The document proposes a load shedding mechanism for intentional islanding of distribution systems with distributed generation. It describes intentional and unintentional islanding, presents a case study on the IEEE 33-bus system with seven distributed generators, and develops an algorithm for load shedding to balance generation and demand in isolated islands. Simulation results show the proposed mechanism can maintain stable frequency and voltage during different islanding scenarios by selectively disconnecting non-critical loads.
2. A Proposed Load Shedding Mechanism for
Enhancing Intentional-Islanding Dynamics of
Distribution Systems
Ahmed M. Elkholy, Hossam A. Abd el-Ghany, Ahmed M. Azmy
Electrical Power and Machines Engineering Department, Faculty of Engineering, Tanta University
ahmed_elkholy@f-eng.tanta.edu.eg, hossam.saleh@f-eng.tanta.edu.eg, azmy@f-eng.tanta.edu.eg
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2
Presenter
Ahmed Elkholy
3. Nineteenth International Middle East Power Systems Conference (MEPCON) - 2017
Introduction Distributed Generation (DG)
Islanding
Mechanism and
Case study
System Description
Load Flow Study
Load Shedding Mechanism
Simulation results
Conclusions
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4. Distributed Generation (DG)
• Small-scale generation usually less than 10 MW
• Usually inserted near load centers
• Many paper discusses DG benefits such as:
• Reduction of transmitted power
• Reduction of power losses
• Enhancement of voltage profile
• The main drawbacks of adding DGs are:
• Probability of instability condition
• Malfunctioning of protection schemes
• Loss of mains (islanding)
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Introduction Case study Simulation result Conclusions
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5. Islanding
Islanding
Unintentional
Intentional
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Introduction Case study Simulation result Conclusions
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Means that are a portion of the electrical
system are isolated from the main grid
intentionally or unintentionally
The system is divided into subsystems by
protective relaying
The algorithm: Detect - disconnected the DG
The system is divided into subsystems by
system operator
6. Intentional-islanding
• Benefits
• This helps to maintain the
service for important clients
• Reduces the restoration time
• Decreases the financial loss
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Introduction Case study Simulation result Conclusions
• Configurations of island
systems
• Facility island
• Lateral island
• Secondary island
• Circuit island
• Substation bus island
• Substation island
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7. Transition-to-island precautions
• Load demand must be satisfied by the generator
• The voltage and frequency level must be within allowed levels
• The voltage and frequency control could be modified to ensure stability
• The system must be equipped with monitoring, information exchange and
control (MIC) equipment's
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Introduction Case study Simulation result Conclusions
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8. Load Shedding
• load shedding means deliberately disconnect loads to supply the
remaining part of the power system at good power quality. the load
shedding can be classified on to
• Under-voltage load shedding (UVLS)
• Under-frequency load shedding(UFLS)
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Definition
Introduction Case study Simulation result Conclusions
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9. System
description
IEEE 33-bus system – 33 bus, 33 load center
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1 2 3 4 6 7 85 9 10 11 12 14 15 1613 17 18
19 20 21 22
23 24 25
262728 29 30 31 3233
Introduction Case study Simulation result Conclusions
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10. Optimal DG location
Seven DG units are added to investigate intentional islanding phenomena
Nineteenth International Middle East Power Systems Conference (MEPCON) - 2017
Bus
Rated capacity
(MVA)
Rated power
factor
Operating
power
(MW)
13 0.25 0.8 0.1738
16 0.5 0.8 0.2914
17 0.25 0.8 0.0816
30 0.25 0.8 0.1789
31 0.5 0.8 0.3888
24 0.5 0.8 0.173
20 0.5 0.8 0.173
Introduction Case study Simulation result Conclusions
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1 2 3 4 6 7 85 9 10 11 12 14 15 1613 17 18
19 20 21 22
23 24 25
262728 29 30 31 3233
11. Load flow study
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0.88
0.9
0.92
0.94
0.96
0.98
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Voltage(pu)
Bus Number
With_DG Without_DG
Voltage profile - losses
Case Active power loss (MW) Reactive power loss (Mvar)
Without DG 0.16249 0.108287
