4.31
Date Code 20170814
Instruction Manual
SEL-700G Relay
Protection and Logic Functions
Group Settings (SET Command)
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
Step 1. Determine the secondary side burdens in ohms for all current
transformers connected to the relay.
Step 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.
Step 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.
The SEL-700G calculates settings TAP1 and TAP2 if the ratio
of maximum (TAPn/INOMn) to minimum (TAPn/INOMn) 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
.
If the ratio of maximum (TAPn/INOMn) to minimum (TAPn/
INOMn) is greater than 7.5, select a different CT ratio to meet
the previous 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. In this
case, repeat Step 2 and Step 3.
Step 4. 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 Step 2 through Step 4.
Step 5. 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 Step 2 through Step 5. 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-700G Series
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