ABB RELION REX640 Technical Manual page 753

Protection and control

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1MRS759142 F

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

• If the positive directions of the winding 1 and winding 2 protection relay

currents are opposite, the

connection examples of "Type 1" are as shown in figures

• If the positive directions of the winding 1 and winding 2 protection relay currents

examples of "Type 2" are as shown in figures

• The default value of the

Technical Manual

CT connection type setting parameter is "Type 2". The connection

CT connection type setting is "Type 1".

CT connection type setting parameter is "Type 1". The

Protection functions