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Date Code 20011026
Applications
5-7
SEL-321/321-1 Instruction Manual
Ground Distance Overcurrent Supervision Setting Criteria
The ground distance elements are supervised by phase and ground fault detector overcurrent
elements. Each zone has an independent phase and ground fault detector overcurrent element.
Before any ground distance element can pick up, the relay must detect phase and residual current
greater than the 50L and 50G settings, respectively. Calculate the phase and residual current for
the most remote ground fault or highest expected ground fault resistance in which the distance
element zone must operate.
Zone 1 Setting
For the relay at Breaker 3 in the system shown in Figure 5.1, the minimum fault condition for the
Zone 1 element is a phase-to-ground fault at Bus C with Line 1 or 4 removed from service. The
fault study shows that for this fault, I
A
= 307 A primary and I
R
= 308 A primary. To ensure that
the 50L1 and 50G1 elements pick up for the calculated fault current, select some current value
below the calculated phase and residual current magnitudes.
This value for the 50L1 setting may be below load. In this example, the value selected for the
50L1 setting is 180 A (approximately 60% of the minimum fault current). The value selected for
the 50G1 setting is 180 A (approximately 60% of the minimum fault current).
Zone 2 Setting
The minimum fault condition for the Zone 2 element is a phase-to-ground fault at Bus D with
Line 1 or 4 removed from service. The fault study shows that for this fault, I
A
= 306 A primary
and I
R
= 202 A primary. To ensure that the 50L2 and 50G2 elements pick up for the calculated
fault current, select some current value below the calculated phase and residual current. This
setting accounts for arc resistance and errors in fault study calculations.
This value for the 50L2 setting may be below load. In this example, the value selected for the
50L2 setting is 180 A (approximately 60% of the minimum fault current). In this example, the
value selected for the 50G2 setting is 120 A (approximately 60% of the minimum fault current).
Zone 3 Setting
The minimum fault condition for the Zone 3 element is a phase-to-ground fault at Bus A with
Line 3 or 6 removed from service. The fault study shows that for this fault, I
A
= 432 A primary
and I
R
= 170 A primary. To ensure that the 50L3 and 50G3 elements pick up for the calculated
fault current, select some current value below the phase and residual current magnitudes.
This value for the 50L3 setting may be below load. In this example, the value selected for the
50L3 setting is 260 A (approximately 60% of the minimum fault current). In this example, the
value selected for the 50G3 setting is 100 A (approximately 60% of the minimum fault current.)
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