ABB RELION 620 Series Technical Manual page 418

Section 4
Protection functions
412
both the external power system (via the machine or the block circuit breaker) and from the
machine itself must be disconnected as fast as possible.
The DC restraint feature should be used in case of an application with a long DC time
constant in the fault currents is present. This fault current may be of a lesser magnitude
(less than rated current) but is unpleasant and tends to saturate the CT and trip the
differential protection for external faults. This feature is effective at moderate through-
currents and ineffective at higher through-currents.
Although the short circuit fault current is normally very large, that is, significantly larger
than the rated current of the machine, it is possible that a short circuit can occur between
phases close to the neutral point of the machine, causing a relatively small fault current.
The fault current fed from the synchronous machine can also be limited due to a low
excitation of the synchronous generator. This is normally the case at the run-up of the
synchronous machine, before synchronization to the network. Therefore, it is desired that
the detection of the machine phase-to-phase short circuits shall be relatively sensitive,
thus detecting the small fault currents.
It is also important that the machine short circuit protection does not trip for external faults
when a large fault current is fed from the machine. To combine fast fault clearance,
sensitivity and selectivity, the machine current differential protection is normally the best
alternative for the phase-to-phase short circuits.
The risk of an unwanted differential protection operation caused by the current
transformer saturation is a universal differential protection problem. If a big synchronous
machine is tripped in connection to an external short circuit, it gives an increased risk of
a power system collapse. Besides, there is a production loss for every unwanted trip of the
machine. Therefore, preventing the unwanted disconnection of machines has a great
economical value.
Recommendations for current transformers
The more important the object to be protected is, the more attention is paid to the current
transformers. It is not normally possible to dimension the current transformers so that they
repeat the currents with high DC components without saturating when the residual flux of
the current transformer is high. The differential protection function block operates
reliably even though the current transformers are partially saturated.
The accuracy class recommended for current transformers to be used with the differential
function block is 5P, in which the limit of the current error at the rated primary current is
1 percent and the limit of the phase displacement is 60 minutes. The limit of the composite
error at the rated accuracy limit primary current is 5 percent.
The approximate value of the actual accuracy limit factor F
CT burden can be calculated on the basis of the rated accuracy limit factor F
1MAC504801-IB E
corresponding to the actual
a
(ALF) at the
n
620 series ANSI
Technical Manual