Functions
2.14 Monitoring Functions
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
171
If a zero sequence or negative sequence current occurs within 10 s after detecting the unbalanced measuring
voltage failure, a short circuit is assumed to exist in the system and the signal „VT FuseFail“ is canceled
immediately. If the zero sequence voltage or the negative sequence voltage exceeds the settable value FFM
V>(min) (address 2911) for longer than 10 s, the signal „VT FuseFail>10s“ (no. 169) is generated. In this
condition, dropout of the signal „VT FuseFail“ can no longer be accomplished by the increase of the zero
sequence current or negative sequence current, but only by the voltages in the zero sequence and negative
sequence system falling below a threshold value. The signal „VT FuseFail“ can also be generated indepen-
dently of the magnitude of the phase currents.
Figure 2-70 Fuse Failure Monitor part 2: detection of the 3-phase measuring voltage failure
A 3-phase failure of the secondary measuring 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 analyzed to recognize the magnitude of the current differential (current differential crite-
rion), cf. figure 2-70.
A 3-pole measuring voltage failure is detected if:
• All 3 phase-to-ground voltages assume a value smaller than the threshold value FFM V<max (3ph)
(address 2913).
• The current differential in all 3 phases is smaller than the threshold value FFM Idiff (3ph)
(address 2914).
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