Note: Descriptions are shown in the official language in which they were submitted.
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PULSE POWER CONTROLLED VACWM SWITCH
BACKGROUND OF THE I NVENT I ON
Field of the Invention:
The disclosed invention is directed to vacuum
switches in general and is particularly directed to a
vacuum switch in which "turn on" is realized by employing
a trigger electrode and "turn off" is accomplished by
employing a pulsed transverse magnetic field.
Description of the Prior Art:
The known prior art is best exemplified by two
10 U.S. Patents 3,~11,070 and 4,021,628.
In U.S. Patent 3,811,070 a laser-initiated
three-electrode type triggered vacuum gap device is pro-
vided. The triggered vacuum gap device comprises a pair
of main arcing electrodes within an insulating vacuum
housing. The triggering electrode is positioned internal
of one of the main electrodes, and is electrically insu-
lated from the associated main electrode. A portion of
the trigyering electrode or the associated main electrode
comprises a material that is saturated with a gas, such as
hydrogen, that is rapidly released when the gas-saturated
portion is heated. It is to be understood that the gas-
saturated material can be a separate piece attached to the
electrode or an integral portion of the electrode. A
voltage potential is applied between the trigger electrode
and its associated main electrode. The applied voltage is
lo~er than the voltage required to cause a breakdown
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between the trigger electrode and the associated main
electrode. To initiate a high~power arc, a laser beam is
projected onto the gas-saturated material through a passage
in the opposite main arcing electrode. This procedure
liberates gas from the gas-saturated material into the
discharge region, between the triggering electrode and the
associated main electrode, producing a low-power arc. The
low-power arc forms guickly between the trigger electrode
and the associated main electrode. When the disclosed
device is operated on an alternating current power line
having an operating frequency of 50 Hz or 60 ~z, the time
duration of the laser beam is very short compared to the
period of the power frequency. The main electrode, within
which the trigger electrode is contained, is constructed
so that a current loop flows through the arc and the
trigger electrode and the associated main electrode to
cause a magnetic force on the arc. The resultant magnetic
force rapidly drives the arc into the interelectrode
region of the main arcing electrodes. The introduction of
the low-power arc into the main interelectrode region
initiates a power arc across the main arcing electrodes.
- When initiating breakdown by a pulsed laser beam
being directed onto the gas-saturated material, the energy
of this laser beam must be sufficient to heat the gas-
saturated material to the point to cause some of the
absorbed gas to be quickly liberated from it.
In one emhodiment of the invention, the laser is
~irected onto the triggering electrode which comprises a
portion saturated with a gas. The trigger electrode is
disposed inside of an opening in the associated main
electrode. The end portion of the trigger electrode,
projecting inside the associated main electrode, is sur-
rounded by an insulating piece that is smaller in diameter
than the inner diameter of the opening through the associ-
ated main electrode. The trigger electrode is also re-
cessed from the primary arcing surface of the associated
main arcing electrode. By this arrangement, metal vapors
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and particles which are dispersed from the main electrode
during power arcing are less likely to be deposited on the
walls of the insulating material causing a shortening path
to exist between the triggering electrode and the asso-
ciated main electrode.
In another embodiment, the gas-saturated mater-
ial is attached to the main electrode wall which has been
beveled so that the laser beam can be focused on the
gas-saturated material, rather than on the triggering
electrode, in order to initiate the low-power arc.
In yet another embodiment, a gas~saturated metal
disc is attached to the trigger electrode and a portion of
the trigger electrode extends through the gas-saturated
metal disc. The gas-saturated metal disc and the trigger
electrodes are electrically insulated from the associated
main electrode by vacuum and a solid high dielectric
material. A portion of the solid insulating material
between the metal disc and the associated main electrode
is undercut to lessen the possibility of this area being
coated with arc-generated metallic products and shorting
the triggering electrode to the associated main electrode.
