ABB RELION REX640, Техническое руководство, Страница: 795 / 1960

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 high, that is, significantly
higher 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 high 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
a
corresponding to the
actual CT burden can be calculated on the basis of the rated accuracy limit factor
F
n
(ALF) at the rated burden, the rated burden S
n
, the internal burden S
in
and the
actual burden S
a
of the current transformer.
F F
S S
S S
a n
in n
in a
= ×
+
+
(Equation 209)
Example 1
The rated burden S
n
of the current transformer 5P20 is 10 VA, the secondary
rated current 5A, the internal resistance R
in
= 0.07 Ω and the rated accuracy
limit factor F
n
corresponding to the rated burden is 20 (5P20). The internal
burden of the current transformer is S
in
= (5A)² × 0.07 Ω = 1.75 VA. The input
impedance of the protection relay at a rated current of 5A is < 20 mΩ. If the
1MRS759142 F Protection functions
REX640
Technical Manual
795