Reliable Impedance Islanding Detection for Power
Distribution Systems with High Motor Penetration
Nguyen Duc Tuyen, Goro Fujita
Shibaura Institute of Technology, 3-7-5, Toyosu, Koto-ku, Tokyo, Japan
Email: m609504@shibaura-it.ac.jp
Abstract— Regarding to dispersed generation field, an islanding is known as a situation that those generators are disconnected
from the utility feeders and a bunch of local loads being powered
by them is isolated. Accordingly, anti-islanding schemes are used
to immediately detect that phenomenon in facilitating the detachment the DG or adaptive controlling of autonomous operation.
Negative-sequence impedance measurement is considered as a
novel method of detecting electrical islands in DG networks.
However, in the case of high induction motor penetration,
this method might malfunction. Using two-round impedance
measurement, this paper deals with that problem while making
typical assessments on how the impedance tolerant values are
selected and how these values vary with network configuration.
The simulation in PSCAD/EMTDC and experiments validate the
methodology.
Keyword Islanding Detection, Impedance Measurement,
Symmetrical Transformation, Clarke Transformation.
I. I NTRODUCTION
Recent high DG penetration into power distribution systems
has created new challenges for protection engineers. One
typical protection configuration that should be considered is
when an open circuit condition causes the creation of an island
of electrical load and generation. In 2003, the IEEE 1547
Standard[1] has the requirements for DG connection to detect
and separate itself from the network within two seconds since
islanding operation may damage existing equipment, human
safety, power reliability, power quality and so forth. However,
in 2011 July, IEEE Standard 1547.4[2] facilitates of islanded
operation provisioning of multi-islanded operation in required
cases and reconnected as smart grid of the future. Research
on islanding detection (ID) using impedance measurement was
reviewed in 2004 [3], and implemented in 2008 by Wrinch et
al.[4]. However, this approach has not investigated thoroughly
in the system with high motor penetration which probably
makes it in a wrong operation. This paper outlines the ID
technique and alleviates the disadvantages.
II. I SLANDING D ETECTION BASED ON
N EGATIVE - SEQUENCE I MPEDANCE M EASUREMENT FOR
F IRST ROUND
The ID method developed in this paper uses the theoretically accurate concept of impedance measurement when
the negative-sequence components of voltage and current are
available. This detection idea is shown in Fig.1. The premise
is that the large impedance difference between an island and a
grid-connected condition has a very low Non Detection Zone
Z DG
Z utility
Z load
E utility
E DG
Fig. 1. Principle of using negative-sequence impedance measurement for
islanding detection
Z 0sys Z 0load
V0
Z -sys
Z -load
V-
E sys
Z +sys
I0
I- V
m
I+
Z mload
I0 ! I ! I+
Z +load
V+
Fig. 2.
Symmetrical-component currents flow in unbalanced-system expanded circuit
(NDZ) for radial systems with stiff network connections, that
is Z utility << (Z DG \ \Z load ).
The unbalanced system is transformed into the symmetrical
system described in Fig.2[5](subscript +, −, 0 represents for
positive-, negative- and zero-sequence components). In this
sys
figure, the system equivalent voltage source, E+
remains
sys
sys
as the only source and the current from the Z0 and Z−
sys
sys
sequences are indeed supplied by the E+ . E+ is used
because the normal resources are considered to generate
only positive-sequence components. From the Fig.2, a current
source that is produced at the load for negative- and zerosequence is a result of I+ being divided up at node Vm . Vm is
the point where three symmetrical currents join together and m
represents mutual components. From the current and voltage
derivation in symmetrical components with this configuration,
the current flows out of the unbalanced load and goes into the
system.
By using Ohm’s law for negative-sequence components
at the point of the unbalanced load or source, the system
sys
Thevenin negative-sequence impedance of system, Z−
can
be calculated as in Eq.(1)[4].
0.060
(1)
Normal Operation
Z +sys
0.020
III. P OSITIVE S EQUENCE I MPEDANCE M EASUREMENT
FOR S ECOND ROUND
Z
DG
+
E sys
As mentioned in the previous section, high motor penetration (around 60% of the utility load) poses limitation to the
reliability of negative-sequence impedance ID. The following
technique could be called the second round of ID based on
positive-sequence impedance measurement. However, because
of the second round is implemented simultaneously, then
it does not increase detection time. After the first round,
the system negative-impedance of post-islanding has been
obtained and it is considered to be smaller to the positivesequence because of the assuming appearance of motors.
