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Modelling and Power quality analysis
of a Gridconnected Solar PV System
S.V. Swarna Kumary*, V.Arangarajan Aman Maung Than Oo, GM Shafiullah, Alex Stojcevski
School of engineering, Faculty of science
Engineering and Built Environment
Deakin University, Waurn Ponds
email: ssrungar@deakin.edu.au
Abstract— Increased concern about global warming coupled
with the escalating demand of energy has driven the
conventional power system to be more reliable one by
integrating Renewable Energies (RE) in to grid. Over the
recent years, integration of solar PV forming a grid connected
PV is considered as one of the most promising technologies to
the developed countries like Australia to meet
the growing
demand of energy. This rapid increase in grid connected
photovoltaic (PV) systems has made the supply utilities
concerned about the drastic effects that have to be
considered
on the distribution network in particular voltage
fluctuations,
harmonic distortions and the Power factor for
sustainable
power generation. However, irrespective of the
fact that the
utility grid can accommodate the variability of load or
irregular solar irradiance, it is essential to study the impact
of grid connected PV systems during higher
penetration
levels as the intermittent nature of solar PV adversely effects
the grid characteristics in meeting the load
demand. Hence,
keeping this in track, this paper examines the gridconnected
PV system considering a residential
network of Geelong region
(38^{◦}.09' S and 144^{◦}.21^{’ }E) and explores the level of impacts
considering summer load profile
with a change in the level of
integrations. Initially, a PV power system network model is
developed in MATLAB/Simulink environment and the
simulations are carried out to explore
the impacts of solar PV
penetration at low voltage
distribution network considering
power quality (PQ) issues such as voltage fluctuations,
harmonics distortion at different load conditions.
Keywords—Renewable energies, Grid connected PV, level of
integration, MATLAB/Simulink
I.
INTRODUCTION
Most of the world’s energy is derived from fossil fuels
especially by burning coal as it has the substantial reserves
of conventional resources. Now a days, generation of
electricity via traditional methods is a challenging issue as it
contributes to greenhouse gas (GHG) emissions and in
fact the continuous usage of these fuels will outstrip the
ability to produce them [1 3].In addition to these facts,
in developed countries like Australia the government has
abandoned plans to shut down some of most dirtiest coal
fired power as part of the Contracts for Closure program
(CFC) to cut down their GHG emissions which amount to
about 1.5 % of GHG emissions, placing it among the top
20 polluting countries in the world. Hence, it is essential to
use alternate energy sources for power generation to meet the
growing demand and as well to conserve nature. In
response to climatic change Australia is developing a suite
of options aimed at delivering more efficient and sustainable
low emission generations. Interest in and production of
renewable energy in Australia has undergone substantial
growth since 2006[4] and among the RE sources, solar PV
has an unpredicted growth in power generation due to its
reliability in power conversion and cost effectiveness [5].
This unpredicted growth in PV market mostly has been
driven by the residential Grid connected PV systems‘A
source of emission free power generation’[6].Gridconnected
photovoltaic (PV) power systems are energized by PV
panels which are connected to the utility grid via an inverter
can upload the excess energy to the grid during average or
low peak demands. Grid connected PV systems reduces the
line losses as the consumer power is generated close to the
load demand. In addition, grid connected system benefits the
utilities economically in delaying the line upgrades by
means of peak load reduction [7]. However, integration of
solar PV into grid has several impacts contributing to
operational problems due to its intermittent nature. Integrating
solar PV effects the functional operation of the power
system network like load/frequency control, load following,
unbalancing of voltage and current levels in the network
and PQ issues including voltage disturbance, poor power
factor, reactive power compensation flicker and harmonic
distortions. Though integration issues/effects are not the
major focus of this paper it is essential to study some of the
effects of the Grid connected PV system that has to be
analyzed for efficient power generation and distribution for
sustainable energy flow [6, 810]. Keeping this in view,
section II explains the effects of Grid connected PV systems
on the distribution network.
The main aim of this paper is to assess the effect
of penetration levels of a grid connected PV system
considering a residential low voltage distribution network in
Geelong. This basic study will explore the impacts of PV
integration on PQ at different solar irradiance levels and daily
load demand based on the summer profile with a change in
the level of PV integrations. Initially, a PV model is
developed using MATLAB and simulations are analyzed
considering different cases with reference to voltage and
harmonic analysis.
II.