A low-power laser beam can then be directed onto the
gas-saturated metal disc to initiate a low-power arc,
which in turn will cause a high-power arc to form between
the main arcing electrodes.
In U.S. Patent 4,021,628 there is provided a
current limiting circuit interrupter having a pair of
relatively movable contacts disposed within an evacuated
enclosure, movable between a closed position completing an
electric circuit and an open position forming an arcing
gap therebetween across which an arc is formed during
circuit interruption, and a magnetic field disposed to
produce a magnetic field transverse to the arc formed
during circuit interruption. A current limiting impedance
can be connected external to and in parallel with the pair
of contacts to provide an alternate path when the arc is
extinguished. Contacts of the vacuum interrupter can be
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formed from material having high current chopping charac-
teristics to facilitate extinction or chopping of the arc
formed during circuit interruption. Tungsten is one
material which exhibits the desired properties. Baffles
can be mounted internal of the vacuum envelope containing
the interrupter contacts to engage the arc during circuit
interruption and enhance arc instability. An axial mag-
netic field can be disposed to be applied to the vacuum
interrupter while the contacts are being separated during
circuit interruption. When the axial magnetic field is
removed, the arc tends to become unstable and this enhances
the interruption ability. As the axial magnetic field is
removed, a transverse magnetic field can be applied causing
rapid arc extinction, before a normal AC current zero.
15During operation, within the first two milli-
seconds of fault current rise, the disclosed vacuum inter-
rupter in the high voltage high current line will be
activated. The vacuum interrupter contacts separate
- rapidly to a relatively large separation. Repulsion coils
or other appropriate apparatus may be required on or
- connected to the contact structure for the desired rapid
separation. When the contacts are widely separated, a
transverse magnetic field is pulse-applied to the vacuum
arc. This magnetic field which is applied transverse to
the initial arc path creates arc instability. The arc
then extinguishes and current is transferred to a parallel
current limiting device. The parallel current limiting
device can be a lightning arrester, resîstor, reactor
bank, or the like. Arc instability can be further enhanced
by the provision of baffles internal to the interrupter
and the use of contact materials, such as tungsten, which
are associated with high cathode spot mobility.
The desirability of large contact separation on
a practical device may necessitate the use of an axial
magnetic field in order to postpone anode spot formation
during contact separation. Under these circumstances the
axial magnetic field will be removed at or near maximum
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contact separation and this would have a tendency to cause
arc ins-tability. Subsequent application of a transverse
field would create the desired current zero.
A vacuum interrupter having the transverse
magnetic field coils can be connected in series with one
or more additional vacuum interrupters. Arcs in the
standard vacuum interrupters would extinguish at the same
instant as the arc in the current limiting vacuum inter-
rupter utilizing the transverse magnetic field. ~owever,
the recovery voltage following forced current zero would
be impressed upon the series gaps-of all the vacuum inter-
rupters. This would provide for high voltage withstand
capability and facilitate operation in a high voltage
circuit. In some instances it may also be desirable to
connect capacitors in parallel with the interrupter con-
tacts. The capacitors enhance arc instability and also
lower the rate of rise of the recovery voltage following
arc current zero. Current interruption in the vacuum in-
terrupter can benefit from the use of parallel capacitors.
The disclosed vacuum interrupter can be used as a current
limiting device on an alternating current circuit or a
direct current circuit. It is advantageous to use a
vacuum interrupter with its separable contacts, which can
carry continuous current of either polarity. This pro-
vides a solution to some of the polarity problems en-
countered in prior art devices. By utilizing the trans-
verse magnetic field to enhance the current chopping
characteristics of the selected vacuum interrupter, a fast
acting current limiting device can be obtained.
