Current interrupter for fault current limiter and method

A current interrupter is provided for use in controlling currents associated with power line faults. The interrupter is of the type having a housing filled with a dielectric fluid such as pressurized sulfur hexafluoride liquid and having electrodes extending through the walls of the housing. Associated with the electrodes is a movable contact member which when closed provides a path for current flow and which is movable to break a circuit. The housing includes a fluid chamber having a passage for fluid communication with the remaining interior of the housing. The movable contact member closes the passage when in its closed position. The method of the invention includes rapidly increasing the pressure in the chamber to move the contact from its closed position. A chemical propellant drives a piston against the fluid in the chamber. The fluid drives the movable contact member to open the passage and to break the current path, causing an arc. The fluid escaping from the chamber flows transverse to the arc thereby increasing the voltage drop of the interrupter.

BACKGROUND OF THE INVENTION 
This invention relates to current interrupters of the type used in 
controlling fault currents associated with transmission lines in power 
distribution systems. More particularly, the invention relates to such 
interrupters employing a housing filled with a dielectric fluid. 
Fast-acting current interrupters are used on power distribution lines for 
current limiting purposes. Fault currents on high voltage lines, due to 
ground shorts, for example, can rapidly become enormous and cause serious 
equipment damage. As transmission voltages rise, there is a continuing 
need in the electric power industry for improved current interrupting 
devices and methods for use in rapidly controlling fault currents. 
Current limiting circuits employ interrupter switches which open to divert 
a fault current through an associated current-suppressive impedance which 
limits the current to a safe level. A common feature of most types of 
interrupters is that arcing occurs between the electrodes when the 
contacts are opened. Since the arc will carry substantially the full fault 
current, the voltage drop between the arcing electrodes must be large to 
successfully divert the fault current into a parallel impedance. It is 
known that submerging the arcing electrodes in a dielectric medium can 
increase the voltage drop. However, the large amounts of energy released 
by arcing electrodes can vaporize or otherwise reduce the effectiveness of 
dielectric fluids. 
Rapid separation of the contact electrodes is desirable to prevent damaging 
current increases. Mechanical or magnetic actuating means for separating 
electrodes generally take on the order of two milliseconds to effect 
separation. Mechanical-type actuators also have the associated problems of 
contact bounce and generally high cost. Chemical explosives have been used 
to break contacts to effect rapid breaking of a circuit. A problem which 
arises when explosive charges are employed to fracture or break 
current-carrying material is that the resultant arcing gap is non-uniform 
and fragmented. This leads to numerous sharp edges and other surface 
features which can result in an efficient arcing environment, producing a 
low voltage drop between the electrodes. Furthermore, such destructive use 
of explosives generally renders the device non-reusable. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the invention to provide a current interrupter and 
method for use in a current limiting circuit which is fast-acting and 
causes a large voltage drop between the arcing electrodes. 
Another object of the invention is to provide such an interrupter and 
method which employs chemical propellants to separate the electrodes. 
Another object of the invention is to provide such an interrupter and 
method which produces a strong transverse flow of dielectric fluid across 
the arcing gap. 
Accordingly, a current interrupter is provided for use in controlling 
currents associated with power line faults. The interrupter includes a 
housing and a pair of electrode members in the housing. A contact member 
is supported for movement in the housing and is movable between a closed 
position and an open position. In the closed position the contact member 
contacts the pair of electrode members to provide a conductive path 
between the electrodes. In the open position the contact member is 
separated from at least one of the electrode members. A chamber is 
provided in the interior of the housing for holding a dielectric fluid. 
There is a passage between the chamber and the interior of the housing. 
The contact member blocks and closes the passage when in its closed 
position. The interrupter further includes means in the housing for 
rapidly increasing fluid pressure in the chamber to move the contact 
member from the closed position to the open position by means of fluid 
pressure. The method of the invention used by the interrupter includes 
increasing the fluid pressure in the chamber by an amount sufficient to 
drive the contact member from the closed position. The increased pressure 
separates the contact member from at least one of the electrode members, 
opening the interrupter. The fluid is then caused to escape the chamber 
through the passage by way of transverse flow through the gap separating 
the contact member and the at least one electrode member from which it is 
separated. An arc in the gap is thus subjected to a transverse flow of 
dielectric fluid.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a first embodiment of a current interrupter 10 is 
shown with a pair of electrode members 12 and 14 fixed in spaced relation 
and extending through the wall of housing 16. The housing 16 is preferably 
formed of a cast electrically insulating material such as epoxy and 
includes reinforcing members 18 for added strength. A contact member 20 is 
supported for movement in the housing by top portion 22. The movable 
support for contact 20 includes a bearing surface 24 against which contact 
support rod 26 slidably engages. Internal spring means 28 in support 26 is 
biased to exert downward pressure. Contact member 20 is thereby urged into 
a closed position to form a bridging contact between electrodes 12 and 14 
to provide a conductive path therebetween. Latch member 30 is biased 
against support rod 26 to engage a shoulder 31 when movable contact 20 is 
moved upwardly to a fully open position. 
