ABB RELION REX640 Technical Manual page 627

1MRS759142 F
Phase-to-phase under-voltage criterion
Monitoring of the magnitude of any phase-to-phase voltage (U
comparing it to setting
During a single-phase earth fault phase-to-phase voltages are not affected, but
during a cross-country fault the phase-to-phase voltages are affected, refer to
Figure 356
nominal phase-to-phase voltage.
Figure 356: Illustration of behavior of residual, phase-to-earth and phase-to-phase
voltages during an evolvement of single-phase earth fault into cross-country fault in
a 10 kV compensated network
The magnitude of phase-to-phase voltage between faulted phases during a cross-
country fault can be estimated using the following equation:
REX640
Technical Manual
XC stage PP V Val during a detected earth fault:
XC stage PP V Val is 0.9xU
. Default setting for
In case all three phase-to-phase voltages (U
XC stage PP V Val , such condition is considered as permanent
setting for
three phase under-voltage condition, not a cross-country fault.
Protection functions
, U or U
AB BC CA
(Equation 135)
, where U is the
n_PP n_PP
, U and U ) are under
AB BC CA
) and
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