4.29
Date Code 20150130 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
CT Arrangements
Use separate relay restraint circuits for each power source to the relay. In the
SEL-787 you may apply two restraint inputs to the relay. You may 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 and C voltage ratings.
➤ Both circuits are radial (no fault current contributions).
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
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-787 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.
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