Siemens SIPROTEC Manual page 98

Functions
2.3 Differential Protection
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 2-29 shows the logic diagram.
The cross-block function also affects all devices since it not only extends the inrush restraint to all three phases
but also sends it to the other devices via the communication link.
Figure 2-29 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 are calculated 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 „seen" by the differential protection system. In the ideal case it is thus equal to the
fault current value. During healthy operation it is small, and in a first approximation, equal to the charging cur-
rent. With active charging current compensation it is very small.
In addition to the evaluation of the measured values of the phases, the differential current for the zero-sequence
current 3I0 is also calculated. I-Diff 3I0 is not transmitted but calculated by means of the phase currents. Each
device calculates its own zero-sequence current. In addition, a zero-sequence current is calculated on the basis
of the transmitted phase currents of the remote end as well. The I-Diff 3I0 is calculated independently for each
device on the base of these values. The such determined I-Diff 3I0 can be allocated as a percentage value of
the nominal operational current to the CFC or to the default display for instance.
The restraining current counteracts the differential current. It is the total of the maximum measuring errors at
the ends of the protected object and is calculated adaptively from the current measured quantities and power
system parameters that were set. For this purpose, the maximum error of the current transformers within the
nominal range and/or the short-circuit current range is multiplied with the current flowing through each end of
the protected object. Consequently, the restraint current always reflects the maximum possible measuring error
of the differential protection system.
The pickup characteristic of the differential protection (Figure 2-30) derives from the restraining characteristic
(45°-curve), that is cut below the setting value I-DIFF>. It complies with the formula
I = I
diff rest
= I-DIFF> + Σ (errors by CT´s and other measuring errors)
I
rest
If the calculated differential current exceeds the pickup limit and the greatest possible measurement error, the
fault must be internal (shaded area in Figure 2-30).
98
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011