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It should be recognized that pickup current flows not only through
but also through one of the primary windings of the through current
However, compared to the operating energy, this quantity of restraint is so small
to be zero.
CURRENT RESTRAINT CIRCUIT
A full wave
transformer. In the
output is directed to the percent slope rheostat
adjusting the rheostat, the percent slope may be varied
an isolatinq transformer, rectified and
telephone tyoe relay.
DIFFERENTIAL
CURRENT CIRCUIT
The differential current
operating signal to the solid state amplifier through a series tuned circuit; and the harmonic restraint
isolating transformer through a parallel resonant filter.
rectified by a full wave bridge prior to being supplied to the sensitive amplifier.
The series resonant circuit is made up of a
which are tuned to pass currents of the fundamental system frequency and to offer high impedance to
Currents of other frequencies.
and can be
to give the desired amount of operate current.
the operating
The parallel resonant trap is made of a 15 microfarad
which are tuned to block fundamental frequency currents while allowing currents of
to pass with relatively little impedance.
harmonic restraint rectifier, and can be adjusted to give the desired amont of harmonic restraint. The
output of the rectifier is paralleled with the through current restraint currents and applied to
restraint
of the sense amplifier.
It will
evident that if the differential current applied to the relay is sinusoidal and of system
will flow mostly in the operating circuit and hence cause the relay to yield an output.
if. however. the differential circuit contains
be restrained from
A
resistor connected across the secondary of the differential
any momentary high voltage peaks which may
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
heavy internal fault currents. this unit will pick up and
unit target will be exposed, to indicate that tripping was through the instantaneous unit.
Because of saturation of the CT's and relay transformers at high fault currents, it is impossible that
less operating currents will be provided from the differential-current
slope tap
imply, and
would supply.
may be falsely restrained. Tripping is assured, however, by the overcurrent unit operation. Pickup
set above the level of differential current produced by maximum magnetizing inrush current.
the relative levels of pickup and speed of operation of the
Trade-Mark of the
l
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rectifier receives the output of the secondary of each through current restraint
relay. the d-c output of all three units are connected in
secondary supplies the instantaneous unit
Resistor
is connected in
of the sense amplifier.
Resistor R2 is connected in parallel on the a-c side of the
by the harmonic currents flowing in the restraint
harmonic restraint will be provided than the actual harmonic content of the
As a result,
Electric Company
Gener a l
located on the front of the relay. By means of'
15 to 40 percent. The
to the
solid state amplifier which
The operating and restraint currents are each
microfarad
capacitor (Cl) and a reactor
The output of the
Pyranol capacitor
l
than a certain percentage of
thus protecting the rectifiers and capacitors
conditions of a high internal fault current, the
unit and the overcurrent unit.
current transformer
it may
is put through
the a-c side of
operate
and a reactor
the relay will
the trip circuit. The
than the percentage
Figure 2
restraint.
assumed
The total
the
the
is applied
limits
damage
unit
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