ABB RELION REX640, Техническое руководство, Страница: 1205 / 1960

Figure 687: 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
with 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 high 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 = "Enabled". However, a 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 688
. In case of earth faults, the estimated fault point resistance includes
the total fault point resistance between the faulty 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 per phase, for example, the arc resistance per phase.
1MRS759142 F Protection related functions
REX640
Technical Manual
1205