Schweitzer Engineering Laboratories SEL-221F, Instruction Manual

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For the relay at Breaker 3 in the system shown in Figure 5.1, the governing fault condition is
a phase-ground fault at Bus D. The fault study revealed that for this fault, IA = 435 A
primary and IR = 410 A primary. To ensure that the 50N and 50G elements pick up for the
calculated fault current, select some current value below the calculated 410 ampere residual
current value for the 50NG setting. This setting accounts for arc resistance and errors in fault
study calculations. This value for the 50NG setting may be below load. In this example, the
value selected for the 50NG setting is 250 A (approximately 60% of the minimum fault
current).
- Setting Limit Check
The primary limit check allows for 50NG settings of 0.25 - 50,000 A. The secondary check
allows for a secondary amp setting of 0.5 - 25 times 51NP, but less than 40 A.
Calculations:
CTR = 200:1
The 50NG setting of 250 A for this example lies within the relay setting limits.
- Relay Word Bits and Other Settings Affected
50NG bit in the Relay Word and all ground distance elements: Relay World bits ZlG, Z2GT,
Z3, and Z3D. REJO bit when REJOE = G.
Low-Set Phase Overcurrent Element Setting (50P)
The 50P element provides 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. The
ideal setting for the 50P element is above load but below minimum fault duty for the most
remote Zone 3 phase fault that the relay must detect.
Although it is not ideal, you can set 50P below load to permit distance element operation for
end-of-line faults with magnitudes below load. When 50P is below load current and all three
potential fuses operate, the relay three-phase LOP function cannot operate. Refer to the
description of LOP logic for more details.
For phase-phase faults, both involved phase current magnitudes must exceed the 50P pickup
threshold before the phase-phase distance elements are allowed to operate. For three-phase
faults, the current magnitude in all three phases must exceed the 50P pickup threshold before
a three-phase fault condition is declared.
For the relay at Breaker 3, the governing fault condition is a phase-phase fault at Bus D. The
fault study revealed that the lowest phase fault current equals 620 A primary for this fault.
5-18
Applications
SEL-221F, -1 Instruction Manual
Date Code 920508