When an islanding occurs, besides system negative-sequence
impedance increases, the positive-sequence impedance also
increases. On the other hand, while separating a bunch of
motors, only negative-sequence impedance of system increases
because of the difference of negative- and positive-sequence
of motors themselves. If system positive-sequence impedance
is measured during the transient, those different phenomena
could be clarified. Clarke transformation is recommended for
an impedance calculation. It provides more data points for
results verification and it can determine impedance without
requiring complete three-phase voltages. And real operators
can be applied to instantaneous voltages and currents. Furthermore, excessive complex number operations tend to introduce
errors to the voltage and current phase angles. The Clarke
transformation has the following form:

(2)
The basic idea of this method is in Fig.3. During the
normal operation, due to connecting to stiff grids, the DG unit
components apparently are integrated in the entire system of
which impedance is quite small. At islanded operation, there
is only DG units with its high impedance. Contrarily, while
disconnecting a motor, those positive-sequence components of
system is kept connecting.
System positive-sequence impedances can be determined
using the following steps:
V+s , I +s
Z +load
Motor Disconnecting
Z +sys
B. Clarke Transformation to Measure Positive Impedance
Z +load
-0.020
-0.060
The positive-sequence impedance of induction motors varies
between and around 0.9 to 1.0 p.u, while the negative-sequence
impedance remains essentially constant at approximately 0.15
p.u. This means the low negative-sequence impedance somehow creates confusion for the threshold window of first round
of ID method because they can indicate a falsely stiff utility.
V+s , I +s
Islanding
E DG
0.000
-0.040
A. Sequence Components for Induction Motors




2 √
−1 −1
V̇A
V̇α
√
  V̇B 
 V̇β  = √1  0
√3 −√ 3
√
6
2
V̇C
V̇
i
h 2 i2
h0
→ V̇αβ0 = [T ] V̇ABC
E sys
Ibeta
0.040
y
sys
Z−
V−
≈−
I−
Vbeta
1st cycle
V+s , I +s
Z +load
6th cycle
Transient
Fig. 3.
Observation window to measure positive-sequence impedance
1) Use the “observation window” that can gather voltage
and current values every 6 cycles as Fig.3. Record the
waveforms of the voltages and currents at the chosen
PoM as ”1st-transient” parameters (V̇s1st , I˙s1st -s represent α, β and 0 components)
2) Record the ”2nd-transient” values (V̇s2nd , I˙s2nd ) in 6
cycles after the 1st one. For the system impedance, the
following system equations can be developed:
V̇s1st = Ėssys − I˙s1st Żssys
V̇s2nd = Ėssys − I˙s2nd Żssys
(3)
where Ėssys and Żssys are the internal system’s positivesequence voltage and system impedance, respectively.
3) Determine the system α − β − 0 impedances based on
the Eq.(3) using the Eq. (4):
sys
Ż+s
=
1st
2nd
V̇+s
− V̇+s
I˙2st − I˙1nd
+s
(4)
+s
4) Assuming again from the first round that the system
impedance to be measured is balanced or symmetrical.
The Clarke transformation will yield:




V̇α
I˙α
i
h
sys
 V̇β  = [T ] Ż+
T −1  I˙β 
V̇
I˙0
0


(5)
Żs 0
0
I˙α
=  0 Żs 0   I˙β 
0
0 Żs
I˙0
As a result, computing either Żα or Żβ is equivalent to
computing positive-sequence component, Ż+ .
IV. E VALUATE R ESULTS
FROM
S IMULATION
The proposed ID technique was tested on IEEE 13-node
system.
A. Islanding
The islanding phenomenon is by opening an utility circuit breaker at the 0.4 instance. Fig.4 shows step changes
of negative-sequence impedance during an islanding and a
motor disconnecting. Before islanding, the negative-sequence
impedance was small and the system’s positive-sequence
Beta Current and Voltage during an motor disconnecting transient
Negative-sequence impedance of system during transients
6
Voltage[kV]-Current[kA]
Impedance[Ohm]
60
40
20
Islanding
Motor Disconnecting
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0
-4
Fig. 7.
