Date Code 20080110 Protection Functions 3-19
SEL-387E Instruction Manual
voltage delta whose reference is “noon,” a wye secondary winding whose direction is at “one
o’clock,” and another whose direction is “9 o’clock” with respect to the direction of the delta.
The CT currents go to relay winding inputs 1, 2, and 3, from left to right as
Figure 3.9 illustrates.
Figure 3.9: Example 1 for WnCTC Selection
The 115 kV delta primary and the 24.9 kV grounded-wye secondary, taken by themselves,
represent a traditional “DABY” two-winding application. This application has wye CTs on the
delta side and delta CTs on the wye side, using the same CT delta connection as the primary of
the transformer. Perform the following simple steps to handle these traditional connections.
1. Establish the line terminal directions. Refer to the line following the transformer
drawings in
Figure 3.9 and note that the delta winding A line terminal direction is at 30
degrees CCW from the A winding direction (vertical), as we would expect for a DAB
connection with ABC phase rotation. The A winding of the 24.9 kV winding is vertical.
Figure 3.9 shows the A winding of the 14.4 kV winding at 120 degrees CCW from
vertical, to make the example more interesting.
2. Adjust the CT connections. In this case the two windings with wye CTs need no
adjustment. The 24.9 kV winding, with DAB CTs, needs a 30-degree correction in the
CCW direction.
Figure 3.9 shows this adjustment in the second line under the
transformer drawings.
3. Select a reference direction for the transformer. You can use one of the three winding
directions as the reference, but this need not be the case. You could establish any of the
12 possible directions, separated by 30 degrees around the complete circle of 360
degrees, as the reference. All three windings would then receive adjustments to correlate
them with this reference. As
Figure 3.9 illustrates, the primary winding direction serves
as reference in the example.
To Next Page
Loading...
