3-18 Protection Functions Date Code 20080110
SEL-387E Instruction Manual
B-A, and C-B in terms of the winding currents. The phase shift produced by each physical type
of delta depends on the system phase rotation.
Note:
The terms “lead” and “lag” refer to the assumed counterclockwise (CCW) rotation of
the phasors for both ABC and ACB phase rotation. “Lead” implies movement in the
CCW direction; “lag” is movement in the clockwise (CW) direction.
In the ABC phase rotation B lags A by 120 degrees and C leads A by 120 degrees. The DAB
connection line current at terminal A is A-B, which in this case is a phasor that leads A winding
current by 30 degrees. For this reason, DAB is often referred to as the “leading connection.”
However, DAB is the leading connection only for ABC phase rotation. In the ACB phase
rotation C lags A by 120 degrees, and B leads A by 120 degrees. Terminal A line current is still
A-B, but current now lags A winding current by 30 degrees.
The DAC connection produces opposite shifts to DAB. In the ABC phase rotation line current
from terminal A is A-C, which lags A winding current by 30 degrees. In the ACB phase rotation
line current A is still A-C, but this result leads A winding current by 30 degrees.
Five-Step Compensation Process
The process of determining W
n
CTC for each winding involves the following five basic steps.
Two examples illustrate important points about the five steps.
1. Establish the phase direction for the terminal-A line current for each three-phase winding
of the transformer. (This step requires transformer nameplate drawings and/or internal
connection diagrams.)
2. Adjust the terminal-A line current direction by the phase shift (if any) of the current
transformer connection. (Reference
Figure 3.8 for this step.)
3. Select any one of the adjusted terminal-A directions from step 2, to serve as the reference
direction. (The relay compensates all other windings to line up with this reference.)
4. Choose a setting for W
n
CTC for each set of winding input currents. This setting is the
number of 30-degree increments needed to adjust each nonreference winding to line it up
with the reference. This number will range from 0 to 12 increments. For ABC phase
rotation, begin at the winding direction and proceed in a CCW direction until reaching
the reference. For ACB phase rotation, begin at the winding direction and proceed in a
CW direction until reaching the reference.
Figure 3.8 shows these compensation
directions.
5. If any winding needs no phase correction (zero degrees), but is a grounded-wye winding
having wye-connected CTs, choose W
n
CTC=12 for that winding, rather than W
n
CTC=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
W
n
CTC 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-
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