GEK-49861
APPLICATION
The SLY81 relay utilizes a three input phase angle comparator for
the phase distance measurement.
The three inputs for the phase A-B
measurements are:
(a) (IA - Ia)
Za1
- T
VAB
(b)
VAB
+
KVABl
(C)
( IA - I
a)
Za1
Operating Quantity
Polarizing Quantity
Overcurrent Supervision
where
TVAB
VAB
+
KVABl
are the currents in the faulted phases.
is
the
base
reach
impedance
with
an
impedance angle of 85°.
is
the
faulted
phase-to-phase
voltage,
multiplied by the restraint setting T.
is the faulted phase-to-phase voltage plus
K
times the positive sequence component of
the faulted phase-to-phase voltage.
K
is a
design constant equal to 0. 3 per unit.
The use of the positive sequence component of voltage in the
polarizing signal results in a relay characteristic of the "variable
mho"
type�
that
is,
the
characteristic
expands
as
the
source
impedance
behind
the
relay
increases,
providing
an
automatic
accommodation of increasing arc impedance.
The positive sequence
component of voltage in the polarizing signal also improves the
directional integrity of the distance measurement.
The third input of
the comparator provides an overcurrent
supervision function.
This input is adjustable so that it can be set
above load current but below the minimum fault current in order to
provide security from mis-operation due to potential failure.
The SLY81 has a characteristic which is adjusted by means of the
setting on the characteristic timer.
For short lines a circular
characteristic is recommended,
but,
for longer lines,
lines with
unusually heavy load transfer, or three terminal lines where very
large reach settings are applied, a lens shaped characteristic is
recommended.
RATINGS
GENERAL
Type
SLY81A
and
SLY81B
relays
are
designed
operation in ambient temperatures between -20° C and
standard C37. 90.
In addition, these relays will not
be damaged if operated in an ambient up to 65°C.
for
continuous
+55°C per ANSI
malfunction nor
The current circuits of the relays which are rated five amperes
( 5 amps} rms will carry 10 amperes continuously and will carry 250
amperes for one second (1 sec).
The current circuits of the relays
4
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