With DG 0.037528 0.02446
Introduction Case study Simulation result Conclusions
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12. Islands configuration- model
1 2 3 4 6 7 85 9 10 11 12 14 15 1613 17 18
19 20 21 22
23 24 25
26 27 28 29 30 31 32 33
Island 2
Island 3
Island 4
Island 1
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Generator Sub-transient
model
Turbine model GAST model
Automatic voltage
regulator model
AC4A model
Introduction Case study Simulation result Conclusions
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13. Load shedding mechanism
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According to the load shedding mechanism, loads will be disconnected
based on the following steps
• All loads that are greater than the generator rating will be disconnected
• Suitable loads are selected to achieve supplying the maximum number
of customers
Island (4) is a detailed example to explain the load shedding process
Introduction Case study Simulation result Conclusions
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14. Load data
No Bus P (MW) Q (Mvar) NO Bus P(MW) Q (Mvar)
1 16 0.06 0.02 7 12 0.06 0.035
2 17 0.06 0.02 8 11 0.045 0.03
3 13 0.06 0.035 9 10 0.06 0.02
4 14 0.12 0.08 10 9 0.06 0.02
5 15 0.06 0.01 11 8 0.2 0.1
6 18 0.09 0.04 12 7 0.2 0.1
Generator data
Bus S (MVA) PF Bus S (MVA) PF
13 0.25 0.8 17 0.25 0.8
16 0.5 0.8 - - -
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Island(4) loads and generation
Active load power is
1.075 MW, which
cannot be supplied by
active power generation
0.8 MW
Introduction Case study Simulation result Conclusions
Load shedding
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15. Scenario
number Load bus numbers for scenarios
Summation of
loads
1 16 17 13 14 15 18 12 11 10 9 0.675
2 16 17 13 14 15 18 12 11 8 - 0.7551
3 16 17 13 14 15 18 12 11 7 - 0.7551
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Load shedding scenarios
The algorithm will choose scenario two for 0.8 MW generation
power because the loads connected at bus 8 is more important
than load center at bus 7
Introduction Case study Simulation result Conclusions
Load shedding
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16. Transition-to-island precautions
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• The load power must be lower than the output power of the generators
to prevent any decrease in the system frequency
• The reactive power of DGs must meet the reactive power of the loads to
prevent any decrease in the system voltage
• Changing the speed controller of the generator
The governor can operate in many modes such as droop control mode for
power-sharing and isochrones mode for the standalone applications
• The AVR is used to maintain the generator output voltage fixed around
the operating value and within permitted limit.
Introduction Case study Simulation result Conclusions
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17. 1 2 3 4 6 7 85 9 10 11 12 14 15 1613 17 18
19 20 21 22
23 24 25
26 27 28 29 30 31 32 33
Island 2
Island 3
Island 4
Island 1
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• The new island configuration
is formed by the line outage
connecting bus 3 with bus 23
• The line disconnection will
happen after 2 s from the
beginning of the simulation
• Load shedding mechanism will
disconnect loads at bus 24 and
bus 25
Bus Load Generator rating
23 0.09 MW, 0.05 Mvar -
24 0.42 MW, 0.2 Mvar 0.5 MVA at 0.8 PF lag.
25 0.42 MW, 0.2 Mvar -
Island (1)
data
Introduction Case study Simulation result Conclusions
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Island (1)
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Output voltage of generator at bus 24 Output frequency of generator at bus 24
Output active power of generator at bus 24 Output reactive power of generator at bus 24
Introduction Case study Simulation result Conclusions
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Island (1)
19. Island (2)
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• The new island
configuration is formed
by the outage of the line
connecting bus 2 with
bus 19
• The line disconnection
will happen after 2 s from
the beginning of the
simulation
• No need for load
shedding
1 2 3 4 6 7 85 9 10 11 12 14 15 1613 17 18
19 20 21 22
23 24 25
26 27 28 29 30 31 32 33
Island 2
Island 3
Island 4
Island 1
Bus Load Generator rating
19 0.09 MW, 0.04 Mvar -
20 0.09 MW, 0.04 Mvar 0.5 MVA at 0.8 PF lag
21 0.09 MW, 0.04 Mvar -
22 0.09 MW, 0.04 Mvar -
Introduction Case study Simulation result Conclusions
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Island (2)
data
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• No need for load shedding for this island
• The frequency will decrease because the
generator have a droop speed
• For increasing the frequency, it is required
