Date Code 20080110 Protection Functions 3-27
SEL-387E Instruction Manual
SLP2 = 50 (50 percent second slope)
IRS1 = 3.0 (limit of slope 1, Restraint current in multiple of tap)
U87P = 10 (unrestrained differential Operate current level, multiple of
tap)
PCT4 = 15 (block operation if fourth harmonic is above 15 percent)
PCT5 = 35 (block operation if fifth harmonic is above 35 percent)
TH5P = OFF (no fifth-harmonic alarm)
DCRB = N (dc ratio blocking disabled)
HRSTR = N (harmonic restraint disabled)
Remember that the O87P setting must yield an operating current value of at least 0.1 • I
N
,
at the least tap. In this case O87P
MIN
= (0.1 • I
N
)/TAP
MIN
= 0.5/1.74 = 0.287. Therefore,
the O87P setting of 0.3 is valid.
The differential unit settings are complete for this specific application. At this point you can also
choose to set backup overcurrent elements which we discuss at the end of this section.
Application Guidelines
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-387E Relay
you may apply two to four 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, C voltage ratings, and core dimensions.
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
1. Determine the secondary side burdens in ohms for all current transformers connected to the
relay.
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
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