ABB ret650, Applications Manual

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6.1.3.9 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 = On 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.
6.1.4 Setting example
SEMOD167745-1 v1
6.1.4.1 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 star 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 intentionally set for √(3)=1.732 times smaller than actual ratio of individual
phase CTs (for example, instead of 800/5 set 462/5) In case the ratio is 800/2.88A,
Section 6
1MRK 504 169-UEN A
Differential protection
84 Transformer protection RET650 2.2 IEC
Application manual