Siemens siprotec 7SD5 User Manual page 95

ever, possible to set the protection in a way that when the permissible harmonic
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 can be limited to a selectable duration. Figure 2-21 shows the logic
diagram.
The cross-block function affects all devices as well, since it extends the inrush restraint
to all three phases.
Figure 2-21
Evaluation of Mea- The evaluation of measured values is performed separately for each phase. Addition-
surement ally, the residual current is evaluated.
Quantities
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
(„seen") 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, in a first ap-
proximation, equal to the charging current. With charging current compensation it is
very low.
The restraining current counteracts the differential current. It is the total of the
maximum measured errors at the ends of the protected object and is calculated from
the actual measured values and power system parameters that were set. Therefore
the highest possible error value of 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. The total value, including the measured internal
errors, is then transmitted to the other ends. This is the reason why the restraining
current is an image of the greatest possible measurement error of the entire differential
protection system.
The pickup characteristic of the differential protection (Figure 2-22) derives from the
restraining characteristic I
DIFF>. It complies with the formula
I
7SD5 Manual
C53000-G1176-C169-1
Logic diagram of the cross-block function for one end
= I-DIFF> + Σ (errors by CT´s and other effects)
rest
(45°-curve), that is cut below the setting value I-
= I
diff rest
2.3 Differential Protection
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