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4.19
Date Code 20081022
Instruction Manual
SEL-787 Relay
Protection and Logic Functions
Basic Protection
4. Choose the W
n
CTC 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 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
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 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
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
shows.
Summary of Contents for SEL-787
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