ABB RELION REX640 Technical Manual page 876

Section 4
Protection functions
870
Start of element START_GFC
I > & I > & TRUE
C A
EARTH_FLT
TRUE
1)
I > & I > & I >
A B C
I > & I > & I > & TRUE
A B C
2)
EARTH_FLT
1) This applies to near three-phase faults. If voltages are still relatively high, all PP releases are given.
2) 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.
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.
U/U
n
Ph V Ph Sel GFC
PP V Ph Sel GFC
Lo A Ph Sel GFC
GUID-E8FA87F7-B5A8-49CB-ADD7-721BC732F1CC V2 EN
Figure 469: Overcurrent/undervoltage characteristics
STARTS_GFC RELEASE PE
CA Fault CA Fault
ABC Fault No fault
ABC Fault ABC Fault
Fault detected
High
sensitivity
Str A Ph Sel GFC
1MRS759142 C
RELEASE PP
CA Fault
ABC Fault
ABC Fault
Normal
sensitivity
I/I
n
REX640
Technical Manual