In the lower portion of housing 16 there is provided a fluid chamber 32 
which is substantially filled with a dielectric fluid 33 of a suitable 
type such as oil or liquid sulfur hexafluoride (SF.sub.6). The remaining 
interior 34 of housing 16 also contains the same dielectric fluid. A 
passage 35 is provided between chamber 32 and the remaining interior 34 of 
housing 16, forming an outlet from the chamber. Passage 35 extends between 
electrodes 12 and 14. Contact member 20 blocks and closes passage 35 when 
in its closed position, being in contact with the liquid in chamber 32, as 
shown in FIG. 1. A movable piston 36 in the lower portion of chamber 32 
provides means in the housing for rapidly increasing the pressure of the 
liquid in the chamber. To drive piston 36, a chemical propellant charge is 
disposed between piston 36 and bottom cap 40 of housing 16. Propellant 38 
can be any suitable chemical explosive of the type which can readily be 
detonated by a signal on wire 42 which extends through cap 40. An example 
of such an explosive would be a firing cap. 
In the upper portion of housing 16, above electrodes 12 and 14, arc runners 
46 and 47 are provided to control the spread of the arcs. To help increase 
the voltage drop of the interrupter, arc runners 46 and 47 can be made of 
a non-linear resistance material which increases resistivity with heating, 
as is well known in the art. As noted above, the housing and chamber 32 
are substantially filled with dielectric fluid. If liquid SF.sub.6 is 
used, the housing is hermetically sealed and pressurized to 300-800 p.s.i. 
to place the SF.sub.6 into a liquid state. A gas pocket 48 is left in the 
top of the housing to permit expansion. 
In operation, the first embodiment interrupter shown in FIG. 1 is installed 
on line in a power distribution system. The external portion of electrodes 
12 and 14 are connected to a power line. A current-suppressive impedance 
(not shown) is connected in parallel with the interrupter. When the 
contact member 20 is in its closed position, as shown in FIG. 1, current 
flows between electrodes 12 and 14 by way of contact 20 and no current 
passes through the parallel impedance. When a line fault occurs, by way of 
a subtantial current path to ground, for example, the current through 
interrupter 10 rises rapidly. Apparatus (not shown) continuously monitors 
the line current to detect such a rapid current rise, indicating a fault. 
When a fault is detected the monitoring apparatus sends a signal over wire 
42 to detonate propellant 38. 
The method of opening the interrupter of this invention is shown most 
clearly in FIG. 2. The igniting of propellant 38 causes piston 36 to be 
rapidly driven upward as indicated by arrow 49 against the liquid 33 in 
chamber 32. This causes an enormous increase in the pressure of the liquid 
which is immediately transmitted against contact 20 by the liquid. The 
pressure is increased by an amount sufficient to overcome the bias of 
spring 28 and drive contact 20 from its closed position, separating it 
from both electrodes 12 and 14 and opening passage 35. Arcs appear in the 
intervening gaps 50 and 52 separating the electrodes and the movable 
contact. As shown in FIG. 3, the gaps extend between the elongated 
surfaces along which the electrodes and contact 20 meet. Immediately 
following separation, the arcs continue to carry substantially the full 
fault current. Once the passage is opened, the liquid flows out of chamber 
32 through passage 35, by way of transverse flow across the arcing gaps 50 
and 52 as indicated by arrows 54. 
Both the dielectric properties of the liquid and the strong transverse 
fluid flow produce a rapid rise in the voltage drop between the arcing 
electrodes. As such, the fault current through interrupter 10 is rapidly 
diverted into the parallel current-suppressive impedance, which then 
controls the fault current. Under normal operating conditions, the arcs in 
gaps 50 and 52 will continue to burn, perhaps extending to adjacent arc 
runners 46 and 47, until a normal current zero in the alternating current 
cycle, at which time the arcs will disappear. As the arc runners are 
heated by the arc, they increase in resistivity thereby increasing the 
voltage drop of the interrupter. Latch 30 prevents movable contact 20 from 
returning to its closed position, once the contact is fully opened. The 
dielectric liquid provides high withstand characteristics which prevent 
arc re-ignition in the presence of substantial recovery voltages. 
