4.62
SEL-787 Relay Instruction Manual Date Code 20081022
Protection and Logic Functions
Voltage-Based Protection
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 while negative-
sequence voltage (V2) and zero-sequence voltage (V0) are both less than 5 V
secondary (VNOM is a relay setting).
Settings
The LOP function has no settings and is always active. You must incorporate
the LOP function in a SEL
OGIC control equation to supervise relay protection
elements (see Example 4.3).
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 an LOP
condition and prevents operation of these elements. For example, when
dropping a wrench on the phase-voltage input fuse holders, the relay LOP
logic accurately determines that this loss of input voltages is an LOP condition
and does not trip (if the LOP Relay Word bit supervises selected tripping
elements, see Example 4.3). If you are using voltage-determined relay
elements for tripping decisions, then blocking these elements is crucial when
the voltage component is no longer valid.
EXAMPLE 4.3 Supervising Voltage-Element Tripping With LOP
To supervise undervoltage by LOP one of the trip equations can be
set as follows:
TR2:= . . . OR ((27P1T OR 27P2T) AND NOT LOP)
Similarly, if you want the additional voltage-affected elements (e.g.,
3PWR1T) to act only when there are correct relaying potentials
voltage, use the following in the equation:
. . . OR ((27P1T OR 27P2T OR 3PWR1T) AND NOT LOP)
You can supervise each element separately or as a group when these
elements occur in the trip equations, as shown in this example.
LOP Monitoring and Alarms
You should take steps to immediately correct an LOP problem so that normal
protection is rapidly re-established. Include the LOP Relay Word bit in an
output contact alarm to notify operation personnel of abnormal voltage input
conditions and failures that can be detrimental to the protection system
performance if not quickly corrected.
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