Siemens SIPROTEC 7SD610 Manual page 208

Functions
2.15 Monitoring Functions
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 analysed to recognise the magnitude of the current differential (current differential crite-
rion), see Figure 2-89.
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) (address 2914).
If such a voltage failure is recognized, the protection functions whose measuring principle is based on under-
voltage are blocked until the voltage failure is removed; afterwards the blocking is automatically reset.
A three-pole measuring voltage failure is also detected without the mentioned criteria if the signal „VT
FuseFail" (No. 170) previously has been generated by an asymmetrical measuring voltage failure. The mea-
suring 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
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 2-90
shows the logic diagram of the measured voltage failure monitoring. A failure of the measured voltage is de-
tected 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".
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SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011