Typical Main Ct Connections For Transformer Differential Protection - ABB RELION 650 Series Applications Manual

1MRK 504 158-UEN A
6.1.3.2

Typical main CT connections for transformer differential protection

Three most typical main CT connections used for transformer differential protection are
shown in figure 28. It is assumed that the primary phase sequence is L1-L2-L3.
Protected
Transformer
Winding
IEC06000549 V1 EN-US
Figure 28: Commonly used main CT connections for Transformer differential protection.
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 28, shall be set
ToObject . If star connected main CTs have their star point away from the protected
transformer this parameter should be set
For delta DAC connected main CTs, secondary currents fed to the IED:
• are increased √3 times (1.732 times) in comparison with star connected CTs
• lag by 30° the primary winding currents (this CT connection rotates currents by 30° in
clockwise direction)
• do not contain zero sequence current component
For DAC delta connected main CTs, ratio shall be set for √3 times smaller than the actual ratio
of individual phase CTs. The "StarPoint" parameter, for this particular connection shall be set
ToObject . It shall be noted that delta DAC connected main CTs must be connected exactly as
shown in figure 28.
For delta DAB connected main CTs, secondary currents fed to the IED:
• are increased √3 times (1.732 times) in comparison with star connected CTs
• lead by 30° the primary winding currents (this CT connection rotates currents by 30° in
anti-clockwise direction)
• do not contain zero sequence current component
Application manual
CT Star
Connected
CT in Delta
DAC Connected
FromObject .
Differential protection
IL1
IL2
IL3
CT in Delta
DAB Connected
en06000549.vsd
Section 6
SEMOD167749-5 v3
L1
L2
L3
71