4.58
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
If setting:
ORDER = OFF
then all of the ground directional elements are inoperable. Note in Figure 4.34
and Figure 4.35 that setting ORDER := OFF effectively makes the neutral
ground and residual ground overcurrent elements nondirectional (the direc-
tional control outputs of Figure 4.34 and Figure 4.35 are continuously
asserted to logical 1).
Petersen Coil Considerations for Setting ORDER. Note in Figure 4.34
that if setting ORDER := P, the residual ground overcurrent elements are not
controlled by the directional control logic (much like when ORDER := OFF).
In such a scenario, where only the wattmetric directional element provides
ground overcurrent element directional control (setting ORDER := P), pre-
sumably there is no bypass around the Petersen coil. With the tuned Petersen
coil in place (and not shorted out by a bypass), very little current flows for a
ground fault. With such low current levels, the neutral-ground overcurrent ele-
ments (referenced in Figure 4.35) are the elements that detect the ground fault,
not the residual-ground overcurrent elements (referenced in Figure 4.34). The
residual ground overcurrent elements (including forward and reverse fault
detectors 50GF and 50GR, respectively; see Figure 4.24) should be set above
any ground fault current level with the Petersen coil in place.
If there is a bypass around the Petersen coil and the bypass is used at times
(i.e., shorting out the Petersen coil), much higher currents can flow for a
ground fault when the bypass is closed. In such a scenario, setting ORDER
should be set something like ORDER := QP or ORDER := QVP (see
Table 4.20). Then, the residual ground elements (Figure 4.34) are controlled
by the directional control logic and provide directional protection for higher
ground fault currents.
50PDIRP—Phase Directional Element Three-Phase Current Pickup
The 50PDIRP setting is set to pick up for all three-phase faults that need to be
covered by the phase overcurrent elements. It supervises the positive-sequence
voltage-polarized directional elements FDIRP and RDIRP (see Figure 4.38).
If the load-encroachment logic is enabled (enable setting ELOAD := Y), then
setting 50PDIRP is not made or displayed, but is fixed internally at:
0.5 A secondary (5 A nominal phase current inputs, IA, IB, IC)
0.1 A secondary (1 A nominal phase current inputs, IA, IB, IC)
Z2F—Forward Directional Z2 Threshold
Z2R—Reverse Directional Z2 Threshold
Z2F and Z2R are used to calculate the Forward and Reverse Thresholds,
respectively, for the negative-sequence voltage-polarized directional elements
(see Figure 4.26 and Figure 4.37).
If enable setting EDIR := Y, settings Z2F and Z2R (negative-sequence imped-
ance values) are calculated and entered by the user, but setting Z2R must be
greater in value than setting Z2F by 0.1 secondary.
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