4.34
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
setting. This is because preference is given to selected directional elements
that operate off of bigger signals (i.e., directional elements corresponding to Q
and V). Setting choice “I” cannot be listed with S, P, or U.
Figure 4.20 shows no control emanating from the Best Choice Ground Direc-
tional Element logic to the directional elements corresponding to S or P
(Figure 4.29, and Figure 4.30, respectively). This Best Choice Ground Direc-
tional Element logic for the directional elements corresponding to S or P is
effectively handled with the “disable inputs” (internal enables DIRQGE and
DIRVE) running into the internal enable logic of Figure 4.25. If neither
DIRQGE nor DIRVE are asserted (and thus their corresponding directional
elements are not enabled), then the internal enable logic of Figure 4.25 is free
to run for the last directional element selected in setting ORDER (if S or P is
the last element listed in setting ORDER).
Setting choice U (ungrounded/high-impedance grounded) can only be listed
by itself (ORDER := U), so Best Choice Ground Directional Element logic is
irrelevant in this case just as it is also irrelevant when Q, V, I, or P are listed by
themselves in setting ORDER.
Directional Elements
Refer to Figure 4.20, Figure 4.21, and Figure 4.26 through Figure 4.31. The
Best Choice Ground Directional Element logic in Table 4.20 determines
which directional element will run.
Note in Figure 4.30 that the incremental conductance directional element out-
puts FDIRC/RDIRC do not propagate to directional outputs FDIRN/RDIRN,
respectively, as do the wattmetric directional element outputs FDIRW/
RDIRW. Incremental conductance elements are used more for alarming pur-
poses than for controlling overcurrent elements for tripping. Incremental con-
ductance elements provide more sensitivity for detecting high-resistance faults
on Petersen coil-grounded systems (as compared to the wattmetric elements).
For more information on the operation and application of incremental conduc-
tance elements for Petersen coil- (resonant) grounded systems, see the paper:
Review of Ground Fault Protection Methods for Grounded, Ungrounded, and
Compensated Distribution System by Jeff Roberts, Hector Altuve, and Daqing
Hou, presented at the 28th Annual Western Protective Relay Conference, Spo-
kane, Washington, October 22–24, 2001.
Directional Element Routing
Refer to Figure 4.20, Figure 4.21, Figure 4.32, and Figure 4.33. The direc-
tional element outputs are routed to the forward (Relay Word bits DIRGF and
DIRNF) and reverse (Relay Word bits DIRGR and DIRNR) logic points and
then on to the direction forward/reverse logic in Figure 4.34 and Figure 4.35.
Loss of Potential
Note if all the following are true:
➤ Enable setting EFWDLOP := Y,
➤ Global setting VSCONN := VS,
➤ A loss-of-potential condition occurs (Relay Word bit LOP
asserts),
➤ And internal enable DIRIE (for channel IN current-polarized
directional element) is not asserted
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