Functionality - ABB REG650 Technical Manual

1MRK 502 034-UEN -
5.3.2
Technical Manual

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 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 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.
An open CT circuit condition creates unexpected operations for Generator
differential protection under the normal load conditions. It is also possible to
damage secondary equipment due to high voltage produced from open CT circuit
outputs. Therefore, it may be a requirement from security and reliability points of
view to have open CT detection function to block Generator differential protection
function in case of open CT conditions and at the same time produce the alarm
signal to the operational personal to make quick remedy actions to correct the open
CT condition.
Generator differential protection GENPDIF is also well suited to generate fast,
sensitive and selective fault clearance, if used to protect shunt reactors or small
busbars
Section 5
Differential protection
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