GE STD15C - Page 4

GEK—45307
2.
The
CT
secondary current
should
not
exceed
the continuous
thermal
rating
of
the
CT secondary
winding.
3.
The
relay
current
corresponding
to
maximum kVA (on
a
forced-cooled
basis)
should not
exceed twice
tap
value, which
is
the
thermal rating of
the relay.
4.
The CT
ratios
should
be
high
enough that the
secondary
currents will
not damage
the relay
under maximum
internal
fault conditions
(refer to RATINGS).
5.
The
relay
current corresponding
to rated
kVA
of
the
power
transformer
(on
self-
cooled
basis) should
not
exceed the
relay tap value
selected
(magnetizing
inrush
night
operate
the
instantaneous overcurrent
unit). If the
transformer
under
consideration
does
not
have
a
self—cooled
rating,
the
transformer
manufacturer should
be
consulted
for the
“equivalent
self—cooled
rating”;
that
is the rating
of
a
self-cooled
transformer
that would
have the
same
magnetizing
inrush
characteristics
as the transformer
being considered.
6. The
current
transformer
tap chosen must
be
able
to
supply
the
relay
with
8
times rated
relay
tap
current,
with
an
error
of less than 20%
of
the total
current.
If
the
current
transformers produce
an error
of
greater
than
20%
at
less
than
8
times
tap
value,
the
harmonic
content of
the
secondary
current
may
be
sufficient
to cause
false
restraint on
internal faults.
7. The CT ratios
should
be
selected
to
provide balanced
secondary
current
on
external
faults. Since
it
is rarely
possible to
match the
secondary currents
exactly
by
selection of
current
transformer ratios,
ratio—matching
taps
are
provided
on
the
relay by
means
of which the
currents may usually
be
matched
within
5%.
When
the
protected
transformer
is
equipped
with
load-ratio
control
it
is
obvious
that a close match
cannot be obtained
at
all points
of the ratio-
changing
range. In this case,
the secondary
currents are
matched
at
the middle
of the
range and the
percentage_differential
characteristic
of
the relay
is
relied
upon to prevent
relay operation
on the unbalanced
current
which
flows
when the
load-ratio
control
is
at the ends
of the range.
8.
In
some applications,
the
power
transformer will
be
connected
to the
high
voltage
or low voltage
system through four
breakers
(as shown in
Figure
1)
as
for example
in
a
ring—bus
arrangement.
In
this case,
the
CT
ratios
must
be
selected
so
that
the secondary
windings
will not
be
thermally
overloaded
on
load current
flowing around
the ring in
addition to
the
transformer
load
current.
It
is recommended that
CTs
on
each of the
two low
voltage
(or
high
voltage)
breakers
be
connected
to a
separate restraining
winding
to assure
restraint
on heavy
through-fault
current
flowing
around the
ring bus.
It is
not
desirable
to
protect
two
parallel transformer
banks with
one
set
of
differential
protection,
since
the
sensitivity of the
protection
would
be
reduced.
In addition,
if
the
banks can be
switched separately,
there
is
a
possibility
of
false
operation on magnetizing
inrush
to
one transformer
bank, causing
a
“sympathetic
inrush”
into the bank
already energized.
In
this
case,
the
harmonics
tend to
flow
between
the banks,
with
the possibility
that there will
be
insufficient
harmonics
in
the
relay
current
to
restrain the
relay.
Typical
elementary
diagrams
for the
STD15C and
STD16C are
illustrated
in Figures
2
and 3.
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