4.2
SEL-311C Relay Instruction Manual Date Code 20060320
Loss-of-Potential, CCVT Transient Detection, Load-Encroachment, and Directional Element Logic
Loss-of-Potential Logic
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.
Note that ILOP disables all distance elements (Figure 3.4–Figure 3.12).
LOP is disabled while 3PO is asserted (breaker open). If an input potential is
lost during this time, LOP will not assert when 3PO deasserts (breaker close)
since the 10 percent drop in V
1
has already occurred. This is the case for
systems using either line-side or bus-side potential transformers.
You may provide a SCADA alarm for bus-side potential transformers with the
following SEL
OGIC
®
expression:
SV1 = 3PO
OUT105 =
!3P59 * SV1T + LOP
See Figure 3.27. Relay Word bit 3P59 asserts when A-phase, B-phase, and
C-phase voltage magnitudes are greater than setting 59P. Setting 59P should
be at least 80 percent of nominal voltage. Relay Word bit 3PO asserts when
the circuit breaker is open. Set SV1PU longer than the reclose open-time
interval. In this expression, if any phase voltage is less than setting 59P while
the circuit breaker is open, or LOP is asserted, the expression is true
(logical 1).
If the output is asserted, check the relay input potentials before closing the
circuit breaker.
In a system using line-side potential transformers, remove SV1T from the
expression. The alarm will assert whenever the line is de-energized and will
clear when the circuit breaker is closed if system voltage is normal. If the
output is asserted when the circuit breaker is closed, check the relay input
potentials.
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, plus all distance elements, are disabled by Relay Word
bit ILOP (see Figure 4.9, Figure 4.10, Figure 4.14, Figure 4.15, and
Figure 3.4–Figure 3.12). The loss-of-potential condition makes these voltage-
polarized directional elements and distance 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 Setting ELOP = Y on page 4.2).
In Figure 4.11, the assertion of ILOP is an additional enable for the channel IP
current-polarized directional element. This directional element is not voltage
polarized and is automatically enabled during LOP conditions if ELOP = Y or
Y1.
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 4.12). These direction forward overcurrent
elements effectively become nondirectional and provide overcurrent
protection during a loss-of-potential condition.
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