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Mansoor Asif
Lecturer
Electronics, Power and Control Group
Electrical Engineering DepartmentSEECS, NUST
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Differential Protection
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1-Introduction
• Protects equipment from fault occurring within theequipment
• Principle
–
The difference of current (magnitude, phase) entering andleaving the equipment is measured
– If the difference exceeds a predetermined value, tripoutput is issued
• Only suitable for equipment's where sending and
receiving end of the equipment are physically near
• Used for equipment protection
– Transformer, Busbar e.t.c.
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2-Dot markings
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• A, B and C are mutually coupled coils
According to Lenz’s law current would flow in coil B in such a direction
So that the flux generated by it oppose the flux generated due to A
• Current enters the dot mark onthe primary coil
• Current should leave the dotmark on the secondary coils
•
If currents are made to entertwo dot marked coils, theirrespective fluxes would add up
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3-Simple Differential Protection
AKA Merz-Price Scheme
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• Current entering and leaving the equipment is
stepped down with help of CT’s
• During the normal load flow current in the
spill path would be zero
•
Current would just keep circulating in the“pilot wire”
When current enters the dot marks on the primary side of CT, it should leave
the similarly marked dot mark on the secondary side and vice versa
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3.1-Behavior during external faults
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• During external faults or through faults the
current entering and leaving the equipment
remains unchanged
• Therefore Diff Protection scheme should
remain stable
• Should not generate trip signal
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3.2-Behavior during internal faults
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• Fault current If,int/n will flow through spill path
• Minimum internal fault current that would
generate a trip signal would be
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3.3-Simple Differential Protection,
Double End Fed: Behavior during
internal faults
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4-Zone of protection of differential
relay
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• Any fault occurring within the two CT’s iswithin zone of differential protection
Differential protection
Should restrain during external faults/through faults
Differential protection
Should trip during internal faults
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5- Actual Behavior of a Differential
Protection Scheme
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IS1 & IS2 Should be lagging instead i.e.
Above the horizontal axis
CT Errors
Ratio Error
Phase Angle Error
Spill current build up as through fault current increases
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5.1-Through fault and stability ratio
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Higher the stability ratio, better is the systems ability to discriminate between
internal and through faults
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5.2.1-Differential Protection Scheme
considering Equivalent Circuit of CT
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6-Percentage Differential Relay
• Simple differential relay can be made more
stable if
– Restraining torque proportional to the through
fault current can be generated
– Operating torque remains proportional to the spill
current
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6-Percentage Differential Relay
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6-Percentage Differential Relay
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6-Percentage Differential Relay
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6.1-Block diagram of Percentage
Differential Relay
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• The relay has two settings
– Pick up setting
– Slope setting
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7-Earth Leakage Protection
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• Due to insulation failure the chassis ofequipment may become live
• As the chassis is connected to earth, so
current will start flowing into earth• Earth current too small for OC relay to operate
• Earth leakage relays or current balance relays
are used to detect earth faults• Can save the personnel when they come in
contact with live unearthed chassis.
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7.1-Earth Leakage Protection for a
Single-Phase load
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7.1-Earth Leakage Protection for a
Three-Phase load
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Problem
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Problem
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