3-16
Differential, Restricted Earth Fault,
Date Code 20020129
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
Selecting the Correct Values of WnCTC for Each Winding
The process of choosing the correct WnCTC setting value for each winding involves a complete
knowledge of the transformer winding connections and phase relationships, the CT connections,
and the system phase rotation (ABC or ACB). The following brief review discusses the nature of
various connections, their phase shifts, and the reference motion for selecting WnCTC based on
system phase rotation.
Winding Connection Review
Figure 3.8 shows the three basic winding connections, consisting of a wye connection and the two
possible delta connections.
Figure 3.8: Winding Connections, Phase Shifts, and Compensation Direction
The wye connection consists of connecting one end of each winding to a common or neutral
point, leaving the other ends of each winding for the line terminals. Because the windings do not
interconnect at the line ends, the line current equals the respective winding current, A, B, or C,
and no phase shift occurs in the line currents with respect to the winding currents. The neutral
point, if it is grounded, permits flow of zero-sequence current components in the windings and
line outputs.
