Schweitzer Engineering Laboratories SEL-387E - Current TAP; Restrained Element Operating Current Pickup; Winding Line-To-Line Voltages

3-22 Protection Functions Date Code 20080110
SEL-387E Instruction Manual
choose W3CTC = 5 as the setting. The process is almost complete, except for a final
check.
5. Ensure that there will be no wye windings with wye CTs and a setting of W
n CTC = 0. In
this case the primary winding is wye-connected and has wye-connected CTs. In step 4
we set W1CTC at zero because Winding 1 was the reference winding. However, this
setting violates the condition that W
n CTC not equal 0. Instead of a zero shift, we must
shift Winding 1 360 degrees by setting W1CTC = 12. This solves the zero-sequence
current problem. The process is now complete. The relay receives the three settings as
W1CTC = 12, W2CTC = 7, and W3CTC = 5.
Winding Line-to-Line Voltages
Enter the nominal line-to-line transformer terminal voltages. If a load tap changer is included in
the transformer differential zone, assume that it is in the neutral position. The setting units are
kilovolts.
Current TAP
The relay uses a standard equation to set TAP n , based on settings entered for the particular
winding. (
n denotes the winding number.)
C•
CTRnVWDGn•3
1000MVA
TAPn
⋅
⋅
=
where:
C = 1 if W
n CT setting = Y (wye-connected CTs)
C =
3 if W n CT setting = D (delta-connected CTs)
MVA = maximum power transformer capacity setting
(must be the same for all TAP
n calculations)
VWDG
n = winding line-to-line voltage setting, in kV
CTR
n = current transformer ratio setting
The relay calculates TAP
n with the following limitations:
• The tap settings are within the range 0.1 • I
N
and 31 • I
N
• The ratio TAP
MAX
/TAP
MIN
≤ 7.5
Restrained Element Operating Current Pickup
The O87P setting range is 0.1 to 1.0; we suggest an O87P setting of 0.3. The setting must be at a
minimum for increased sensitivity but high enough to avoid operation because of steady-state CT
error and transformer excitation current. The setting must also yield an operating current greater
than or equal to 0.1 • I
N
, when multiplied by the smallest of TAP1 through TAP3. Stated another
way,
O87P
MIN
≥ (0.1 • I
N
) / TAP
MIN