Post on 23-Apr-2020
Contemporary Engineering Sciences, Vol. 11, 2018, no. 22, 1069 - 1084
HIKARI Ltd, www.m-hikari.com
https://doi.org/10.12988/ces.2018.8237
Study and Analysis of Anti-Islanding Protection for
Grid-Connected Photovoltaic Central of Ghardaïa
Mohamed Redha Rezoug
Departement of Electrical Engineering
Université Kasdi Merbah Ouargla 30000, Algeria
Rachid Chenni
MoDERNa Laboratory Mentouri
University of Constantine1, Constantine 25000, Algeria
Djamel Taibi
Departement of Electrical Engineering
Université Kasdi Merbah Ouargla 30000, Algeria
Copyright © 2018 Mohamed Redha Rezoug, Rachid Chenni and Djamel Taibi. This article is
distributed under the Creative Commons Attribution License, which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
This paper aims at providing a simulation study of the nature of the Islanding
phenomenon occurs in the Grid-Tied PV (photovoltaic) System and the different
methods to detect it which permits us to protect the system. We are going to resort
to a number of devices to put into practice these methods including over-current
and under-current (OI/UI) relays, over-voltage and under-voltage (OV/UV) relays
and over-frequency and under-frequency (OF/UF) relays. The protection is
examined in two different circumstances, when the PV system is completely
disconnected from the electric power grid and also during the occurrence of some
various grid faults. These faults can lead to the decoupling of the inverter and
cause of unintentional islanding phenomenon that has dangerous consequences
particularly on equipment and on grid maintenance personnel.
Keywords: PV system, Defaults, Islanding, protection, Grid-Tied PV
1070 Mohamed Redha Rezoug et al.
1 Introduction
The huge consumption of the fossil energy resources in addition to its high cost of
extraction lead for a need to adapt to new modes of energy production and
consumption. Through the implementation of various PV (Photovoltaic) power
plants, we offer a sustainable source of renewable energy for all energy-based
sectors [1].
The integration of new technologies of information and communication to the
actual power grids turns them into Smart Grids capable of responding to different
changes and requirements for the long term. Those grid requirements help in
detecting any harmful defaults protect the equipment and ensure safety of the grid
maintenance personnel [1], [7]. The islanding phenomenon occurs in Grid-Tied
PV systems can be intentional or non-intentional (accidental) and forms a
destructive effect on the whole PV system. In order to face such a phenomenon,
reliable and effective AI (anti-islanding) protection methods should be developed
[5], [12]. This paper provides a simulation study under a Matlab/Simulink
environment of a passive method for islanding detection. This anti-islanding
technique is based on the monitoring of voltage, current and frequency. For our
method to be functional, we compare directly the islanding detection time(s) in
different scenarios of anti-islanding relays such as over-current and under-current
(OI/UI), over-voltage and under-voltage (OV/UV) and over-frequency and under-
frequency (OF/UF). [1] All these scenarios evaluate the performance of AI relays
and then determine which one of the anti-islanding methods is the most effective
and appropriate so as to work on Grid-tied PV system [11].
The work was carried out at the Ghardaïa photovoltaic central in south Algeria at
a latitude of 32°24N and a longitude of 3°48E with an altitude of 566m. The land
is trimmed with a 10-hectare extent.
This area is characterized by a solar irradiation which reaches in summer 900 to
1000W/m² and a Saharan climate whose conditions are very severe given the high
temperatures and sand storms to which the southern regions are subjected.
The central has a rated power of approximately 1100 kWp (kW peak), distributed
as follows (see Figure 1):
Under field 105 KWC monocrystalline silicon fixed structure.
Under field 98.7 KWC in polycrystalline silicon fixed structure.
Under field 105 KWC in monocrystalline structured silicon.
Under field 98.7 KWC in polycrystalline silicon structure motorized.
Under field 100.8 KWC in thin layer (Telluride of cadmium Cd-Te) fixed
structure.
Subfield 100.11 KWC amorphous silicon fixed structure.
Under field 255 KWC in monocrystalline silicon fixed structure.
Under field 258.5 KWC in polycrystalline silicon fixed structure.
Study and analysis of anti-islanding protection 1071
Figure 1: The different types of PV technologies in the PV central in Ghardaïa
2 The Problematic of Islanding
In an electric power grid and in the presence of decentralized energy production
(DEP), particularly of photovoltaic installations, it appears a phenomenon called
“Islanding” [4], [3]. It happens when a sub-grid having one or more DEPs is
disconnected from the main grid, these DEPs continue supplying local loads.
