Earth-Fault Protection - ABB REX 521 Technical Reference Manual, Standard Configurations

1MRS 751802-MUM
9.1.3.2.
Protection Relay
Technical Reference Manual, Standard Configurations
For more information about the phase overcurrent blocking signals between the first
and the third application example, please refer to the overview diagram on page 134
(Fig. 9.3.5.-1).
The operation of the low-set overcurrent stage 3I> of the relay can be based on a
definite- or on an inverse-time characteristic. The operation characteristic is selected
within the settings of the low-set overcurrent function block. When the definite time
characteristic has been selected, the operate time of the relay is current independent.
On the contrary, when the inverse-time characteristic is in use, the operate time is a
function of the fault current level: the greater the fault current, the shorter the operate
time. Therefore, the operate time is short at close faults. The definite-time
characteristic can be used for obtaining the constant-time grading steps over a wide
current range and it offers faster tripping times than the inverse time protection at
low multiples of the current settings.
The settings for the over current protections can be found in:
Main menu\Protection\3I>\ ... \3I>>>\Setting group 1\. The
parameters "Operation mode", "Start current", "Operate time" and "Time
multiplier" for the function can be set in this setting group.
For example, in Main menu\Protection\3I>\ ... \3I>>>\Control
setting\ the "Drop-off time" and the length of the trip signal, "Trip pulse", can
be set.
The data of the three last operations, start or trip, are recorded and the values of the
most recent operation can be found in: Main menu\Protection\3I>\ ...
\3I>>>\Recorded data1\...3\.

Earth-fault protection

The neutral current can be measured either via a set of three-phase current
transformers in a residual connection, like in this example, or via a core-balance
current transformer. The residual connection can be used in the cases of high earth-
fault currents and moderate sensitivity requirements. In solidly earthed networks or
in networks earthed via a low-resistance resistor or low-impedance coil, the earth-
fault current is high enough to guarantee the sufficient accuracy of the residual
current connection for measuring the earth-fault current. The accuracy of the
residual current connection depends on electrical similarity of the current
transformers. To secure selectivity and stability at high fault current levels, current
transformers with sufficient repeatability are recommended, especially, if the high-
set stage is to operate instantaneously.
The earth-fault protection includes three stages. In the application two stages are in
use, the low-set Io> stage and the high-set Io>> stage. The low-set stage fulfills the
sensitivity requirements of the earth-fault protection and can be given definite-time
or inverse-time characteristic. The high-set stage fulfills the operate-time
requirements and can be used for instantaneous operation.
Alternatively the high-set stages Io>> and Io>>> can be used instead of Io> and
Io>> in the example above, when the Io> stage can be an alarming stage for high
resistance faults. Note that in this application example the CBFP signal has to be
taken out of use for this stage. With the default settings, the CBFP is in use.
REX 521
107