06 differential protection

DIFFERENTIAL

PROTECTION

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

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


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


Id = I1 – I2

I1 I2


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.


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


Percentage Restrained Differential Protection


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.




Operating characteristic



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.



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.


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


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


06 differential protection

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