ABB RELION Series Technical Manual page 496

Section 5
Protection related functions
490
GUID-359785B9-1D24-4751-B018-2225F04D7A2F V2 EN
Figure 253: Fault loop impedance for a three-phase fault loop ("ABC Fault")
The three-phase fault distance is calculated with a special measuring element using
positive-sequence quantities. This is advantageous especially in case of non-
transposed (asymmetric) lines, as the influence of line parameter asymmetry is
reduced. If the line is non-transposed, all the phase-to-phase loops have different fault
loop reactances. The use of positive-sequence quantities results in the average value
of phase-to-phase loop reactances, that is, the most representative estimate in case of
three-phase faults.
The fault distance calculation algorithm for the three-phase fault loop is defined by
using settings Load Com PP loops and Enable simple model. Options for the selection
are "Disabled" or "Enabled".
Load compensation can be enabled or disabled with setting Load Com PP loops. The
load compensation should be disabled only if the ratio between the fault current and
load current is large or when the value of the fault distance estimate for the short circuit
fault is required from each shot of an autoreclosing sequence.
The fault distance calculation is most accurate when the calculation is made with the
fault loop model. This model requires positive sequence impedances of the protected
feeder to be given as settings. If these settings are not available, valid impedance
values can be calculated also without the fault loop model with setting Enable simple
model = "TRUE". However, valid distance estimate, that is, the conversion of
measured impedance (''electrical fault distance'') into a physical fault distance
requires accurate positive sequence impedance settings.
Estimation of fault resistance in different fault loops
The fault point resistance value provided by the impedance calculation is available in
recorded data Flt point resistance and it depends on the applied fault loop as shown in
Figure 254. In case of earth faults, the estimated fault point resistance includes the
total fault point resistance between the faulted phase and earth, for example, the arc
and earthing resistances. In case of phase-to-phase faults, the estimated fault point
resistance is half of the total fault point resistance between the phases. In case of a
three-phase fault, the estimated fault point resistance equals the total fault point
resistance as per phase value, for example, the arc resistance per phase.
1MRS758755 A
REC615 and RER615
Technical Manual