Islanding can be intentional or accidental. Indeed, during a maintenance operation
on the power grid, the disconnection of the grid may lead to the islanding of the
generator. Since the loss of the grid is voluntary, the islanding is known and can
be put off-voltage by the operating personnel. Unintentional islanding linked to an
accidental disconnection of the grid is of great interest. This situation highlights
the dangers of maintaining a voltage in the islanded grid and can generate risks on
[7], [11]:
Electrical equipment during high drifts of voltage or frequency.
The generators when the protections are reset (false coupling)
People near equipment or during maintenance operations.
It is therefore essential to detect any islanding situation and to reduce the running
time of the system. This situation must be detected in order to:
To avoid feeding a fault or leaving a faulty system running.
1072 Mohamed Redha Rezoug et al.
To avoid feeding the islanding with an abnormal voltage or frequency.
To enable automatic reclosing systems to operate.
Also, the development of anti-islanding protection which is sensitive and reliable
is very important to encourage the integration of the distributed power generation
system (DPGS) in the electrical grid and avoid its untimely launching [3] - [6].
The design of PV inverters will be influenced by the requirements of the power
grid, including the anti-islanding (AI) requirement which is considered the most
technically difficult.
3 Method of Detecting Islanding
There are several methods of detecting islanding (or detection of loss of the main
grid). These methods can be divided into three categories: passive methods, active
methods, and methods of using communications between the main grid and the
PV inverter [12], [9].
3.1 Passive methods
They are based on the monitoring of the parameters of voltage and frequency or
their characteristics “harmonic, speed of change” etc. These methods require a
definition of the thresholds. If the preset threshold is exceeded, the inverter is
therefore disconnected. They are simple and easy to install with low currents and
do not need any additional materials. They do not cause disturbances to the grid or
inverters and have a rapid detection time. However, they are a great disadvantage
regarding to the definition of a threshold [1], [10], [2].
3.2 Active Methods
They are based on the injection at the output of the inverter (or the grid) of small
disturbances that can deviate a magnitude and thus detect more quickly the
islanding. However, the fact that harmonic currents are injected at the connection
node can cause variations in voltage, power or resistance of the grid and thus
degradation in the quality of the energy supplied. They are inefficient in the case
of several inverters in parallel (possible unjustified disconnection).
3.3 Methods Using Means of Communication
They are based on communication between the PV plant and the grid. They are
very fast allowing the non-degradation of the quality of the energy supplied. They
are therefore, very effective but their major disadvantage is their high cost and
that they are difficult to implant and need communication infrastructures as well.
4 Simulation of Anti-Islanding Protection Systems
In our work, we present a simulation study on a Matlab / Simulink environment of
a passive method of islanding detection which is based on the monitoring of the
Study and analysis of anti-islanding protection 1073
parameters related to the voltage on the DC side and the point of connection to the
PCC (Point of Common Coupling) grid, Current and frequency [7], [11], [4]. This
function is provided by the implementation of anti-islanding AI relays in the PV
system such as over-voltage and under-voltage (OV / UV), over-current and
under-current (OI / UI) relays and over-Frequency and under-frequency (OF /
UF). The simulation model of the PV system studied here is shown in Figure2.
Figure 2: Simulation model of the Grid-Tied PV System
Two cases are studied:
1st case: islanding is produced when the circuit breaker CB1 is open.
2nd case: islanding is produced when various grid faults are produced at a
distance of 8 Km away of the Point of Common Coupling PCC.
4.1 Passive Methods
An islanding state is simulated when the three-phase circuit breaker CB1 is
opened at time (t) = 0.3s and is closed at time (t) = 0.45s. In this case, we study
three scenarios depending on the connected load to the PV system.
Scenario 1: Local load is higher than local generated power.
Scenario 2: Local load is equal to the local generated power.
Scenario 3: Local load is lower than local generated power.
The implementation scheme of the various AI relays: OVdc, UVdc, OC, UC, OV,
UV, OF, UF are shown in the following Figures.
