ABB RELION RET670, Applications Manual

Page: 183 / 1016

Fast and sensitive detection of ground faults in a power transformer winding can be
obtained in solidly grounded or low impedance grounded networks by the restricted
earth fault protection. The only requirement is that the power transformer winding is
connected to ground in the star point (in case of wye-connected windings) or through a
separate grounding transformer (in case of delta-connected windings).
The low impedance restricted ground fault protection REFPDIF (87N) is a winding
protection function. It protects the power transformer winding against faults involving
ground. Observe that single phase-to-ground faults are the most common fault types in
transformers. Therefore, a sensitive ground fault protection is desirable.
A restricted ground fault protection is the fastest and the most sensitive protection, a
power transformer winding can have and will detect faults such as:
• ground faults in the transformer winding when the network is grounded through an
impedance
• ground faults in the transformer winding in solidly grounded network when the
point of the fault is close to the winding star point.
The restricted ground fault protection is not affected, as a differential protection, with
the following power transformer related phenomena:
• magnetizing inrush currents
• overexcitation magnetizing currents
• load tap changer
• external and internal phase faults which do not involve ground
• symmetrical overload conditions
Due to its features, REFPDIF (87N) is often used as a main protection of the
transformer winding for all faults involving ground.
7.3.2.1 Transformer winding, solidly grounded
M13048-3 v10
The most common application is on a solidly grounded transformer winding. The
connection is shown in Figure
62 .
1MRK 504 163-UUS A
Section 7
Differential protection
Transformer protection RET670 2.2 ANSI 177
Application manual