Typical Main Ct Connections For Transformer Differential Protection - ABB RET650 Applications Manual

1MRK 504 169-UEN A
6.1.4.2
Protected
Transformer
Winding
IEC06000549 V1 EN-US
Figure 31: Commonly used main CT connections for Transformer differential protection.
Transformer protection RET650 2.2 IEC
Application manual
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.

Typical main CT connections for transformer differential protection

Three most typical main CT connections used for transformer differential
protection are shown in figure 31. It is assumed that the primary phase sequence is
L1-L2-L3.
CT Star
Connected
For star connected main CTs, secondary currents fed to the IED:
• are directly proportional to the measured primary currents
• are in phase with the measured primary currents
• contain all sequence components including zero sequence current component
For star connected main CTs, the main CT ratio shall be set as it is in actual
application. The "StarPoint" parameter, for the particular star connection shown in
figure 31, shall be set ToObject. If star connected main CTs have their star point
away from the protected transformer this parameter should be set FromObject.
For delta DAC connected main CTs, secondary currents fed to the IED:
CT in Delta
DAC Connected
Section 6
Differential protection
IL1
IL2
IL3
CT in Delta
DAB Connected
en06000549.vsd
SEMOD167749-5 v3
L1
L2
L3
85