to change the speed droop (R) with a
relation between the change in frequency
and the change in generator output power
Introduction Case study Simulation result Conclusions
• Generator at bus 20 must increase its
output by 38.2% to meet the load demand
• This causes the generator frequency to be
0.983 pu when the speed droop (R) is 0.047
• Speed droop is modified to 0.02 to increase
generator frequency to 0.993 pu
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Island (2)
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Output voltage of generator at bus 20 Output frequency of generator at bus 20
Output active power of generator at bus 20
Introduction Case study Simulation result Conclusions
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Island (2)
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• The new island configuration
is formed by the outage of
the line connecting bus 6
with bus 26
• The line disconnection will
happen after 2 s from the
beginning of the simulation
• Load shedding mechanism
will disconnect load at buses
26, 27 and 30
Island (2)
data
1 2 3 4 6 7 85 9 10 11 12 14 15 1613 17 18
19 20 21 22
23 24 25
26 27 28 29 30 31 32 33
Island 2
Island 3
Island 4
Island 1
Bus Load Generator rating
30 0.2 MW, 0.6 Mvar 0.25 MVA at 0.8 PF lag
31 0.15 MW, 0.07 Mvar 0.5 MVA at 0.8 PF lag
32 0.21 MW, 0.1 Mvar -
29 0.12 MW, 0.07 Mvar -
33 0.06 MW, 0.04 Mvar -
28 0.06 MW, 0.02 Mvar -
26 and 27 0.06 MW, 0.025 Mvar -
Introduction Case study Simulation result Conclusions
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Island (3)
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Output frequency of generators at buses 30 and 31
Output power of generators at buses 30 and
Output voltage of generators at buses 30 and 31
Introduction Case study Simulation result Conclusions
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Island (3)
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• The new island configuration is formed by the
outage of the line connecting bus 6 and bus 7
• The line disconnection will happen after 2 s
from the beginning of the simulation
• Load shedding mechanism will disconnect
load at bus 9, 7 and 10
Island (4) data
1 2 3 4 6 7 85 9 10 11 12 14 15 1613 17 18
19 20 21 22
23 24 25
26 27 28 29 30 31 32 33
Island 2
Island 3
Island 4
Island 1
Bus Load Generator rating
16 0.06MW, 0.02 Mvar 0.5 MVA at 0.8 PF lag
17 0.06 MW, 0.02 Mvar 0.25 MVA at 0.8 PF lag
13 0.06 MW, 0.035 Mvar 0.25 MVA at 0.8 PF lag
14 0.12 MW, 0.08 Mvar -
15 0.06 MW, 0.01 Mvar -
18 0.09 MW, 0.04 Mvar -
12 0.06 MW, 0.035 Mvar -
11 0.045 MW, 0.03 Mvar -
10 0.06 MW, 0.02 Mvar -
9 0.06 MW, 0.02 Mvar -
8 0.2 MW, 0.1 Mvar -
7 0.2 MW, 0.1 Mvar -
Introduction Case study Simulation result Conclusions
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Island (4)
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Output frequency of generators at buses 30 and 31 Rotor-angle difference between the generators in island (4)
Introduction Case study Simulation result Conclusions
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Island (4)
26. Conclusions
• The dynamics of intentional islanding is extensively investigated
• A load shedding algorithm is maintaining the service for the
maximum number of customers
• Four island cases are extensively studied to prove high reliability
gained from intentional islanding
• The load shedding mechanism improved the transition
dynamics
• The frequency is regulated to match network frequency by
modifying the governor mode or droop characteristic to
facilitate the restoring of the system
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Introduction Case study Simulation result Conclusions
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Editor's Notes
IEEE Std 1547.4™-2011
facility island system is formed from generation and load normally served within a customer facility
Lateral island formed from load normally served from a lateral on a distribution circuit
Secondary island one or more DR and multiple customers connected to the secondary side of one distribution transformer
Circuit island an island is formed from load normally served from a single distribution circuit
Substation bus island an island is formed from load normally served from a single bus within a substation, though multiple buses may be used to serve loads from the substation
Substation island an island is formed from load normally served from a single substation. This island may be used when the distribution substation is out of service
IEEE Std 1547.3-2007 provides guidance on MIC for DR
For DR island systems that include part of the area EPS, MIC from the DR to the area EPS operator is likely to be required.
DR island systems with multiple DR may require communications among the DR. Load monitoring and control may be employed to manage the island systems