SUMMARY OF THE INVENTION
The present invention is directed to a vacuum
switch comprising an evacuated envelope, said evacuated
envelope being comprised of a cylindrical side member and
two end caps, a first stationary main arcing electrode
disposed within said evacuated envelope, said arcing
electrode having first and second opposed major surfaces,
a first stationary electrode stem affixed to the first
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surface of said first arcing electrode and extending
entirely through one of said end caps, an aperture extend-
ing entirely through the stationary electrode stem along
its main axis, said aperture continuing entirely through
said first main arcing electrode fro:m its first major
surface to its second major surface, a second main arcing
electrode disposed within said evacuated envelope, said
second main arcing electrode having first and second
opposed major surfaces, said second major surface of said
second main arcing electrode facing said second major
surface of said first main arcin~ electrode, said first
and said second main arcing electrodes being spaced apart
a predetermined distance, said predetermined distance
defining a first arcing gap, a second electrode stem
affixed to the first major surface of said second arcing
electrode and extending entirely through the other end
cap, said second main arcing electrode and said second
electrode stem being movable relative to said first main
arcing electrode, thereby providing means for setting said
first predetermined arcing gap across which the switch
will operate is set, a trigger electrode disposed within
said aperture within said stationary electrode stem, said
trigger electrode extending to close proximity with the
first main arcing electrode, said trigger electrode being
~5 electrically insulated from said stationary electrode
stem, said trigger electrode being electrically insulated
from said first main arcing electrode by a vacuum space,
said vacuum space comprising a second arcing gap, said
second arcing gap being less than said first arcing gap,
means for applying a direct current voltage pulse between
said trigger electrode and the stationary electrode, and
means for commutating the current from the vacuum switch.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present inven
tion reference should be had to the following detailed
discussion and drawings of which:
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Fig. l is a schematic view of a vacuum switch
embodying the teachings of this invention; and
Fig. 2 is a schematic view of the vacuum switch
OI Fig. 1 with typical "turn on" and "turn off" electrical
circuitry connected thereto.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to Fig. l, there is shown a
vacuum switch lO. The vacuum switch 10 is comprised of a
highly evacuated tubular envelope consisting o~ a cylin-
drical side member 12 of a suitable electrically insulat-
ing glass or ceramic and metallic end caps 14 and 16
closing off the ends 18 and 20 of the side member 12.
Suitable seals 22 are provided between the end caps 14 and
16 and the side member 12 to render the inside of the
evacuated envelope formed by side member 12 and end caps
14 and 16 vacuum tight. The vacuum in the evacuated
envelope, under normal operating conditions, is at least
10 4 so that the mean-free path of electrons within the
~` envelope will be longer than the potential breakdown
distance within the envelope.
Disposed within the evacuated envelope is a first
- main arcing electrode 24. The first main arcing electrode
24 has ~irst and second substantially parallel opposed
major surfaces 26 and 28, respectively, and an edge portion
30.
A first electrode stem 32 is affixed to arcing
electrode 24 by, for example, welding or brazing end 34 of
the stem to the central portion of surface 26 of the
arcing electrode 24.
The electrode stem 32 extends away from the
arcing electrode 24, vertically as shown in Fig. 1, and
passes through end cap 14 with end 36 of the electrode
stem 32 being outside of the evacuated envelope. A suit-
able seal 38 as, for example, a glass seal in end cap 14
at the point where the stem 32 passes through end cap 14
ensures the integrity of the vacuum within the envelope.
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As shown in Fig. 1, the arcing electrode 24 -
electrode stem 32 structure is stationary within the
envelope.
The first main arcing electrode 24 normally will
consist of copper and chromium and the copper will range
from, by weight percent, 25% to 75% and the chromium will
range from, by weight percent, 25% to 75%.
The first electrode stem 32 normally will consist
of copper. The stem 32 may have a cladding, not shown, of
a refractory metal or stainless steel disposed about its
outer surface 40.
There is an aperture 42 extending entirely
through the first main arcing electrode 24 from surface 28
to surface 26 and the aperture 42 continues entirely
through the first electrode stem 32 terminating at surface
44 of the stem which is outside of the evacuated envelope.
