4.19
Date Code 20150130 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
secondary winding whose direction is at “one o’clock” with respect to the
direction of the delta. The CT currents go to relay winding inputs 1 and 2 from
left to right as Figure 4.8 illustrates.
Figure 4.8 Example 1 for WnCTC Selection
The 115 kV delta primary and the 24.9 kV grounded-wye secondary 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.
Step 1. Establish the line terminal directions.
Refer to the Xfmr Phase Relationship in Figure 4.8 and note
that Terminal A of Winding 1 is 30 degrees CCW from the
vertical line, as we would expect for a DAB connection with
ABC phase rotation. Terminal A of Winding 2 is in phase with
the vertical line.
Step 2. Adjust the CT connections.
In this case, the primary winding with wye CTs need no
adjustment. The 24.9 kV winding, with DAB CTs, needs a
30-degree correction in the CCW direction. Figure 4.8 shows
this adjustment under Xfmr + CT Phase Relationship.
Step 3. Select a reference direction for the transformer.
You can use either one of the two 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
with complete circle of 360 degrees, as the reference. Both
windings would then receive adjustments to correlate them
with this reference. As Figure 4.8 illustrates, Winding 1
direction serves as reference in this example.
Xfmr
Phase
Relationship
Xfmr + CT
Phase
Relationship
CTComp: W1CTC = 0 W2CTC = 0
Terminal A
Vertical
Line
A
A
A
(REF.)
CT
Dy1 Connection (ABC Rotation)
115 kV
24.9 kV
A
(DAB)
C
B
Y
1
2
A
B
C
DAB
(Winding 2)(Winding 1)
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