of the main line. As a result, the calculated value is stored in the recorded data
Equivalent load Dis.
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
unearthed 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 SCEFRFLO in earth faults in unearthed
networks is that the earth-fault current of the network corresponding to a solid
fault exceeds the pre-fault load current; that is the
Equation 51
is valid.
Flt to Lod Cur ratio
I
I
ef Rfault
Load
= ≥
=( )0
1
(Equation 51)
This ratio is estimated by SCEFRFLO and stored in the recorded data Flt to Lod Cur
ratio together with the fault distance estimate.
In case of unearthed 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 533
.
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-earth fault if the individual
earth faults are located at the same feeder.
Figure 251
shows the phase-to-phase
fault loop model. The following impedances are measured and stored in the
recorded data of SCEFRFLO.
Flt point resistance
R
fault
=
2
(Equation 52)
Flt loop resistance R
R
fault
= +
1
2
(Equation 53)
Protection related functions
1MRS758755 C
502 REC615 & RER615
Technical Manual
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