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Figure 2-208
Fuse failure monitoring Part 2: Detection of the 3-phase measuring voltage failure
A 3-phase failure of the secondary measured voltages 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 current 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 crite-
rion), see Figure 2-208.
A three-pole measuring voltage failure is detected if:
• All 3 phase-to-earth voltages are smaller than the threshold FFM U<max (3ph) (address 2913),
• The current differential in all 3 phases is smaller than the threshold FFM Idelta (3p) (address 2914).
• All 3 phase current amplitudes are larger than the minimum current Iph> (address 1502) for impedance
measurement of the distance protection.
If such a voltage failure is recognized, the distance protection and all other functions that operate on the basis
of undervoltage (e.g. also weak infeed tripping) are blocked until the voltage failure is removed; thereafter the
blocking is automatically reset. Differential protection and O/C emergency operation are possible during the
voltage failure, provided that the differential protection or the time overcurrent protection are parameterized ac-
cordingly (refer also to Sections 2.3 and 2.16).
A three-pole measuring voltage failure is also detected without the mentioned criteria if the signal „VT
FuseFail" (No. 170) previously has been generated by an asymmetrical measuring voltage failure. The mea-
suring voltage failure is still detected in this state if the three phase-to-earth voltages subsequently fall below
the threshold value FFM U<max (3ph) (address 2913 ).
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
Functions
2.24 Monitoring Functions
427
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