GE STD15C - INTRODUCTION; DESCRIPTION; APPLICATION

GEK-45
307
TRANSFORMER
DIFFERENTIAL
RELAYS
WITH
PERCENTAGE
AND
HARMONIC RESTRAINT
STD15C
and
STD16C
INTRODUCTION
Relays
of
the
STD
type
are
transformer
differential
relays
provided
with
the
features
of percentage
and
harmonic restraint.
A
static decision
unit
controls
a
small
telephone-type
relay
that
provides
the
contact
output.
Percentage
restraint
permits
accurate
discrimination
between
internal
and
external
faults
at
high
current,
while
harmonic
restraint
enables
the
relay
to
distinguish,
by
the
difference
in
waveform,
between
the
differential
current
caused
by
an
internal
fault,
and
that
of
transformer
magnetizing
inrush.
DES
CR1
PTI
ON
Each
Type
STD
relay
is
a
single-phase
unit.
The
Type
STD15C
relay is designed
to
be
used
for
the
protection
of
two-winding
power
transformers
and
has
two
through-
current
restraint
circuits
and
one
differential—current
circuit.
The
Type
STD16C
relay
is
designed
for
use
with
three—winding
power
transformers
and
has
three
through-current
restraint
circuits
and
one
differential—current
circuit.
It
may
also
be
used
for
four-circuit
transformer
protection
(see
Figure
1)
when
only
three
circuits
require
through-current
restraint,
while
the
fourth circuit,
being
the
weakest,
needs
no
through-current
restraint.
APPLICATION
The
current
transformer
ratios
and
relay
taps
should be
selected
to
obtain
the
maximum
sensitivity
without
risking
thermal
overload
of
the
relay
or
current
transformer
(CT),
or
the
possibility
of
misoperation.
Therefore,
current
transformer
ratios
in
the
various windings
of
the
power
transformer
should
be
selected
with
the
following
points
in
mind:
1.
The
lower
the
relay
tap
and
the
lower
the
CT
ratio
selected, the
higher
will
be
the
sensitivity.
However, the
lowest
CT
ratio
and
the
lowest
relay
tap
may
not
be
compatible
with
some
of
the
following
restrictions.
Where
a
choice
is
available
of
increasing
either
the
CT
ratio
or
the
relay
tap,
it
is
desirable
to
increase
the
CT
ratio
in
preference
to
the
relay
tap.
Since
the
relay
burden is
likely
to
be small
compared
to
the
lead
burden,
increasing
the
CT
ratio
tends
to
improve
the
relative
performance
of
the
CT5,
as
a
result
of
reducing
the
maximum
secondary
fault
current and
increasing
the
accuracy
of
the
CTs.
These
1nstrucron5
do
not
purport
to cover
i.11
details
or
variations
in
equipaent
nez
e provide
for
every
possible
ctingency
to
be
met in
connection
with
installation,
operation
or
IlntnCe. Should
further
nforma
ton
be
desi red
or
should
parti
cular
probleme arise
which
are
not
covered
ziWficiently
for
the
purchaser’s
purposes,
the
matter
should
be
referred
to the
General
Electric
Company.
To
the
extent
required
the
products
described herein
meet
applicable
ANSI,
IEt
andN
standards,
but
no
such
assurance
is
given
with
respect
to
local
codes
and
ordinances because
they
varç greatly.
3