Schweitzer Engineering Laboratories SEL-321 - Negative-Sequence Overcurrent Settings; Negative-Sequence Time-Overcurrent Settings

5-14 Applications Date Code 20011026
SEL-321/321-1 Instruction Manual
Calculations: CTR = 200:1
51NP secondary amps =
150
200
075
A primary
A= .
secondary
E51N = Y
YY
Y
51NP = 0.75
0.750.75
0.75
51NC = U3
U3U3
U3
51NTD = 2.00
2.002.00
2.00
51NRS = Y
YY
Y
51NTC = 32QF
32QF32QF
32QF
E50N = N
NN
N
- Relay Elements and Other Settings Affected
Relay elements 51NT and 51NP depend on the 51NP, 51NTD, 51NC and 51NTC settings.
50N1, 50N2, 50N3, 50N4, 67N1, 67N2, 67N3, 67N4, 67N2T, 67N3T, 67N4T elements.
Negative-Sequence Overcurrent Settings
Negative-Sequence Time-Overcurrent Settings
The negative-sequence time-overcurrent element provides current dependent, time delayed
clearance of faults along the protected line and provides backup protection for remote terminals.
Because the measure of negative-sequence current varies with system switching configuration,
fault location, and fault resistance, a complete fault study is necessary to determine the minimum
pickup setting, appropriate time dial, and curve characteristic. When the element is directionally
controlled, consider only faults in front of the line terminal for coordinating purposes.
The negative-sequence overcurrent element can provide backup for phase-to-phase or
phase-to-ground faults. For this example, the negative-sequence element provides backup for
phase-to-phase faults. Phase-to-ground faults need only be considered for coordination purposes.
For the relay at Breaker 3, a phase-to-phase fault at Bus C with Line 1 or 4 open dictates the
sensitivity required for the negative-sequence time-overcurrent pickup. The negative-sequence
(3I
2
) current magnitude for this fault is 736 A primary. To allow for fault resistance, a value of
60% of 736 A is selected as the negative-sequence time-overcurrent element pickup. The pickup
(51QP), time dial (51QTD), and curve family (51QC) selections are assumed to coordinate with
the remaining negative-sequence and residual time-overcurrent elements of the example system.
Instantaneous/Definite-Time Negative-Sequence Setting
The negative-sequence instantaneous/definite-time overcurrent elements are not required for this
application. The negative-sequence overcurrent elements are disabled by setting E50Q = N.
Calculations: CTR = 200:1
51QP secondary amps =
440
200
220
A primary
A= .
secondary