GE STD15C - BURDEN ON CTs

GEK-45307
where
B
STD
relay
total
burden
(see
Table
I)
N
= number
of
turns
in
bushing
CT
e
bushing
CT
resistance
per turn,
milliohms
(at
maximum
expected
tempera
ture)
f
busing
CT
resistance
per
lead,
milliohms
(at
maximum
expected
temperature)
R
one-way
control
cable
lead
resistance
(at
maximum
expected
temperature)
TABLE
I
Total
Burden
for
60 Cycle
Relays
STD
TAPS
AMPS
8
X
TAP
AMPS
BURDEN
OHMS
(B)
MIN
P.U.
AMPS
2.9
23.2
0.180
0.87
3.2
25.6
0.156
0.96
3.5
28.0
0.140
1.05
3.8
30.4
0.120
1.14
4.2
33.6
0.112
1.26
4.6
36.8
0.096
1.38
5.0
40.0
0.088
1.50
8.7
69.6
0.048
2.61
2.
Determine
CT
secondary
current
for
8
times
tap
setting.
=
8
x
STD relay
tap
rating
(Note:
For
the
location
of
fault
assumed,
all
the fault
current
is
supplied
by
one CT,
so
that
CT
current
and
relay
current
are
the
same,
regardless of
whether
the
CTs
are
connected
in
wye
or
delta.)
3.
Determine
secondary
CT
voltage
required
at
8
times
tap
setting.
Esec
=
IZ
4.
From excitation
curve
of
particular
tap
of
current
transformer
being
used,
determine
excitation
current
IE.
corresponding to
this
secondary
voltage,
Esec.
5.
Determine
the
percent
error
in
each
CT
by the
expression:
%
error
=
1 E
X
100
Is
This
should
not
exceed
20%
of any
set
of CTs.
If
it
does,
it
will be
necessary
to
choose
a
higher
tap
on that
set
of CTs,
and repeat
the
calculations
on selection
of
relay
taps,
mismatch
error,
and percent
ratio
error.
7