9.35
Date Code 20080213 Instruction Manual SEL-351A Relay
Setting the Relay
Settings Explanations
Use caution when selecting CTs for saturation conditions in firmware
revisions released prior to November 2002 (see Appendix A: Firmware and
Manual Versions). If you apply the SEL-351A in high fault current situations,
such as in power plant auxiliary buses with as much as 40000 A of line-to-line
fault current, current transformers used with the SEL-351A should meet the
following criterion:
Equation 9.1
This ensures a two-cycle trip of an instantaneous element set at 80 A. The
following examples show how the criterion is used.
Example 1: Maximum Fault Current With an 80 A Instantaneous Setting
Maximum fault current in terms of primary CT and ANSI voltage rating,
burden in ohms, and X/R ratio is:
Equation 9.2
Equation 9.2 is an actual-value equation derived from Equation 9.1 above
An SEL-351A phase instantaneous overcurrent element is to be set at
80 amps. The relay will be used with a C400, 400:5 current transformer with a
0.50 Ω ohm total burden. The X/R ratio is 20. Determine the maximum fault
current for dependable operation.
The burden is primarily from the CT windings and external leads to the
SEL-351A (the SEL-351A has a negligible burden):
Equation 9.3
where:
I
f
= maximum fault current in per unit of CT rating
Z
b
= the CT burden in per unit of standard burden
X/R = the X/R ratio of the primary fault circuit
262.5
X
R
----1+
⎝⎠
⎛⎞
I
f
Z
b
• • ≥
where:
I
MAX
= the maximum primary fault current for line-to-line fault
CT
RATING
= the CT primary rating in amperes
Z
B
= the total CT secondary burden in ohms
ANSI = the ANSI voltage classification of CTs
300 feet full-circuit run of #10 AWG (1.0 Ω /1000-ft) 0.30
CT winding of 80 turns at 0.0025 Ω/turn 0.20
Total burden 0.50 Ω
I
MAX
262.5
1
X
R
----+
⎝⎠
⎛⎞
-------------------
ANSI
100 Z
B
•
----------------------
CT
RATING
• • =
I
MAX
262.5
1
X
R
----+
⎝⎠
⎛⎞
-------------------
ANSI
100 Z
B
•
----------------------
CT
RATING
• • =
262.5
120+()
--------------------
400
100 0.50 Ω•
--------------------------------
400• • 40000 A==
Courtesy of NationalSwitchgear.com
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