4.77
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
When a switch or breaker closes, the three poles may not close at the same
time, creating a momentary current unbalance condition. To avoid any possi-
ble operation of the ungrounded/high-impedance grounded element for this
momentary current unbalance condition, use the EDIRIV SEL
OGIC to over-
ride this condition, as in the following example.
EDIRV := SV01T OR ....
SV01PU := 0.05
SV01DO := 0.00
SV01 := 52A
Directional Control
Provided by Torque
Control Settings
For most applications, the level direction settings DIR1 through DIR4 are
used to set overcurrent elements direction forward, reverse, or nondirectional.
Table 4.25 shows the overcurrent elements that are controlled by each level
direction setting. Note in Table 4.25 that all the time-overcurrent elements
(51_T elements) are controlled by the DIR1 level direction setting. See
Figure 4.34, Figure 4.35, Figure 4.40, and Figure 4.41.
Suppose that the Level 1 overcurrent elements should be set as follows:
67P1P direction forward
67G1P direction forward
51P1T direction forward
51N1T nondirectional
51G1T direction forward
To accomplish this, the DIR1 setting is “turned off,” and the corresponding
SEL
OGIC torque-control settings for the above overcurrent elements are used
to make the elements directional (forward or reverse) or nondirectional. The
required settings are:
DIR1 = N (“turned off”; see Figure 4.34, Figure 4.35, Figure 4.40, and
Figure 4.41)
50P1TC = DIRPF (direction forward; see Figure 4.2)
50G1TC = DIRGF (direction forward; see Figure 4.2)
51P1TC = DIRPF (direction forward; see Figure 4.6)
51N1TC = 1 (nondirectional; see Figure 4.8)
51G1TC = DIRGF (direction forward; see Figure 4.9)
This is just one example of using SEL
OGIC control equation torque control
settings to make overcurrent elements directional (forward or reverse) or non-
directional. This example shows only Level 1 overcurrent elements (controlled
by level direction setting DIR1). The same setting principles apply to the other
levels as well. Many variations are possible.
Load-Encroachment
Logic
The load-encroachment logic (see Figure 4.54) and settings are enabled/ dis-
abled with setting ELOAD. If the Group setting VNOM := OFF, then ELOAD
is not available. See Table 4.3 for more details on the VNOM setting.
The load-encroachment feature allows certain elements (system backup, phase
directional, etc.) to be set without regard for load levels. For example, to
obtain necessary system backup sensitivity, you may want to set the imped-
ance element reach very long. Because of the long reach setting, the phase dis-
tance element would pick up during heavy load.
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