EFFECTS OF GRID CONNECTED PV SYSTEMS ON
DISTRIBUTION NETWOK
As per AS4777 standard, the nominal AC voltage of
230V at the point of supply in single phase line to neutral
and 400 V in three phase line to line with a tolerance of
10% 6% and a frequency of 50Hz has to be
maintained at low voltage distribution network side [11].
Australasian Universities Power Engineering Conference, AUPEC 2014, Curtin University, Perth, Australia, 28 September – 1 October 2014
1
Grid connected PV system causes several operational
problems due to the intermittent nature of the solar PV
and the bidirectional power flow in accommodating the
excess power produced by the Solar PV during low load
demands. Hence, it is essential to study the impacts to
estimate the level of impact to maintain the As4777
standards. Following are some of the issues that have to
be considered in analyzing the performance of any grid
connected PV systems.
A.
Over Voltage/Reverse Flow
Excess power generated by the solar PV should be
properly accommodated by the grid for consistent power
flow to avoid the worst case scenarios like power outage.
For instance, the excess power generated by the solar PV
during lower demands should export the active power to the
grid which could result in over voltage or reverse power
flow affecting the utility grid and household appliances
leading to other safety and protection challenges.
B.
Voltage Fluctuations
The intermitted nature of the solar PV is one of
the reasons for voltage fluctuations in grid connected PV
systems Irregular solar irradiance caused by the passing
clouds, PV installation area, and the selected angle of
incidences/reflections also plays a major role in driving
the system to instability by means of voltage fluctuations.
This irregular fluctuations causes voltage flicker /flicker
noise, overloading problems, line losses and network losses
in the distribution network.
C.
Power factor
Grid operated PV systems usually operate at unity
power factor and the power produced by the PV units is
active/real power. In addition to this active /real power
supplied by the solar PV, the grid still has to still
supply the reactive power. During this process the
regular power flow of the system may have the adverse
effect due to the insufficient reactive power and may
decrease the power implying insufficient transmission.
D.
Harmonics
Harmonic distortion is one of the major effect that has
to be considered during the operation of grid connected
PV systems. Inverters used for conversion of DC current
to AC current, inject voltage harmonics and current
harmonics to the system and will result in power
harmonics. As the number of inverters increases the
system becomes more unstable and unreliable due to
the overheating in capacitor banks and transformers.
III.
GRID CONNECTED PV POWER SYSTEM NETWORK MODEL
Fig 1 shows the solar PV power system model developed
in MATLAB to analyze the voltage variations and the
Harmonics for the considered location in Geelong. To
investigate the various impacts of the proposed grid
connected PV system model a rigorous study has to
be done on the model considering level of solar
irradiance and load demand. This section describes the
proposed distribution network model developed in
MATLAB, the typical load profile, solar profile used
in analyzing the model. .
Fig.1 Solar PV Power system model.
.
Australasian Universities Power Engineering Conference, AUPEC 2014, Curtin University, Perth, Australia, 28 September – 1 October 2014
2
A.
Grid source
A three phase source block available in MATLAB is used
as a grid source in this network model. The power source
block specifications used in this analysis are base voltage
22kV, frequency 50 Hz, three phase short circuit level 100
MVA.
B.
Low voltage distribution transformer
A low voltage distribution transformer (DT) with star
delta configuration is considered in this network model. The
DT specifications with respective to winding voltage ratio,
impedance ratio, and transformer rated capacity are
considered as 22 kV/400 V, 4.5%, and 100 KVA
respectively.
C.
2kv feeder
G. Load Profile
The Fig.2 depicts the typical residential daily load
profile of particular area in Geelong. As per load profile,
the daily peak load of each house is considered around
0.865 kW and similarly, minimum average load during
sun shine hours is considered around 0.488 kW.
Therefore considering 30 houses in each load group, the
total peak load, and minimum average load for each load
group is considered around 25.95 kW, and
14.64 kW respectively.
A 22KV feeder is used between the utility source and
the high voltage end of the transformer .The assumed
length of the line per phase is 1000m and with respect to
the positive sequence values of resistance, inductance, and
capacitance of the line are considered to be 0.1153 Ω/km,
1.05 mH/km and 11.3 nF respectively.
D. Solar PV power system
H. Solar profile
Fig. 2. Load profile.
