Schweitzer Engineering Laboratories SEL-787 - Figure 4.7 Winding Connections, Phase Shifts, and Compensation Direction

4.16
SEL-787 Relay Instruction Manual Date Code 20081022
Protection and Logic Functions
Basic Protection
Figure 4.7 Winding Connections, Phase Shifts, and Compensation Direction
The wye connection consists of connecting one end of each winding to a
common or neutral point, leaving the other ends of each winding for the line
terminals. Because the windings do not interconnect at the line ends, the line
current equals the respective winding current, A, B, or C, and no phase shift
occurs in the line currents with respect to the winding currents. The neutral
point, if it is grounded, permits flow of zero-sequence current components in
the windings and line outputs.
There are two possible delta connections. In determining WnCTC, it is
essential to know not only that the CTs or transformer windings are connected
in delta but in which delta. In this manual we call these delta connections
DAB and DAC. In the DAB connection the polarity end of the A winding
connects to the nonpolarity end of the B winding, and so on, to produce the
delta. In the DAC connection the polarity end of the A winding connects to the
nonpolarity end of the C winding, and so on, to produce the delta. In
Figure 4.7 an arrowhead indicates the polarity end of each winding.
These arrangements involve a connection point between two windings at each
line terminal; the line currents are not the same as the winding currents, but
are in fact the phasor difference between the associated winding currents.
Therefore, the line currents will shift in phase by some amount with respect to
the winding currents. In the DAB connection the line currents from the A, B,
and C line terminals are, respectively, A-B, B-C, and C-A in terms of the
winding currents. In the DAC connection the line currents from the A, B, and
C line terminals are, respectively, A-C, 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.
A
Connection Line Outputs
Connection Phase Shift
ABC Rotation
ACB Rotation
WYE
DAB
DAC
(Xfmr. or CT)
A, B, C
A—B, B—C, C—A
A—C, B—A, C—B
A
C
B
C
B
0˚
30˚ CCW
30˚ CW
0˚
30˚ CW
30˚ CCW
A
C
B
A
B
C
B
C
A
B
C
WnCTC
(CCW)
WnCTC
(CW)
A
WnCTC
Direction