08/0205/02/031 105/02/031 1

Differential Protection Principle

Protection & Contrôle / Application

Rev. A JM, September 2004

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

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

08/0205/02/034 4

Differential Protection Principle

R I diff=0

Nominal current through the protected equipment

• I Diff = 0 : No tripping

08/0205/02/035 5

Differential Protection Principle

R I diff=0

Through fault current

• I Diff = 0 : No tripping

08/0205/02/036 6

Differential Protection Principle

R I diff=0

Internal Fault

• I Diff = 0 : Tripping

08/0205/02/037 705/02/037 7

High Impedance Principle

Protection & Contrôle / Application

Rev. A JM, September 2004

08/0205/02/038 8

TC saturéM

RCT

ZM

RCT2RL 2RL

A

M

ZM

RCT

2RL

2RL

RCT

08/0205/02/039 9

RCT

ZM

RCT

ZM

2RL 2RL

A

M

M

08/0205/02/0310 10

RCT

ZM

RCT

ZM

2RL 2RL

TC saturé

A

M

M

08/0205/02/0311 11

RCT

ZM

RCT

ZM

2RL 2RL

A

M

M

08/0205/02/0312 12

RCT

ZM

RCT

ZM

2RL 2RL

TC saturé

A

M

M

08/0205/02/0313 13

RCT

ZM

RCT

ZM

2RL 2RL

A

M

M

08/0205/02/0314 14

RCT

ZM

RCT

ZM

2RL 2RLTC saturé

A

M

M

08/0205/02/0315 15

RCT

ZM

RCT

ZM

2RL 2RL

A

M

M

08/0205/02/0316 16

RCT

ZM

RCT

ZM

2RL 2RL

TC saturé

A

M

M

08/0205/02/0317 17

RCT

ZM

RCT

ZM

2RL 2RL

A

M

M

08/0205/02/0318 18

RCT

ZM

RCT

ZM

2RL 2RL

TC saturé

A

M

M

08/0205/02/0319 19

RCT

ZM

RCT

ZM

2RL 2RL

A

M

M

08/0205/02/0320 20

M

RCT

ZM

RCT2RL 2RL

A

M

RCT

2RL

2RL

RCT

ZM

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

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

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

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

08/0205/02/0325 25

Current Transformer Saturation

RCT

ZM

RCT

ZM=0

IM

VS

IM

VS

08/0205/02/0326 26

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

08/0205/02/0327 27

RCT

ZM

RCT

ZM

2RL 2RL

A

RS

M

M

RCT

2RL

2RL

RCTVset

Current Transformer Saturation

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

08/0205/02/0329 29

A

RCT

ZM

RCT

ZM

2RL 2RL

2RL

RCT

2RL

RCT

RS

M

M

Vset

08/0205/02/0330 30

2RL

RCT

2RL

RCT

M

A

RCT

ZM

RCT

ZM

2RL 2RL

RS

M

Vset

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

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

08/0205/02/0333 33

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

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

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)

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

08/0205/02/0337 3705/02/0337 37

Percentage Restraint (Biased)Principle

Protection & Contrôle / Application

Rev. A JM, September 2004

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

08/0205/02/0339 39

Biased Differential Scheme

I2I1

I1 - I2

Trip

no Trip

Mean ThroughCurrent

I1 - I2

DifferentialCurrent

I1

2

I2+

08/0205/02/0340 40

Biased Differential Scheme

Trip

no Trip

I1 - I2

I2I1

I1 - I2

Mean ThroughCurrent

DifferentialCurrent

I1

2

I2+

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