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
Fuse failure monitoring Part 2: Detection of three-phase measuring voltage failure
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 Figure 2-53.
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
VT Fuse-
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
VT FuseFail
(No. 170) and
VT FuseFail>10s
(No. 169) on the protection func-
tions is described in the following section “Effect of the measuring voltage failure”.
Additional Measured Voltage Failure Monitoring
Failure: Voltage absent
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.
Functions
2.7 Monitoring Functions
136 SIPROTEC 4, 7VK61, Manual
C53000-G1176-C159-5, Edition 05.2018
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