Functions
2.24 Monitoring Functions
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
428
The effect of the signals „VT FuseFail“ (No. 170) and „VT FuseFail>10s“ (No. 169) on the protection
functions is described in the following section „Effect of the measuring voltage failure“.
Additional Measured Voltage Failure Monitoring „Fail U absent“
If no measuring voltage is available after power-on of the circuit breaker (e.g. because the voltage transformers
are not connected), the absence of the voltage can be detected and reported by an additional monitoring func-
tion. Where circuit breaker auxiliary contacts are used, they should be used for monitoring as well. Figure 2-
209 shows the logic diagram of the measured voltage failure monitoring. A failure of the measured voltage is
detected if the following conditions are met at the same time:
• All 3 phase-to-earth voltages are less than FFM U<max (3ph),
• At least 1 phase current is larger than PoleOpenCurrent or at least 1 breaker pole is closed (can be set),
• No protection function has picked up,
• This condition persists for a settable time T V-Supervision (default setting: 3 s).
The time T V-Supervision is required to prevent that a voltage failure is detected before the protection picks
up.
If this monitoring function picks up, the indication „Fail U absent“ (no. 168) will be issued. The effect of
this monitoring indication will be described in the following section „Effect of the Measuring Voltage Failure“.
Figure 2-209 Logic diagram of the additional measured voltage failure monitoring „Fail U absent“
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