ELEC 423
Distribution of Power System
Power Distribution Protection
Adel Gastli
Adel Gastli
CONTENTS
•Objective of Distribution System Protection
1
•Overcurrent Protection Devices
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2
•Coordination of Protective Devices
3
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Adel Gastli
Five components of protection systems
Current and voltage transformer to step down the high voltages and currents of the
electrical power system to convenient levels for the relays to deal with,
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Protective relays to sense the fault and initiate a trip, or disconnection,
Circuit breakers to open/close the system based on relay and autorecloser commands;
Batteries to provide power in case of power disconnection in the system,
Communication channels to allow analysis of current and voltage at remote terminals
of a line and to allow remote tripping of equipment.
Transducer
• Sensor & transformer
• Current transformer (CT)
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&
Potential transformer (VT, PT)
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POWER SYSTEM PROTECTION
Main idea is to remove faults as quickly as possible
while leaving as much of the system intact as possible
Fault sequence of events
1. Fault occurs somewhere on the system, changing the system
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currents and voltages
2. Current transformers (CTs) and potential transformers (PTs)
sensors detect the change in currents/voltages
3. Relays use sensor input to determine whether a fault has
occurred
4. If fault occurs relays open circuit breakers to isolate fault
The protections are often classified according to the object that they
protect. An example is shown in following figure. If a failure occurs
within an indicated area in figure, this area should be isolated from
the rest of the network.
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Protective relays
A protective relay is a device designed to trip a circuit breaker when a
fault is detected. The first protective relays were electromagnetic
devices, relying on coils operating on moving parts to provide detection
of abnormal operating conditions such as over-current, over-voltage,
reverse power flow, over- and under- frequency.
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COORDINATION OF PROTECTIVE DEVICES
What is coordination?
Overcurrent protective devices can be coordinated selective or
nonselective.
Selective coordination: localization of overcurrent condition to restrict
outages to the circuit or equipment affected, accomplished by choice of
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overcurrent protective devices and their ratings or settings (i2t).
Nonselective coordination: occurs when a fault on a branch circuit
opens not only the branch-circuit overcurrent device but also the feeder
overcurrent device.
• Nonselective systems are installed unknowingly
• Causes needless power outage in portions of the electrical system
that should not be affected.
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COORDINATION OF PROTECTIVE DEVICES
The fault on the branch-circuit
may trips both the branch-circuit CBM CBB
breaker and the feeder circuit 100A 20A
breaker if the curves are set as CBB CBM
shown in this figure.
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In this case the protection is not
fully coordinated.
Time
Overlap
As a result, power to the panel is zone
cut off, and circuits that should 0.9s
0.5s
not be affected are now off.
3kA 5kA
Current
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COORDINATION OF PROTECTIVE DEVICES
In general, when the protective device
characteristics do not intersect, the
system is fully coordinated. 100A 20A
If they touch in the bottom or anywhere
(overlap zone), then they are not fully
coordinated.
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20A 100A
As rule of thumb, if the branch fuses are
rated not more than half of the main
Time
feeder fuse, then in most of the cases we
have a fully coordinated system. So the
main protective device should be rated
twice the largest branch protective device.
Current
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COORDINATION OF PROTECTIVE DEVICES
Circuit breaker coordination:
The ground fault pickup time delay
feature is useful for circuit breaker
coordination. In the following scheme,
upstream breaker “A” has been set to
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.2s and downstream breakers “B” and
“C” have been set to .1s.
A ground fault occurring in the circuit
supplied by “B” will trip the “B” breaker
without disturbing “A” or “C”..
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COORDINATION OF PROTECTIVE DEVICES
IMPORTANT NOTE:
We must be careful when large motors are part 33kA
of the distribution system to be protected.
During short-circuit, large motors will behave as
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generators for a short time period until they stop
3kA
completely (regenerative breaking) and will 36kA
contribute to further increase of the short circuit
current in the point of fault. M
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CIRCUIT BREAKERS
This figure shows one line diagram
that has three molded case circuit
breakers in series: from the main
1200A to the 100A branch circuit
with the 400A feeder in between.
The other circuit breakers on the
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one-line diagram supply other
circuits and loads.
The fault current path from the
power source is depicted by the red
arrows/lines superseded on the
one-line diagram.
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CIRCUIT BREAKERS
These curves illustrate the time current
characteristics for the 1200A, 400A, and 100A
circuit breakers.
The instantaneous trip (IPU) settings for each of
these three molded case circuit breakers are
provided on the one-line diagram.
The 100A circuit breaker has a non-adjustable
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instantaneous trip setting and the curve is as
depicted.
The 400A circuit breaker has an instantaneous
trip set at 10 times its amp rating (10X) which is
10 times 400A or 4000A.
The 1200A circuit breaker has an instantaneous
trip set at six times its amp rating (6X) which is
six times 1200A rating or 7200A..
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CIRCUIT BREAKERS
Remember that when the curves of two circuit
breakers cross over in their instantaneous trip
region, then the drawing indicates that the two
circuit breakers do not coordinate for fault
currents greater than this cross over point.
For instance, interpreting the coordination
curves for the 100A circuit breaker and the
400A circuit breaker: their curves intersect in
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the instantaneous region starting at
approximately 3600A.
That means for a fault current greater than
3600A on the load side of the 100A circuit
breaker, the 400A circuit breaker will open as
well as the 100A circuit breaker.
This demonstrates a lack of coordination and
results in a "cascading effect" that will cause a
partial blackout.
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CIRCUIT BREAKERS
This curve also shows that for any fault
greater than approximately 6500 amps
on the load side of the 100A circuit
breaker, the 400A and 1200A circuit
breakers will open as well as the 100A
circuit breaker.
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The reason: for a fault of or greater than
6500A, all three of these circuit breakers
are in their instantaneous trip region.
Both the 400A and 1200A circuit
breakers can unlatch before the 100A
circuit breaker clears the fault current.
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CIRCUIT BREAKERS
How does this affect the electrical system?
Look at the one-line diagram next.
For any fault current greater than
approximately 6500A on the load side of the
100A circuit breaker, the 1200A and 400A
circuit breakers open as well as the 100A
circuit breaker.
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The yellow shading indicates that all three
circuit breakers opened - 100A branch circuit,
400A feeder and the 1200A main.
In addition, all the loads fed by the other
circuit breakers, denoted by the hash shading,
are blacked out unnecessarily.
This is due to the lack of coordination between
the 100A, 400A and 1200A circuit breakers.
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