ABB RELION REX640 Technical Manual page 956

Protection functions
Start of element
Z
AB
EARTH_FLT
Z
BC
EARTH_FLT
Z
CA
EARTH_FLT
Z
AB
Z
AB
EARTH_FLT
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 monitored data signals.
Cross-country fault detection function for high-impedance earthed networks
In isolated and high-impedance earthed networks, the overvoltages produced by a
single-phase earth fault may lead to a cross-country fault, where two simultaneous
single-phase earth faults are present in the system but in different locations and
phases. The cross-country fault is hazardous because the magnitude of the fault
current through earth can be as high as in the case of a two-phase short circuit
fault. The protection must therefore remove the cross-country fault quickly without
sacrificing the selectivity. This can be done with a phase preference logic which is
activated by the cross-country fault detection function. The phase preference logic
transforms the fault into an ordinary single-phase earth fault by selectively tripping
one of the faulted feeders.
The cross-country detecting function can become active only if the network is either
isolated or high-impedance earthed (
selection "From input" and binary input SYS_EARTHING is active).
The fault detection is based on monitoring two quantities.
• Residual current
• Phase-to-phase voltages
A cross-country fault is detected if:
• An earth fault is detected, EARTH_FLT = TRUE.
AND
8
This applies to near three-phase faults. If voltages are still relatively high, all PP releases are
given, depending on the impedances seen inside the GFC zone.
9
This applies to near three-phase faults with EF indication. If voltages are still relatively high, all or
some PE and PP releases (except ABC Fault) are given, depending on the impedances seen inside
the GFC zone.
956
< &
< &
< &
8
< & Z < & Z <
BC CA
< & Z < & Z <&
BC CA
9
START_GFC STARTS_GF
C
TRUE AB Fault
TRUE BC Fault
TRUE CA Fault
TRUE ABC Fault
TRUE ABC Fault
System grounding GFC = "High impedance" or
1MRS759142 F
RELEASE_P RELEASE_P
E P
AB Fault AB Fault
BC Fault BC Fault
CA Fault CA Fault
No fault ABC Fault
No fault ABC Fault
REX640
Technical Manual