Hitachi Relion 670 Series Applications Manual page 78

Section 6
Differential protection
• 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:
• magnetizing inrush currents
• overexcitation magnetizing currents
• load tap changer
• external and internal phase faults which do not involve earth
• symmetrical overload conditions
Due to its features, REFPDIF is often used as a main protection of the transformer winding for all
faults involving earth.
6.1.3.1 Transformer winding, solidly earthed
The most common application is on a solidly earthed transformer winding. The connection is shown
in Figure 31.
IdN/I
IEC16000117 V1 EN-US
Figure 31:
6.1.3.2 CT earthing direction
To make the restricted earth fault protection REFPDIF operate correctly, the main CTs are always
supposed to be star -connected. The main CT's neutral (star) formation can be positioned in either
72
REFPDIF
I2P
ISI
Connection of the low impedance Restricted earth fault function REFPDIF for a
directly (solidly) earthed transformer winding
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1MRK506375-UEN Rev. N
M13048-3 v10
Protected
winding
IEC16000117-1-en.vsdx
M13048-29 v14
Railway application RER670
Application manual