1 Impact of Islanding and Resynchroniza?on on Distribu?on Systems Authors: Dr. Vijay Sood Damanjot Singh Kush Duggal IEEE 2011 Electrical Power and Energy Conference
2 Contents • Introduction
• System Description
 Rural Radial Distribution System
 Urban Meshed Distribution System
• System Studies
 Fault on Source Bus
 Load Rejection
 Three phase fault within the Islanded System
• Grid Resynchronization
 Single phase temporary fault at the DG Bus
 Three phase temporary fault at the DG Bus
• Conclusion
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3 Introduc?on • Electrical grid was designed during the 19-20th centuries and it was mostly
radial and had centralized generation.
• Due to environmental compliance, energy conservation and better
operational efficiencies, it has become necessary to transform the electric
grid into the so-called “smart grid”.
• Importance of DG is increasingly accepted by utilities but it brings some
serious challenges like load forecasting and stability
• One positive feature of DG is that it can supply power in islanded mode.
(Islanding refers to a condition in which the DG system continues to supply
power to some critical loads even when the main electric grid is
disconnected from the system)
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4 • According to IEEE Std. 1547-2003, intentional or planned, islanding is a
topic still under consideration by many utilities.
• Some utilities do not allow intentional islanding at this time because of the
safety hazards imposed by islanding since lines may still be live when they
are assumed to have been disconnected
• Another reason for not allowing islanding is to prevent abnormal voltages
and frequency excursions on the system.
• However, planned islanding if it is used prudently can enhance reliability
by serving critical loads when the main grid has been disconnected. IEEE 2011 Electrical Power and Energy Conference
5 System Descrip?on • Two electric distribution systems modeled in EMTP-RV simulation
package:
 Rural Radial Distribution System
 Urban Meshed Distribution System
• Both systems are integrated with a DG using a Synchronous Generator at
the Point of Common Coupling (PCC).
• The DG is assumed to be a synchronous machine with a capacity of 10
MVA.
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6 Rural Radial Distribu?on System IEEE 2011 Electrical Power and Energy Conference
7 • 33 kV distribution system.
• Typical radial system that consists of 3 AC loads of 17.5 MVA and a nonlinear DC load of 3 MW (i.e. saw-mill).
• DG is installed at Bus 4 which is the point of common coupling (PCC).
• When system suffers a fault at the main feeder, protection will open breaker
B1 which will create a micro-grid with DG alone providing the feed.
• This scenario will create an islanded micro-grid and loads within it will
have to be re-adjusted.
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8 Urban Meshed Distribu?on System IEEE 2011 Electrical Power and Energy Conference
9 • This system has feeds from two sides besides the internal feed in the form
of a DG.
• The supply to the system loads is based on the concept of an open ring
system, in which two different feeders supply the two halves of the ring.
The ring can be opened at one point by a disconnecting device.
• Protection of the power system equipment and safety of personnel poses a
major threat due to potential synchronization problems.
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10 System Studies • According to IEEE Std. 1547-2003 [5], as soon as main source is
disconnected from the grid due to a fault, all DGs connected to the electric
grid should also be disconnected due to safety hazards and risk of system
failure.
• The simulations show the effect of non compliance of the IEEE Std.
1547-2003 and out of phase resynchronization of the DGs to the grid.
• Following test studies were performed only on the radial distribution
system.
• This was due to the fact that without a proper frequency/voltage
controller combined with load shedding, the urban meshed distribution
system in the islanded mode will not be able to maintain the IEEE Std.
1547-2003 specified limits.
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11 Fault On Source Bus (b) Voltage at DG Bus (pu)
1.1
1.1
1
1
0.9
0.9
Voltage
Voltage
(a) Voltage at Bus 1(pu)
0.8
0.8
0.7
0.7
0.6
0.6
0.5
5
6
7
0.5
8
5
voltage at the DG bus also does
not stabilize after the fault as shown in
Fig. (b).
1.2
1
0.8
Active Power
Active Power
7
(c) Active Power at DG Bus (MW)
• The
(c) Active Power at Source Bus (MW)
0.6
0.4
1
0.8
0.6
0.4
0.2
0
6
0.2
5
6
7
• When a fault occurs at Bus 1, the
protection relay disconnects the main
source from the system. Then the DG
alone will supply power to the loads in
the 8system.
8
0
5
6
7
• Active power from the main source
bus is reduced to zero when it is
disconnected from the system shown in
(c). 8
Time (sec)
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12 • System frequency starts to
decrease.
• This is because the DG output
(10 MW) is less than the total
demand due to which the
machine slows down to 0.90 pu,
which is out of the allowable
limits.
• Hence the system is
dynamically unstable, and the
DG should be disconnected to
prevent any damage.
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13 Load Rejec?on • When the frequency of the system
goes below 0.97 pu a load rejection/
shedding routine is applied to balance
the generation and the load.
• Islanding occurs at 6 s and the
frequency of the system starts to
drop.
• The under frequency protection acts
and an AC load is disconnected from
the system.
• The frequency of the system is
constant after the load shedding and
its value is 1.01 pu.
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14 Three phase fault within the Islanded System • . To further test the stability of
the system, a 4 cycle three phase
fault is applied at 8 s within the
islanded system.
• Voltage at the DG bus reduces to
0.30 pu but comes back to 0.87 pu
within 8 cycles.
• Due to under voltage protection,
a load of 2.5 MW is further
rejected.
• Frequency drops down to 0.92
pu but comes back to 1.02 pu.
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15 Grid Resynchroniza?on  Single phase Temporary fault at DG Bus
• A 6 cycle single phase fault is created at
DG Bus at 8 s.
• Once the fault is cleared, the system
becomes stable after a few transients
• There is no loss of synchronism between
the DG and the main source.
• During the fault, the voltage at the DG bus
reduces to 0.78 pu but it comes back to 0.98
pu once the fault is cleared.
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16  Three phase temporary fault at DG Bus • System does not stabilize after the
fault which means that there is a loss
of synchronism.
• Loss of synchronism is because of
the miss-match between the voltages
and/or the frequency of the main and
the DG sources.
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17 • After the fault, frequency of the
system does not remain constant
and goes out of the allowed limits.
• Also, the voltage at the source
bus and the DG bus are in
synchronism before the fault but
after the fault this synchronism is
lost.
• DG is unable to resynchronize
itself to the grid after a 3 phase
fault
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18 Conclusion • The ability of DG to form an islanded system upon separation of the main
grid may be beneficial to provide continuity of service if suitable measures
are taken.
• The old adage of disconnecting the DG upon separation from the main grid
is too protectionist.
• However, formation of an island requires care and suitable a load shedding
and Var compensation strategy may be required.
• Further care is required when resynchronization back to the main grid is
desired.
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19 • Questions?
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