1. Performance of the Proposed IDM
 UL 1741 Testing
The active load power is adjusted to set the inverter at
25%, 50%, 100%, and 125% of the rated output power of
the inverter. The reactive power has been adjusted
between 95% and 105% of the balanced condition (unity
power factor loading) in 1% steps.
 Effect of Load Switching
The proposed islanding detection method is tested for
load switching in the grid-connected operation mode.
Similar to the old load, the new load is switched at t=2 s
and disconnected at t=3 s. Three cases are simulated in
this test. In all cases, the load real power is equal to 100
kVA but the power factor is 0.8 lead, 1.0 and 0.8 lag.
 Multiple-DG Operation Mode
A Novel Hybrid Islanding Detection Method combination of SMS and Q-f for Islanding
Detection of Inverter- Based DG
Shahrokh Akhlaghi
Department of Electrical and Computer Engineering, Amirkabir University of Technology, Tehran, Iran
shahrokh.akhlaghi@gmail.com
Abstract
In this paper a novel hybrid method for islanding detection
of inverter- based distributed generation is proposed. This
algorithm which is a combination of Slip mode frequency-
shift (SMS) and reactive power versus frequency (Q−f) as
active methods detects the islanding phenomena by
forcing the DG lose its stable operation and drift the
frequency out of the allowed normal range of the
frequency relays. A simple passive islanding detection
scheme that relies on frequency relays such as under/over
frequency protection (UFP/OFP) would be sufficient to
detect the moment of islanding. For demonstrating the
performance of the proposed method, it is evaluated
under the IEEE 1547, UL 1741 anti-islanding test
configuration and multiple-DG operation. The studies
reported in this paper are based on time-domain
simulations in the MATLAB/Simulink. The proposed hybrid
method detects islanding more efficiently for loads with
high quality factor (Qf); also it operates accurately in
condition of load switching and does not interfere with the
power system operation during normal conditions.
Simulation results prove that the proposed method
decreases the time of islanding in comparison to previous
methods. In addition, the technique represents to be
robust under multiple-DG operations.
Introduction
Islanding is a condition in which a portion of the
utility system that contains both load and distributed
resources remains energized while isolated from the
remainder of utility system [1].
 Islanding can occur due-to intentional or unintentional
events.
The unintentional islanding of distributed generation
(DG) which can occur due to faults could cause
negative impacts on distribution systems such as
power-quality problems, equipment damage and
even it can be dangerous to utility workers.
 Most of previous studies used SMS method for
islanding detection of inverter base DG since it is easy
to implement. However, the constant power
controlled inverter that is equipped with the SMS IDM
performs poorly.
The Q−f droop curve method forces the DG lose its
stable operation once an islanding condition occurs.
Although, these methods take a lot of time to detect
islanding, and also they are not able to detect
islanding for loads with high quality factors.
Islanding Detection Techniques
System and DG Interface Model
 System under study consists of a distribution network
displayed by a source behind impedance, a load is
displayed in terms of R-L-C and a 100 kW inverter-based
DG is connected to the Point of common coupling (PCC).
 The DG is implemented and designed to operate as a
constant power source by adjusting the controller’s
active and reactive reference values to fixed pre-
specified values with no grid supporting capability [2].
The d−q synchronous reference frame is used to control
the DG interface control variables. The a-b-c three-phase
DG output currents are measured and transformed using
Park’s transformation into its d−q components.
 Influence of Load Quality Factor
The IEEE Standard 929 proposes the use of Qf<2.5 as test
condition. However, The UL 1741 test specifies that an
islanding detection method must succeed in detecting the
islanding phenomenon within 2 s for RLC loads with
Qf<1.8.
Conclusions
In this paper, a new hybrid islanding detection method
for inverter based DGs is proposed. The proposed
method is chosen such that the DG maintains its stable
operation while the grid is connected and loses its
stability once an islanding condition occurs. With a DG
equipped with the proposed method, the OUF is
adequate for efficiently and precisely detecting islanding.
The proposed technique has been studied for the
inverter-based DG unit under the UL 1741 test
conditions, multiple-DG operation mode, load switching
conditions and also with various load quality factors.
Based on simulation results, it is obvious that the
proposed method is capable of detect islanding within
the minimum standard time for loads with high quality
factor and different conditions.
Reference
[1] IEEE Standard for Interconnecting Distributed Resources
Electric Power Systems, IEEE Std. 1547-2003, Jul. 2003.
[2] H. H. Zeineldin, “A (Q-f) droop curve for facilitating Islanding
detection of inverter-based distributed generation,” IEEE Trans.
Power Electron., vol. 24, no. 3, pp. 665–673, Mar. 2009.
[3] H. H. Zeineldin and J. L. Kirtley, “A simple technique for
islanding detection with negligible non-detection zone,” IEEE
Trans. Power Del., vol. 24, no. 2, pp. 779–786, Apr. 2009.
+
-
+
-
fg
fm
f
m
)
2
sin( u

fg
+ -
+
-
PI
control
+
+
++
Vsd
Vsq
Current Regulator
Iq
abc
to
dq
I*
dref
Id
I*
qref
DGI
Amplitude
&
Phase
angle
m

