Identification - ABB Relion Technical Manual

Section 6
Differential protection
6.4.1
6.4.2
132

Identification

Function description
Generator differential protection
Functionality
Short circuit between the phases of the stator windings causes normally very large fault
currents. The short circuit gives risk of damages on insulation, windings and stator iron
core. The large short circuit currents cause large forces, which can cause damage even to
other components in the power plant, such as turbine and generator-turbine shaft.
The task of Generator differential protection GENPDIF (87G) is to determine whether a
fault is within the protected zone, or outside the protected zone. If the fault is internal, the
faulty generator must be quickly tripped, that is, disconnected from the network, the field
breaker tripped and the power to the prime mover interrupted.
To limit the damage due to stator winding short circuits, the fault clearance must be as fast
as possible (instantaneous). If the generator block is connected to the power system close
to other generating blocks, the fast fault clearance is essential to maintain the transient
stability of the non-faulted generators.
Normally, the short circuit fault current is very large, that is, significantly larger than the
generator rated current. There is a risk that a short circuit can occur between phases close
to the neutral point of the generator, thus causing a relatively small fault current. The fault
current can also be limited due to low excitation of the generator. Therefore, it is desired
that the detection of generator phase-to-phase short circuits shall be relatively sensitive,
detecting small fault currents.
It is also of great importance that the generator differential protection does not trip for
external faults with large fault currents flowing from the generator. To combine fast fault
clearance, as well as sensitivity and selectivity, the generator differential protection is
normally the best choice of protection for phase-to-phase generator short circuits. A
negative-sequence-current-based internal-external fault discriminator can also be used to
determine whether a fault is internal or external. The internal-external fault discriminator
not only positively discriminates between internal and external faults, but can
independently detect minor faults which may not be detected (until they develop into more
serious faults) by the "usual" differential protection based on operate-restrain
characteristic.
IEC 61850 IEC 60617
identification identification
GENPDIF
I >
d
SYMBOL-NN V1 EN
1MRK 502 048-UUS A
ANSI/IEEE C37.2
device number
87G
Technical manual