2.3 Differential Protection
97
7SD5 Manual
C53000-G1176-C169-1
an initial saturation of current transformers only takes place after the expiration of at
least one integration interval (
1
/
4
cycle) that commenced with the occurrence of a fault.
When the power line is switched on, the pickup value of the charge comparison is au-
tomatically redoubled for a period of approximately 1.5 s. This is to prevent from mal-
function caused by transient current in the CT secondary circuit due to remanence of
the CTs (e.g. during auto-reclosure). This current would simulate a charge value which
is not found in the primary quantities.
Each phase is subject to the charge comparison. Therefore an internal fault (evolving
fault) in a different phase after the external fault occurred is registered immediately.
The functional limitation of the charge comparison is reached in the less probable case
that an internal fault (evolving fault) appears after occurrence of an external fault with
considerable current transformer saturation in the same phase. This must then be de-
tected by the actual differential protection.
Furthermore the charge comparison is influenced by charge currents from lines and
shunt currents from transformers (steady-state and transient) that also cause a charge
difference. Therefore the charge comparison is, as aforesaid, a function suited to com-
plete the differential protection ensuring a fast tripping for high-current short-circuits.
Normally, the charge comparison is set higher than the nominal current. For charge
comparison, it is irrelevant whether the charging current compensation is activated or
not.
Blocking/Inter-
bl o cki n g
The distance protection, provided that it is available and configured, automatically
takes over as protection function if the differential protection is blocked by a binary
input signal. The blocking at one end of a protected object affects all ends via the com-
munications link (interblocking). If the distance protection is not available or ineffec-
tive, and if overcurrent protection has been configured as emergency function, all
devices automatically switch to this emergency mode.
Please keep in mind that the differential protection is phase-selectively blocked at all
ends when a wire break is detected at one end of the protected object. The message
„Wire break“ appears only on the device in which the wire break has been detected.
All other devices show the phase-selective blocking of the differential protection by
displaying dashes instead of the differential and restraint current for the failed phase.
In the case of a phase-selective blocking of the differential protection due to wire
break, the distance protection, even if it is available and configured, does not take over
the protection function for the failed phase.
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