1074 Mohamed Redha Rezoug et al.
Figure 3: Simulation model of over/under Vdc relay
Figure 4: Simulation model of over/under Frequency relay
Figure 5: Simulation model of over/under Current relay
Study and analysis of anti-islanding protection 1075
Figure 6: Simulation model of over/under Voltage relay
4.2 Simulation result
In Figures (7, 8, 9, 10) there is a representation the variations of the voltage Vdc,
the efficient value of the voltage and current and the variation of frequency for the
three scenarios during islanding.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8200
250
300
350
400
450
500
550
600
650
700
Time (S)
Vdc voltage during islanding of PV system (Volts)
Reference Vdc voltage
Local load greater than local generation
Local load matches with local generation
Figure 7: The voltage Vdc during islanding for the different local loads
1076 Mohamed Redha Rezoug et al.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
0.5
1
1.5
2
2.5
3x 10
4
Time (S)
RMS voltage during islanding at point PCC-B2 (Volts)
Local load greater than local generation
Local load matches w ith local generation
Local load less than local generation
Figure 8: The effective value of voltage during islanding for the different local
loads
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
5
10
15
20
25
30
Time (S)
RMS current during islanding at point PCC-B2 (A)
Local load greater than local generation
Local load matches with local generation
Local load less than local generation
Figure 9: The effective value of current at the PCC point during islanding for
different values of the local load
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.849.5
50
50.5
51
51.5
52
Time (S)
Frequencey during islanding at point PCC-B2 (Hz)
Local load greater than local generation
Local load matches with local generation
Local load less than local generation
Figure 10: The variation of frequency during islanding for the different local loads
Study and analysis of anti-islanding protection 1077
Figures (11, 12, 13) represent the measured voltage of reference Vdc, the current
with reference Id, measured Id and Iq, the compound voltage Vab-Vsc (V) at the
output of the inverter, instant voltage and current at the point of PCC and the
power at point PCC during islanding for the case of a higher local load than the
local generated power (scenario 1).
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8480
500
520
540
560
580
600
620
640
660
Time (S)
Vdc reference voltage
Vdc measure voltage
Figure 11: Voltage Vdc reference and measured Vdc during islanding (load is
higher than local generated power)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
Time (S)
Id reference (pu)
Id measure (pu)
Iq measure (pu)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1000
-500
0
500
1000
Time (S)
Vab- VSC (Volts)
Figure 12: The compound voltage Vab-Vsc (V) during islanding
1078 Mohamed Redha Rezoug et al.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-4
-2
0
2
4x 10
4
Time (S)
Voltage at PCC-B2 (Volts)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-40
-20
0
20
40
Time (S)
Current in PCC-B2 (A)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-500
0
500
1000
Time (S)
Power at PCC-B2 (Kw)
Figure 13: Voltage of phase Va, line current and power at the point PCC during
islanding
A graphic representation of the detection time of different anti-islanding AI relays
for test case 1 (the circuit breaker in open position) is shown in Figure 14.
When comparing the performance of AI relays by detection time, we notice that
[11]:
The OC relay has a very longer detection time in the two cases of equal and
higher load than the local generated power. On the contrary, in the case the load is
lower than the local generated power, the detection time is short.
The UV relay is the fastest in detecting the islanding with respect to the other
AI relays and it has the same detection time for the three load levels.
UVdc took a long detection time in cases of higher and lower loads and failed
to detect in the case of load equal to the local generated power.
UC, OV, UF, OVdc completely failed to detect islanding in the three load-
level cases.
Study and analysis of anti-islanding protection 1079
Figure 14: Anti-islanding relays detection time in the case the circuit breaker is in
open position
4.3 Test case 2 faults that occurs in the power grid
Different grid faults (single-phase-to-ground, phase-to-phase, phase-to-phase-to-
ground, three-phase and three-phase-to-ground faults) lasting for 150 ms were
simulated at 8 Km away from the PCC, Figure 15.
Figure 15: Simulation model of PV system during islanding created by defaults
The simulation model for sub-fields 7 and 8 is shown in Figure 16. Two circuit
breakers CB3 and CB4 are implemented at the output of the inverter.
1080 Mohamed Redha Rezoug et al.
Figure 16: Simulation model of sub-fields 7 and 8 with the implementation of
controlled circuit-breakers
Figure 17 shows the comparison of detection times of the AI relays for test case 2
if different defaults occur in the grid. We notice that:
The UV relay has the best performance as it is noted in the study of the first
case. It is the fastest.
