Schweitzer Engineering SEL-387-0, Instruction Manual

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SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
Step 4. The SEL-387 calculates settings TAP1 through TAP4 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
(0.1–31) • I
N
.
Step 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
Step 2 through Step 5 .
Step 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
N
. If necessary, reselect the CT ratios and repeat
Step 2 through Step 6 .
Step 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
Step 2 through Step 7 . This
selection criterion helps reduce the likelihood of CT saturation
for a fully offset fault current signal.
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.