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Islanding Operations for the Distribution
Systems with Dispersed Generation Systems
Cheng-Ting Hsu
Department of Electrical Engineering
Southern Taiwan University of Technology
Tainan, Taiwan
Chao-Shun Chen
Department of Electrical Engineering
National Sun-Yat Sen University
Kaohsiung, Taiwan
1
Outline
Introduction
Description of Study System and DGS
Load Models
Load Shedding Schemes
Transient Stability Analysis of Islanding System
Conclusion
2
Introduction

DGS are growing quickly due to the environmental issue and
most of DGS have smaller installation capacity so that they
will be connected to the distribution system.

It is possible to have the islanding operation, although it is
prohibited by utility since it may endanger the safety of the
equipment and utility staff. However, it is also a feasible
condition because the probability of power failure can be
reduced if the utility can solve the problems.

This paper investigates the operation feasibility for the
islanding system with different types and control schemes of
DGS by executing transient stability. Also, different load
models and load shedding schemes are applied to know their
impact on the islanding system.
3
Description of Study System and DGS
4
TW  0.5R 3U 2Cp(, ) / 
5
f ref
f
+ -
 max
f

Tdp s 
1


Kp 1


Tip s 1  Tdp s 

PID control scheme
0

CP
PM
 min
6
Load Models
CP, RCI and RCIM load models are applied in this paper
1. CP: constant power
2. RCI: RCI load model is the combination of the residential,
commercial and industrial type customers. The load
composition at each bus of the feeder can be applied to the
static RCI load model derived by the EPRI to know the
variation of the load on the voltage and frequency deviations.
3. RCIM: The RCIM load model is composed of the typical
dynamic model of induction motor and the static RCI load
model.
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Load Models
The percentages of the load composition at different buses
Residential (24.1%)
Commercial (11.5%)
Industrial (64.4%)
Load
P
(MW)
Q
(MVAR)
A/C
R
IL
A/C
FIL
RFG
IM
IM
A/C
FIL
R
L1
1.7
0.42
72
18
10
46
39
9
6
56
20
21
3
L2
1.94
0.46
67
19
14
65
22
9
4
80
10
7
3
L3
1.31
0.2
58
18
24
40
45
10
5
45
25
23
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Load
P
(MW)
Q
(MVAR)
A/C
R
IL
A/C
FIL
RFG
IM
IM
A/C
FIL
R
L4
1.5
0.41
66
17
17
45
39
10
6
40
25
30
5
L5
1.5
0.37
56
24
20
50
32
13
5
60
20
17
3
L6
1.8
0.42
56
24
20
40
42
10
8
70
15
11
4
Load
P
(MW)
Q
(MVAR)
A/C
R
IL
A/C
FIL
RFG
IM
IM
A/C
FIL
R
L7
2.7
0.71
71
17
12
60
20
13
7
85
10
4
1
L8
1.31
0.2
71
22
7
45
33
15
7
85
10
4
1
L9
2.33
0.6
66
12
22
65
18
12
5
50
20
24
6
A/C : air conditioner load
IM : induction motors
Residential (42.1%)
Commercial (53%)
Residential (29.4%)
Industrial (4.9%)
Commercial (19.1%)
Industrial (51.5%)
IL : incandescent lighting RFG : refrigerator load
R : resistive load
FIL : fluorescent and incandescent lighting
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Load Shedding Schemes
1. Low frequency relay load shedding
Step
Frequency (Hz)
Shedding
Amount (MW)
1
59
2
2
58.8
1
3
58.6
1
4
58.4
1
5
58.2
1
6
58
0.5
2. Frequency decay-rate load shedding
Pstep 
2 H sys df 2 H sys


