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Figure 2-84
Logic diagram of the fuse failure monitor with zero and negative sequence system
Three-Phase Mea-
A three-phase failure of the secondary measured voltages can be distinguished from
suring Voltage
an actual system fault by the fact that the currents have no significant change in the
Failure "Fuse
event of a failure in the secondary measured voltage. For this reason, the current
Failure Monitor"
values are routed to a buffer so that the difference between present and stored current
values can be analysed to recognise the magnitude of the current differential (current
differential criterion). A three-pole measuring voltage failure is detected when
• All three phase-to-earth voltages are smaller than the threshold FFM U<max
• the current differential in all three phases is smaller than a threshold value FFM
If no stored current values are present (yet), the current magnitude criterion is resorted
to. Figure 2-85 shows the logic diagram of the 3-phase measured voltage failure mon-
itoring. A three-pole measuring voltage failure is detected when
• All three phase-to-earth voltages are smaller than the threshold FFM U<max
• All three phase current amplitudes are greater than a fixed set noise threshold (40
If such a voltage failure is recognized, the protection functions that operate on the
basis of undervoltage are blocked until the voltage failure is removed; afterwards the
blocking is automatically removed. The O/C emergency operation is possible during
7SD610 Manual
C53000-G1176-C145-4
(3ph),
Idelta (3p) and
(3ph),
mA).
2.15 Monitoring Functions
205
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