ABB RELION 670 Series Technical Manual page 309

1MRK 502 066-UUS B
A-G
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Figure 148: The different measuring loops at phase-to-ground fault and phase-to-phase fault
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 (
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 is implemented. A 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.
7.8.7.3 Phase-selection element
The operation of the phase-selection element is primarily based on current change criteria (i.e.
delta quantities) with significantly increased dependability. To handle this, there is also a phase
selection criterion operating in parallel which bases its operation only on voltage and current
phasors.
This 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
Technical manual
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B-G C-G
B-G C-G
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CVTtype ) is introduced in order to inform the algorithm about the type
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A- B B - C C-A
Impedance protection
Zone 1
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