Step 1. Calculate the Operating Slope S
i
.
The operating slope
S
i
is the ratio of the differential current (
I
op
) caused by the sum of all the mismatch
hanger (this can be compensated for in the tap calculation). You may choose to include
the no load taps may be adjusted without the relay taps
should be 10% * (n-1) where
The following errors are not included in the
operating slope value. (These sources of mismatch do not
It will offset the slope characteristic from the origin in the operate direction on the operate versus
and the
maximum slope
In the example, the tap factors were calculated
tap changer
position is included as
S
I
= (5% NLTC) + (10% LTC) + (10% CT) + (4% Relay) = 29%
sources versus the through current. The following sources of
I
op
current must be taken into account:
No load tap c
this source of mismatch if it is possible that
being reset.
Load tap changer.
Tap mismatch (assumed to negligible due to the fine tap adjustment available in the relay).
Quality of the CTs (If the saturation factor calculated previously is greater than 0.5, additional margin
is recommended. For SF<0.5, we assume that the CTs perform within the 10% limit defined in the
ANSI Accuracy Class. When accounting for CT error, the total CT error
n is the number of restraint current inputs (2 for two restraint relay, 3 for three restraint relay). If more
than one CT is nearing the performance limit, the errors tend to cancel.
Relay measuring errors. 4% applies to the BE1-CDS240 (2% per input circuit).
S
i
vary with through current so they affect the minpu setting only.)
Transformer excitation current
IE
(assumed to be less than 4% of the self-cooled rating). This
mismatch does not vary with through current (load) so it tends to not add to the slope mismatch line.
restraint characteristic (see Figure D-10).
Unmonitored loads “
Iunmon
” such as station service or small capacitor banks in the differential zone
add to the constant excitation current.
When the saturation factor exceeds 0.5 on any of the CTs, CT saturation is likely. For this condition, the
BE1-CDS240 improves security by delaying restrained tripping by two cycles when the transient monitor
function detects operate (differential) current that is a result of CT saturation. For applications where the
saturation factor is greater than 0.5, additional slope margin is recommended. For applications where the
saturation factor exceeds 1.0, severe distortion is likely at high fault current
setting (60%)
is recommended. For more information, see Setting Note 4.
on system nominal ratings so the no load
a source of mismatch.
OFFSET
i
o
OFFSET
i
o
o
I
S
S
1
pu
min
M
I
S
pu
min
S
pu
min
M
AC
AB
M
Choose slope setting S. The tripping slope S must be greater than S
Step 2.
current. Refer to Figure D-2, Slope and Operating Margin. A
slope margin of 10%
is
recom
i
to provide a safety margin
at the differential tripping characteristic knee point (intersection of the minimum pickup tripping
characteristic and the slope-tripping characteristic). This safety margin is required to
accommodate the additional mismatch caused by the excitation current and the unmonitored load
mended.
D-18
BE1-CDS240 Settings Calculations
9365200990 Rev F
Summary of Contents for BE1-CDS240
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Page 292: ...ii BE1 CDS240 Human Machine Interface 9365200990 Rev F This page intentionally left blank ...
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Page 349: ...Figure 12 5 Horizontal Rack Mount Front View 9365200990 Rev F BE1 CDS240 Installation 12 5 ...
Page 361: ...Figure 12 17 Typical DC Connection Diagrams 9365200990 Rev F BE1 CDS240 Installation 12 17 ...
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