Siemens SIPROTEC Manual page 428

Functions
2.24 Monitoring Functions
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".
Logic diagram of the additional measured voltage failure monitoring „Fail U absent"
Figure 2-209
428
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011