Identification - ABB REG650 Technical Manual

Section 6
Differential protection
6.3.1
6.3.2
114

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 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 felt (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 502 043-UUS B
ANSI/IEEE C37.2
device number
87G
Technical Manual