4.18
SEL-787 Relay Instruction Manual Date Code 20150130
Protection and Logic Functions
Basic Protection
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 sequence 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 sequence. In the ACB phase sequence
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
sequence line current from terminal A is A-C, which lags A winding current
by 30 degrees. In the ACB phase sequence 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.
For an additional resource, see the “Winding Compensation Settings
Worksheet” (SEL_WCTC_R1_0.xls, available on the SEL-787 Product
Literature CD).
Step 1. Establish the phase direction for the terminal-A line voltage for
each three-phase winding of the transformer. (This step
requires transformer nameplate drawings and/or internal
connection diagrams.)
Step 2. Adjust the terminal-A line voltage direction for each set of
input currents by the phase shift (if any) of the current
transformer connection. (Reference Figure 4.7 for this step.)
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.)
Step 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 sequence, begin at the winding direction and
proceed in a CCW direction until reaching the reference. For
ACB phase sequence, begin at the winding direction and
proceed in a CW direction until reaching the reference.
Figure 4.7 shows these compensation directions.
Step 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 4.8 illustrates the first example. This is a two-winding transformer with
a DAB delta primary and secondary connected in grounded wye. The
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 “Dy1” connection. This means the
transformer has a high-voltage delta whose reference is “noon” and a wye
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