Vacuum envelope for current limiter

An improved envelope for a vacuum device circuit interrupter of the type having a pair of relatively movable electrodes within a vacuum envelope and including magnetic field arc suppression. The envelope includes end portions which support the electrodes and an intermediate insulating portion which surrounds and is spaced from the electrodes. Means for producing a magnetic field is outside the envelope. The insulating portion is of a shape which provides a greater distance between the electrodes and the surrounding insulating portion in the direction in which an interelectrode arc is deflected by the magnetic field. In the preferred embodiment the insulating portion is substantially cylindrical in shape with a pair of protruding side tubes. The interrupter envelope is oriented so the arc is deflected down the inside of the side tubes.

BACKGROUND OF THE INVENTION 
The invention relates to current interrupters of the vacuum-type for use in 
controlling fault currents associated with transmission lines in power 
distribution systems. In particular, the invention relates to the shape of 
the vacuum envelope employed in the current interrupter portion of a 
current limiting circuit. 
Increases in electric power demand has led utility systems to use higher 
voltages in the transmission of power. Fault currents, due to ground 
shorts for example, can rapidly become enormous on high voltage power 
distribution lines and can cause serious equipment damage. Therefore, as 
transmission voltages rise, there is a continuing need in the electric 
power industry for improved current limiting devices capable of rapidly 
controlling fault currents. 
Current limiters generally employ a current-suppressive impedance in 
parallel with some type of current interrupter. The interrupter opens 
under a fault condition and the current is diverted through the impedance, 
which limits the current to a safe level. A vacuum-type current 
interrupter generally comprises a pair of relatively movable electrodes in 
a vacuum envelope. The electrodes can be placed in electrical contact to 
provide a free path for current flow. Means are associated with the 
interrupter to separate the electrodes when a fault current is detected. 
When the electrodes separate, arcing occurs across the gap between the 
electrodes as soon as the last point of contact has been broken. Since the 
arc continues to carry substantially the full fault current it becomes 
necessary to extinguish the arc if the current is to be successfully 
diverted through the current-suppressive impedance. 
In the past it has been possible in alternating current power systems to 
permit the interelectrode arc in an interrupter to burn until a normal 
current zero is reached, at which time the arc disappears. Reignition of 
the arc is prevented if the dielectric strength across the electrode gap 
is sufficient to withstand the subsequent transient voltages. In present 
high-voltage lines, fault currents can build up to such a high value that 
even a single current half-cycle can cause damage to the transmission 
system. Instead, it is necessary to immediately suppress the arc. One 
method of extinguishing the arc is by application of a transverse magnetic 
field across the interelectrode gap of the vacuum device. The magnetic 
field causes a space charge in front of the anode which in turn causes a 
large voltage drop across the interrupter. This high voltage can then be 
used to force the current into the parallel current-suppressive impedance. 
Arcing which occurs in vacuum causes the release of a cloud of metallic 
vapor containing conductive ions of electrode material. This metallic 
vapor forms a metallic deposit upon any surface it reaches. After repeated 
interruptions, this deposit can build into a continuous current path which 
can seriously degrade interrupter performance. For example, if the arc is 
driven by the magnetic field into the wall of the envelope when it is 
coated with metallic arc deposit, an arcing path will likely arise from 
one electrode to the wall and then back to the other electrode. This can 
prevent arc extinction because the magnetic field cannot divert an arc 
carrying current in a direction parallel to the lines of magnetic force. A 
magnetic field creates a Hall electric field which diverts an arc in the J 
X B direction and, unless the arc cuts across the lines of magnetic force, 
there is no resultant diverting force. When arcing initiates between the 
electrodes and the envelope wall, the arc tends to align itself with the 
magnetic field lines. If this happens, the arc is unaffected by the 
magnetic field and will continue to burn. It is therefore highly 
undesirable to have arcing proceed between the electrodes and the envelope 
wall. 
It is preferable to have the wall of the vacuum envelope spaced far from 
the electrodes so as to discourage arcing between the electrodes and the 
wall. Most commonly this is done by increasing the diameter of the 
envelope. This greatly increases the cost of the interrupter. It also 
necessitates larger separation between the exterior magnetic field coils 
used to suppress the arc. Interelectrode field strength is thereby 
lowered, resulting in reduced performance. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the invention to provide a vacuum-type current 
interrupter employing magnetic arc suppression which has improved 
performance. 
