Sel 551 - Settings Explanations

4.18
SEL-551 Relay Instruction Manual Date Code 20110408
Setting the Relay
Settings Explanations
Settings Explanations
Note that most of the settings in the settings sheets that follow include
references for additional information. The following explanations are for relay
settings (accessed under the SET command) that do not have reference
information anywhere else in the instruction manual.
Identifier Labels
The SEL-551 has two identifier labels:
➤ the Relay Identifier (RID)
➤ the Terminal Identifier (TID)
The Relay Identifier is typically used to identify the relay or the type of
protection scheme. Typical Terminal Identifiers include an abbreviation of the
substation name and line terminal.
The relay tags each report (event report, meter report, etc.) with the Relay
Identifier and Terminal Identifier. This allows you to distinguish the report as
one generated for a specific breaker and substation.
RID and TID settings may include the following characters:
➤ 0–9
➤ A–Z,
➤ -, /, ., space
Current Transformer
Ratios
Phase and neutral current transformer ratios are set independently. If IN is
connected residually with IA, IB, and IC, then set CTR and CTRN the same. For
example, for a CT ratio of 600/5, set CTR = 120.
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 the SEL-551
firmware revisions prior to SEL-551-R507-Vf Z001001-D20020828 and
SEL-551-R108-Vr Z001001 D20020828 (see Appendix A: Firmware and
Manual Versions). If you apply the SEL-551 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-551 should meet the
following criterion:
Equation 4.1
This ensures a two-cycle trip of an instantaneous element set at 80 A. The
following examples show how the criterion is used.
where:
I
f
= the 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 current
262.5
X
R
----1+
⎝⎠
⎛⎞
≥ I
f
Z
b
• •