A PV system with a combination of three different
groups of solar PV is used as a unique solar PV power
system in analyzing the proposed model. The rated capacity
of each group of solar PV system at Standard Test
Condition (STC) is 31.9 kW. Each PV system consists of
around 110 numbers of solar PV modules with
configuration of 5 solar PV modules connected in series per
string and 22 strings connected in parallel. The maximum
voltage (Vmp), and maximum current (Imp) of each solar
PV module is considered as 54.7 V and 5.58 A
respectively. In each group of solar PV system, a Voltage
Source Inverter (VSI) of rated capacity 50 KVA is
considered with the implementation of current control
scheme [12].
E.
Low voltage (400 V) distribution feeder
The above mentioned three solar PV groups are
connected in low voltage distribution feeder with a length
of 600 meters in distance. From the figure 1 it can be
noticed that group 1 (solar PV 1) is connected very close to
the transformer, group 2 (solar PV 2) is connected around
300 meters apart from first solar PV group, and similarly
group 3 (solar PV 3) is connected around 300 meters
apart from second PV group. The distribution feeder
specification parameters are like; resistance 0.646 Ω/km,
inductance 0.24 mH/km, and capacitance C=0.07 nF/km.
F.
Load group
Three load groups are considered in this model and
the maximum capacity of load in each group is assumed
to be around 25.95 kW which is equivalent to residential
load of 30 houses.
As per daily solar irradiance level during summer days,
for the selected location in Geelong, the daily maximum solar
irradiance level is considered around 871.5 w/m^{2 }at peak
sun shine condition and similarly, a minimum average solar
irradiance of 319.8w/m^{2 }is considered for this study.
IV.RESULTS AND DISCUSSION
A study of voltage and harmonics analysis has been
carried out for the developed PV power system network
model considering different solar irradiance and different
load conditions. Three different cases are used in analyzing
the voltage scenario with respective to the PV integration in
to the grid and two cases are considered in analyzing the
Total Harmonic Distortion (THD).
Case A: Voltage analysis with only Grid (without PV
integration)
The voltage analysis has been carried out in LV network
at minimum average load condition (14.64 kW) for each load
group. Initially, the simulation has been done by
considering only grid connected system (without PV
integration).Fig.3 clearly shows the behavior of voltage
level at three different bus nodes A1, A2, and A3. From the
simulation results it was observed that the voltage level at bus
node A1, which is close to the distribution transformer is
around 240V whereas the voltage at bus node A2, and A3 is
observed as 232 V and 220 V respectively. From this voltage
level graph it can be clearly stated that the voltage is in
decreasing order from bus node A1 to A3. The reason
behind the voltage drop is due to the increase in the
impedance values with respective to the line distance length of
line.
.
Australasian Universities Power Engineering Conference, AUPEC 2014, Curtin University, Perth, Australia, 28 September – 1 October 2014
3
Fig. 3
.
Bus voltage levels without PV int
e
Case B: Voltage analysis at maximum PV
g
minimum average load
The voltage analysis has been carried ou
t
Fig. 4
.
Vo
Case C: Voltage analysis at minimum PV
g
with peak load
The voltage analysis has been
minimum solar irradiance (319.8 W/m2)
(25.95 kW) conditions. As per voltage
Fig.5, it can be seen th at the vol
t
decreasing mode, from bus node
A
minimum solar irradiance, the power
g
each group of solar PV is around 9.7
5
only around 37% penetration with re
fe
load level. The voltage level at bus no
d
216 V whereas at bus node A2 and A1
i
and 236 V respectively which is same
results in without PV integration conditio
n
Fig. 5. Bus voltage levels with minimum PV integr
a
e
gratio
n
g
eneration with
t
at maximum
Solar irradiance (871.5W/m
2
)
(14.64 kW) conditions. The
indicates the voltage level i
n
results, the voltage level at
bu
246 V as compared to bus no
at A2 around 236 V respec
t
graph shown in Fig.4 that the
v
trend from bus node A1
t
irradiance, the power generati
o
PV is around 27.8 kW whi
c
considering for minimum av
e
kW. Due to this excessive g
e
flow causing voltage rise at
b
significant as compared to other
ltage analysis at maximum PV generation and minimum average loa
d
g
eneration
carried out at
and peak load
level graph in
t
age level is in
A
1 to A3. At
g
eneratio
n
from
5
kW which is
fe
rence to peak
d
e A3 is around
i
s around 220 V
as the case of
n
.
a
tio
n
and peak load.
