Schweitzer Engineering Laboratories SEL-787 - Page 103

4.25
Date Code 20081022 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
at low currents and greater tolerance for CT saturation on
external high-current faults. The settings are as follows:
Remember that the O87P setting must yield an operating
current value of at least 0.1 • I
NOM
, at the least tap. In this case
O87P
MIN
= (0.1 • I
NOM
)/TAP
MIN
= 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.
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, C voltage ratings, and
core dimensions.
➤ 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.
O87P = 0.3 (Operate current pickup in multiple of
tap)
SLP1 = 25 (25 percent initial slope)
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)
HRSTR = Y (harmonic restraint enabled)
HBLK = Y (harmonic blocking enabled)