ABB RELION REX640, Техническое руководство, Страница: 1200 / 1960

Equivalent load Dis
U
U
d real
d tap d
=
=
( )
( , ) 1
(Equation 297)
U
d(real)
The actual maximum voltage drop of the feeder
U
d(tap,d=1)
The fictional voltage drop, if the entire load would be tapped at the end (d=1)
of the feeder (not drawn in
Figure 684
). The calculation of this value requires
data from the DMS system.
Alternatively, the setting
Equivalent load Dis can be determined by conducting a
single-phase earth-fault test (R
fault
= 0 Ω) at that point of the feeder where the
maximum actual voltage drop takes place. This point is typically located at the end
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 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 the
Equation 298
is valid.
Flt to Lod Cur ratio
I
I
ef Rfault
Load
= ≥
=
( ) 0
1
(Equation 298)
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
Table 93
.
Protection related functions 1MRS759142 F
1200 REX640
Technical Manual