GE STD16C - PERCENT SLOPE SETTING

GE:K-45307
PERCENT
SLOPE
SETTING
The proper
percent
slope
required
is
determined
by the
sum
of:
a.
The
maximum
range
of manual
taps
and
the
load—ratio-control,
or automatic
tap
changing
means,
in percent.
b.
The
maximum
percent
of
mismatch
of
the
relay
taps.
Set
the desired
percent
slope
by
means
of
R3
(See
Figure
6A).
The
percentage
slope
setting
selected
should
be
greater
than
the
ratio
of
maximum
total
error
current
to
the
smaller
of the
through
currents.
In general,
if the
total
error
current
does not
exceed
20%,
the
25%
setting
is
used.
If
it
exceeds
20%,
but
not
35%,
the
40%
setting
is
used.
If the
movable
lead
is
used
(as
in
Figure
1,
for
example)
the
percent
slope
setting
should
be
chosen
about
twice
as high,
since
the movable
lead
provides
no restraint.
EXAMPLE
(REFER
TO FIGURE
4)
I.
Determination
of
CT
Turns
and
STD Relay
Tap
Settings
1.
Transformer
and
Line
A
B
C
2.
Maximum
I
= 3750/
J3
(Line
kV)
19.7
49.5
157
3.
100%
Ip
= 3000/
J
(Line
kV)
15.7
39.6
125
4.
Assume
CT
turns
(N)
20
20
60
5.
Maximum
I
secondary
(less
than
5a)
0.98
2.47
2.62
6.
100%
I
secondary
0.79
1.98
2.08
7.
CT connections
Delta
Wye
Delta
8.
Relay
Current
for
100%
I
Sec.
1.37
1.98
3.60
Select
a relay
tap
for
one
of the
line
currents
and
calculate
what
the
currents
in
other
lines
would
be if
they
were
increased
in
the
same
ratio.
If
any
current
is
greater
than
V3
times
any other,
the
8.7
tap
should
be
chosen
for it,
and
new
ideal
relay
taps
calculated
for
the
other
lines.
9.
Ideal
Relay
Taps
(Set
C
8.7)
3.31
4.78
8.7
10.
Try
Relay
Taps
3.2
4.6
8.7
11.
Check
Mismatch
Error
Ratio
of
Taps
on
Lines
B-A
=
1.43
Ratio
of
Sec. Lines
Currents
1.98
-
1.37
—
1.44
Mismatch
1.44
- 43
=
0.7%
Ratio
of Taps
on
LinesC-B
-!.j-
=
1.89
Ratio
of
Sec.
Line
Currents
=
1.82
1.89
-
1.82
Mismatch
1
8
=
3.8%
8