ABB RELION 670 Series Technical Manual page 348

Section 7
Impedance protection
The use of full scheme technique gives faster operation time compared to switched schemes
which mostly uses a pickup of an overreaching element to select correct voltages and current
depending on fault type. Each distance protection zone performs like one independent distance
protection function with six measuring elements.
Transients from CVTs may have a significant impact on the transient overreach of a distance
protection. At the same time these transients can be very diverse in nature from one type to the
other; in fact, more diverse than can be distinguished by the algorithm itself in the course of a few
milliseconds. So, a setting is introduced in order to inform the algorithm about the type of CVT
applied and thus providing the advantage of knowing how performance should be optimized, even
during the first turbulent milliseconds of the fault period.
There are two types of CVTs from the function point of view, the passive and the active type,
which refers to the type of ferro-resonance suppression device that is employed. The active type
requires more rigorous filtering which will have a negative 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 relation to this. 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.
To avoid overreach and at the same time achieve 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/mho
characteristic. There are no specific reach settings for this circular zone. It uses the normal
quadrilateral/mho 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/mho
zone.
7.9.7.3 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-ground 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.
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