The current interrupter of FIGS. 1-3 takes advantage of the 
pressure-transmitting properties of the dielectric liquid to effect rapid 
electrode separation. The use of a chemical propellant as the driving 
force insures both rapid operation and high pressures. The construction 
provides for a strong crossflow of dielectric liquid at right angles to 
the resultant arcs which helps cool the electrodes and increases the 
voltage drop between the arcing electrodes. The interrupter is completely 
safe, with the explosion fully contained within the housing. Furthermore, 
the interrupter is reusable with only replacement of the chemical 
propellant and the contact head 20, and the resetting of movable contact 
20 required. 
Another embodiment of the invention is shown in FIGS. 4 and 5. In this 
embodiment, interrupter 59 includes a housing 60 which is substantially 
cylindrical in shape and encloses a pair of electrodes 61 and 62 supported 
by end walls 65 and 66. The cylindrical side walls 63 of housing 60 are 
preferably formed of molded epoxy or ceramic or another suitable 
insulating material. Electrodes 61 and 62 extend through side walls 63 
providing protruding portions 67 and 68, respectively, for interconnection 
with a power line. Each electrode has a substantially cylindrical portion 
shown most clearly in FIG. 5 extending inwardly from the end walls coaxial 
with the axis 69 of housing 60. Cylindrical portion 70 of electrode 62 
encloses a chamber 71. Electrode 62 further includes an opening 72 at its 
upper end forming a passage between chamber 71 and the interior 73 of 
housing 60. As in the first embodiment, the interior 73 of the housing, 
including chamber 71, is filled with a suitable dielectric fluid such as 
oil or liquid SF.sub.6. To minimize the amount of liquid which must be 
moved during opening, resilient compressible foam members 74 are provided 
near passage 72 within housing 60. Foam members 74 are preferably formed 
of a closed cell polyurethane material. 
Cylindrical portion 75 of electrode 61 movably supports a contact member 
76. Contact 76 is slidably disposed in an opening 77 through the base of 
electrode 61, and is thereby supported for movement. When contact 76 is in 
its closed position, as illustrated with solid lines in FIG. 3, it fits 
within opening 72 in electrode 62, blocking and closing passage 72. In the 
closed position contact 76 provides a conductive path between electrodes 
61 and 62. Contact 76 is also movable to a fully open position resting 
against stops 78, as illustrated with broken lines in FIG. 4, and in FIG. 
5. In its open position, contact 76 is separated from only one electrode 
(62) and remains in contact with electrode 61. When in either position, 
contact member 76 remains in conductive contact with electrode 61 and thus 
serves as a movable contact portion of electrode 61. In effect, contact 76 
functions as a movable electrode member interconnected with the power line 
by means of electrode 61. Electrode 62 functions as a fixed electrode from 
which the movable electrode formed by electrode portion 61 and contact 
portion 76 is separable to induce current interruption. 
As in the first embodiment, piston 80 provides means in the housing for 
rapidly increasing the pressure of the liquid in chamber 71. A chemical 
propellant 81 is provided between piston 80 and housing cap 82 for driving 
piston 80 against the liquid upon a signal over wire 83. 
In operation, the interrupter of FIGS. 4 and 5 is installed on line in the 
same manner as the first embodiment. Electrode portions 67 and 68 are 
connected to a power line in parallel with an impedance. During normal 
operations contact 76 is in its closed position and current flows freely 
between electrodes 61 and 62. When a rapid rise in current through the 
interrupter indicates a line fault, the previously-mentioned actuating 
means sends a detonation signal over wire 83, initiating the opening of 
the interrupter. Chemical propellant 81 is ignited, driving piston 80 
against the liquid in chamber 71 to rapidly increase the pressure of the 
liquid. The resultant increase in pressure drives contact 76 upwardly in 
the direction of arrow 84, breaking the continuous current path and 
causing arcing between electrode 62 and both contact 76 and electrode 61. 
As before, the pressurized liquid in chamber 71 opens passage 72 causing 
the liquid to escape into the remainder of housing 60 by way of transverse 
flow across the arcing gap separating the electrodes. Evantually, contact 
76 is driven to a rest position against stops 76. Foam members 74 are 
compressed as the liquid from chamber 71 enters the housing, reducing back 
pressure and absorbing some of the pressure pulse in the housing. Because 
the foam members are located near the point where liquid from chamber 71 
enters interior 73, the volume of liquid moved in the housing is 
minimized. It is intended that sufficient friction will exist between 
contact 76 and opening 77 in electrode 61 to prevent return of the contact 
to its closed position. Alternatively, suitable latching means could be 
provided, as in the first embodiment. 
