Section 4
1MAC050144-MB C
Protection functions
328
615 series ANSI
Technical Manual
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 machine and/or the block circuit breaker) and from the
machine itself must be disconnected as fast as possible.
The DC restrain feature should be used in case of application where very long DC time
constant in the fault currents. This fault current may be of lesser magnitude (less than rated
current) but are unpleasant and tend to saturate the CT and operate the differential
protection for external faults. This feature is thus effective at moderate through currents
and ineffective at higher through currents.
Although normally the short circuit fault current is very large, i.e. significantly larger than
the machine rated current it is possible that a short circuit can occur between phases close
to the neutral point of the machine, thus causing a relatively small fault current. The fault
current fed from the synchronous machine itself can also be limited due to low excitation
of the synchronous generator. This is normally the case at running up of the synchronous
machine, before synchronization to the network. Therefore it is desired that the detection
of machine phase-to-phase short circuits shall be relatively sensitive, thus detecting small
fault currents.
It is also of great importance that the machine short circuit protection does not trip for
external faults, when large fault current is fed from the machine. In order to combine fast
fault clearance, sensitivity and selectivity the machine current differential protection is
normally the best alternative for phase-to-phase short circuits.
The risk of unwanted operation of the differential protection, caused by current
transformer saturation, is as universal differential protection problem. If the big
synchronous machine is tripped in connection to an external short circuit this will first give
an increased risk of power system collapse. Besides that there will be a production loss for
every unwanted trip of the machine. There is therefore a great economic value to prevent
unwanted disconnection of machines.
Recommendations for current transformers
The more important the object to be protected, the more attention has to be paid to the
current transformers. It is not normally possible to dimension the current transformers so
that they repeat 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 as follows:
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