Schweitzer Engineering Laboratories SEL-749M - Figure 4.17 Power Factor Elements Logic

4.28
SEL-749M Relay Instruction Manual Date Code 20080918
Protection and Logic Functions
Voltage-Based Protection
Figure 4.17 Power Factor Elements Logic
Loss-of-Potential
(LOP) Protection
The SEL-749Msets Relay Word bit LOP (loss-of-potential) upon detecting a
loss of relay ac voltage input such as that caused by blown potential fuses or
by the operation of molded-case circuit breakers. Because accurate relaying
potentials are required by certain protection elements (undervoltage 27
elements, for example), you can use the loss-of-potential function to supervise
these protection elements.
The relay declares a loss-of-potential when there is more than a 20 percent
drop in the measured positive-sequence voltage (V1) with no corresponding
magnitude nor angle change (above a pre-determined threshold) in positive-
sequence (I1), negative-sequence (I2), or zero-sequence currents (I0).
If this condition persists for 60 cycles, then the relay latches the LOP Relay
Word bit at logical 1. The relay resets LOP when the positive-sequence
voltage (V1) returns to a level greater than 0.43 • VNOM/PTR while negative-
sequence voltage (V2) and zero-sequence voltage (V0) are both less than 5 V
secondary (VNOM and PTR are relay settings).
Settings
The loss-of-potential function has no settings and is always active. You must
incorporate the LOP function in a SEL
OGIC control equation in order to
supervise relay protection elements (see Example 4.7).
LOP Impact on Other Protection Elements
Undervoltage and directional power elements require accurate relaying
potentials for correct operation. It is critical that the relay detects a loss-of-
potential condition and prevents operation of these elements. For example,
when a technician drops a wrench on the phase-voltage input fuse holders, the
relay LOP logic accurately determines that this loss of input voltages is a loss-
of-potential condition and does not trip (if the LOP Relay Word bit supervises
Measured
Power Factor
RUNNING
PF Leading
55LDAP
55LGAP
55A
0
55AD
Relay
Word
Bits
Relay
Word
Bit
Settings
55LDTP
55LGTP
55T
0
55TD
Courtesy of NationalSwitchgear.com