ABB RELION REX640 Technical Manual page 1108

Section 5
Protection related functions
1102
Alternatively, the setting Equivalent load Dis can be determined by conducting a
single-phase earth-fault test (R
maximum actual voltage drop takes place. This point is typically located at the end of
the main line. 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 the used algorithm is based on zero-
sequence "Io based" or negative-sequence "I2 based" current. 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.
With 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
I
ef Rfault
Flt to Lod Cur ratio
=
GUID-C5AD8C22-0D26-4C65-BCBE-6612E954C29F V1 EN
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 an 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 so the
resistance should not be higher than a few hundred ohms. Also, a low value of Flt to
Lod Cur ratio causes inaccuracy and affects the quality of the fault distance estimate.
The inaccuracies affecting the calculated fault distance estimate are reported in the
recorded result quality indicator value Flt Dist quality in
Earth-fault distance calculation in case of compensated (resonant earthed)
networks where an earth-fault current raising resistor is used
In case of compensated (resonant earthed) networks where an earth-fault current
raising resistor is used, the setting Flt Loc Det mode is equal to "Comp, switched R".
The algorithm for calculation of earth-fault distance requires a measurable change in
the measured phase currents and phase-to-earth voltages during an earth fault. Such a
change can be accomplished by switching on the resistor of the PAW winding (Power
Auxiliary Winding) of the compensation coil during the fault.
= 0 Ω) at that point of the feeder where the
fault
Equation 264 is valid.
( ) 0
=
1
≥
I
Load
1MRS759142 C
(Equation 264)
Table 1044.
REX640
Technical Manual