Schweitzer Engineering Laboratories SEL-351, Instruction Manual

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4-2 Loss-of-Potential, Load Encroachment, and Directional Element Logic Date Code 20001006
SEL-351 Instruction Manual
Loss-of-potential is declared (Relay Word bit LOP = logical 1) when a 10% drop in V
1
is detected,
with no corresponding change in I
1
or I
0
. If the LOP condition persists for 60 cycles, it latches in.
LOP resets (Relay Word bit LOP = logical 0) when V
1
returns above 50 V secondary and
•
V
0
is less than 5 V secondary (wye-connected voltages) {150 V voltage inputs}.
V
0
is less than 10 V secondary (wye-connected voltages) {300 V voltage inputs}.
•
V
2
is less than 5 V secondary (delta-connected voltages).
The loss-of-potential enable setting, ELOP, does not enable or disable the LOP logic. It just routes
the LOP Relay Word bit to different logic, as shown in Figure 4.1 and explained in the remainder
of this subsection.
Setting ELOP = Y or Y1
If setting ELOP = Y or Y1 and a loss-of-potential condition occurs (Relay Word bit LOP asserts
to logical 1), negative-sequence voltage-polarized, zero-sequence voltage-polarized, and positive-
sequence voltage-polarized directional elements are disabled (see Figure 4.8, Figure 4.9,
Figure 4.14, and Figure 4.15). The loss-of-potential condition makes these voltage-polarized
directional elements unreliable. Thus, they are disabled. The overcurrent elements controlled by
these voltage-polarized directional elements are disabled also (unless overridden by conditions
explained in the following Setting ELOP = Y discussion).
In Figure 4.10, the assertion of LOP is an additional enable for the channel IN current-polarized
directional element. This directional element is not voltage polarized.
In Figure 5.6, if setting ELOP = Y1 and LOP asserts, keying and echo keying in the permissive
overreaching transfer trip (POTT) logic are blocked.
Setting ELOP = Y
Additionally, if setting ELOP = Y and a loss-of-potential condition occurs (Relay Word bit LOP
asserts to logical 1), overcurrent elements set direction forward are enabled (see Figure 4.11 and
Figure 4.16). These direction forward overcurrent elements effectively become nondirectional and
provide overcurrent protection during a loss-of-potential condition.
As detailed previously, 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.
Setting ELOP = N
If setting ELOP = N, the loss-of-potential logic still operates (Relay Word bit LOP asserts to
logical 1 for a loss-of-potential condition) but does not disable any voltage-based directional
elements (as occurs with ELOP = Y or Y1) or enable overcurrent elements set direction forward
(as occurs with ELOP = Y).
Courtesy of NationalSwitchgear.com