ABB RELION Series, Technical Manual

Page: 494 / 804

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
GUID-CABC3779-DAB9-4785-8451-DCCC8EAA034F V3 EN
(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 514 .
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 252 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
GUID-F8007C95-5C0B-4FBA-B724-0BC45E64841F V3 EN
(Equation 52)
Flt loop resistance R
R
fault
= +
1
2
GUID-CB02F4ED-8E75-4C30-BB25-3F4984D75FC8 V3 EN
(Equation 53)
Flt loop reactance Flt phase reactance X
= =
1
GUID-049B892B-9031-4361-843B-7102E6A4DA17 V3 EN
(Equation 54)
Section 5
1MRS758755 A
Protection related functions
488 REC615 and RER615
Technical Manual