Date Code 20011026 Applications 5-21
SEL-321/321-1 Instruction Manual
Original Loss-of-Potential Logic
The relay discriminates between faults (which may reduce voltage magnitudes to nearly zero)
and loss-of-potential conditions. The following equation shows the original LOP detection logic
for the relay:
SET LOP = [NOT(50QF + 50QR) * 59QL]: Detects the presence of negative-sequence
voltage in the absence of negative-
sequence current
+ [NOT(59PL) * NOT(50M)]: Detects the absence of positive-sequence
voltage in the absence of current above
the 50M setting
Where:
59QL ≡ Negative-sequence overvoltage detector
50QF ≡ Low-set negative-sequence overcurrent detector
50QR ≡ Low-set negative-sequence overcurrent detector
59PL ≡ Positive-sequence overvoltage detector
50M ≡ Medium-set phase overcurrent element
For one or more blown PT fuses, the relay declares an LOP condition when the measured
negative-sequence voltage exceeds the 59QL setting and the negative-sequence current is below
the 50QF and 50QR settings. These conditions must exist for the LOPD time delay for the LOP
element to set.
For three blown PT fuses, the relay declares an LOP condition when the measured positive-
sequence voltage is below the 59PL and the low-set phase overcurrent detector (50M) is not
picked up.
You determine how the relay uses loss-of-potential detection by selecting the ELOP setting. The
ELOP setting choices and their associated results are:
•
N Relay sets LOP element when LOP condition is detected using original LOP logic.
Distance and directional elements are not blocked.
Set the LOP element in a SEL
OGIC
®
control equation to indicate condition, if
desired.
•
Y Relay sets LOP element when LOP condition is detected.
Distance elements are blocked and directional elements default forward.
Set the LOP element in a SEL
OGIC control equation to indicate the LOP condition,
if desired.
The 50QF, 50QR, and 59QL elements are required to be set above the normal expected system
unbalance.
The setting for the 50M pickup value requires that the element must never be picked up for load
current conditions. The setting must also be below the minimum fault current level for which the
relay is required to trip following an LOP condition. In this example, the application should
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