In addition, when the setting EF algorithm Sel is equal to “Load modelling”, the EF
algorithm Cur Sel setting determines whether zero-sequence “Io based” or negative-
sequence “I2 based” current based algorithm is used. The difference between “Io based”
and “I2 based” methods is that “I2 based” does not require the Ph capacitive React and Ph
leakage Ris settings. In case of “Io based”, these settings are needed to compensate for the
influence of the line-charging capacitances of the protected feeder. This improves the
accuracy of the fault location estimate when fault resistance is involved in the fault.
Under certain restrictions, the “Load modelling” algorithm can also be applied to
ungrounded networks. In this case the EF algorithm Cur Sel setting should be set to “Io
based” and thus Ph capacitive React and Ph leakage Ris settings must be determined.
The prerequisite for the operation of 21FL in ground faults in ungrounded networks is that
the ground-fault current of the network corresponding to a solid fault exceeds the pre-fault
load current; that is the
Equation 143 is valid.
Flt to Lod Cur ratio
I
I
ef Rfault
Load
= ≥
=( )0
1
GUID-CABC3779-DAB9-4785-8451-DCCC8EAA034F V3 EN (Equation 143)
This ratio is estimated by 21FL and stored in the recorded data Flt to Lod Cur ratio together
with the fault distance estimate.
In case of ungrounded network, sufficient fault current magnitude resulting in Flt to Lod
Cur ratio >1 can be achieved, for example, with proper switching operations in the
background network, if possible, which increase the fault current. If the faulty feeder is re-
energized after the switching operation, a new estimate for the fault distance can be
obtained. Fault resistance decreases the fault location accuracy and the resistance should
not be too high, the maximum is a few hundred ohms. Also low value of Flt to Lod Cur
ratio causes inaccuracy and affects the quality of fault distance estimate. Considered
inaccuracies affecting the calculated fault distance estimate are reported in the recorded
result quality indicator value Flt Dist quality in
Table 711.
Fault loops “AB Fault”, “BC Fault” or “CA Fault”
Fault loops “AB Fault”, “BC Fault” or “CA Fault” are used for phase-to-phase short
circuit faults as well as in the case of a two-phase-to-ground fault if the individual ground
faults are located at the same feeder. Figure 413 shows the phase-to-phase fault loop
model. The following impedances are measured and stored in the recorded data of 21FL.
Flt point resistance
R
fault
=
2
GUID-F8007C95-5C0B-4FBA-B724-0BC45E64841F V3 EN (Equation 144)
Flt loop resistance R
R
fault
= +
1
2
GUID-CB02F4ED-8E75-4C30-BB25-3F4984D75FC8 V3 EN (Equation 145)
Section 5 1MAC059074-MB A
Protection related functions
810 615 series ANSI
Technical Manual
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