GE STD15C - THROUGH-CURRENT RESTRAINT ADJUSTMENT

GE
K-45
307
The
following
expression
shows
the
relationship
between
the
percent
second
harmonic,
the
DC
component, and
the
by-pass
current.
%
Second
Harmonic
=
0.212
‘DC
100
0.4511
+ 0.51Dc
Figure
16
is
derived
from
the
above
expression.
It
shows
the
percent
second
harmonic
corresponding
to
various
values
of
by—pass
current
(ii)
for
a
constant
DC
set
at
4.0
(0.8)
amperes.
The
relay
is
calibrated
with
a
composite
RMS
current of
two
times
tap
value. When
properly
set,
the
relay
will
restrain with
greater
than
20%
second
harmonic,
but
will
operate
with
second
harmonic
equal
to
20%
or
lower.
With
the
DC
ammeter
(12)
set
at
4.0
(0.8) amperes,
the
auxiliary
relay
should
just
begin
to
close its
contacts
with
gradually
increasing
bypass
current
(‘i)
at
a
value
of
4.5—5.5
(0.9—
1.1)
amperes.
This
corresponds
to
19-21%
second
harmonic
(see
Figure
16),
providing
a
2%
tolerance
at the
set
point
to
compensate
for
normal
fluctuations
in
pickup.
It
should
be
noted
that
the
current
magnitude
in
the
rectifier
branch
(12)
is
slightly
influenced
by
the
application
of
by-pass
current
(Ii)
and
should
be
checked
to
make
sure
it
is
maintained
at
its
proper
value.
If
harmonic
restraint
is
found
to
be
out
of
adjustment,
it may
be
corrected
by
adjusting
rheostat
R2.
(See
CAUTION
at
beginning
of
INSTALLATION
PROCEDURES
section.)
THROUGH-CURRENT
RESTRAINT
The
through-current
restraint,
which
gives
the
relay
the
percentage
differential
or
percent
slope
characteristics,
is
shown in
Figure
7.
It
may
be
checked
and
adjusted
using
the
circuit
illustrated
in
Figure
15,
with
S2
closed
to
position
B.
Ammeter
I
reads
the
differential
current
and
13
reads
the
smaller
of
the
two
through—
currents.
r-
CAUTION
These
currents
should
be
permitted
to
flow
for
only
a
few
seconds
at
a
time,
with
cooling
periods
between
tests;
otherwise,
the
coils
will
be
overheated
-
NOTE:
The
percent
slope
tolerance
is
10%
of
nominal,
all
in
the
plus
direction.
This
is
to
ensure
that
the
slope
characteristic
never
falls
below
set
point
value.
In
testing
STD16C
relays,
the
setting
should
be
checked
with
switch
S4
first
in one
position
and
then
the
other,
thus
checking
all
the
restraint coils. With
the
current
tap
plugs
in
5.0
(1.0)
ampere
position
and
the
percent slope
set
in
the
40%
position,
the
relay
should
just
pick
up
for
values
of
the
1j
and
13
currents
indicated
in
Table
V
(VA).
Repeat
with
the
percent
slope
tap
set
in
the
25%
and
15%
positions.
If
any
one
of
these
set
points
is
found
to
be
other
than
as
prescribed,
the
adjustment
may
be
made
made
by
adjusting
R3.
It
should
be
noted
that
the
current
magnitude
in
the
through-current
branch
(13)
is
slightly
influenced by
the
application
of
differential
current
(Ii)
and
should
be
checked
to
make
sure
that
it
is
maintained
at
its
proper
value.
TABLE
V
TABLE
VA
%
SLOPE
AMPERES
—-
-
-
SETTING
13
40
30
12.0-13.2
40.0—44.0
25
30
7
.5-
8.3
25.0—27.5
15
30
4.5-
5.0
15
.0—16.5
1RUE
SLOPE
SLOPE
AIPERES
TRUE
SLOPE
(11/13
x 100)
SETTING
13
I
(1/3
X
100)
40
6
2.4-2.64
40-44
25
6
1
.5—1
.66
25—27.5
15
6
0
.9—1
.00
15—16.5
20