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5.2
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Functions
Current and Voltage Source Selection
➤
➤
Circuit Breaker Failure Protection on page 5.146
➤
Synchronism Check on page 5.158
Current and Voltage Source Selection
The SEL-421 has two sets of three-phase current inputs (IW and IX) and two sets
of three-phase voltage inputs (VY and VZ), as shown in
. Currents IW
and IX are also combined internally (COMB = IW + IX) on a per-phase basis and
made available as the line-current option for protection, metering, etc. You can
select the current and voltage sources for a wide variety of applications, using the
global settings in
. The SEL-421 provides five default
application settings (ESS := N, 1, 2, 3, or 4) that cover common applications (see
). When you set ESS := Y, you can set the current and voltage sources
for other applications (see
and
). ESS settings examples are
given later in this subsection.
Current Source Switching
through
show the basic application of some of these set-
tings.
shows an alternate breaker that can be substituted for the main
breaker (bus switching details not shown). Normally, current IW (main breaker)
is used as the line-current source. But, if the alternate breaker substitutes for the
main breaker, then current IX is used as the line-current source, instead. SEL
OGIC
setting ALTI controls the switching between currents IW and IX as the line-cur-
rent source (assert setting ALTI to switch to designated alternate line current
ALINEI := IX). Alternate line-current source settings ALINEI and ALTI are not
used often and thus are usually set to NA. Setting ALTI is automatically hidden
and set to NA if ALINEI := NA (no line-current switching can occur).
NOTE:
If a current source is set to
"combine" (e.g., LINEI := COMB), then
the current transformer ratios for the
respective IW and IX secondary
circuits have to be the same (i.e. group
settings CTRW = CTRX). Starting in
firmware R318, the current
transformer ratios can be different
when the current source is set to
"combine" (e.g. LINEI := COMB).
shows combined currents IW and IX (see COMB = IW + IX in
) set for line protection, metering, etc. (LINEI := COMB). To combine
these currents correctly inside the relay to produce the effective line current,
when the CT ratios are different, the relay divides IX by TAPX before adding IX
to IW. The relay automatically calculates TAPX from the CTRW and CTRX set-
ting values (TAPX = CTRW/CTRX).
Figure 5.1
Current and Voltage Source Connections for the SEL-421 Relay
SEL-421 rear panel
(partial)
COMB = IW + IX
(combined
current
source
for line)
Current
source
(for breaker
and/or
for line)
IAW IBW ICW
IW
IAX IBX ICX
IX
Current
source
(for breaker
and/or as
alternate
for line)
VAZ VBZ VCZ
VZ
Voltage source
(alternate)
VAY VBY VCY
VY
Voltage source
Summary of Contents for SEL-421-4
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