GE STD16C - CURRENT TRANSFORMER RATIO ERROR

GEK-45307
For Delta-connected
CTs
Tap
Current
=
LnL
N
where
N is
the
number
of CT secondary
turns.
4. Check
the
matching
of
relay
currents
to
relay
taps,
to keep the
mismatch
error
as low as possible.
Calculate
the
percentage
of
mismatch
as
follows: on
two—winding
transformers,
determine
the
ratio of
the two relay currents
and the
tap values
selected.
The
differences
between
these ratios, divided
by
the
smaller
ratio, is
the percent
of mismatch.
The
mismatch normally
should not
exceed
5%.
For
three-winding
transformers,
the percent of
mismatch error
should be
checked
for
all combinations
of currents or
taps.
If taps cannot
be selected
to keep this
percentage
error within
allowable
limits,
it will
be
necessary to
choose a
different
CT ratio on
one or
more
lines,
to obtain a better
match between
relay currents
and
relay taps.
5.
Check to see
that the sum
of
the
relay currents
that
will
be
applied to
the
relay for
a
fault
at
the
terminals of the power
transformer
is
less than 220
amperes
RMS for
1
second.
If the
period
during
which a fault current
flows
in
the
relay can
be
definitely limited
to a shorter
time,
a
higher
current can be
accommodated
in accordance with
the
relation:
(Amperes)?
x
seconds
48,400
Also
check that the
suni
of
the
multiples of
tap current
on an
internal
or
external
fault
do
not
exceed
150.
CURRENT
TRANSFORMER
RATIO
ERROR
The
CT ratio
error must be
less
than 20% at
8
times relay rated
tap
current.
This
is based on
the
instantaneous
unit
being
set at its
normal setting,
which
is
8
times
tap
rating.
If
the
instantaneous
unit
pickup
is
raised
above
this value,
the
20%
figure
must
be
reduced,
as
described
in the CHARACTERISTICS
section.
The
calculations
listed below
are for the
worst—fault condition,
as
far as CT
performance
is concerned,
which is an internal
ground
fault
between
the
CT
and
the
transformer
winding, with none
of
the
fault
current
supplied through
the neutral
of
the
protected
transformer.
1.
Determine
the
burden
on each CT, using
the
following
expressions:
a. For
Wye-connected
CTs
=
+ Ne+2f
+ 2R
Ohms
(Equation
1)
b. For Delta-connected
CTs
Z
= 2B
+ Ne
+
2f
+ 2R Ohms
(Equation
2)
1000
6