Siemens Siprotec 7VK61 Manual page 137

Three-Phase Measuring Voltage Failure "Fuse Failure Monitor"
A three-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 analysed to recognise the magnitude of the current differential (current differential crite-
rion). A three-pole measuring voltage failure is detected when
• All three phase-to-earth voltages are smaller than the threshold FFM U<max (3ph),
the current differential in all three phases is smaller than the threshold FFM Idelta (3p)
•
If no stored current values are present (yet), the current magnitude criterion is resorted to. Figure shows the
logic diagram of the 3-phase measured voltage failure monitoring. A three-pole measuring voltage failure is de-
tected when
• All three phase-to-earth voltages are smaller than the threshold FFM U<max (3ph),
• All three phase current amplitudes are greater than a fixed set noise threshold (40 mA).
If such a voltage failure is recognized, the protection functions that operate on the basis of undervoltage are
blocked until the voltage failure is removed; Thereafter the blocking is automatically removed.
The „Fuse-Failure-Monitor" is only active if the three-phase currents and voltages are connected. Otherwise,
this function is disabled and the associated parameters and indications are hidden .
Additional Measured Voltage Failure Monitoring
If no measuring voltage is available after power-on of the device (e.g. because the voltage transformers are
not connected), the absence of the voltage can be detected and reported by an additional monitoring function.
Where circuit breaker auxiliary contacts are used, they should be used for monitoring as well. Figure 2-49
shows the logic diagram of the measured voltage failure monitoring. A failure of the measured voltage is de-
tected if:
• 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 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).
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 annunciation 168 „Fail U absent" is output.
The additional monitoring function for measured voltage failure is only active if the three-phase currents and
voltages are connected. Otherwise, this function is disabled and the associated parameters and indications are
hidden .
SIPROTEC, 7VK61, Manual
C53000-G1176-C159-3, Release date 05.2009
Functions
2.7 Monitoring Function
137