3.17
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
Step 3. With the winding in Step 2 as reference, determine the required
compensation settings for all other windings. Use odd matrices
for compensating wye-windings. Avoid the use of even
matrices when possible.
Refer to Appendix H: Protection Application Examples for more details on
application of these guidelines in determining correct compensation settings.
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 TAPn, based on settings entered for
the particular winding. (n denotes the winding number.)
Equation 3.3
The relay calculates TAPn 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 TAP4. Stated
another way,
Equation 3.4
Restraint Slope Percentage
Example:
The current transformer error, e, is equal to ±10 percent. In per unit:
e = 0.1
where:
C = 1 if WnCT setting = Y (wye-connected CTs)
C = 3 if WnCT setting = D (delta-connected CTs)
MVA = maximum power transformer capacity setting
(must be the same for all TAPn calculations)
VWDGn = winding line-to-line voltage setting, in kV
CTRn = current transformer ratio setting
TAPn
MVA 1000•
3 VWDGn CTRn• •
----------------------------------------------------------- C• =
O87P
MIN
0.1 I
N
•
TAP
MIN
------------------------
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