ABB Relion REG670, Technical Manual

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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
(retained from REL 531) 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.
7.4.7.3 Phase-selection element
The operation of the phase-selection element is primarily based on current change
criteria. The current change criteria itself can however only be relied on for a short
period following the fault inception (during what we will call the current change
phase). Subsequent switching in the network may render the change in current invalid.
So, naturally, the phase-selection element also operates on continuous criteria.
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. This is a substantial difference compared
to the previous phase selection function in the 500- and 600-series (that is,
FDPSPDIS). 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-phase-earth fault and the function will
eventually use phase-to-phase zone measurements also here.
1MRK502052-UEN B
Section 7
Impedance protection
239
Technical manual