Typical Main Ct Connections For Transformer Differential Protection - ABB REG670 Applications Manual

1MRK 502 071-UEN -
7.1.4.2
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
• 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, 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 noted that irrespective of the main CT connections (star or delta) on-line
reading and automatic compensation for actual load tap changer position can be used
in the IED.

Typical main CT connections for transformer differential protection

Three most typical main CT connections used for transformer differential protection
are shown in figure 54. It is assumed that the primary phase sequence is L1-L2-L3.
Section 7
Differential protection
137