Siemens SIPROTEC 4 7VK61 Manual page 136

Functions
2.7 Monitoring Functions
the primary side which cannot be distinguished from a measuring voltage failure in the secondary circuit (not
represented in the logic diagram).
[lo_7vk6-ffm-mcl-02, 1, en_GB]
Figure 2-53
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 analyzed to recognize the magnitude of the current differential (current differen-
tial criterion), see
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) (Adresse 2914).
A three-pole measuring voltage failure is also detected without the mentioned criteria if the signal
Fail (No. 170) previously has been generated by an asymmetrical measuring voltage failure. The measuring
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 ).
The effect of the signals
tions is described in the following section "Effect of the measuring voltage failure".
Additional Measured Voltage Failure Monitoring
If no measuring voltage is available after power-on of the circuit breaker (e.g. because the voltage trans-
formers are not connected), the absence of the voltage can be detected and reported by an additional moni-
toring function. Where circuit breaker auxiliary contacts are used, they should be used for monitoring as well.
136
Fuse failure monitoring Part 2: Detection of three-phase measuring voltage failure
Figure 2-53.
VT FuseFail (No. 170) and VT FuseFail>10s (No. 169) on the protection func-
Failure: Voltage absent
VT Fuse-
SIPROTEC 4, 7VK61, Manual
C53000-G1176-C159-5, Edition 05.2018