GE STD15C - Page 13

GEK—45307
Any
current of
distorted,
nonsinusoidal
waveform
may be
considered
as being
composed
of
a
DC
component
plus
a
number
of
sine-wave
components
of different
frequencies;
one
of
the
fundamental
system
frequency,
and the
others,
called
“harmonics,”
having
frequencies
which
are
2,
3,
4, 5,
etc.,
times
the fundamental
frequency.
The
relative
magnitudes
and
phase
positions
of the
harmonics
with reference
to
the
fundamental
determine
the
waveform.
When analyzed
in this
manner,
the
typical
fault-current
wave
is
found
to contain
only
a very
small
percentage
of harmonics,
while
the
typical
magnetizing-inrush—current
wave
contains
a
considerable
amount.
The high
percentage
of harmonic
currents
in the magnetizing-inrush—current
wave
afford
an excellent
means
of
distinguishing
it electrically
from the
fault-current
wave.
In the
Type
STD
relays,
the
harmonic
components
are
separated
from
the
fundamental
component
by
suitable
electric
filters.
The harmonic
current
components
are
passed
through
the restraining
circuit
of
the
relay,
while
the fundamental
component
is
passed
through
the operating
circuit.
The
DC component
present
in both
the
magnetizing-inrush—
and offset-fault—current
waves
is
largely
blocked
by the
auxiliary
differential-current
transformer
inside
the relay,
and
produces
only
a
slight
momentary
restraining
effect.
Relay
operation
occurs
on differential-current
waves
in
which
the
ratio
of harmonics
to fundamental
is lower
than a
given
predetermined
value,
for
which
the
relay
is
set
(e.g.
an
internal
fault—current
wave),
and
is restrained
on
differential—current
waves
in
which the
ratio
exceeds
this value
(e.g.
magnetizing-inrush—current
wave).
BURDENS
Burdens
are shown
in
Table
III
and IV.
Burdens
and minimum
pickup
values
are
substantially
independent
of
the
percent
slope settings,
and
are
all approximately
100%
power factor.
Figures
given
are
burdens
imposed
on
each
current
transformer
at
5.0
amperes.
TABLE
III
ZERO
OPERATING
CIRCUIT
*
RESTRAINT
CIRCUIT
TA
G
RESTRAINT
60
CYCLE RELAYS
60
CYCLE
RELAYS
SETTIN
PICKUP
BURDEN
IMPEDANCE BURDEN
IMPEDANCE
AMPS
AMPS
VA
OHMS
VA
OHMS
2.9
0.87
3.2
0.128
1.3
0.052 —
3.2
0.96
2.7
0.108 1.2
0.048
3.5
1.05
2.4
0.096 1.1
0.044
3.8
1.14
2.0
0.080
1.0
0.040
4.2
1.26
1.9
0.076
0.9
0.036
4.6
1.38
1.6
0.064 0.8
0.032
5.0
1.50
1.5
0.060
0.7
0.028
8.7
2.61
0.7
0.028 0.5
0.020
*
Burden of
operating
coil is
0
under normal
conditions.
Burden
of 50-cycle
relay is
the same
or slightly
lower.
TABLE
IV
DC
CONTROL CIRCUIT
BURDEN
RATED VOLTS
48
125
250
48
110
220
MILLIANPS
140
105
88
140
96
80
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