Siemens SIPROTEC 7SD610 Manual page 70

Functions
2.3 Differential Protection
The charges of both ends of the protected object are added in the same way as done with the current phasors
of the differential protection. Thus the total of the charges is available at both ends of the protected zone.
Immediately after a fault has occurred in the protected zone, a charge difference emerges. For high fault cur-
rents which can lead to saturation of current transformers, a decision is thus reached before the saturation
begins
The charge difference of external faults is theoretically equal to zero at the beginning. The charge comparison
protection function immediately detects the external fault and blocks its own function. If saturation begins in one
or more current transformers which limit the protected zone, the before-mentioned function remains blocked.
Thus possible differences resulting from the saturation are excluded. Generally it is assumed that an initial sat-
uration of current transformers only takes place after the expiration of at least one integration interval (
that commenced with the occurrence of a fault.
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.
Blocking/interblocking
The differential protection can be blocked via a binary input. The blocking at one end of a protected object
affects the other end via the communications link (interblocking). If the overcurrent protection is configured as
an emergency function, both devices will automatically switch over to this emergency operation mode.
Please keep in mind that the differential protection is phase-selectively blocked at both 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. The other device shows the phase-selective blocking of the differential pro-
tection by displaying dashes instead of the differential and restraint current for the failed phase.
70
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011
1
/ cycle)
4