Schweitzer Engineering Laboratories SEL-387E, Инструкция по эксплуатации

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Date Code 20020129 Differential, Restricted Earth Fault, 3-17
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
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 3.8 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.
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 WnCTC 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 WnCTC 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.