4.28
SEL-787 Relay Instruction Manual Date Code 20150130
Protection and Logic Functions
Basic Protection
For this example, we have selected a two slope, variable-
percentage differential characteristic for maximum sensitivity
at low currents and greater tolerance for CT saturation on
external high-current faults.
The minimum error for selecting SLP1 for this application is
determined as follows:
➢ Excitation current (4%)
➢ CT accuracy (3%)
➢ No-Load Tap Changer (NLTC) (5%)
➢ Load Tap-Changer (LTC) (0%)
➢ Relay accuracy (±5% or ±0.02 • INOM, whichever is
largest) (5%)
The CTs have been evaluated for maximum through fault and
found to be unlikely to saturate severely. So, SLP2 does not
have to be set higher than normal.
The settings are as follows:
Remember that the O87P setting must yield an operating
current value of at least a minimum of 0.1 • INOMn/TAPn,
where n = 1, 2. In this case, O87P = minimum
((0.1 • INOMn)/TAPn) = 0.5/3.14 = 0.159. 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.
O87P = 0.3 (Operate current pickup in multiple of
tap)
SLP1 = 15 (15 percent initial slope)
SLP2 = 50 (50 percent second slope)
IRS1 = 6.0 (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)
HRSTR = Y (harmonic restraint enabled)
HBLK = Y (harmonic blocking enabled)
SLP1
MIN
%
17
200 17–
-------------------------
100 9.3%==
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