The UC relay also set a short detection time of islanding for the 4 faults:
phase-to-phase, phase-to-phase-to-ground, three-phase and three-phase-to-ground
and it were late for the single-phase-to-ground fault.
The UF relay was very late in detecting all faults.
The OC, OV, OF, OVdc and UVdc relays completely failed to detect
islanding for all types of defaults.
Study and analysis of anti-islanding protection 1081
Figure 17: Anti-islanding relays detection time in the case of different grid faults
To compare the detection times of the AI relays in both cases: the case of the
circuit-breaker in open position with the three scenarios of variation of the local
load and the case of the occurrence of different types of faults at point 8Km, the
simulation results are shown in the Table 1.
It gives the results of the theoretical simulation of the AI detection methods
considering: the state of the relays and the time (s) for detecting insularity by relay
protection. The results of the electrical grid monitoring are presented for both
cases: in case of simulation 1, when the three-phase circuit breaker CB1 is in the
open position for scenarios where the power generated locally by the photovoltaic
central is higher, less and roughly balanced with the power of the consumers
connected to Local level; And in case of simulation 2, for scenarios where
different types of defects are simulated in the utility public grid.
The minimum current relay (OC), does not meet the conditions for detecting an
abnormal operating situation in any of these cases. The maximum frequency relay
(OF), is only activated in case 1, when the power of locally connected consumers
is less than or equal to the power generated locally. The maximum voltage relay
(OV), is activated only in case 2. Right after the isolation conditions and three-
phase faults in the mains supply, the voltage drops, the current increases and the
frequency changes.
* Status of the relay;
** Detection time (s) of anti-islanding condition.
1082 Mohamed Redha Rezoug et al.
Table 1: Detection time of anti-islanding relays
Test
case scenario
Type of anti-islanding detection method
OC UC OV UC OF UF OVdc UVdc
* ** * ** * ** * ** * ** * ** * ** * **
1.
CB1
open
Higher load
than
production
1
0.0
2
0 - 0 - 1
0.0
1
1
0.1
0
0 - 0 - 1
0.1
6
Equal load to
the
production
1
0.1
6
0 - 0 - 1
0.0
1
1
0.0
7
0 - 0 - 0 -
Lower load
than
production
1
0.1
8
0 - 0 - 1
0.1
3
1
0.0
6
0 - 0 - 1
0.2
0
2.
Fault
in the
grid
Single phase
to ground 0 - 1
0.0
7
0 - 1 0
.01
0 - 1
0.1
7
0 - 0 -
Isolated
biphase 0 - 1
0.0
3
0 - 1
0.0
1
0 - 1
0.4
5
0 - 0 -
Biphased to
ground 0 - 1
0.0
3
0 - 1
0.0
1
0 - 1
0.3
2
0 - 0 -
isolated
three-phase 0 - 1
0.0
3
0 - 1
0.0
1
0 - 1
0.2
5
0 - 0 -
Three-phase
to ground 0 - 1
0.0
3
0 - 1
0.0
1
0 - 1
0.3
8
0 - 0 -
5 Conclusion
This paper presents the results of a simulation study concerning the islanding
phenomenon occurs in a photovoltaic PV system. Due to the harmful effects of
islanding on the equipment and on the maintenance personnel as well, we study
the different methods for its detection. Three methods are analyzed (passive,
active and communication between grid and PV inverter) in an attempt to
elaborate a passive protection method consisted of monitoring the parameters of
voltage, current and frequency of DC and AC sides in the PV system by
implementing different anti-islanding relays at fixed points on the PV installation.
Two cases are tested, the first when circuit-breaker is in open position and the
second by creation of various defaults at a point away from the Point of Common
Coupling PCC.
Study and analysis of anti-islanding protection 1083
By comparing the anti-islanding relays detection time, the simulation results allow
us to determine the most efficient relays in each case. These results are of great
importance for the creation of AI protection devices for Grid-Tied PV Systems.
Acknowledgements: Any collaboration with central GHARDAÏA
Nomenclature
AI: Anti-islanding
CB: Circuit breaker
OC: Maximum current relay
OF: Maximum frequency relay
OV: Maximum voltage relay
PV: Photovoltaic
UC: Minimum current relay
UF: Minimal frequency relay
UV: Minimal voltage relay
VSC: Voltage Source Converter
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Received: February 24, 2018; Published: April 23, 2018