Siemens siprotec 7SA6 User Manual page 446

Functions
Fuse Failure
Monitor
(Non-Symmetrical
Voltages)
Fuse Failure
Monitor
(Three-Phase)
Voltage
Transformer
Secondary m.c.b.
Trip Circuit
Supervision
6-264
Note:
The current summation monitoring is only in service if the earth current of the protect-
ed feeder is connected to the fourth current measuring input (I
The settings of the fuse failure monitor for non-symmetrical measured voltage failure
(single- or two-phase) must be selected such that on the one hand reliable pick-up of
the monitoring is ensured in the case of loss of a single-phase voltage (address  
))0 8! PLQ ), while on the other hand a pick-up due to earth faults in an earthed
system is avoided. In accordance with this requirement, address   ))0 ,
PD[ must be set sufficiently sensitive (below the smallest fault current due to earth
faults). In address   )86( )$,/ 021, the fuse failure monitor can be switched
2)) e.g. during non symmetrical testing.
In address   ))0 8 PD[ SK the minimum voltage threshold is set. If the
measured voltage drops below this threshold and a simultaneous current jump which
exceeds the limits according to address   ))0 ,GHOWD S is not detected
while all three phase currents are greater than the minimum current required for the
impedance measurement by the distance protection according to address 
,SK!, a three phase measured voltage failure is recognized.
If a miniature circuit breaker for voltage transformers (VT mcb) is installed in the
secondary circuit of the voltage transformers, the status is sent, via binary input, to
the device informing it about the position of the VT mcb. If a short-circuit in the
secondary side initiates the tripping of the VT mcb, the distance protection function
has to be blocked immediately. Otherwise a trip by the distance protection due to the
lack of measured voltage while load current is possible. The blocking must be faster
than the first stage of the distance protection.This requires an extremely short
reaction time for VT mcb (≤ 4 ms for 50 Hz, ≤ 3 ms for 60 Hz nominal frequency). If
this cannot be ensured, the reaction time is to be set under address   7 PFE
Note that the fast trip of Zone 1 is delayed by the setting in  . Unless absolutely
necessary the setting should be zero. Alternatively the internal Fuse Failure Monitor
can be used (see above).
The number of circuits to be monitored was set during the configuration in address
 7ULS&LUF6XSHUY (Section 5.1). If the trip circuit supervision is not used at all,
the setting 'LVDEOHG must be applied there.
The trip circuit supervision can be switched 21 or 2)) in address  )&7
7ULS6XSHUY. The number of binary inputs that shall be used in each of the
monitored circuits is set in address  1R RI %, If the marshalling of the binary
inputs required for this function does not correspond to the previously selected type of
monitoring, a corresponding alarm is issued ("7ULS& 3URJ)$,/" with the number of
the faulty monitoring circuit).
The trip circuit failure alarm is delayed by a fixed period of approximately 1 s to 2 s in
the case of monitoring with two binary inputs. The alarm delay in the event of
monitoring with one binary input can be set in address  $ODUP 'HOD\. If 7SA6
is the only device connected in the trip circuit, a delay of 1 s to 2 s is sufficient as the
trip circuit supervision is not active during a detected system fault. If, however, trip
contacts from other devices are connected in parallel in the trip circuit, the fail alarm
must be delayed such that the longest trip command duration can be reliably bridged.
) for earth currents.
4
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7SA6 Manual
C53000-G1176-C133-1