The function can operate correctly with only two phase-to-earth voltages if the
residual voltage is also measured and connected to the source UTVTR which
provides the phase-to-earth voltages to the function. In this case, the missing
phase-to-earth voltage is correctly calculated internally in the relay. If the measured
residual voltage is connected to SCEFRFLO input URES,
Phase voltage Meas
can be
set to "Accurate".
When the
Phase voltage Meas
setting is set to "Ph-to-ph without Uo" and only
phase-to-phase voltages are available (but not Uo), only short-circuit measuring
loops (fault loops “AB Fault”, “BC Fault”, “CA Fault” or “ABC Fault”) can be measured
accurately. In this case, the earth-fault loops (fault loops “AG Fault”, “BG Fault” or
“CG Fault”) cannot provide correct fault distance estimates and the triggering of the
function in case of an earth fault is automatically disabled. The situation is similar if
only two phase-to-earth voltages are measured and the measured residual voltage
is not connected to SCEFRFLO.
5.7.7 Application
The main objective of the feeder terminals is a fast, selective and reliable operation
in faults inside the protected feeder. In addition, information on the distance to
the fault point is very important for those involved in operation and maintenance.
Reliable information on the fault location greatly decreases the downtime of the
protected feeders and increases the total availability of a power system.
SCEFRFLO provides impedance-based fault location. It is designed for radially
operated distribution systems and is applicable for locating short circuits in all
kinds of distribution networks. Earth faults can be located in effectively earthed and
low-resistance/low-reactance earthed networks. With certain limitations, SCEFRFLO
can also be used for locating an earth fault in unearthed distribution networks.
Configuration example
A typical configuration example for SCEFRFLO triggering is illustrated in
Figure
691
where external triggering is applied, that is,
Calculation Trg mode
is set to
“External”. The OPERATE signal from non-directional overcurrent function PHLPTOC
is used to provide an indication of a short circuit fault. The OPERATE signal from
the directional earth-fault function DEFLPDEF is used to provide an indication of an
earth fault at the protected feeder.
SCEFRFLO with the autoreclosing function
When SCEFRFLO is used with the autoreclosing sequence, the distance estimate
from the first trip is typically the most accurate one. The fault distance estimates
from successive trips are possible but accuracy can be decreased due to inaccurate
load compensation. During the dead time of an autoreclosing sequence, the load
condition of the feeder is uncertain.
The triggering of SCEFRFLO can also be inhibited during the autoreclosing
sequence. This is achieved by connecting the signal ACTIVE from the autoreclosing
function DARREC, which indicates that the autoreclosing sequence is in progress,
to the BLOCK input of SCEFRFLO. Blocking of the SCEFRFLO triggering is indicated
during the autoreclosing sequence when the load compensation or steady-state
asymmetry elimination is based on the delta quantities. This applies to the short
circuit faults when
Load Com PP loops
is set to "Enabled" or, to earth faults, when
EF algorithm Sel
is set to "Load compensation" or "Load modelling".
1MRS759142 F
Protection related functions
REX640
Technical Manual
1217
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