4.20
SEL-787 Relay Instruction Manual Date Code 20150130
Protection and Logic Functions
Basic Protection
Step 4. Choose the WnCTC settings for the 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 WnCTC setting is zero for both
windings. The CT connections themselves perform exactly the
right correction without additional help from the relay. The
process is nearly complete.
Step 5. As a final step, ensure that no wye-connected winding having
wye-connected CTs is set at WnCTC = 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
WnCTC will eliminate the zero-sequence. In this example there
is no wye-connected winding with wye-connected CTs. The
selection is complete. The relay receives the settings as
W1CTC = 0 and W2CTC = 0 (alternately, you can set
ICOM ;= N because no compensation is required from the
relay).
Example 2 for WnCTC Selection
Figure 4.9 illustrates the second example. This is another two-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-787.
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. This transformer is
therefore a “Yd7” connection type. We assume ABC phase rotation. The CT
currents go to relay winding inputs 1 and 2 from left to right as Figure 4.9
shows.
Step 1. Establish the line terminal directions.
Figure 4.9 shows these phase directions under Xfmr Phase
Relationship. Based on the transformer designation, the
terminal direction are shown at “noon” and “7 o’clock.”
Step 2. Adjust the transformer winding directions based on the CT
connections. Neither windings need correction, because they
both have wye-connected CTs.
Step 3. Select a reference direction. In this example we have chosen the
delta secondary winding as reference.
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