ABB RELION REX640 Technical Manual page 716

Section 4
Protection functions
710
contributions from 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 operate 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
actual CT burden can be calculated on the basis of the rated accuracy limit factor F
(ALF) at the rated burden, the rated burden S
burden S of the current transformer.
a
S S
+
in n
F F
= ⋅
a n
S S
+
in a
GUID-2E56F98D-99C3-4BF3-B0B0-3CCE8FEF3782 V1 EN
corresponding to the
a
, the internal burden S
n in
1MRS759142 C
n
and the actual
(Equation 161)
REX640
Technical Manual