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 reference manual.
3.5.1.3 Setting example
Introduction
Differential protection for power transformers has been used for decades. In order to
correctly apply transformer differential protection proper compensation 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 select proper 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, often designed
for such typical delta connections, set the ratio as 800/5 in the IED. At the same time
the power transformer vector group shall be set as Yy0 because the IED shall not
internally provide any phase angle shift compensation. The necessary phase angle shift
compensation will be provided externally by delta connected main CT. All other
settings should have the same values irrespective of main CT connections. It shall be
Section 3 1MRK504116-UUS C
IED application
108
Application manual
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