4.3.3.5 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 main CTs are designated as "Type 1" and "Type 2". In case
the earthings of the current transformers on the phase side and the neutral side are
both either inside or outside the area to be protected, the setting parameter CT
connection type is "Type 1".
If the earthing of the current transformers on the phase side is inside the area to be
protected and the neutral side is outside the area to be protected or if the earthing
on the phase side is outside the area and on the neutral side inside the area, the
setting parameter CT connection type is "Type 2".
GUID-63BD73B4-7B60-4354-9690-E96C0A8076C7 V1 EN
Figure 227: Connection of the current transformers of Type 1. The connected
phase currents and the neutral current have opposite directions at
an external earth-fault situation.
1YHT530004D05 D Section 4
Protection functions
615 series 427
Technical Manual
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