Another object of the invention is to provide such an interrupter in which 
the walls of the vacuum envelope in the regions toward which the arc is 
deflected are a large distance from the electrodes. 
Accordingly, a current interrupter is provided for rapidly interrupting 
currents associated with power line faults having an evacuated envelope 
and a pair of relatively movable electrodes within the envelope. The 
evacuated envelope has spaced end portions and an intermediate insulating 
portion which is preferably substantially cylindrical in shape and sealed 
to the end portions. Each of the electrode members is supported within the 
envelope by one of the end portions of the envelope. The electrodes are 
relatively movable into and out of conductive contact. When separated 
under fault conditions, arcing occurs between the electrodes. Means is 
provided for producing a magnetic field between the electrodes when 
separated to deflect the arc toward the surrounding insulating portion to 
extinguish the arc. The insulating portion of the envelope is spaced from 
and surrounds the electrode members and is shaped so that the spacing 
between electrodes and the insulating portion is greatest in the region 
where the arc is deflected by the magnetic field. Thus the arcing distance 
between the electrodes and the surrounding insulating portion is increased 
.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a first embodiment of a current interrupter 20, having 
electrodes 22 and 24, is connected in parallel with a current limiting 
circuit. The current interrupter includes an evacuated envelope 26 having 
spaced end portions 28 and 30 and an intermediate insulating portion 32. 
Insulating portion 32 is substantially cylindrical in shape. End caps 28 
and 30 are suitably sealed to intermediate insulating portion 32 to form a 
vacuum enclosure. The envelope is evacuated so as to insure a mean-free 
electron path sufficient to prevent gaseous breakdown in the 
interelectrode gap. For this purpose the pressure in envelope 26 should be 
lower than approximately 10.sup.-4 torr. 
Electrodes 22 and 24 are each supported by one of the end caps of the 
envelope. Electrode 22 includes supporting portion 33 and electrode 24 
includes supporting portion 34. Both electrodes extend along a first axis 
which forms the axis of cylindrical insulating portion 32. The first axis 
also forms an axis of movement along which the electrodes are relatively 
movable within the envelope. They can be moved into mutual contact to 
complete an electrical path or separated to induce current interruption. 
Supporting portion 34 of electrode 24 extends through and is suitably 
sealed to end cap 30. Supporting portion 33 of electrode 22 is attached to 
the lower end of bellows 36, as shown in FIG. 1. The upper end of bellows 
36 is then suitably sealed to end cap 28, permitting electrode 22 to be 
movable within the envelope while maintaining a vacuum therein. A cover 35 
protects bellows 36. An actuator (not shown) coupled to end 37 of 
electrode 22 serves to move the electrode into and out of contact with 
electrode 24. 
Intermediate insulating portion 32 is spaced from and surrounds electrodes 
22 and 24. As noted above, it is substantially cylindrical in shape having 
an axis running vertically, as shown in FIGS. 1-3, forming the 
aforementioned first axis. Insulating portion 32 also includes a pair of 
protruding tubular portions 38 and 40 extending radially from opposite 
sides of the cylindrical portion. The tubular protruding portions each 
form an interior concavity in envelope 26 on opposite sides of insulating 
portion 32. Both side tubes 38 and 40 extend along a common second axis 
which is substantially perpendicular to the first axis of portion 32 noted 
above. In the preferred embodiment, side tubes 38 and 40 are cylindrical 
and together with cylindrical insulating portion 32 form a cross pattern 
of intersecting cylinders. The length of side tubes 38 and 40 is optional. 
Long side tubes provide a greater surface on which to accumulate metallic 
deposits, but they require a strengthened insulating portion 32 and are 
more expensive and cumbersome. Short side tubes are less expensive and 
provide the desired increase in arcing length between the electrodes and 
the envelope but the interrupter will have a shorter life. In the 
preferred embodiment, side tubes 38 and 40 provide an overall transverse 
dimension somewhat greater than the length of envelope 26. 