SPWM
dV
qV
PI
control
ωLf
ωLf
+
-
Iqref
Idref
f







 
ff
ff
CosSin
SinCos


Phase Angel
Transformation
P
control
+
-
Pref
P
Q
control
Power Regulator
Q
+-
Qref
SMS
+K1
+
f
K2
Q-f droop curve
1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2
57
58
59
60
61
62
63
64
Time (sec)
frequency(Hz)
Case 4
Case 3
Case 7
Case 5
Case 6
Case 1
Case 2
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
59.2
59.4
59.6
59.8
60
60.2
60.4
60.6
60.8
61
time (sec)
frequency(Hz)
Case 8
Case 9Case 4
Case 5
Case 3
PCC frequency for UL 1741 testing with the
SMS IDM
PCC frequency for UL 1741 testing with the
Proposed Method
1.5 2 2.5 3 3.5
0.94
0.95
0.96
0.97
0.98
0.99
1
1.01
1.02
1.03
1.04
time (sec)
PCCVoltage(p.u)
Load=100 kVA & p.f=0.8 lead
Load=100 kVA & p.f=1
Load=100 kVA & p.f=0.8 lag
1.5 2 2.5 3 3.5
59.8
59.9
60
60.1
60.2
60.3
time (sec)
frequency(Hz)
Load=100 kVA & p.f=1
& p.f=0.8 lead
& p.f=0.8 lag
PCC voltage during a load switching event Frequency of the Voltage at the PCC
Lfilter
Lg Rg
Grid
L RC
VPCC
+ Vdc -
IDG
CB
Network
Old
Load
DG Unit
VT
Cdc
L RC
Inverter-Based DG
New
Load
PCC frequency for the multiple-DG
operation mode
Lfilter
Lg Rg
Grid
L RC
VPCC
+ Vdc -
Inverter
IDG
CB
Network
DG Unit
VT
Cdc
Inverter-Based DG2
Load
+ Vdc -
Inverter
DG Unit
Cdc
Inverter-Based DG1
Effect of Load Switching Multiple-DG Operation Mode
1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4
58
58.5
59
59.5
60
60.5
61
61.5
62
62.5
63
Time (sec)
frequency(Hz)
Qf=1.77Qf=1
Qf=4.21Qf=3
Qf=0.5
PCC frequency for different values of Qf With
the proposed method
PCC frequency for different values of Qf with
SMS IDM
1.5 2 2.5 3 3.5
60
60.5
61
61.5
62
Time (sec)
Frequency(Hz)
Qf=8.1
Qf=6.38
Qf=1.77
Qf=3
Qf=4.21
PCC frequency for the different values of Qf
with the Q-f IDM
frQfC (μF)L (H)R (Ω)
60.070.5575.40.01222.304
60.1111500.00612.304
601.7720370.003452.304
60.12334520.002032.304
604.2148500.001452.304
606.3873500.0009572.304
608.193300.0007542.304
LOAD PARAMETERS FOR DIFFERENT VALUES Qf
1.5 2 2.5 3 3.5
59.5
60
60.5
61
Time (sec)
Frequency(Hz)
Qf=6.38
Qf=3
Qf=1.77
Qf=4.21
Qf=8.1
Lfilter
Lg Rg
Grid
L RC
VPCC
+ Vdc -
Inverter
IDG
CB
Network
DG Unit
VT
Cdc
Inverter-Based DG
Load
Inverter Parameters
8000 HzSwitching frequency
900 VInput DC Voltage
2.1 mHFilter Inductance
480 VVoltage (Line to Line)
100 kWDG Output Active Power
Grid Parameters
60 HzFrequency
0.3056 mHGrid Inductance
0.012 ΩGrid Resistance
Load Parameters
2.304 ΩResistance
0.00345 mHInductance
2037 μFCapacitance
Inverter, System, Load and DG Parameters
System Under Study
1.8 1.9 2 2.1 2.2 2.3 2.4 2.5
58
58.5
59
59.5
60
60.5
61
61.5
62
62.5
63
time (sec)
frequency(Hz)
Load with 59.3 Hz resonant frequency
Load with 60 Hz resonant frequency
Load with 60.5 Hz resonant frequency
The proposed islanding
detection method is further
tested in a system with
multiple DGs. Two identical
DGs, each with a 100 kW
rated output power, are
connected at the PCC.
fm-fg=3 Hzθm=10SMS method
K2=1.5K1=-0.025Q-f droop curve
Proposed Method Parameters
DG interface control for constant power controlled
equipped with proposed method