Date Code 20001006 Loss-of-Potential, Load Encroachment, and Directional Element Logic 4-19
SEL-351 Instruction Manual
Directional Elements
Refer to Figure 4.13, Figure 4.14, and Figure 4.15.
If enable setting ELOP = Y or Y1 and a loss-of-potential condition occurs (Relay Word bit LOP
asserts), the negative-sequence voltage-polarized and positive-sequence voltage-polarized
directional elements are disabled (see Figure 4.14 and Figure 4.15).
Refer to Figure 4.1 and accompanying text for more information on loss-of-potential.
Note in Figure 4.13 and Figure 4.15, that the assertion of internal enable 32QE (for the negative-
sequence voltage-polarized directional element) disables the positive-sequence voltage-polarized
directional element. The negative-sequence voltage-polarized directional element has priority 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 4.15 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 4.13 and Figure 4.16.
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 4.17 and Figure 4.18.
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 4.14 and Figure 4.15), 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.
Courtesy of NationalSwitchgear.com
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