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08/0205/02/031 105/02/031 1
Differential Protection Principle
Protection & Contrôle / Application
Rev. A JM, September 2004
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08/0205/02/032 2
Differential Protection
• Protection will not trip if the fault is outside the zone between the CT
• Protection will trip if the fault is inside the zonebetween the CT
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08/0205/02/033 3
Differential Protection
Used where:
• Protection co-ordination is difficult / not possibleusing time delayed elements
• Fast fault clearance is critical
Generators,Transformers,Lines,Cables,Busbars,Motors
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08/0205/02/034 4
Differential Protection Principle
R I diff=0
Nominal current through the protected equipment
• I Diff = 0 : No tripping
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08/0205/02/035 5
Differential Protection Principle
R I diff=0
Through fault current
• I Diff = 0 : No tripping
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08/0205/02/036 6
Differential Protection Principle
R I diff=0
Internal Fault
• I Diff = 0 : Tripping
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08/0205/02/037 705/02/037 7
High Impedance Principle
Protection & Contrôle / Application
Rev. A JM, September 2004
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TC saturéM
RCT
ZM
RCT2RL 2RL
A
M
ZM
RCT
2RL
2RL
RCT
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RCT
ZM
RCT
ZM
2RL 2RL
A
M
M
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08/0205/02/0310 10
RCT
ZM
RCT
ZM
2RL 2RL
TC saturé
A
M
M
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08/0205/02/0311 11
RCT
ZM
RCT
ZM
2RL 2RL
A
M
M
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08/0205/02/0312 12
RCT
ZM
RCT
ZM
2RL 2RL
TC saturé
A
M
M
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08/0205/02/0313 13
RCT
ZM
RCT
ZM
2RL 2RL
A
M
M
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08/0205/02/0314 14
RCT
ZM
RCT
ZM
2RL 2RLTC saturé
A
M
M
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08/0205/02/0315 15
RCT
ZM
RCT
ZM
2RL 2RL
A
M
M
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08/0205/02/0316 16
RCT
ZM
RCT
ZM
2RL 2RL
TC saturé
A
M
M
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08/0205/02/0317 17
RCT
ZM
RCT
ZM
2RL 2RL
A
M
M
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08/0205/02/0318 18
RCT
ZM
RCT
ZM
2RL 2RL
TC saturé
A
M
M
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08/0205/02/0319 19
RCT
ZM
RCT
ZM
2RL 2RL
A
M
M
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08/0205/02/0320 20
M
RCT
ZM
RCT2RL 2RL
A
M
RCT
2RL
2RL
RCT
ZM
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08/0205/02/0321 21
Through Fault with CT saturationThrough Fault with CT saturation
RCT
ZM
RCT
ZM=0
2RL 2RL
TC saturé
RCT
2RL
2RL
RCT
A
M
M
CT saturation
False tripping
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08/0205/02/0322 22
RCT
ZM
RCT
ZM=0
2RL 2RL
TC saturé
RCT
2RL
2RL
RCT
A
RS
M
M
Through Fault with CT saturationThrough Fault with CT saturation
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08/0205/02/0323 23
RCT
ZM
RCT
ZM=0
2RL 2RL
TC saturé
RCT
2RL
2RL
RCT
A
RS
M
M
Stabilising resistor
Through Fault with CT saturationThrough Fault with CT saturation
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08/0205/02/0324 24
Above knee point :
l IM increases a lot
l => Zm can be considered zero
KneePoint
Current Transformer Saturation
IM
VS
Small increaseof IM
High increaseof IM
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08/0205/02/0325 25
Current Transformer Saturation
RCT
ZM
RCT
ZM=0
IM
VS
IM
VS
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Before CT saturation
Current Transformer Saturation
2RLRCT
ZM
2RL RCT
ZM
2RLRCT
ZM
2RL RCT
A
RS
A
RSZM=0
IM
VS
IM
VS
During CT saturation
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08/0205/02/0327 27
RCT
ZM
RCT
ZM
2RL 2RL
A
RS
M
M
RCT
2RL
2RL
RCTVset
Current Transformer Saturation
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08/0205/02/0328 28
RCT
ZM
RCT
ZM=0
2RL 2RL
RCT
2RL
2RL
RCT
A
RS
M
M
ZM = 0(CT "short circuited" )Vset
Current Transformer Saturation
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A
RCT
ZM
RCT
ZM
2RL 2RL
2RL
RCT
2RL
RCT
RS
M
M
Vset
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08/0205/02/0330 30
2RL
RCT
2RL
RCT
M
A
RCT
ZM
RCT
ZM
2RL 2RL
RS
M
Vset
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08/0205/02/0331 31
M
A
RCT
ZM
RCT
ZM
2RL 2RL
RS
M
Vset
Metrosil may be requiredfor voltage limitation
2RL
RCT
2RL
RCT
M
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08/0205/02/0332 32
2RL
RCT
Internal fault with CT saturationInternal fault with CT saturation
RCT
ZM
RCT2RL 2RLM
A
RS
ZM = 0(CT "short circuited" )
ZM=0
RCT
2RLM
Vset
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IM
VS
IM
VS
RCT
ZM
RCT
ZM
2RL 2RL
A
RS
Icc / N
Icc / N
External fault with CT saturationExternal fault with CT saturation
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08/0205/02/0334 34
RCT
ZM
RCT
ZM
2RL 2RL
A
RS
IM
VS
IM
VS
Icc / N
Icc / N
External fault with CT saturationExternal fault with CT saturation
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08/0205/02/0335 35
High Impedance Principle
Based on Current operated relay with an externalstabilising resistor
• Requires matched current transformers of low reactance design, typically class X or equivalent
• Equal CT ratios
• Non-linear resistor may be required to limit voltageacross relay circuit during internal faults
• Suitable for zones up to 200 - 300 metres (typically)
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08/0205/02/0336 36
High Impedance Principle
Application difficulties :
• Spill Current (magnetising current difference) throughrelay circuit for faults outside of the protected zone
• Current transformer mismatch (turn difference)
• Current transformer insufficient knee point voltage
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08/0205/02/0337 3705/02/0337 37
Percentage Restraint (Biased)Principle
Protection & Contrôle / Application
Rev. A JM, September 2004
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08/0205/02/0338 38
Trip
no TripMean Through
Current
I1 - I2
I2I1
I1 - I2
DifferentialCurrent
I1
2
I2+
Trip
no Trip
Mean ThroughCurrent
I1 - I2
DifferentialCurrent
I1
2
I2+
I2I1
I1 - I2
Biased Differential Scheme
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08/0205/02/0339 39
Biased Differential Scheme
I2I1
I1 - I2
Trip
no Trip
Mean ThroughCurrent
I1 - I2
DifferentialCurrent
I1
2
I2+
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08/0205/02/0340 40
Biased Differential Scheme
Trip
no Trip
I1 - I2
I2I1
I1 - I2
Mean ThroughCurrent
DifferentialCurrent
I1
2
I2+
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08/0205/02/0341 41
Biased Differential Scheme
Trip
no Trip
I1 - I2
Differential Current
Mean Through Current
I2I1
I1 - I2
Mean ThroughCurrent
DifferentialCurrent
I1
2
I2+I1
2I2+
I1 I2+
Differential Current = 2 X Mean Through Current