Schweitzer Engineering Laboratories SEL-387-0 Instruction Manual Download

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3.31

Date Code 20050919

Instruction Manual

SEL-387-0, -5, -6 Relay

Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements

Temperature Measurement (SEL-387-5 and SEL-387-6 Relays)

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

Step 1. Determine the secondary side burdens in ohms for all current

transformers connected to the relay.

Step 2. Select the CT ratio for the highest-rated winding (e.g., CTR1)

by considering the maximum continuous secondary current,
I

, based on the highest MVA rating of the transformer.

For wye-connected CTs, the relay current, I

REL

, equals I

. For

delta-connected CTs, I

REL

equals

√

3 • I

. Select the nearest

standard ratio such that I

REL

is between 0.1 • I

and 1.0 • I

secondary, where I

is the relay nominal secondary current,

1 A or 5 A.

Step 3. Select the remaining CT ratios (e.g., CTR2–CTR4) by

considering the maximum continuous secondary current, I

for each winding.

Typically, the CT ratio is based on the rated maximum MVA of
the particular winding. If this rating is much smaller than the
rating of the largest winding, you can violate the tap ratio limit
for the SEL-387 (see

Step 4

and

Step 5

). As before, for wye-

connected CTs I

REL

equals I

. For delta-connected CTs I

REL

equals

√

3 • I

. Select the nearest standard ratio such that I

REL

is between 0.1 • I

and 1.0 • I

A secondary.

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

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

. 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.