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 becausethe IED shall not internally provide any phase angle shift compensation. Thenecessary phase angle shift compensation will be provided externally by deltaconnected main CT. All other settings should have the same values irrespective ofmain CT connections.6.1.4.2 Typical main CT connections for transformer differential protectionSEMOD167749-5 v3Three most typical main CT connections used for transformer differentialprotection are shown in figure 31. It is assumed that the primary phase sequence isL1-L2-L3.L1IL1IL1-IL2IL2-IL3IL3-IL1IL1-IL3IL2-IL1IL3-IL2IL1IL2IL3ProtectedTransformerWindingCT StarConnectedCT in DeltaDAC ConnectedCT in DeltaDAB ConnectedIL2IL3L2L3en06000549.vsdIEC06000549 V1 EN-USFigure 31: 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 componentFor star connected main CTs, the main CT ratio shall be set as it is in actualapplication. The “StarPoint” parameter, for the particular star connection shown infigure 31, shall be set ToObject. If star connected main CTs have their star pointaway from the protected transformer this parameter should be set FromObject.For delta DAC connected main CTs, secondary currents fed to the IED:1MRK 504 169-UEN A Section 6Differential protectionTransformer protection RET650 2.2 IEC 85Application manual