The aperture 42 has a common axis through the electrode 24
and the electrode stem 32 and this axis is substantially
perpendicular to surface 28 of electrode 24.
A triggering electrode 46, preferably of copper,
- is disposed within the aperture 42 and extends from above
surface 44 of electrode stem 32 to an area in close prox-
imity to surface 28 of electrode 24. Tip 48 of triggering
electrode 46 does not, however, extend to or beyond surface
28 of electrode 24.
The gap 47 between tip 48 of triggering electrode
46 and wall 49 of aperture 42 through the electrode 24 is
approximately 1 mm.
The triggering electrode 46 is electrically
insulated from electrode stem 32 by an electrically insu-
lating material 50, as for example alumina.
In addition to electrically insulating the
triggering electrode 46 from the electrode stem 32, the
alumina preserves the integrity of the vacuum within the
evacuated envelope.
The electrical insulating material extends from
above iurface 44 of the stem 32 to a point in proximity to
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the arcing electrode 24. The tip 48 of the triggering
electrode 46 is electrically insulated from the arcing
electrode 24 only by the vacuum or arc gap between the two
which~ as sta-ted above, is approximately 1 mm.
A second main arcing electrode 52 is disposed
within the evacuated envelope. The second arcing elec-
trode 52 will normally have the same composition as the
first main arcing electrode 24.
The second main arcing electrode 52 has first
and second substantially parallel opposed major surfaces
54 and 56, respectively, and an edge portion 58.
A second electrode stem 60 is affi~ed to arcing
electrode 52 by, for example, welding or brazing end 62 of
the stem to the central portion of surface 54 of the
arcin,g electrode 52. The second electrode stem 60 will
normally be comprised of copper and may have a cladding,
not shown, of a refractory metal or stainless steel dis-
posed about its outer surface 61.
The electrode stem 60 e~tends away from the
arcing electrode 52, vertically as shown in Fig. l, and
passes through a bellows 64 and extends outside of the
evacuated envelope.
The bellows 64 is normally of stainless steel.
The bellows 64 is sealed into the end cap 16 and
there are seals 66 where the stem 60 passes through the
bellows to preserve the integrity of the vacuum within the
evacuated envelope.
There is an arc shield 68 to protect the bellows
64 from any arc blasts or metal particles. The arc shield
68 may be of copper, copper and stainless steel or of the
same composition as the main arcing electrodes.
The first main arcing electrode 24 and the
second main arcing alectrode 52 are vertically aligned
within the evacuated envelope.
The first arcing electrode 24 is stationary with
the electrode stem 32 sealed in place in the end cap 14.
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The second arcing electrode 52 is movable verti-
cally relative to the first arcing electrode 24 by means
of the bellows 64.
The potential for relative movement between the
two main arcing electrodes 24 and 52 affords a method of
setting an arc gap 70 between surace 28 of first main
arcing electrode 24 and surface 56 of second main arcing
electrode 52.
The arc gap 70 will vary from 1 to 2 cm depending
upon the voltage that is to be corducted between the two
arcing electrodes 24 and 52 when the vacuum switch 10 is
"turned on".
With reference to Fig. 2, a load circuit 78
consisting of a load 80 electrically connected between the
first arcing electrode stem 32 and the second arcing
electrode stem 60 by an electrical conductor 82, and a DC
power source 84 in the load circuit 78 connected electri-
cally in series with the load 80. The load circuit 78 is
outside of the evacuated envelope.
For purposes of explanation, it will be assumed
that the arc gap between the two main arcing electrodes 24
and 52 is 1 cm and that the arc gap between the tip 48 of
the triggering electrode 46 and wall 49 of the aperture in
the first main arcing electrode 24 is 1 mm. It will be
further assumed that the load 80 has a 25 ohm resistance,
the DC power source is a 50 kV source, that the conductor
82 has a line impedance, denoted at 86, of 10 ~H and that
the circuit current is 2 kA.
