Start of element START_GFC STARTS_GF
C
RELEASE_P
E
RELEASE_P
P
I
B
> & I
C
>
TRUE BC Fault No fault BC Fault
I
C
> & I
A
>
TRUE CA Fault No fault CA Fault
I
A
> & I
B
> &
EARTH_FLT
TRUE AB Fault AB Fault AB Fault
I
B
> & I
C
> &
EARTH_FLT
TRUE BC Fault BC Fault BC Fault
I
C
> & I
A
> &
EARTH_FLT
TRUE CA Fault CA Fault CA Fault
I
A
> & I
B
> & I
C
4
TRUE ABC Fault No fault ABC Fault
I
A
> & I
B
> & I
C
> &
EARTH_FLT
5
TRUE ABC Fault ABC Fault ABC Fault
In case of a high-impedance earthed network, a phase preference logic can be
added to complement the phase selection in case of a cross-country fault. This can
affect the RELEASE_PE signals.
In case of a low-impedance earthed network, a faulted loop selection logic can be
added to complement the phase selection in case of a two-phase-to-earth fault.
This can affect the RELEASE_PE and RELEASE_PP signals.
Faulted phase selection, the voltage-dependent overcurrent method
The "Voltdep overcur" method combines the overcurrent and undervoltage
conditions. The phase current amplitude is compared to the
Str A Ph Sel GFC
threshold setting but a lower current
Lo A Ph Sel GFC
threshold is valid during
an undervoltage condition. This increases the sensitivity of the fault detection. The
amplitude of phase-to-earth and phase-to-phase voltages is compared to the
Ph V
Ph Sel GFC
and
PP V Ph Sel GFC
settings. In case of a three-phase fault, the phase-
to-phase voltages are monitored and compared to the
PP V Ph Sel GFC
setting.
Earth-fault recognition is always required to release a phase-to-earth
impedance-measuring element.
4
This applies to near three-phase faults. If voltages are still relatively high, all PP releases are
given.
5
This applies to near three-phase faults with EF indication. If voltages are still relatively high, all PE
and PP releases (except ABC Fault) are given.
1MRS759142 F Protection functions
REX640
Technical Manual
951
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