4.3.4.6 Application
An earth-fault protection using an overcurrent element does not adequately protect
the transformer winding in general and the star-connected winding in particular.
The restricted earth-fault protection is mainly used as a unit protection for the
transformer windings. LREFPNDF is a sensitive protection applied to protect the
star-connected winding of a transformer. This protection system remains stable for
all the faults outside the protected zone.
LREFPNDF provides higher sensitivity for the detection of earth faults than the
overall transformer differential protection. This is a high-speed unit protection
scheme applied to the star-connected winding of the transformer. LREFPNDF is
normally applied when the transformer is earthed solidly or through low-impedance
resistor (NER). LREFPNDF can be also applied on the delta side of the transformer
if an earthing transformer (zig-zag transformer) is used there. In LREFPNDF, the
difference of the fundamental component of all three phase currents and the
neutral current is provided to the differential element to detect the earth fault in the
transformer winding based on the numerical stabilized differential current principle.
Connection of current transformers
The connections of the primary current transformers are designated as "Type 1" and
"Type 2".
• If the positive directions of the winding 1 and winding 2 protection relay
currents are opposite, the
CT connection type
setting parameter is "Type 1". The
connection examples of "Type 1" are as shown in figures
Figure 425
and
Figure
426
.
• If the positive directions of the winding 1 and winding 2 protection relay currents
equate, the
CT connection type
setting parameter is "Type 2". The connection
examples of "Type 2" are as shown in figures
Figure 427
and
Figure 428
.
• The default value of the
CT connection type
setting is "Type 1".
1MRS759142 F
Protection functions
REX640
Technical Manual
753
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