4.21
Date Code 20081022 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
current problem. The process is now complete. The relay
receives the two settings as W1CTC = 12 and W2CTC = 7.
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.)
where:
The relay calculates TAPn with the following limitations:
➤ The tap settings are within the range 0.1 • I
NOMn
and
6.2 • I
NOMn
➤ 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
NOM
, when multiplied by the smallest of TAP1 and TAP2. Stated
another way,
O87P
MIN
≥ (0.1 • I
NOM
) / TAP
MIN
The above restriction applies to each winding sides (irrespective of TAP
magnitudes) when 1A and 5A nominal CTs are used in same relay.
Restraint Slope Percentage
Example:
The current transformer error, e, is equal to ±10 percent. In per unit:
e=0.1
The voltage ratio variation of the power transformer load tap-changer, LTC, is
from 90 percent to 110 percent. In per unit:
a=0.1
C = 1 if WnCT setting = Y (wye-connected CTs)
C = 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•
3VWDGCTRn••
------------------------------------------------------
C•=
3
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