Siemens siprotec 7SA6 User Manual page 439

Fuse Failure
Monitor
(Non-Symmetrical
Voltages)
7SA6 Manual
C53000-G1176-C133-1
In the event of measured voltage failure due to a short circuit or broken conductor in
the voltage transformer secondary circuit certain measuring loops may mistakenly see
a voltage of zero, which due to the load current may result in an unwanted pick-up or
even trip.
If a VT miniature circuit breaker (mcb) with correspondingly adjusted auxiliary contacts
is not available, but instead e.g. fuses are used, the fuse failure monitor may be
activated. Naturally, it is also possible to use voltage transformer mcb and fuse failure
monitor at the same time.
The non-symmetrical measured voltage failure is characterized by its voltage
unsymmetry with simultaneous current symmetry. In Figure 6-133 the logic diagram
of the fuse failure monitor during unsymmetrical failure of the measured voltage is
shown.
If there is substantial voltage unsymmetry of the measured values, without
unsymmetry of the currents being registered at the same time, this indicates the
presence of a non-symmetrical failure in the voltage transformer secondary circuit.
The unsymmetry of the voltage is detected by the fact that either the zero sequence
voltage or the negative sequence voltage exceed a settable value ))0 8! PLQ . The
current is assumed to be sufficiently symmetrical, if both the zero sequence as well as
the negative sequence current are below the settable threshold ))0 , PD[ .
As soon as this state is recognized, the distance protection and all other functions that
operate on the basis of undervoltage (e.g. also weak infeed tripping, undervoltage
protection) are blocked. The immediate blocking demands current flow in at least one
of the phases. The distance protection may be switched over to definite time
overcurrent emergency operation if the overcurrent protection was configured
accordingly (refer to Section 6.9).
The fast blocking may not occur as long as one phase is without voltage due to a
single-pole dead time condition before auto-reclosure, as the non-symmetry of the
measured values arising in this state is due to the switching state of the line and not
due to a failure in the secondary circuits. Accordingly, the fast blocking is disabled
when the line is tripped single-pole (internal information "1pole open" in the logic
diagram).
If a zero sequence or negative sequence current is detected within approximately 10 s
after recognition of this criterion, the protection assumes a short-circuit and removes
the blocking by the fuse failure monitor for the duration of the fault. If on the other hand
the voltage failure criterion is present for longer than approx. 10 s, the blocking is
permanently activated (latching of the voltage criterion after 10 s). Only once the
voltage criterion is removed by correction of the secondary circuit failure, will the
blocking automatically reset, thereby releasing the blocked protection functions again.
Functions
6-257