1
Impedance [Ohm]
Voltage[kV]-Current[kA]
1st cycle
th
6 cycle
0
-1
Voltage Vbeta
Current Ibeta
0.38
0.4
0.42
0.44
6th cycle
0.38
0.4
0.42
0.46
0.48
0.5
0.52
Impedance [Ohm]
Zsys
+
200
Zsys
mean
+
100
0
6
8
10
12
14
16
18
20
Value number
Fig. 6.
Zsys
mean
+
1000
-500
-2000
-3500
2
4
6
8
10
12
14
16
18
20
Positive-impedance of system in case of motor disconnecting
as more than 20[Ω] after islanding. The second round demonsys
strates that the island’s positive-sequence impedance, Z+
was
bigger than the negative-sequence one. This is understandable
since the motor is still connected after the islanding. In
comparison to the low system negative-sequence impedance
measured by the first round (around 4[Ω] as in Fig.4), this
step change is the evidence of an islanding phenomenon.
B. Induction Motor Disconnecting
The motor disconnecting is at the 0.4-instance as in Fig.4.
The negative-sequence impedance change is also significant
as of the case of an islanding. For the second round, positivesequence impedance is obtained in the similar way as described in previous subsection. And the calculated data is
shown on Fig.8. Consequently, the data in Table II are based
on the current and voltage waveforms of Vβ and Iβ shown
in Fig.7. Subsequent to the transient, the positive-sequence
impedance apparently is remained in small values. Table II
synthesizes the entire statistics.
sys
The first round gave the negative-sequence impedance, Z−
as more than 36[Ω] after islanding. The second round shows
sys
the system’s positive-sequence impedance, Z+
is still small
(8.12[Ω]) which is supposed to be higher in the case of
islanding. This value is still larger than the system’s negativesequence impedance before opening the motor which is shown
in Fig.4. Therefore, the difference in those impedances of both
rounds indicates that the islanding is not actually occurring.
Positive-impedance of system in islanding case
TABLE II
AND Iβ FROM OBSERVATION WINDOW IN CASE OF ISLANDING
t
0.41
0.51
0.54
Zsys
+
TABLE I
Vβ
0.52
Value number
Fig. 8.
Vβ and Iβ in case of islanding
300
4
0.5
2500
-5000
400
2
0.48
5000
4000
0.54
impedance was a bit higher because of the connecting motor. Subsequent to islanding, negative-sequence impedance
increases to much higher values. That quick change brings
promise to the speed of detection.
In the second round of impedance measurement, V̇β and
I˙β waveforms are shown in Fig.5. From these waveforms,
the parameter values of Clarke transformation are extracted
by using an observation window. The investigated discrete
values of voltage V̇β1st in the instance range [0.41 ÷ 0.43]
with the step 0.001 and after six cycles of the 60Hz-system,
those values of V̇β2nd in the instance range [0.51 ÷ 0.53].
The corresponding discrete values of current I˙β1st and I˙β2nd
at the similar instance are also obtained. As a result, there
are 21 impedance values to get the average positive-sequence
impedance. Then the calculated data is shown on Fig.6. Then,
the synthesized results is in Table I.
sys
The first round gave the negative-sequence impedance, Z−
-100
0.46
Vβ and Iβ in case of motor disconnecting
Time [s]
Fig. 5.
0.44
Time [s]
Beta Current and Voltage during an islanding transient
-2
0.36
1st cycle
-2
Negative-sequence impedance of system
2
Current Ibeta
2
-6
0.36
Time [s]
Fig. 4.
Voltage Vbeta
4
Vβ
1.4180
0.4708
Iβ
0.0283
0.0049
sys
Z+
sys
Z−
42.6843
20.0221
Vβ
AND Iβ FROM OBSERVATION WINDOW IN CASE OF OPENING MOTOR
t
0.41
0.51
Vβ
4.4782
4.8045
Iβ
0.0733
0.0776
sys
Z+
sys
Z−
8.1185
36.7681
3000
1
Zdiscrete
positive
2
Esitmated Z+ [Ohm]
1
2
Zdiscrete
mean
positive
2000
MeanValue:163.06
1000
0
-1000
V,I
-2000
Fig. 11.
