Overcurrent Protection of
Radial Lines
Prof. Hossam Talaat
CONTENTS
1. Fundamentals of Radial Distribution Systems
2. Protective Devices in Radial Systems
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1. Fundamentals of Radial Distribution Systems
1.1 Features of Radial Distribution Systems
1. Simple Structure (Power flows in a unidirectional manner)
2. Low Installation Cost
3. Ease of Maintenance
4. Significant Voltage Drop
5. Low Reliability
6. Simple Protection Scheme
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1.2 Radial distribution system
Main Feeder Fuse
Lateral Dist. Transf.
X CB, CT
& OCR R Recloser
I>
Equipment
Fuses
Sectionalizing
Substation Fuses
2 3 5 6 8
X R 1
R 4 7
I>
Downstream
Upstream
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1.3 Objectives of Radial System Protection
1. Fast isolation of temporary faults (Recloser) without long load
interruption. Temporary faults represents 90% of system faults.
(Videos)
2. Clearing of permanent faults with minimum area isolated.
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2. Protective Devices in Radial systems
1. Overcurrent Relays (with Circuit Breakers)
2. Load Break Switches (LBS)
3. Reclosers
4. Medium Voltage Fuses
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2.1 Overcurrent Relays (other topics)
Ground (Earth Fault) overcurrent relay
• Ground relay monitors the value of ground current “Ig“ (= 3 Io).
• The advantage of ground OC relay is the discrepancy between
the value of Ig during normal system operation, usually Ig < 0.1
If.l., and its value during ground faults (LG & LLG), Ig = 5-20 If.l..
• Accordingly, the current setting of the ground OC relay is
usually set to 30% to 50% of the setting of the phase relay,
which improves the sensitivity of the protection system.
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Two-Set Overcurrent relay
• The low set (I>), usually inverse time-current, deals with
overload and low short circuit currents.
• High set (I>>), usually instantaneous, delas with high short
circuit currents.
• A common practice is to set
the instantaneous relay to
detect 125% of the max fault
current at the far end bus to
cover about 80% of the
upstream part of the line.
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Symbols of
Protection
Devices
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2.2 Load Break Switch (LBS)
May be found in the Ring Main Unit (RMU)
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2.3 Automatic Recloser
• It serves as an automatic circuit breaker that detects faults,
isolates the affected section, and attempts to restore service
by re-energizing the circuit after a brief delay.
• A popular operation sequence for recloser is 2 fast operations
(instantaneous) followed by 2 delayed operations.
• The fast operations allow temporary faults to self-clear
• The delayed operations allow downstream fuses to clear
permanent faults.
• This technique improves the overall performance and
reliability of the radial distribution system. 11
A Commercia Automatic Recloser
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Typical Recloser
Time Characteristics
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2.4 Medium Voltage Fuses
Fuse is a device that carries current through
fuse element that melts due to self-heating
resulting from excessive current and initiates Fuse-Link
the interruption of the current.
Fuse-Base
(Holder)
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HV Fuse-Link Ratings
Fuse Current (A)
Load Current Interruption
In I3 I1
In = Rated Current
I1 = Breaking Capacity (max breaking current)
I1 > Max short circuit current at fuse-link location
I3 = Min breaking current
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Standard Current Ratings of HV Fuses
6.3, 10, 16, 20, 25, 40, 50
63, 80, 100, 125, 160, 200 A
Egyptian Standard
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Egyptian Standard –
Suitable Fuses for Transformers
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Fuse Time-Current Characteristics
Melting Arcing
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High Voltage Fuse Classification
Current-Limiting Expulsion Fuse
Fuse
•Quickly limit •Simple &
fault current Economic
•Expensive •Cost effective
•Used for •Used for less
sensitive sensitive
equipment equipment
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Current-limiting vs Expulsion Fuses Supplementary
Operating Principle Topic
Current-Limiting Fuse Expulsion Fuse
• The high current causes • After the fuse element
the fuse element melting melts, the arc produces
gas inside the fuse tube,
before the first peak of
which is designed to
the fault current. Upon expel (push out) the arc
melting, the fuse gases. The force of these
introduces resistance into gases helps to blow out
the circuit so rapidly that the arc, breaking the
circuit completely
the current stops rising
and instead is forced
quickly to zero
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Current-limiting vs Expulsion Fuses Supplementary
Current & Voltage Waveforms Topic
21
Operation of Current-limiting Fuse Supplementary
Topic
22
Minimum Melting
Curves of Slow
Expulsion Fuse
(T)
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