06 Differential Protection
Transcript of 06 Differential Protection
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DIFFERENTIAL
PROTECTION
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Is one of the most sensitive and effective methods of providing
protection against faults.
Detection and trip for internal faults.
Capable of detecting very small magnitudes of fault currents.
All current differential protection have problems to overcome if the
CT:s saturate (and they often do) on external faults.
Differential Protection
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Unit protection:
Can not be used as back-up protection for other lines, transformers, etc.
Absolute selective
Communication between the line terminals with sufficient capacity
Factors to be considered:
Unavailability of communication
Current transformer saturation
Line charging current
Differential Protection
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In theory, differential protection is a straightforward application of
Kirchhoff's first law.
The differential-relay current will be proportional to the vector
difference between the currents entering and leaving the protected
circuit.
Id = I1 – I2
I1 = current entering the protected object
I2 = current leaving the protected object
If the differential current exceeds the relay’s pickup value, the relay
will operate.
Id > I pickup Operation
Id < I pickup Non operation
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Id = I1 – I2
I1 I2
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Used as short circuit and earth-fault protection for busbars,
transformers, cables, reactors, generators, etc.
Different modes of stabilization for external faults are used for
differential relays in different applications and with different
measuring principles.
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Idiff= IA
- IB
IA IB
I diff
Ibias=
IA+ IB
2
IA IB
I bias
I diff CT-saturation
Normal condition
I bias
Vector comparison
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Differential Protection
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Percentage Restrained Differential Protection
Differential Protection
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The differential current required to operate this relay is a variable
quantity, owing to the effect of the restraining coil.
The differential current in the operating coil is proportional to I1 – I2,
and the equivalent current in the restraining coil is proportional to
(I1 + I2) / 2, since the operating coil is connected to the midpoint of
the restraining coil.
The ratio of the differential operating current to the average
restraining current is a fixed percentage, which explains the name
of this relay.
Differential Protection
Percentage Restrained Differential Protection
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Percentage Restrained Differential Protection
Operating characteristic
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Percentage Restrained Differential Protection
Since the percentage-differential relay has a rising pickup
characteristic as the magnitude of the through current
increases, the relay is restrained against operating
improperly.
The advantage in this relay is that it is less likely to operate
incorrectly than a differentially connected overcurrent relay
at external faults.
Maximum security for external faults is obtained when all
CT:s have the same ratio.
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Percentage Restrained Differential Protection
Only one operating coil per phase is required, but one restraint
winding for each phase of each circuit is necessary.
Normally, one restraint winding is connected to each circuit that
is a major source of fault current.
Feeders and circuits with low fault-current contribution may be
paralleled on a single restraint winding.
The required current to operate the relay is proportional to the
current flowing in the restraint windings.
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Differential Protection
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Differential Protection
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Differential Protection
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Requirement at Internal Faults
Protected part of the
Power System
High impedance differential circuit
Zdiff
Internal fault
I1
I2
I1
I2
+
Knee point voltage > I * Zop diff
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Requirement at External Faults
Protected part of thePower System
High impedance differential circuit
Zdiff
I 1I2
External fault
SaturatedCurrent transf.ZIT
Idiff
Idiff
=ZIT
ZIT Zdiff+
shall be < i op
Differential Protection