Schweitzer Engineering Laboratories SEL-387E, Инструкция по эксплуатации

Страница: 73 / 506

Date Code 20020129 Differential, Restricted Earth Fault, 3-19
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
Figure 3.9 shows the A winding of the 14.4 kV winding at 120 degrees CCW from
vertical, to make the example more interesting.
2. Adjust the CT connections. In this case the two windings with wye CTs need no
adjustment. The 24.9 kV winding, with DAB CTs, needs a 30-degree correction in the
CCW direction. Figure 3.9 shows this adjustment in the second line under the
transformer drawings.
3. Select a reference direction for the transformer. You can use one of the three 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 the complete circle of 360
degrees, as the reference. All three windings would then receive adjustments to correlate
them with this reference. As Figure 3.9 illustrates, the primary winding direction serves
as reference in the example.
4. Choose the WnCTC 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 WnCTC 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 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 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.