Siemens SIPROTEC 7SD610 Manual page 68

Functions
2.3 Differential Protection
Figure 2-18 shows a simplified logic diagram. The condition for the inrush restraint is examined in each device
in which this function has been activated. The blocking condition is also effective at the other device.
Figure 2-18
Since the inrush restraint operates individually for each phase, the protection is fully operative when the trans-
former is switched onto a single-phase fault, where an inrush current may be flowing through one of the undis-
turbed phases. It is, however, also possible to set the protection in such a way that when the permissible har-
monic content in the current of only one single phase is exceeded, not only the phase with the inrush current
but also the remaining phases of the differential stage are blocked. This cross-block function can be limited to
a selectable duration. Figure 2-19 shows the logic diagram.
The cross-block function also affects both devices since it not only extends the inrush restraint to all three
phases but also sends it to the other device via the communication link.
Figure 2-19 Logic diagram of the cross-block function for one end
Evaluation of the measured quantities
The evaluation of measured values is performed separately for each phase. Additionally, the residual current
is evaluated.
Each device calculates a differential current from the total of the current phasors that were formed at each end
of the protected zone and transmitted to the other ends. The differential current value is equal to the value of
the fault current that is registered by the differential protection system. In the ideal case it is equal to the fault
current value. In a healthy system the differential current value is low and similar to the charging current.
68
Logic diagram of the inrush restraint for one phase
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011