4-32 Loss-of-Potential, Load Encroachment, and Directional Element Logic Date Code 20020426
SEL-351-5, -6, -7 Instruction Manual
over the positive-sequence voltage-polarized directional elements in controlling the phase
overcurrent elements. The negative-sequence voltage-polarized directional element operates for
unbalanced faults while the positive-sequence voltage-polarized directional element operates for
three-phase faults.
Note also in Figure 4.21 that the assertion of ZLOAD disables the positive-sequence voltage-
polarized directional element. ZLOAD asserts when the relay is operating in a user-defined load
region (see Figure 4.2).
Directional Element Routing
Refer to Figure 4.19 and Figure 4.22.
The directional element outputs are routed to the forward (Relay Word bits 32QF and 32PF) and
reverse (Relay Word bits 32QR and 32PR) logic points and then on to the direction
forward/reverse logic in Figure 4.23 and Figure 4.24.
Loss-of-Potential
Note if both
the following are true:
• Enable setting ELOP = Y,
• A loss-of-potential condition occurs (Relay Word bit LOP asserts),
then the forward logic points (Relay Word bits 32QF and 32PF) assert to logical 1, thus, enabling
the negative-sequence and phase overcurrent elements that are set direction forward (with settings
DIR1 = F, DIR2 = F, etc.). These direction forward overcurrent elements effectively become
nondirectional and provide overcurrent protection during a loss-of-potential condition.
As detailed previously (in Figure 4.6 and Figure 4.21), voltage-based directional elements are
disabled during a loss-of-potential condition. Thus, the overcurrent elements controlled by these
voltage-based directional elements are also disabled. But this disable condition is overridden for
the overcurrent elements set direction forward if setting ELOP = Y.
Refer to Figure 4.1 and accompanying text for more information on loss-of-potential.
Direction Forward/Reverse Logic
Refer to Figure 4.19, Figure 4.23, and Figure 4.24.
The forward (Relay Word bits 32QF and 32PF) and reverse (Relay Word bits 32QR and 32PR)
logic points are routed to the different levels of overcurrent protection by the level direction
settings DIR1 through DIR4.
Table 4.4 shows the overcurrent elements that are controlled by each level direction setting. Note
in Table 4.4 that all the time-overcurrent elements (51_T elements) are controlled by the DIR1
level direction setting.
In most communications-assisted trip schemes, the levels are set as follows (see Figure 5.4):
Level 1 overcurrent elements set direction forward (DIR1 = F)
Level 2 overcurrent elements set direction forward (DIR2 = F)
Level 3 overcurrent elements set direction reverse (DIR3 = R)
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