4.19
Date Code 20081022 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
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.
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.
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