Siemens siprotec 7SD5 User Manual

2.22 Monitoring Functions
383
7SD5 Manual
C53000-G1176-C169-1
In case of negative phase rotation, the indication „Fail Ph. Seq.“ (No. 171) is
issued.
Unsymmetrischer
Messspannungsau-
sfall „Fuse-Failure-
Monitor“
In the event of measured voltage failure due to a short-circuit or a 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 pickup
or even trip.
If fuses are used instead of a secondary miniature circuit breaker (VT mcb) with con-
nected auxiliary contacts, then the „fuse failure monitoring“ can detect problems in the
voltage transformer secondary circuit. Of course, the miniature circuit breaker and the
„fuse failure monitor“ can be used at the same time.
The asymmetrical measured voltage failure is characterized by its voltage asymmetri-
cal with simultaneous current symmetry. Figure 2-166 depicts the logic diagram of the
„fuse failure monitors“ during asymmetrical failure of the measured voltage.
If there is substantial voltage asymmetry of the measured values, without asymmetry
of the currents being registered at the same time, this indicates the presence of an
asymmetrical failure in the voltage transformer secondary circuit.
The asymmetry of the voltage is detected by the fact that either the zero sequence
voltage or the negative sequence voltage exceed a settable value FFM U>(min). 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 FFM I< (max) .
In non-earthed systems, the zero-sequence system quantities are no reliable criterion
since a considerable zero-sequence voltage occurs also in case of a simple earth fault
where a significant zero-sequence current does not necessarily flow. Therefore, the
zero-sequence voltage is not evaluated in such networks but only the negative-se-
quence voltage (parameter SystemStarpoint).
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) are blocked. The
immediate blocking demands that current flows in at least one of the phases. The dis-
tance protection can be switched to differential protection and/or O/C emergency op-
eration, provided that these functions are parameterized accordingly (refer also to
Sections 2.3 and 2.14).
The fast blocking may not occur as long as one phase is without voltage due to a
single-pole dead time condition, 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 sec-
ondary circuits. Accordingly, the fast blocking is disabled when the line is tripped
single-pole (internal information „1 pole 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 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.
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Siemens siprotec 7SD5 Specifications

General

Brand	Siemens
Model	siprotec 7SD5
Category	Relays
Language	English