Schweitzer Engineering Laboratories SEL-351-5, Инструкция по эксплуатации

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9.38
SEL-351-5, -6, -7 Relay Instruction Manual
Date Code 20070117
Setting the Relay
Settings Explanations
CT Sizing
Sizing a CT to avoid saturation for the maximum asymmetrical fault is ideal,
but not always possible. This requires a CT ANSI voltage classification
greater than (1 + X/R) times the burden voltage for the maximum symmetrical
fault current, where X/R is the reactance-to-resistance ratio of the primary
system.
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-351 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-351 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 ,
An SEL-351 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-351 (the SEL-351 has a negligible burden):
where:
I
f
is the maximum fault current in per unit of CT rating
Z
b
is the CT burden in per unit of standard burden
X/R is the X/R ratio of the primary fault circuit
262.5
X
R
---- 1 +
⎝ ⎠
⎛ ⎞
I
f
Z
b
• •
≥
where:
I
MAX
is the maximum primary fault current for line-to-line fault
CT
RATING
is the CT primary rating in amperes
Z
B
is the total CT secondary burden in ohms
ANSI is 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
• • =