ABB RELION REX640, Техническое руководство, Страница: 628 / 1960

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