A triggering circuit 90 is connected outside of
khe evacuated envelope between electrode stem 32 and the
trigger electrode 46.
The triggering circuit 90 consists of an elec-
trical conductor g2 connecting a switch 94 and a capacitor
96 electrically in series and a DC power source 98 and an
electrical resistor 100 electrically in parallel with the
capacitor 96.
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For the purposes of explanation, assume the
capacitor 96 to be of 30 kV, the power source 98 to be a
30 kV power source and the resistor to have a value of 1
megohm.
A "turn-off" circuit 110 consists of a commutat-
ing capacitor 112 electrically connected in parallel
across the switch hy an electrical conductor 114 between
stem electrode 32 and stem electrode 60 outside of the
evacuated envelope.
In conjunction with "turn-off" c rcuit 110 a
hollow tube coil 116 is disposed around the switch ou-tside
of the evacuated envelope.
The hollow tube coil is preferably of copper.
The hollow tube coil 116, when electrically
pulsed, generates a magnetic field of 200 mT at a buildup
rate o~ 6 kT/sec.
The operation of the vacuum switch of Figs.
and 2 is as follows.
For "turn on", the switch 94 is closed and a
voltage pulse from DC power supply 98 and capacitor 96 is
- applied between the tip 48 of trigger electrode 46 and the
stationary main arcing electrode 24 or the stationary or
first electrode stem 32. This causes a low-energy vacuum
breakdown to be initiated in the gap 47 between the sta-
tionary main arcing electrode 24 and the trigyer electrode46.
Assuming that a voltage exists in load circuit
7~, from DC voltage source 84, between or across main
arcing electrode 24, the stationary electrode, and main
arcing electrode 52, the discharge between the triggering
electrode 46 and the stationary main arcing electrode 24
will initiate a breakdown in the main gap 70 causing a
vacuum arc and thus current flow between the main arcing
electrode 24 and main arcing electrode 52.
To shut the current off, circuit 110 is used in
conjunction with coil 116. This is accomplished by apply-
ing a pulsed transverse magnetic fi31d to the vacuum arc.
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The hollow tube coil 116 is energized thereby
building up a magnetic field of 100 to 200 mT at a rate of
6 kl'/sec. transverse to the arc between main arcing elec-
trodes 24 and 52.
The transverse magnetic fi01d commutates the
current, which has been flowing in the direction indicated
by arrow 11~, from the vacuum switch into the parallel
commutating capacitor 112 in circuit 110. The current
will flow in the direction indicated by arrow 120.
As the commutated current charges the capacitor
112, the voltage across the capacitor 112 will increase
until it approaches the source voltage of DC power supply
84 and current will cease to flow.
The critical parameters for current commutation
as taught by this invention are the intensity of the
transverse magnetic field, the time derivative of the
magnetic field and the value of the parallel capacitor
112.
The commutated current is proportional to the
intensity of the magnetic field and to the time derivative
of the magnetic field.
The time derivative of the magnetic field, i.e.,
how fast the magnetic field is built up is the more impor-
~ant parameter of the two.
The current to be commutated is approximately
proportional to the square root of the time derivative of
the magnetic field.
The current to be commutated is also a function
of the square root of the capacitance.
The main arcing electrode should also be of such
a design that the vacuum arc burns in a diffuse mode.
One of the main advantages of the vacuum switch
OI this invention is that the gap 70 can be varied by
adjusting the position of the movable electrode 52 relative
to the stationary electrode 24 and, therefore, the vacuum
s~itch can be made to operate over a wide range of voltages.
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For the switch shown in Figs. 1 and 2 with a 10
~f capacitor connected electrically in parallel across it,
a magnetic field of approximately 200 mT is required to
commutate currents of from 2 to 5 kA into the capaci'_or
causing a recovery voltage of approximately 500 kV.
