ABB ret650, Applications Manual

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6.2 Low impedance restricted earth fault protection
REFPDIF
IP14640-1 v6
6.2.1 Identification
M14843-1 v6
Function description IEC 61850
identification
IEC 60617
identification
ANSI/IEEE C37.2
device number
Restricted earth fault protection, low
impedance
REFPDIF
IdN/I
SYMBOL-AA V1 EN-US
87N
6.2.2 Application
IP14941-1 v1
M13940-3 v12
A breakdown of the insulation between a transformer winding and the core or the
tank may result in a large fault current which causes severe damage to the windings
and the transformer core. A high gas pressure may develop, damaging the
transformer tank.
Fast and sensitive detection of earth faults in a power transformer winding can be
obtained in solidly earthed or low impedance earthed networks by the restricted
earth fault protection. The only requirement is that the power transformer winding
is connected to earth in the star point (in case of star-connected windings) or
through a separate earthing transformer (in case of delta-connected windings).
The low impedance restricted earth fault protection REFPDIF is a winding
protection function. It protects the power transformer winding against faults
involving earth. Observe that single phase-to-earth faults are the most common
fault types in transformers. Therefore, a sensitive earth fault protection is desirable.
A restricted earth fault protection is the fastest and the most sensitive protection, a
power transformer winding can have and will detect faults such as:
• earth faults in the transformer winding when the network is earthed through an
impedance
• earth faults in the transformer winding in solidly earthed network when the
point of the fault is close to the winding star point.
The restricted earth fault protection is not affected, as a differential protection, with
the following power transformer related phenomena:
1MRK 504 169-UEN A
Section 6
Differential protection
Transformer protection RET650 2.2 IEC 93
Application manual