GE STD15C - DIFFERENTIAL-CURRENT CIRCUIT; OVERCURRENT UNIT

GEK—45307
rheostat
(R3)
located
on
the
front
of
the
relay.
By
means
of
adjusting
the
rheostat,
the
percent
slope
may
be
varied
from
15%
to
40%.
The
output
is
put
through
an
isolating
transformer,
rectified,
and
directed
to
the
sensitive
solid
state
amplifier
that
controls
the
telephone-type
relay.
DIFFERENTIAL-CURRENT
CIRCUIT
The
differential-current
transformer
secondary
supplies
1)
the
instantaneous
unit
directly;
2)
the
operating
(tripping)
signal
to
the
solid—state
amplifier
through
a
series-tuned
circuit;
and
3)
the
harmonic
restraint
isolating
transformer
through
a
parallel
resonant
filter.
The
operating
and
restraint
currents
are
each
rectified
by
a
full—wave
bridge
prior
to being
supplied
to
the
sensitive
sense
amplifier.
The
series
resonant
circuit
is
made
up
of
a
5
microfarad
capacitor
(Cl)
and
a
reactor
(Li)
that
are
tuned
to
pass
currents
of
the
fundamental
system
frequency
and
to
offer
high
impedance
to
currents
of
other
frequencies.
Resistor
Ri
is
connected
in
parallel
on
the
AC
side
of
the
operate
rectifier,
and
can
be
adjusted
to give
the
desired
amount
of
operate
current.
The
output
of
the
rectifier
is
applied
to
the
operating
circuit
of
the
sense
amplifier.
The
parallel
resonant
trap
is
made
of
a 15
microfarad
capacitor
(C2)
and
a
reactor
(L2)
that
are
tuned
to
block
fundamental
frequency
currents
while
allowing
currents
of
harmonic
frequencies
to
pass
with
relatively
little
impedance.
Resistor
R2
is
connected
in
parallel
on
the
AC
side
of
the
harmonic
restraint
rectifier,
and
can
be adjusted
to
give
the
desired
amount
of
harmonic
restraint.
The
output
of
the
rectifier
is
paralleled
with
the
through-current
restraint
currents
and
applied
to
the
restraint
circuit
of
the
sense
amplifier.
It
will
be
evident
that
if
the
differential
current
applied
to
the
relay
is
sinusoidal
and
of
system
frequency,
it
will
flow
mostly
in
the
operating
circuit
and
hence
cause
the
relay
to
yield
an
output.
If,
however,
the
differential
circuit
contains
more
than
a
certain
percentage
of
harmonics,
the
relay
will
be
restrained
from
operating
by the
harmonic
currents
flowing
in
the
restraint
circuit.
A
ThyriteR
resistor
connected
across
the
secondary
of
the
differential-current
transformer
limits
any
momentary
high—voltage
peaks
which
may
occur,
thus
protecting
the
rectifiers
and
capacitors
from
damage,
without
materially
affecting
the
characteristics
of
the
relay.
OVERCURRENT
UNIT
The
instantaneous
unit
is
a hinged—armature
relay
with
a
self—contained
target
indicator.
On
extremely
heavy
internal
fault
currents,
this
unit
will
pick
up
and
complete
the
trip
circuit.
The
instantaneous
unit
target
will
be exposed,
to
indicated
that
tripping
was
through
the
instantaneous
unit.
Because
of
saturation
of
the
CTs
and
relay
transformers
at
high
fault
currents,
it
is
possible
that
less
operating
currents
will
be
provided
from
the
differential—
current
transformer
than
the
percentage
slope
tap
would
imply,
and
more
harmonic
restraint
will
be
provided
than
the
actual
harmonic
content
of
the
fault
current
would
supply.
As
a
result,
under
conditions
of
a
high
internal
fault
current,
the
main
unit
may
be
falsely
restrained.
Tripping
is
assured,
however,
by
the
15