GE STD15C - Percent Ratio Error; IA - Repeat - CT Turns and Relay Tap Setting

GE
K—45
307
Ratio
of
Taps
on Lines
C-A
2.72
Ratio of
Sec.Line Currents
= 2.63
Mismatch
2.72 -2.63
=
(All
are
less than
5%; therefore 1
mismatch error
is
not
excessive)
12. Check
that
the sum
of the
maximum relay currents
is
less than 220 amps
for
1
second, and therefore,
short-time rating
of relay
is
not exceeded.
II. Percent Ratio
Error
ASSUME (all
measured at
their maximum
One-way
CONTROL
CABLE RESISTANCE
Bushing
A CT
resistance
per turn
B
II
C
II
II
Bushing A CT
resistance
per lead
B
II II
II
II
Ii
II II
expected
temperatures)
(R)
= 0.284
ohms
Ce) =
4 rnilliohms
(e)
2.5
Ce)
= 2.3
(f)
= 75
niilliohms
(f)=525
(f)
= 18.6
1.
Burdens
on
CTs,
using
Equation
1
or Equation
2 from page
6.
a.
Line
A, Z
= 2
(0.156)
+ (20x4)
+
(2.0 x 75)
1000
= 0.312
+ 0.205
+ 0.568
=
1.085
+ 2 (.284)
b. Line
3,
Z = 0.096
+ (20
x
2.5)
+ (2.0 x 52.5)
1000
A
T.o85
25.6
27.8
1
.00
3.4%
+ 0.568
B
0.8
36.8
29.4
50
136%
C
0.833
69.6
58.0
0.5
0.8%
Exciting
current
on line
B is too high;
should
improve CT
performance.
IA - Repeat
— CT
Turns
and
Relay
Tap
Setting
try
higher tap
on CT
to
1. 100%
2.
Try
CT
turns
(necessary to change
C
also
for proper matching)
3. 100% 1 secondary
4.
Relay Current
5. Ideal
Relay
Taps (Set
C
8.7)
6.
Use Relay
Taps
7.
Mismatch
Error is
less
than
5%
= 0.096 + 0.138
+ 0.568
0.80
c.
Line C,
Z 2 (0.048)
+ (60
x
2.3)+(2
x 18.6)
+ 0.568
= 0.096
+ 0.180
+
0.568 0.833
2. Impedance,
ohms
3.
8
times
tap,
amperes
4. E 5
CT voltage require
(IZ)
5.
JE
required,
from excitation
curve
6. % Ratio Error
15.7
39.6
125
20
40
80
0.79 0.99 1.56
1.37
0.99
2.70
4.40
3.19
8.7
4.6
3.2
8.7
9