Schweitzer Engineering Laboratories SEL-387E, Инструкция по эксплуатации

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3-18 Differential, Restricted Earth Fault, Date Code 20020129
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
5. If any winding needs no phase correction (zero degrees), but is a grounded-wye winding
having wye-connected CTs, choose WnCTC=12 for that winding, rather than WnCTC=0.
This setting will remove zero-sequence current components from the relay currents to
prevent false differential tripping on external ground faults. (All non-zero values of
WnCTC remove zero-sequence current.)
Example 1 for WnCTC Selection
Figure 3.9 illustrates the first example. This is a three-winding transformer with a DAB delta
primary and two lower voltage secondaries connected in grounded wye. Two windings have
wye-connected CTs. The higher voltage secondary winding has DAB delta-connected CTs. We
assume ABC phase rotation. Using the “hour of the clock” convention for specifying transformer
connections, the transformer is a “Dy1y9” connection. This means the transformer has a high-
voltage delta whose reference is “noon,” a wye secondary winding whose direction is at “one
o’clock,” and another whose direction is “9 o’clock” with respect to the direction of the delta.
The CT currents go to relay winding inputs 1, 2, and 3, from left to right as Figure 3.9 illustrates.
Figure 3.9: Example 1 for WnCTC Selection
The 115 kV delta primary and the 24.9 kV grounded-wye secondary, taken by themselves,
represent a traditional “DABY” two-winding application. This application has wye CTs on the
delta side and delta CTs on the wye side, using the same CT delta connection as the primary of
the transformer. Perform the following simple steps to handle these traditional connections.
1. Establish the line terminal directions. Refer to the line following the transformer
drawings in Figure 3.9 and note that the delta winding A line terminal direction is at 30
degrees CCW from the A winding direction (vertical), as we would expect for a DAB
connection with ABC phase rotation. The A winding of the 24.9 kV winding is vertical.