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Section 7
Differential protection
178
time. Fast fault clearance of this fault type is therefore of greatest importance to limit
the damages and thus the economic loss.
To limit the damages in connection to stator winding short circuits, the fault clearance
time must be as fast as possible (instantaneous). Both the fault current contributions
from the external power system (via the generator and/or the block circuit breaker) and
from the generator itself must be disconnected as fast as possible. A fast reduction of
the mechanical power from the turbine is of great importance. If the generator is
connected to the power system close to other generators, 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 fed from the generator itself can also be limited due to low excitation
of the generator. This is normally the case at running up of the generator, before
synchronization to the network. Therefore, it is desired that the detection of generator
phase-to-phase short circuits shall be relatively sensitive, thus detecting small fault
currents.
It is also of great importance that the generator short circuit protection does not trip for
external faults, when large fault current is fed from the generator. In order to combine
fast fault clearance, sensitivity and selectivity the Generator current differential
protection GENPDIF (87G) is normally the best choice for phase-to-phase generator
short circuits.
The risk of unwanted operation of the differential protection, caused by current
transformer saturation, is a universal differential protection problem. If the generator is
tripped in connection to an external short circuit, this can first give an increased risk of
power system collapse. Besides that, there can be a production loss for every unwanted
trip of the generator. Hence, there is a great economic value to prevent unwanted
disconnection of power generation.
The generator application allows a special situation, where the short circuit fault
current with a large DC component, can have the first zero crossing of the current, after
several periods. This is due to the long DC time constant of the generator (up to 1000
ms), see figure 66.
GENPDIF (87G) is also well suited to give fast, sensitive and selective fault clearance,
if used for protection of shunt reactors and small busbars.
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
1MRK 502 071-UUS A
Application manual
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