ABB RELION REX640 Technical Manual page 628

Protection functions
where
U
PP_XC
with minimum expected cross-country fault current according to
Example 2:
U
•
Maximum earthing Ris = 15 ohm
•
Maximum earthing Ris_i = 10 ohm
•
X
•
R
•
Using the above parameters U
kV/20 kV=~0.97xU
in supply voltage, the setting
In this way unnecessary activations of phase-to-phase undervoltage condition can
be prevented.
Figure 357
to-phase voltage magnitude according to
as a function of fault resistance. Note that fault resistance is due to earthing
resistances of the fault locations only, and its value equals the average of fault
resistances R
ohm.
628
= Estimate of the highest phase-to-phase voltage during cross-country fault
= 20 kV
n_PP
= 2.2 ohm
k
= 0.2 ohm
k
). However, taking additionally into account normal variations
n_PP
illustrates fault current magnitude according to
and R in both fault locations. U
F1 F2
equals 19.3 kV in order of magnitude (19.3
PP_XC
XC stage PP V Val can be selected 0.9xU
Equation 136
n_PP
1MRS759142 F
(Equation 136)
Equation 136
(18.0 kV).
n_PP
Equation 134
and phase-
, during a cross-country fault
= 20 kV, X = 2.2 ohm, R
k
Technical Manual
.
= 0.2
k
REX640