GE STD15C - PERCENTAGE DIFFERENTIAL CHARACTERISTICS; HARMONIC RESTRAINT CHARACTERISTICS

GEK—45
307
If
ratio
matching
taps
are
chosen
so
that rated
CT
current
is
not
greater
than
the
tap
rating
on
a
self-cooled
basis,
the overcurrent
unit will
not
pick
up
on
magnetizing
inrush.
If
CT
currents
are
greater
than
tap
rating,
there
is
danger
that
the
unit
may
pick
up,
especially
on
small
transformer
banks.
If
this
happens,
it
is
recommended
that
the
CT
ratio
or
relay
tap
setting
be
increased,
rather
than
increasing
the
pickup
of
the
overcurrent
unit.
If
the
overcurrent
setting must
be
raised,
the
requirements
on
CT
error
will
be
more
stringent,
in
accordance
with
the
following
equation:
E
20
- (2.5)
(P-8)
where
E
= CT
error
current
in
percent,
at
pickup
of
the
overcurrent
unit
P
=
Pickup
of
overcurrent
in
multiples
of
tap
setting.
PERCENTAGE
DIFFERENTIAL
CHARACTERISTICS
The
percentage
differential
characteristics
are
provided
by
through—current
restraint
circuits.
In
addition
to
the
operating
circuit,
which is
energized
by
the
differential
current
of
the
line
current
transformers,
the
relay is
equipped
with
a
restraining
circuit
that
is
indirectly
energized
by
the
transformer
secondary
currents.
For
the
relay
to
operate,
the
current
transformer
secondary
currents
must
be
unbalanced
by
a
certain
minimum
percentage,
determined
by
the
relay
slope setting
(as
shown
in
Figure
7).
This
characteristic
is
necessary
to
prevent
false
operation
on
through-fault
currents.
High
currents
saturate
the
cores
of
the
current
transformers
and
cause
their
ratios
to
change,
with
the
result
that
the
secondary
currents
become
unbalanced.
Percentage
restraint
is
also
required
to
prevent
operation
by
the
unbalanced
currents
caused
by
imperfect
matching
of
the
secondary
currents,
as
previously
described
under
Determination
of
CT
Turns
and
STD
Relay
Tap
Settings.
HARMONIC
RESTRAINT
CHARACTERISTICS
At
the
time
a
power transformer
is
energized,
current
is
supplied
to
the
primary
that
establishes
the
required
flux
in
the
core.
This
current
is
called
magnetizing
inrush,
and
in
the
primary
winding
flows
only
through
the
current
transformers.
This
causes
an
unbalance
current
to
flow in
the
differential
relay, which
would
cause false
operation
if
means
were
not
provided
to
prevent
it.
Power
system
fault
currents
are
of
a
nearly
pure
sine
waveform,
plus
a
DC
transient
component.
The
sine
waveform
results
from
sinusoidal
voltage
generation
and
nearly
constant
circuit
impedance.
The
DC
component
depends
on
the
time
in
the
voltage
cycle
at
which
fault
occurs,
and
upon
circuit
impedance
magnitude
and
angle.
Transformer
magnetizing-inrush
currents
vary
according
to
the
extremely
variable
exciting
impedance
resulting
from
core
saturation.
They are
often
of
high
magnitude,
occasionally
having
an
RMS
value
with
100% offset,
approaching
16
times
full-load
current
for
worst
conditions
of
power
transformer
residual
flux
and
point-
of-circuit
closure
on
the
voltage
wave.
They
have
a very
distorted
waveform
made
up
of
sharply
peaked
half-cycle
loops
of
current
on
one
side
of
the
zero
axis,
and
practically
no
current
during
the
opposite
half
cycles.
The
two
current waves
are
illustrated
in
Figure
8.
12