Siemens siprotec 7SD5 User Manual page 385

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 removed.
Differential protection and O/C emergency operation are possible during the voltage
failure, provided that the differential protection and/or the time overcurrent protection
are parameterized accordingly (refer also to Sections 2.3 and 2.14).
Additional Mea- If no measuring voltage is available after power-on of the device (e.g. because the
sured Voltage voltage transformers are not connected), the absence of the voltage can be detected
Failure Monitoring and reported by an additional monitoring function. Where circuit breaker auxiliary con-
tacts are used, they should be used for monitoring as well. Figure 2-167 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 three phase-to-earth voltages are smaller than FFM U<max (3ph),
• At least one phase current is larger than PoleOpenCurrent or at least one
• No protection function has picked up,
• This condition persists for a settable time T V-Supervision (default setting: 3 s).
This time T V-Supervision is required to prevent that a voltage failure is detected
before the protection picks up.
If a failure is detected by these criteria, the indication 168 „Fail U absent" is
output, and the device switches to emergency operation (see Section 2.14).
Figure 2-167 Logic diagram of the additional measured voltage failure monitoring
7SD5 Manual
C53000-G1176-C169-1
breaker pole is closed (can be set),
2.22 Monitoring Functions
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