Insulating portion 32 is formed of insulating material such as glass or 
ceramic so as to prevent shorting between metal end caps 28 and 30. Side 
tubes 38 and 40 may likewise be formed of insulating material or they may 
be formed of metal. In the embodiment shown in FIGS. 1 and 2, the side 
tubes are formed of metal suitably sealed to insulating portion 32 at the 
intersections thereof. Use of metal for side tubes 38 and 40 provides 
additional strength. As shown in FIG. 2, tubes 38 and 40 are closed at 
their respective ends by separate metal caps 42 and 44. 
Circuit interrupter 20 also includes means for producing a magnetic field 
between electrodes 22 and 24 to extinguish the arc. Such means include 
field coils 47 and 48 disposed outside envelope 26. The coils are 
positioned on opposite sides of insulating portion 32 aligned 
perpendicular to side tubes 38 and 40 to provide for minimum coil 
separation. The coils preferably have an independent power supply 50 and 
are energized by switch 52. The magnetic field created by coils 47 and 48 
has lines of magnetic force which extend transverse to the first axis of 
cylindrical insulating portion 32 and substantially perpendicular to the 
second axis along side tubes 38 and 40. Such a transverse field deflects 
the arc which arises between the electrodes after separation toward the 
surrounding walls of envelope 26. 
Arc extinction is known to occur when the conductive plasma in the 
interelectrode gap ceases to provide a conductive path between electrodes. 
The conductive plasma includes electrons and ions of cathode material 
which are emitted from the cathode and travel to the anode. Under a strong 
magnetic field the trajectories of the ions and electrons can be bent 
sufficiently to prevent their reaching the opposite electrode. With a 
sufficiently strong field, substantially all ions and electrons are 
prevented from crossing the interelectrode gap and the arc is 
extinguished. Representative ion emission path from electrode 22 under the 
influence of a magnetic field are shown at 54 in FIG. 2. A diagrammatic 
representation in FIG. 2 shows the directions of the various forces 
involved. The current is flowing in the direction j and the magnetic field 
B is directly into the page. The current and magnetic field cause an 
electric field E to arise in the j X B direction as shown. If the 
transverse field E is sufficiently strong, the ions will be successfully 
diverted toward the surrounding walls of vacuum envelope 26. Before the 
arc disappears it is driven to the right as are the metallic acing vapors. 
Tubular portion 38 is positioned to correspond substantially with the 
region where the arc is deflected by the magnetic field. The arc and the 
associated metallic vapors are deflected down side tube 38. In that way 
the distance between the electrodes and the surrounding insulating portion 
is increased in the region where the arc is deflected. Arcing between the 
electrodes and the envelope is thereby inhibited. 
In an alternating current system, a fault may be detected with the current 
flowing in either direction. If at the moment of separation the electrodes 
have reversed polarity to that in FIG. 2, the field diagram will be 
changed to shows the current j flowing upward. Electrode 24 would then be 
the instantaneous cathode and, with the magnetic field still into the 
page, the transverse electric field E would point to the left. That would 
tend to drive the arc down the interior concavity within tubular portion 
40. 
Metal side tubes 38 and 40 are lined with insulating material. The lining 
is in the form of tubular insulating members 45 and 46 which are placed 
within tubular portions 38 and 48, respectively. The insulation eliminates 
conductive paths around the interior concavities. Insulating members 45 
and 46 are closed at their ends to cover metal caps 42 and 44, as shown in 
FIG. 2. An alternative construction could employ insulating members open 
at both ends to leave the metal caps exposed. When employing relatively 
long side tubes, the ends need not be protected with insulation since 
arcing between the electrodes by way of the distant metal caps is 
exceedingly unlikely. 
The first embodiment of a current interrupter described above is adapted to 
be placed on a power line in parallel with a current-suppressive circuit 
which will rapidly reduce excessive current flow. A representative example 
of such a circuit comprises resistor 56 in parallel with capacitor 58. 
When the current is diverted from the interrupter into this parallel 
circuit the resistor and capacitor provide an impedance which maintains 
the current flowing therethrough within safe limits. 
In operation, current interrupter 20 is placed on a power distribution line 
with electrodes 22 and 24 in mutual contact. Line current flows freely 
between the electrodes and no current is diverted into the parallel 
current-suppressive load. When a line fault occurs, by way of a 
substantial current path to ground for example, the current passing 
through interrupter 20 increases rapidly. Apparatus (not shown) 
continuously monitors line current to detect rapid rises in current flow 
indicating a fault. When a fault is detected, such apparatus sends a 
signal to the actuator for moving electrode 22 described above. The 
electrodes are rapidly separated causing arcing between the electrodes. 
