV) cell. It acts as an direct DC generator [1]. PV cells can be
connected in series and/or parallel form to obtain the required output voltage and
output current. Many PV cells together form a solar array [2].
The sunshine about 3000hour/year produces solar radiation of 5.4 kWh/m2
per day in an annual average shows the immense potential of solar energy [3].
Solar energy supply systems called as PV systems, supply power in stand-alone
mode or grid-connected mode. In stand-alone mode, the generated supply power
from solar energy is directly given to any electrical gazette [4]. The photovoltaic
technology has an installed capacity of 2000MW in 2006 and increased to 300
GW by the end of 2016[5]. A large number of PV modules are connected to form
large standalone and grid connected power generation plant [6]. As years go by
International Journal of Pure and Applied MathematicsVolume 119 No. 16 2018, 1659-1669ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/
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the installation cost of PV systems are dropped to 50%[7]. To obtain maximum
power from solar energy maximum power point tracking (MPPT) is necessary [8].
Based on the revolution of earth around the sun, in PV array a nonlinear i–v
characteristics is obtained. To track the maximum power MPPT is used. Based on
the factors, illumination level and temperature [9]. The output power of PV array
varies. It is challenging to distinguish the global maximum power point (MPP)
because of completely change in environment and shadowing conditions [10].
Line commutated inverter is the first grid connected inverter [11]. International
standard (IEC61727) and the present standards EN61000-3-2, IEEE1547 and the
U.S. National Electrical Code (NEC)690 are considered while using inverter for
grid connections [12].
In Grid connected PV systems, Gird-connected voltage source inverters
(VSIs) are generally applied. Also, Pulse width modulation technique is used in
grid connected inverters control technique. The important function is a specific
value stabilization of dc-bus voltage of the inverter because the PV modules
output voltage varies with temperature, irradiance, and the effect of maximum
power-point tracking (MPPT). Also, the energy should be fed from the PV
modules into the utility grid by inverting the dc current into a sinusoidal
waveform synchronized with utility grid [13]. Fig.1 shows the complete blocks
from PV system to grid connection [2].
Fig.1 Block diagram
PV Modelling
A PV cell model is shown in fig.2. The operational characteristics is
similar to pn diode. The PV cell working depends on surface temperature and
solar radiations. The solar cell equivalent circuit can be represented using single
diode or double diode. But single diode equivalent gives required accuracy and
can be used for the development of explicit models [4].
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Fig.2 Single-diode equivalent solar cell
In fig.2 Iph is photocurrent, Rsh is shunt resistance in PV cell, Rs is series resistance
in PV cell, V and I are the output voltage and output current of a PV cell
respectively. The photocurrent is temperature dependent and is given in equation
(1)
𝐼𝑝ℎ = (𝐼𝑝ℎ)𝑇1+ 𝐾0(𝑇 − 𝑇1) (1)
where, T1 represents normal standard temperature.
(𝐼𝑝ℎ)𝑇1= (𝐼𝑠𝑐)𝑇1
𝐺
𝐺𝑆𝑇𝐶 (2)
𝐾0 =(𝐼𝑠𝑐)𝑇2−(𝐼𝑠𝑐)𝑇1
𝑇2−𝑇1 (3)
The PV current is directly proportional to irradiance. The saturation current in PV
cell is given by equation (4),
𝐼0 = (𝐼0)𝑇1∗ (
𝑇
𝑇1)
3𝑛⁄
∗ 𝑒𝑞(𝐸𝑔)𝑇1
𝑛𝑘(
1
𝑇−
1
𝑇1) (4)
(𝐼0)𝑇1=
(𝐼𝑠𝑐)𝑇1
𝑒𝑞(𝑉𝑜𝑐)𝑇1
𝑛𝑘𝑇1 −1
(5)
Boost Converter
Boost converter is used for DC to DC converter. For many applications,
the available DC voltage from a battery is not sufficient to drive much powered
devices. To boost the available DC voltage to the required voltage, boost
converter circuit is used. Fig. 3 shows the boost converter basic circuit. Switching
transistors are the main components. Switching transistors can be power
MOSFET or bipolar power transistors. The selection of switching transistor is
based on switching speed, cost, required voltage and current level. Based on the
magnetic field around the inductor, the transistor switching operation happens.
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Fig.3 Boost Converter Circuit
Maximum Power Point Tracker (MPPT)
Fig.4 MPPT Flow Diagram
In grid-connected PV systems, MPPT is now popularly used. For a set of
environmental conditions, MPPT maximize the power output and intern the array
efficiency is maximized. For maximum power tracking, many researchers have
come with different algorithms. Among them perturb and observe (P&O) method
tops in the literature. Fig.4 shows the MPPT approach based on P& O algorithm
[15]. PV cell output current and voltage is given as input for the algorithm. This
algorithm follows many iterations. After every iteration, the system compares the
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current and previous power value with some constant. Based on the comparison,
the algorithm adjusts the duty cycle either negative or positive to the current duty
cycle.
Voltage Source Inverter
The inverter in power electronics represents power conversion from dc to
ac. The input to the inverter can be dc current or voltage source. If the input is a
dc voltage source, then it is voltage source inverter (VSI). Fig. 5 shows the three-
phase voltage source inverter [16]. The DC bus supplies current to the inverter.
The inverter switches on and off based on the step change in the input dc current.
If the power flow is from dc bus to ac load, the average magnitude of dc current
remains positive,
Fig.5 Voltage Source Inverter
Grid Integration
Fig. 6 Simulink Model of 600V Grid
Fig. 6 shows the matlab simulink model of a 600V grid which is supplied with a
voltage source inverter. Fig.7 shows the integration of voltage source inverter to
the utility grid.
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Fig. 7 Integration of VSI to utility grid
Result and Discussions
The simulation model considered is the 100kW PV array connected with a
25KV grid through a DC-DC boost converter with MPPT tracking and a three-
phase voltage source converter using Matlab Simulink. Fig. 8a shows the input
irradiation and temperature at which the test results are taken. Analysis is done by
varying the temperature and irradiation (from 1000 to 500), and the voltage
stability of the DC bus plotted.
Fig.8a Input irradiation and Temperature
Fig.8b Voltage stability @changing irradiance
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Fig.9 VSC Control Parameters
Fig.10 PV array Characteristics
Fig 8b shows the voltage stability of the dc bus voltage at reduced irradiation
level. This affects the power delivered to the Grid however the grid integration
parameters are maintained constant. Fig. 9 VSC control parameters Id ref, Id, Iq
variation with time. Fig.10 shows the characteristics of PV array such as
irradiation, temperature, voltage. Fig 11 gives the duty cycle.Fig12 and 13 shows
the output voltage of Grid and voltage source inverter respectively and they both
provide a constant output voltage.
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Fig.11 PV array Duty cycle Fig.12 Va(Grid)
Fig 13.Output voltage at VSI
Conclusion
In this work, a simple single phase grid connected photovoltaic array is
analysed for obtaining an efficient synchronisation and power. The basic P&O
method is used for Maximum power point tacking which makes the response of
the boost converter fast and effective. The stability of the DC output voltage is the
most important factor for the steady Grid supply that is achieved by the MPPT
algorithm. The output voltage of the VSI is controlled for effective Grid
integration. The irradiation fluctuation can be adapted easily by this proposed
system and Grid connection of low power sources are made easy and effective.
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