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5-6
Applications
Date Code 20011026
SEL-321/321-1 Instruction Manual
The Zone 3 quadrilateral element setting is based upon the same requirement established for the
Zone 3 mho elements. The reactive reach is set the same as the mho distance setting. The
resistive reach determines sensitivity for high resistance faults on a radial basis. The Zone 3
resistive reach must be set greater than the Zone 2 overreach from the remote end, in this case
Breaker 4. Assuming the same criteria for the relays at Breaker 4, a conservative setting is to set
the Zone 3 resistive reach at 120% of the remote Zone 2 setting. For this example, the Zone 3
resistive reach is set for 120% of 50
Ω
primary or 60
Ω
primary.
Phase Distance Overcurrent Supervision Setting Criteria
The 50PP elements provide fault detector supervision of the phase distance elements and must
pick up for all fault conditions where a phase distance element is expected to operate. Each
phase distance element is supervised by independent fault detectors.
The ideal setting for the 50PP element is above load but below minimum fault duty for the most
remote three-phase or phase-to-phase fault that the distance element zone must detect. Although
it is not ideal, you can set 50PP below load to permit distance element operation for end-of-line
faults with magnitudes below load.
The 50PP elements are set based upon the phase-to-phase current for both three-phase and
phase-to-phase faults. For three-phase faults, the phase current magnitude must be multiplied by
the square root of three to determine the phase-to-phase current magnitude. Phase-to-phase
currents for phase-to-phase faults can be derived directly from a fault study. In either case, the
lowest current magnitude must be used for the fault detector.
Zone 1 Setting
For the relay at Breaker 3, the governing fault condition for the Zone 1 element is a
phase-to-phase fault at Bus C with Line 1 or 4 removed from service. The fault study shows that
the lowest phase-to-phase fault current equals 850 A primary for this fault. To ensure that the
50PP1 element picks up for the calculated fault current, select some current value below 850 A.
In this example, the value selected for the 50PP1 setting is 510 A (approximately 60% of the
minimum fault current).
Zone 2 Setting
The governing fault condition for the Zone 2 elements is a phase-to-phase fault at Bus D with
Line 1 or 4 removed from service. The fault study shows that the lowest phase-to-phase fault
current equals 743 A primary for this fault. To ensure that the 50PP2 element picks up for the
calculated fault current, select some current value below 743 A. In this example, the value
selected for the 50PP2 setting is 445 A (approximately 60% of the minimum fault current).
Zone 3 Setting
The governing fault condition for the Zone 3 elements is a phase-to-phase fault at Bus A with
Line 1 or 4 removed from service. The fault study shows that the lowest phase-to-phase fault
current equals 817 A primary for this fault. To ensure that the 50PP3 element picks up for the
calculated fault current, select some current value below 817 A. In this example, the value
selected for the 50PP3 setting is 490 A (approximately 60% of the minimum fault current).
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