7.1.3.10
Switch onto fault feature
M15266-348 v5
The Transformer differential function in the IED has a built-in, advanced switch onto
fault feature. This feature can be enabled or disabled by the setting parameter
SOTFMode
. When
SOTFMode
=
Enabled
this feature is enabled. It shall be noted that
when this feature is enabled it is not possible to test the 2
nd
harmonic blocking feature
by simply injecting one current with superimposed second harmonic. In that case the
switch on to fault feature will operate and the differential protection will trip. However
for a real inrush case the differential protection function will properly restrain from
operation.
For more information about the operating principles of the switch onto fault feature
please read the Technical Manual.
7.1.4
Setting example
SEMOD167745-1 v1
7.1.4.1
Introduction
SEMOD167720-5 v6
Differential protection for power transformers has been used for decades. In order to
correctly apply transformer differential protection proper compensation is needed for:
•
power transformer phase shift (vector group compensation)
•
CT secondary currents magnitude difference on different sides of the protected
transformer (ratio compensation)
•
zero sequence current elimination (zero sequence current reduction) shall be done.
In the past this was performed with help of interposing CTs or special connection
of main CTs (delta connected CTs). With numerical technology all these
compensations are done in IED software.
The Differential transformer protection is capable to provide differential protection for
all standard three-phase power transformers without any interposing CTs. It has been
designed with assumption that all main CTs will be wye connected. For such
applications it is then only necessary to enter directly CT rated data and power
transformer data as they are given on the power transformer nameplate and differential
protection will automatically balance itself.
These are internal compensations within the differential function. The
protected power transformer data are always entered as they are given
on the nameplate. Differential function will by itself correlate
nameplate data and properly select the reference windings.
However the IED can also be used in applications where some of the main CTs are
connected in delta. In such cases the ratio for the main CT connected in delta shall be
1MRK 504 163-UUS A
Section 7
Differential protection
Transformer protection RET670 2.2 ANSI
145
Application manual
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