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2 Functions
The asymmetrical measured voltage failure is characterized by its voltage asymmetry
with simultaneous current symmetry. Figure 2-84 shows the logic diagram of the „Fuse
Failure Monitor" 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 these systems but only the negative se-
quence voltage (address207 SystemStarpoint).
As soon as this is detected, all functions that operate on the basis of undervoltage are
blocked. The indication "„VT FuseFail"" (No. 170) is output. The immediate block-
ing demands that current flows in at least one of the phases. The differential protection
can be switched to emergency operation, provided that these functions are parame-
terized accordingly (refer also to Sections 2.9).
The immediate blocking must not occur as long as one phase is without voltage due
to single-pole dead time condition as the asymmetry 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 circuit. Accordingly, the immediate 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.
204
7SD610 Manual
C53000-G1176-C145-4
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