Field coils 47 and 48 are energized by closing switch 52 causing a strong 
magnetic field to arise between the electrodes driving the arc down one of 
the side tubes 38 and 40 as described above. When the arc is extinguished, 
the fault current is diverted into the parallel current-suppressive 
impedance which maintains the line current within safe limits. 
Referring to FIG. 3, an alternative embodiment of a current interrupter is 
shown having a vacuum envelope according to the invention. In this 
embodiment the envelope 59 is formed with end caps such as those in the 
first embodiment suitably sealed to intermediate insulating portion 60. 
The envelope is evacuated as described above. Electrodes 62 and 64 are 
relatively movable along a first axis of portion 60 extending vertically 
through the electrodes as shown in FIG. 3. Actuating means and the 
external circuitry including the means for producing a transverse magnetic 
field are the same as for the first embodiment. 
Intermediate insulating portion 60 is formed of a suitable insulating 
material such as ceramic or glass. The main body of insulating portion 60 
is substantially cylindrical in shape. Protruding tubular portions 66 and 
68 are integral with the main body. They project from opposite sides of 
portion 60 along a second axis transverse to the first axis of portion 60. 
As in the first embodiment, side tubes 66 and 68 are preferably 
cylindrical. The one-piece construction is less expensive than using 
separate metal side tubes lined with insulation, but it has been found to 
be more fragile. 
The surfaces of the interior concavities within side tubes 66 and 68 
include baffle members 70 and 72. Baffles 70 are in the form of 
trough-shaped tubular segments arrayed in opposing pairs suspended from 
the interior of the side tubes. The baffles 70 provide a segmented tubular 
path along the aforementioned second axis of the side tubes. Baffles 70 
are formed of either metal or an insulating material and are supported by 
members 74 likewise formed of either metal or insulating material. The 
spacing between upper and lower baffle members 70 is preferably comparable 
with the interelectrode gap when electrodes 62 and 64 are separated, as 
shown in FIG. 3. 
The ends of side tubes 66 and 68 are protected by baffles 72 which project 
angularly into the concavities. Baffles 72 may be formed of metal or 
insulating material attached to the walls of the concavities. Baffles 72 
may also be formed by convoluting the interior of the insulating portion 
at the ends of the side tubes into angular projections. 
Operation of the interrupter shown in FIG. 3 is the same as that in the 
first embodiment. Upon application of the transverse magnetic field the 
interelectrode arc is driven down one of the side tubes. Baffles 70 and 72 
are adapted to collect the deposits of metallic arcing vapors and thereby 
to prevent formation of continuous current paths within the side tubes. 
Without such protection the interior surfaces of the side tubes will tend 
to accumulate continuous metallic deposits. After repeated interruptions 
these deposits can coat the interior concavities. This particularly is a 
problem when the metallic coating approaches the entrance to the side 
tubes because it will tend to encourage arcing from one electrode to the 
insulating wall and then back to the other electrode. It is the purpose of 
baffles 70 and 72 to break up any current paths along the envelope wall. 
For this purpose baffles 70 are of greater importance than baffles 72 in 
that current paths are less likely to be formed and cause fewer problems 
along the most distant wall portion of the envelope. 
Other embodiments of current interrupters are possible within the scope of 
the invention. The insulated lining of the metal side tubes in the first 
embodiment could be protected by baffles of the type shown in FIG. 3. 
Likewise, the interrupter of FIG. 3 having a one-piece insulating portion 
could be employed without baffles. For direct-current applications an 
interrupter according to the invention would have only a single protruding 
side tube with an interior concavity in the region where the arc is 
deflected by the transverse magnetic field. Finally, although it is 
preferable for the protruding side portions to be tubular in shape, other 
shapes could be employed. Protrusions in the form of hemispheres, for 
example, would provide some additional arcing distance between the 
electrodes and the surrounding envelope wall and thus achieve the improved 
performance desired. 
There has been provided a current interrupter of the vacuum type employing 
magnetic arc suppression which has improved performance. Space between the 
electrodes and the surrounding envelope walls is increased in the 
direction of arc deflection without the need for increasing the overall 
diameter of the envelope. In that way the magnetic field coils can be 
positioned in relatively close mutual proximity.