Siemens SIPROTEC 7UT613 series Manual page 284

2 Functions
Figure 2-116 Logic diagram of the "fuse failure monitor" with zero and negative sequence system (simplified).
Three-Phase Mea-
suring Voltage
Failure "Fuse
Failure Monitor"
284
is permanently activated (latching of the voltage criterion after 10 s). Only 10 s after
the voltage criterion has been removed by correction of the secondary circuit failure,
will the blocking automatically reset, thereby releasing the blocked protection func-
tions again.
The release „Current Criterion" for „VT FuseFail" is determined by assignment VT
SET according to the measuring location selection.
Figure 2-116 shows the assignment to measuring location 1 or side 1 with one as-
signed measuring location. That means that, for example, during assignment to side
2, whereby measuring location 2 and measuring location 3 are assigned to side 2,
evaluation of the addresses 1122 PoleOpenCurr.M2 and 1123
PoleOpenCurr.M3) occurs.
A three-phase failure of the secondary measured voltage 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 sampled
current values are routed to a buffer, so that the difference between the present and
stored current values can be analysed to recognise the magnitude of the current dif-
ferential (current differential criterion). Decisive are the connected voltages and cur-
rents of the measured locations or sides are used that are assigned voltages.
A three-pole voltage failure is detected if
• all three phase–earth voltages are smaller than the threshold FFM UMEAS<,
• the current differential in all three phases is bigger than in the preset expected value
and
• all three phase current amplitudes are greater than the corresponding side/measur-
ing location set residual current I-REST for the detection of a closed breaker.
If such a voltage failure is recognised, the protection functions are blocked until the
voltage failure is removed; afterwards the blocking is automatically removed. For
7UT613/63x Manual
C53000-G1176-C160-2