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4.26
SEL-787 Relay
Instruction Manual
Date Code 20081022
Protection and Logic Functions
Basic Protection
As a rule of thumb, CT performance will be satisfactory if the CT secondary
maximum symmetrical external fault current multiplied by the total secondary
burden in ohms is less than half of the C voltage rating of the CT. The
following CT selection procedure uses this second guideline.
CT Ratio Selection for a Multiwinding Transformer
1. Determine the secondary side burdens in ohms for all current
transformers connected to the relay.
2. Select the CT ratio considering the maximum continuous
secondary current, I
HS
, based on the highest MVA rating of the
transformer. For wye-connected CTs, the relay current, I
REL
,
equals I
HS
. For delta-connected CTs, I
REL
equals
√
3 • I
HS
.
Select the nearest standard ratio such that I
REL
is between 0.1 •
I
NOM
and 1.0 • I
NOM
A secondary, where I
NOM
is the relay
nominal secondary current, 1 A or 5 A.
3. Select the remaining CT ratios (e.g., CTR2) by considering the
maximum continuous secondary current, I
LS
. As before, for
wye-connected CTs I
REL
equals I
LS
. For delta-connected CTs
I
REL
equals
√
3 • I
LS
. Select the nearest standard ratio such that
I
REL
is between 0.1 • I
NOM
and 1.0 • I
NOM
A secondary.
4. The SEL-787 calculates settings TAP1 and TAP2 if the ratio
TAP
MAX
/TAP
MIN
is less than or equal to 7.5. When the relay
calculates the tap settings, it reduces CT mismatch to less than
1 percent. Allowable tap settings are in the range (0.1–
6.2) • I
NOM
.
5. If the ratio TAP
MAX
/TAP
MIN
is greater than 7.5, select a
different CT ratio to meet the above conditions. You can often
do this by selecting a higher CT ratio for the smallest rated
winding, but you may need to apply auxiliary CTs to achieve
the required ratio. Repeat steps 2 through 5.
6. Calculate the maximum symmetrical fault current for an
external fault, and verify that the CT secondary currents do not
exceed your utility standard maximum allowed CT current,
typically 20 • I
NOM
. If necessary, reselect the CT ratios and
repeat steps 2 through 6.
7. For each CT, multiply the burdens calculated in step 1 by the
magnitude, in secondary amperes, of the expected maximum
symmetrical fault current for an external fault. Select a nominal
accuracy class voltage for each CT that is greater than twice the
calculated voltage. If necessary, select a higher CT ratio to meet
this requirement, then repeat steps 2 through 7. This selection
criterion helps reduce the likelihood of CT saturation for a fully
offset fault current signal.
Please note that the effective C voltage rating of a CT is lower than the
nameplate rating if a tap other than the maximum is used. Derate the CT C
voltage rating by a factor of ratio used/ratio max.
Содержание SEL-787
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