3-20 Protection Functions Date Code 20080110
SEL-387E Instruction Manual
4. Choose the W
n
CTC settings for all three windings. Because Winding 1 is the reference,
we need no adjustment; the setting is W1CTC = 0. Note that the adjusted Winding 2
inputs coincide exactly with the reference direction; we need make no adjustment for the
24.9 kV winding either. Therefore, the setting is W2CTC = 0. As mentioned earlier,
these two windings represent a classical DABY application. We can see this from the
fact that the W
n
CTC setting is zero for both windings. The CT connections themselves
perform exactly the right correction without additional help from the relay. The final
winding inputs still reside at the “8 o’clock” position and need adjustment to the
reference at “11 o’clock.” Beginning at the Winding 3 direction, the compensation
direction is CCW until arrival at the reference. This compensation requires nine
increments of 30 degrees (or 9 “hours”) in the CCW direction. We therefore set
W3CTC = 9. The process is nearly complete.
5. As a final step, ensure that no wye-connected winding having wye-connected CTs is set
at W
n
CTC = 0 (uncompensated). Were this the case, zero-sequence currents could
appear in these relay inputs but in no others, and a possible false trip could occur for
external ground faults. Any non-zero value of W
n
CTC will eliminate the zero sequence.
In this example the only wye winding with wye CTs is Winding 3, which we have
compensated by W3CTC = 9. The selection is complete. The relay receives the three
settings as W1CTC = 0, W2CTC = 0, and W3CTC = 9.
Example 2 for WnCTC Selection
Figure 3.10 illustrates the second example. This is another three-winding transformer, for which
we have chosen rather unusual winding phase relationships in order to show the flexibility of the
winding compensation feature in the SEL-387E Relay.
The transformer has a 115 kV primary winding that is wye connected, with wye-connected CTs.
The 34.5 kV secondary winding is DAB connected, but designated with the A line terminal at the
“7 o’clock” position with respect to the primary A line terminal. It has wye-connected CTs. The
12.47 kV winding is another wye-connected winding, but with delta-connected CTs. The CTs
connect in a DAC delta rather than DAB. However, this poses no problem for the relay.
Figure 3.10 shows the A line terminal at the “4 o’clock” position with respect to the primary A
line terminal. This transformer is therefore a “Yd7y4” connection type. We assume ABC phase
rotation. The CT currents go to relay winding inputs 1, 2, and 3, from left to right as
Figure 3.10
shows.
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