Case D: Harmonics analysis
with peak load
The THD analysis is ca
r
irradiance (871.5 W/m
2
)
a
conditions. The level of v
o
distortions have been analy
z
at far end of feeder and sim
i
near to LV distribution tr
a
shows the harmonic analy
s
A1.From the results it can
voltage and current harm
o
around 3.19% and 3.84%
voltage and current harm
o
3.13% respectively. At ma
x
the PV generation of arou
n
of solar PV contributes h
i
node A3 as compared to
bu
of current harmonics are
compared to voltage har
m
and minimum average load
result analysis in Fig 4.
n
each bus node. As per the
u
s node A3 is high around
de at A1 around 232 V and
t
ively. As per voltage level
v
oltage level is in increasing
t
o A3. At maximum solar
on
from each group of solar
c
h is in excess level when
e
rage load of around 14.64
e
neration, the reverse power
b
us node A3 which is more
bus nodes.
d
.
at maximum solar irradiance
r
ried out at maximum solar
a
nd peak load (25.95 kW)
o
ltage and current harmonics
z
ed at bus node A3 which is
i
larly, at bus node A1 which is
a
nsformer. Fig 6 and Fig 7
s
is results at nodes A3 and
be seen that THD values of
o
nics at bus node A3 are
whereas at bus node A1 the
o
nics are around 2.37% and
x
imu
m
solar irradiance level,
n
d 27.8 kW from each group
i
ghe
r
harmonics level at bus
u
s node A1. The THD level
high in both cases as
m
onics level
Australasian Universities Power Engineering Conference, AUPEC 2014, Curtin University, Perth, Australia, 28 September – 1 October 2014
4
. .
Fig. 6. THD analysis at bus node A3.
Fig. 7. THD analysis at bus node A1.
Case E: Harmonic analysis at minimum solar
irradiance with peak load
The THD analysis is also carried out at minimum
solar irradiance (319.8 W/m^{2}) and peak load (25.95
kW) conditions. Fig 8.and Fig 9. shows the harmonic
analysis results and from the results it can be seen
that the THD values of voltage and current harmonics
at bus node A3 is around 2.26% and 2.91% whereas
at bus node
A1 it is around 1.18% and 1.35% respectively. The
results clearly indicate that the THD values are higher
at far end of the feeder (bus node A3) as compared to
transformer near bus node A1.From the above two cases
it can be clearly stated that THD values of current and
voltage distortions at minimum solar irradiation
condition is less compared to maximum PV generation
condition with peak load.
Fig. 8 .THD analysis at bus node A3.
Fig. 9 .THD analysis at bus node A1.
Australasian Universities Power Engineering Conference, AUPEC 2014, Curtin University, Perth, Australia, 28 September – 1 October 2014
5
IV CONCLUSIONS
This study is developed a simulation model to analyze the
power quality attributed such as voltage fluctuations and
harmonic injection in the low voltage distribution network
with the integration of solar PV systems. From the voltage
analysis results the following conclusions are drawn.
Considering case‘A’ it can be seen that the voltage level
at far end of the feeder (bus node A3) is less as compared
to bus node which is near to the distribution transformer.
The level of drop in the voltage level at far end of the
feeder depends on the distribution network length or
distance and impedance level. From the case ‘B’ results it
can be stated that there is significant voltage rise at far end
bus node feeder as compared to other bus nodes due to
reverse power flow with excessive PV generation from
solar PV group at minimum load condition. Case ‘C’
results concludes that the voltage level at far end feeder of
bus node is less as compared to other bus nodes, which is
mimicking case ‘A’ results.
From the THD analysis results the following
conclusions are drawn. At maximum PV generation as well
as at minimum PV generation considering peak load
condition it can be observed that the level of harmonics of
voltage and current harmonics are high at far end feeder
of bus node as compared to bus node near distribution
transformer. The only difference is that the THD values of
current and voltage harmonics are in reduced level at
minimum solar PV generation as compared with the
maximum solar PV generation condition. This is due to
cumulative contribution of harmonics from more number of
PV inverters used during maximum solar PV generation
conditions. However irrespective of the case selected, the
maximum voltage deviation and the harmonics are within the
tolerance level as per the AS777 standard. This research
study will be helpful for utilities and customers in future to
estimate the level of impacts of PQ factors in distribution
network while integrating large scale PV in to the network.
In this context, future work will extended this analysis to
investigate the model with storage under higher
penetrations and lower load demands to optimize the
provision of power from PV system and support the
network for sustainable energy generation and distribution.
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https://www.researchgate.net/publication/286651421_Modelling_and_power_quality_analysis_of_a_gridconnected_solar_PV_system
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