Siemens SIPROTEC 7SD80 Manual page 172

Functions
2.14 Monitoring Functions
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; afterwards the blocking is automatically removed. The definite time
overcurrent protection as emergency function is possible during voltage failure, provided that the time overcur-
rent protection is parameterized accordingly (refer to Section 2.4).
A 3-pole measuring voltage failure is also detected in the absence of these criteria if the signal „VT
FuseFail" (no. 170) was previously generated due to an unbalanced measuring voltage failure. In this con-
dition, measuring voltage failure is still detected if the 3 phase-to-ground voltage subsequently falls below the
threshold value FFM V<max (3ph) (address 2913).
The effect of the signals „VT FuseFail" (no. 170) and „VT FuseFail>10s" (no. 169) on the protection
function is described in the following section „Impact of the Measuring Voltage Failure".
Additional Measuring Voltage Failure Monitoring „Fail V absent"
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. If
the circuit-breaker auxiliary contacts are used, they should be used for monitoring as well. Figure 2-71 shows
the logic diagram of the measuring voltage failure monitoring. A failure of the measuring voltage is detected if
the following conditions are met at the same time:
• All 3 phase-to-ground voltages are smaller than FFM V<max (3ph),
• At least 1 phase current is larger than PoleOpenCurrent or at least 1 circuit-breaker pole is closed (set-
table)
• 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 V absent" 168 is generated. The effect of this mon-
itoring indication is described in the following section „Impact of the Measuring Voltage Failure".
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SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011