2.19 Monitoring function
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7SA522 Manual
C53000-G1176-C155-3
the voltage criterion has been removed by correction of the secondary circuit failure,
will the blocking automatically reset, thereby releasing the blocked protection func-
tions again.
Figure 2-141 Logic diagram of the fuse failure monitor with zero and negative sequence system
Three-Phase Mea-
suring Voltage
Failure "Fuse
Failure Monitor"
A three-phase failure of the secondary measured voltage can be distinguished from
an actual system fault by the fact that the currents have no significant change in the
event of a failure in the secondary measured voltage. For this reason, the sampled
current values are routed to a buffer, so that the difference between the present and
stored current values can be analyzed to recognize the magnitude of the current dif-
ferential (current differential criterion). A three-pole voltage failure is detected if
• all three phase–earth voltages are smaller than the threshold ))08PD[SK,
• the current differential in all three phases is smaller than the threshold ))0
,GHOWDS, and
• all three phase current amplitudes are greater than the minimum current ,SK! for
impedance measurement by the distance protection.
If no stored current values are present (yet), the current magnitude criterion is resorted
to. A three-pole system voltage failure is detected in this case if
• all three phase–earth voltages are smaller than the threshold ))08PD[SK,
• all three phase current amplitudes are smaller than the minimum current ,SK! for
impedance measurement by the distance protection, and
• all three phase current amplitudes are greater than a fixed set noise threshold (40
mA).
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