Functionality - ABB REG650 Technical Manual

Section 6
Differential protection
6.4.2
122

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 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.
Adaptive frequency tracking is included to ensure proper operation of the generator
differential protection during varying frequency conditions.
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
busduct.
1MRK 502 048-UEN A
Technical manual