ABB RELION REX640 Technical Manual page 1106

Section 5
Protection related functions
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Figure 596: Fault loop impedance for phase-to-earth fault loops "AG Fault", "BG
Fault" or "CG Fault"
Setting Flt Loc Det mode is used for selecting between different algorithms for the
calculation of the earth-fault distance. In case of compensated (resonant earthed)
networks where an earth-fault current raising resistor is used, setting Flt Loc Det mode
= "Comp, switched R" should be selected. In all other cases of system neutral point
earthing (effectively earthed, low-resistance, low-reactance or unearthed networks),
setting "Non-comp" should be selected.
Earth-fault distance calculation in case of non-compensated networks
In case of non-compensated networks (effectively earthed, low-resistance, low-
reactance or unearthed networks), the setting Flt Loc Det mode is equal to "Non-
comp". Furthermore, the earth-fault distance calculation algorithm is selected with
setting EF algorithm Sel. Options for the selection are "Load compensation" and
"Load modelling". For the correct operation of both algorithms, there should not be
any zero-sequence current sources, for example, earthing transformers, in front of the
protection relay.
The "Load compensation" algorithm uses symmetrical components to compensate for
the effect of load on the measured voltages and currents. In case of radial feeders, this
algorithm should be selected with low-impedance/effectively earthed systems where
the fault current is fed from one side only and there are no in-feeds along the protected
line.
The "Load modelling" algorithm takes into account the effect of the load in the
measured currents and voltages by considering it in the fault loop model. In case of
radial feeders, this algorithm can be applied with low-impedance/effectively earthed
systems where the fault current is fed from one side only. The "Load modelling"
algorithm has been especially designed for unearthed systems.
The "Load modelling" algorithm requires the Equivalent load Dis setting, that is, an
equivalent load distance, as an additional parameter. The derivation and meaning of
this parameter is illustrated in
distributed along the feeder, resulting in the actual voltage drop curve as seen in the
middle part of Figure 597.
Figure 597, where the load is assumed to be evenly
1MRS759142 C
REX640
Technical Manual