ABB Relion REG670, Техническое руководство

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impact on operate times. However, this will be evident primarily at higher source
impedance ratios (SIRs), SIR 5 and above, or close to the reach limit.
The IEC 60044-5 transient classification is of little or no use in this connection. It is
not primarily the damping of transients that is important; it is the frequency content of
the transients that is decisive, i.e. how difficult it is to filter out the specific frequency.
So, even if two CVTs, one passive and the other active type, comply with the same
transient class, the active type requires more extensive filtering in order to avoid
transient overreach.
For avoiding overreach and at the same time achieving fast operate times, a
supplementary circular characteristic that includes some alternative processing is
implemented. One such circular characteristic exists for every measuring loop and
quadrilateral characteristic. There are no specific reach settings for this circular zone.
It uses the normal quadrilateral zone settings to determine a reach that will be
appropriate. This implies that the circular characteristic will always have somewhat
shorter reach than the quadrilateral zone.
7.3.7.3 Phase-selection criteria
The operation of the phase-selection element is primarily based on current change
criteria (i.e. delta quantities), with significantly increased dependability. Naturally,
there is also a phase selection criterion operating in parallel which bases its operation
only on voltage and current phasors.
The phase-selection element can, owing to the current change criteria, distinguish
faults with minimum influence from load and fault impedance. In other words, it is not
restricted by a load encroachment characteristic during the current change phase. This
significantly improves performance for remote phase-to-earth faults on heavily
loaded lines. One exception however is three-phase faults, for which the load
encroachment characteristic always has to be applied, in order to distinguish fault
from load.
The continuous criteria will in the vast majority of cases operate in parallel and carry
on the fault indication after the current change phase has ended. Only in some
particularly difficult faults on heavily loaded lines the continuous criteria might not be
sufficient, for example, when the estimated fault impedance resides within the load
area defined by the load encroachment characteristic. In this case, the indication will
be restricted to a pulse lasting for one or two power system cycles.
Phase-to-phase-earth faults (also called double earth faults) will practically always
activate phase-to-phase zone measurements. Measurement in two phase-to-earth
loops at the same time is associated with so-called simultaneous faults: two earth
faults at the same time, one each on the two circuits of a double line, or when the zero
sequence current is relatively high due to a source with low Z0/Z1 ratio. In fact, in
these situations zone measurement will be released both for the related phase-to-earth
loops and the phase-to-phase loop simultaneously. On the other hand, simultaneous
faults closer to the remote bus will gradually take on the properties of a phase-to-
Section 7
1MRK502052-UEN B
Impedance protection
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Technical manual