Fig. 9.
0
5
Esitmated Z+ [Ohm]
Negative-sequence Impedance [Ohm]
15
20
25
30
35
40
45
50
Positive-sequence impedance in islanding by experiment
Configuration of experiment system
50
0.5
Z- Islanding
0.45
10
Z- Motor Disconnecting
0.4
Zdiscrete
positive
Zdiscrete mean
40
positive
30
20
Mean Value: 9.08
10
0.35
0
0.3
0.25
Circuit Breakers
open
0.2
0.02
0.04
0.06
0.08
0.1
0.12
0
5
10
15
20
25
30
35
40
45
50
Fig. 12. Positive-sequence impedance in motor disconnecting by experiment
0.14
Time
Fig. 10.
Negative-sequence impedance of experimental model
V. E VALUATE R ESULTS FROM E XPERIMENT
The three-phase experiment system is set up as described
in Fig.9. At first, the utility supply power for two IMs.
Simultaneously, the synchronous DG is initiated. Using the
synchroscope, the DG unit is synchronized to supply a part
of two IMs’ power demand. Subsequently, the experiment is
focused on opening CB1 and CB2 imitating an islanding and
motor disconnecting, respectively.
The negative-sequence impedances in two events are
shown on Fig.10. Initial conditions of two events are a
bit different from each other which make their negativesequence impedance are not tightly identical. Both curves
show negative-sequence impedances step up quickly. Those
magnitudes keep fluctuating during operation. Fortunately,
their mean value can be determined. It should be noted that
magnitude of the negative-sequence impedance is too small because the measured negative-sequence voltage is much smaller
compare to measured negative-sequence current. During gridconnected operation, the magnitude is about 0.3[Ω], then in
the case of islanding it increases to 0.37[Ω]. In the case of
motor disconnecting, it comes up with higher values which is
about 0.45[Ω] and the negative-sequence impedance gets less
fluctuation than the islanding case does due to the connection
to stiff grid. This phenomenon affirms some disconnection
happened, then in order to distinguish the difference between
two events, the proposed second round need to be presented.
Under the same data processing as the previous simulation
analysis, the estimated positive-sequence impedance in the
islanding case is shown in Fig.11. In this case, for a more
precise calculation, 55 discrete points subsequent to islanding
(6-cycle distance) are synthesized to get the mean value of
estimated positive-sequence impedance. Estimated positivesequence impedance of system in the motor disconnecting
case is presented in Fig.12. Those figures match the proposed
ideas. In the case of islanding, because of disconnection of stiff
utility, the system positive-sequence impedance reaches a very
high subsequent value compared to grid-connected mode’s. On
the contrary, in the case of motor disconnecting, the positivesequence impedance stays at small value which is due to
the parallel connection with the stiff utility. Those values
can be predetermined accurately based on through analysis
of system power flow. And the load of islanded entity should
be considered seriously.
VI. C ONCLUSION
A method of detecting electrical islands in DG networks
with high motor penetration was presented. In this method,
the first round measures negative-sequence impedance of distribution system while at the second round, only the positivesequence impedance is measured during transient periods.
The theoretical and experimental analysis with promising
characteristics has been proven.
R EFERENCES
[1] IEEE-SA-Standards-Board, IEEE STANDARD for interconnecting distributed resources with electric power systems, IEEEStd 1547 Std., 2003.
[2] I. SCC21, IEEE Guide for Design, Operation, and Integration of Distributed Resource Island Systems with Electric Power Systems, IEEEStd
1547.4 Std., 2011.
[3] S. Xu Wilsun, K.Mauch, “An assessment of distributed generation islanding detection methods and issues for Canada,” Natural Resources Canada,
Tech. Rep., 2004.
[4] J. Wrinch and M.Nagpal, “Negative sequence impedance based islanding
detection for distributed generation (NSIID),” in Electric Power Conference EPEC 2008. IEEE Canada, 2008, pp. 1–6.
[5] M. C.Wrinch, “Negative sequence impedance measurement for distributed
generator islanding detection,” Ph.D. dissertation, niversity of British
Columbia, 2008.