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Functions
2.3 Differential Protection
When energizing a line, the pickup value of the charge comparison is automatically doubled for a period of ap-
proximately 1.5 s. This is to prevent from malfunction caused by transient currents in the CT secondary circuit
due to remanence of the CTs (e.g. during auto-reclosure). This current would simulate a charge value in the
primary circuit.
Each phase is subject to the charge comparison. Therefore an internal fault (sequential fault) in a different
phase after the external fault occurred is detected immediately. The functional limitation of the charge compar-
ison is reached in the less probable case that an internal fault (sequential fault) appears after the occurrence
of an external fault with considerable current transformer saturation in the same phase. This must be detected
by the current comparison stage in the differential protection.
Furthermore the charge comparison is influenced by charge currents from lines and shunt currents from trans-
formers (steady-state and transient) that also cause a charge difference. Therefore the charge comparison is,
as aforesaid, a function suited to complete 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/interblocking
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 communications link (interblocking). If the distance protection is not available or
ineffective, and if overcurrent protection has been configured as emergency function, all devices automatically
switch to 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" is only generated at 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, 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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SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
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