Schweitzer Engineering SEL-387-0, Instruction Manual

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3.23
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
Application Guideline
It is vital that you select adequate current transformers for a transformer
differential application. Use the following procedure, based on ANSI/IEEE
Standard C37.110-1996,
IEEE Guide for the Application of Current
Transformers Used for Protective Relaying Purposes .
CT Arrangements
Use separate relay restraint circuits for each power source to the relay. In the
SEL-387 you can apply two to four restraint inputs to the relay. You can
connect CT secondary windings in parallel only if both circuits meet the
following criteria:
➤
They are connected at the same voltage level.
➤
Both have CTs that are matched in ratio, C voltage ratings, and
core dimensions.
➤
The paralleled CTs are applied on outgoing radial loads.
CT Sizing
Sizing a CT to avoid saturation for the maximum asymmetrical fault current is
ideal but not always possible. Such sizing requires CTs with C voltage ratings
greater than (1 + X/R) times the burden voltage for the maximum symmetrical
fault current, where X/R is the reactance-to-resistance ratio of the primary
system.
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
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
N
and 1.0 • I
N
A
secondary, where I
N
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
LS
,
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
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
N
and 1.0 • I
N
A secondary.