 m0
60
dt
60
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Transient Stability
Analysis of Islanding System
The utility network is disconnected from the distribution
substation at 16 cycles. To investigate the effects of the DGS on
the islanding distribution network, three operation scenarios are
selected for transient stability analysis. Besides, different load
models and load shedding schemes as described above are
applied in the computer simulation.
Case A :Islanding system with GTG alone
In this case study, the WG is out of service and the GTG is
operated alone. The initial active and reactive power outputs of
GTG are 10MW and -0.3Mvar. Also, the total load demands of
the distribution feeders are 16.2MW and 3.8Mvar.
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This figure shows the voltage responses of the islanding
system without considering the load shedding. It is found
that the voltage responses of the islanding system are almost
recovered to the nominal value finally. However, the voltages
have ever dropped to the values of 0.86, 0.87 and 0.91pu for
the CP, RCI and RCIM load models respectively. The RCIM
load model gives the better dynamic responses than the CP
and RCI load models.
11
This figure gives the frequency responses of the islanding
system without considering the load shedding. Without
executing the load shedding, the frequencies of the islanding
system decline very quickly and reach an unacceptable value for
any kind of the load model even the GTG has increased its
mechanical input power to the maximal value. For the CP load
model, it produces the largest frequency decay rate because the
constant load demand is assumed during the transient period.
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CP: 3steps4MW
RCI: 2steps3MW
RCIM: 2 steps3MW
This figure shows the frequency responses of the islanding
system with considering the under-frequency load shedding.
Two shedding steps with a total amount of 3MW load are
executed to recover the islanding system frequency to 59.5Hz
for the RCI and RCIM models. For the CP load model, three
shedding steps with a total amount of 4MW load are necessary
to restore the frequency.
13
CP: 5.46MW
RCI: 4.72MW
RCIM: 3.61MW
This figure shows the frequency responses of the islanding
system with considering the frequency decay rate load shedding.
After the tripping of the utility, the frequency decay rates are 5.9,
5.1 and 3.9 Hz/sec for CP, RCI and RCIM load models. The
total shedding loads are therefore calculated as 5.5, 4.7 and
3.6MW for CP, RCI and RCIM models. The frequencies recover
quickly after the load shedding have been executed.
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Case B: Islanding system with WG alone
For study case B, the WG is operated to generate active power
while the GTG is considered as a synchronous condenser to
regulate the voltage by its excitation system. Also, a capacitor
bank with rated capacity of 3.5Mvar is installed to provide the
reactive power absorbed by the WG. The initial active power
outputs for the WG is 10MW.
15
This figure gives the frequency responses of the islanding
system without considering the load shedding schemes. It is
found that the frequency responses are worse than the case A
because the WG has provided the constant power output of
10MW. It is also observed that the islanding system collapsed
very quickly for all kinds of load models.
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CP: 6 steps (6.5MW)
RCI: 5 steps (6MW)
RCIM: 5steps (6MW)
This figure shows the frequency responses with considering the
under-frequency load shedding scheme. Six shedding steps with
a total amount of 6.5MW are executed for CP load model.
However, the frequency kept rising to an unacceptable level due
to over load shedding and constant active power output of WG.
On the other hand, five shedding steps with a total amount of
6MW load are executed to recover the islanding system
frequency to 59.5Hz for the RCI and RCIM models.
17
This figure gives the blade angle responses of the wind turbines
with considering the under-frequency load shedding and pitch
controller. The initial blade angles are operated at 5 degree to
produce 10MW mechanical power output. Due to the action of
pitch controller, the blade angle reduces to 0° to result in the
variation of the mechanical power from 10MW to 12.85MW.
Finally, the blade angles keep at 1.05°, 1.21°and 1.32° for the CP,
RCI and RCIM load models.
18
CP: 3 steps (4MW)
RCI: 3 steps (4MW)
RCIM: 3 steps (4MW)
This figure gives the frequency responses of the islanding system
with considering the under-frequency load shedding and pitch
controller. The frequency has ever declined to a minimal value of
58.4Hz. Three shedding steps have been executed for all load
models to recover the system frequency. After the load has been
tripped, the frequencies of the islanding system reach the
maximum value of 60.8, 60.2 and 60.4Hz for the CP, RCI and
RCIM load models. With the proposed pitch controller to regulate
the blade angle, the frequency can be recovered very well.
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Case C: Islanding system with
WG and GTG
In this case study, the WG and GTG are operated at
the same time. The initial active power outputs for
the GTG and WG are 6MW and 4MW respectively.
The utility has provided the 6.2 MW active power
and 3.3 Mvar reactive power to the distribution
feeders.
20
This figure gives the frequency responses of the islanding system
without considering the load shedding schemes. After the
disconnection of utility, the frequencies of the islanding system
decline very quickly and reach the minimal values of 58.4,
58.6and 58.8Hz for CP, RCI and RCIM models respectively. Due
to the governor action of the GTG, the frequencies begin to rise
and maintain at 58.8Hz even without executing load shedding.
21
CP: 2steps (3MW)
RCI: 1steps (2MW)
RCIM: 1steps (2MW)
In this case study, the frequencies have reached the setting of load
shedding schemes. This figure gives the frequency responses of
the islanding system with considering the under-frequency load
shedding scheme. For the CP load model, two shedding steps with
a total amount of 3MW load are tripped and the frequency is
restored to 59.4Hz. On the other hand, step one load shedding is
executed only to recover the frequency to 59.3Hz for the RCI and
RCIM models.
22
CP: 5.46MW
RCI: 5.42MW
RCIM: 4.02MW
The above figure gives the frequency of the islanding system with
considering the frequency decay rate load shedding scheme. The
frequency decay rates after the tripping of utility are 5.7, 5.4 and
4.0 Hz/sec and the shedding loads are therefore calculated as 5.5,
5.4 and 4MW for CP, RCI and RCIM models respectively. It can
be found that the frequencies of the islanding system can be
maintained well for all the load models after the load shedding has
been executed.
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Conclusion

The load models have a great impact on the dynamic response
of the islanding system and the amount of load shedding. The
CP load model has resulted in greater discrepancy than the RCI
and RCIM models. With considering the static and dynamic
characteristics of load, the RCIM load model should present the
most accurate simulation results.

The islanding operation is difficult for the WG to be operated
alone even different load shedding schemes have been
considered. However, it is feasible for the WG with the
proposed frequency-based pitch controller. By executing the
suitable load shedding, the islanding systems with a GTG alone
or the combination of WG and GTG can also be operated safely.

It is concluded that the power islanding operation is feasible if
the suitable load shedding schemes and proper DGS control
schemes are applied.
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