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GE K-45 307
The followin g expressi on shows the relations hip between the percent second harmoni c,
the DC compon ent, and the by-pass current.
% Second Harmon ic
=
0.212 'DC
100
0.4511
+
0.51Dc
Figure
16
is
derived
from
the
above
expressi on.
It
shows
the
percent second
harmoni c correspo nding to various values of by—pass current (ii) for a constant DC
set at 4.0 (0.8) amperes .
The relay is calibrate d with a compos ite RMS current of two times tap value.
When
properly set,
the relay will
restrain with greater than 20% second harmoni c,
but
will operate with second harmoni c equal to 20% or lower.
With the DC ammete r (12)
set at 4.0
(0.8)
amperes ,
the
auxiliary
relay
should
just
begin
to
close
its
contacts with graduall y increasi ng bypass current ('i) at a value of 4.5—5.5 (0.9—
1.1) amperes .
This correspo nds to 19-21% second harmoni c (see Figure 16), providin g
a 2% toleranc e at the set point to compen sate for normal fluctuati ons in pickup.
It
should be noted that the current magnitu de in the rectifier branch (12) is slightly
influenc ed by the applicat ion of by-pass current (Ii) and should be checked to make
sure it is maintain ed at its proper value.
If harmoni c
restraint is
found
to be out of adjustm ent,
it may be correcte d by
adjustin g
rheostat
R2.
(See
CAUTION
at
beginnin g
of
INSTALLATION
PROCEDURES
section.)
THROUGH-CURRENT RESTRAINT
The through -current restraint , which gives the relay the percenta ge differen tial or
percent slope characte ristics, is shown in Figure 7.
It may be checked and adjusted
using the circuit illustrate d in Figure 15, with S2 closed to position B.
Ammete r
I
reads
the differen tial
current and
13
reads
the
smaller of the
two
through —
currents .
CAUTION
a
a
-
NOTE:
The
percent
slope
toleranc e
is
10%
of
nominal ,
all
in
the
plus
direction .
This is to ensure that the slope characte ristic 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
checkin g
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
indicate d in Table V (VA).
Repeat with the percent slope tap set in the 25% and 15%
position s.
If any one of these set points is found to be other than as prescrib ed,
the adjustm ent may be made made by adjustin g R3.
It should be noted that the
current magnitu de in the through -current branch (13)
is slightly influenc ed by the
applicat ion of differen tial current (Ii) and should be checked to make sure that it
is maintain ed at its proper value.
TABLE V
TABLE VA
% SLOPE
AMPERES
—-
-
-
SETTING
13
40
30 12.0-13. 2
40.0—4 4.0
25
30
7 .5- 8.3
25.0—2 7.5
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
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