The embodiment of FIGS. 4 and 5 provides for a pair of relatively movable 
electrode members. One electrode is electrode 61 together with contact 76, 
and the other is electrode 62. The electrodes are relatively movable into 
mutual contact when in their closed position to complete a current path. 
The electrodes are separable by means of liquid flowing through passage 
72, as described above. 
The embodiment shown in FIGS. 4 and 5 provides for large voltage drops 
between the electrodes in a manner similar to the first embodiment. Unlike 
the first embodiment, the contact member becomes separated from ohly one 
of the two electrodes causing a single break in the current flow. The 
cylindrical shape of the electrodes provides additional arcing surface. 
Resilient foam members 74 reduce the volume of fluid which must be 
transported by the force of the explosive, thus conserving energy. Re-use 
of the interrupter will generally require replacement of movable contact 
76 as well as the propellant charge. 
Another embodiment of the invention is shown in FIG. 6. Interrupter 89 of 
this embodiment has a substantially cylindrical housing 90 having side 
walls 91 formed of an insulating material and end walls 92 and 94. In this 
embodiment, a pair of electrode members 95 and 96 are supported by end 
walls 92 and 94. As in the embodiment of FIGS. 4 and 5, electrodes 95 and 
96 have substantially cylindrical portions 97 and 98, respectively, 
extending inwardly and coaxial with axis 99 of housing 90. Cylindrical 
portions 97 and 98 are substantially the same as portions 75 and 70 shown 
in FIG. 5. Portion 97 of electrode 95 movably supports a contact member 
100 in an opening 102, providing a sliding contact. Portion 98 of 
electrode 96 encloses a chamber 104 and has an opening 105 forming a 
passage between chamber 104 and the interior 106 of housing 90. As in 
previous embodiments, interior 106 and chamber 104 are substantially 
filled with a suitable dielectric fluid such as oil or liquid SF.sub.6. 
When contact 100 is in its closed position, as illustrated with solid 
lines in FIG. 6, it fits within opening 105 of electrode 96, blocking and 
closing the passage. To provide means for rapidly increasing the pressure 
of the liquid in chamber 104 to move contact member 100, a piston 110 is 
disposed in chamber 104. A chemical propellant 112 is provided between 
piston 110 and the bottom cap 113 of housing 90 to drive the piston 
against the liquid. Wire 114 carries the detonation signal. As before, 
closed-cell resilient foam members 115 minimize the volume of liquid 
transported by the force of the explosive. 
In addition to the pair of electrodes 95 and 96, a third electrode 116 is 
disposed in housing 90. Electrode 116 also has an opening 117 therethrough 
which is larger than opening 102 of electrode 95. Consequently, third 
electrode 116 remains electrically separated from both the other 
electrodes when contact member 100 is in its closed position. When contact 
member 100 is shifted to its fully open position, as illustrated with 
broken lines in FIG. 4, enlarged portion 118 engages opening 117 causing 
electrode 95 to be electrically interconnected with third electrode 116. 
Movement of contact 100, therefore, breaks one connection and forms 
another. 
Use of this interrupter is illustrated in FIG. 7. Connection points 119, 
120 and 121 represent the protruding portions of electrodes 95, 116 and 
96, respectively. The pair of switches 122 and 123, with a mechanical 
linkage 124 between them, represent the dual switching function of the 
interrupter of FIG. 4. After the opening of switch 122, which occurs when 
contact member 100 moves upward and is separated from electrode 96, switch 
123 is closed, connecting points 119 and 120. This permits insertion into 
the line of an additional element such as fuse 125. Resistor 126 in FIG. 7 
represents the parallel current-suppressive impedance employed in 
current-limiting circuits. 
Operation of the embodiment of FIG. 6 is according to the same method as 
the embodiment shown in FIGS. 4 and 5. The interrupter is installed on 
line with portions 119 and 121 connected to the power line. During normal 
current flow, contact member 100 remains in its closed position connecting 
electrodes 95 and 96. As before, apparatus (not shown) monitors line 
current and when a rapid rise in current indicates a fault, the apparatus 
initiates the opening method of the invention. Chemical explosive 112 is 
detonated by a signal on line 114, causing piston 110 to be forced against 
the liquid in chamber 104. Pressurized liquid immediately exerts pressure 
on contact 100, driving the contact from opening 105 and initiating arcing 
between contact 100 and electrode 96. As the liquid in chamber 106 
escapes, there is a transverse flow across the arcing gap separating the 
electrodes, increasing the voltage drop between the arcing electrodes. 
Resilient foam members 115 absorb the initial pulse pressure in the 
housing as in the previous embodiment. The force provided by the 
propellant is sufficient to drive contact 100 up to its fully open 
position, establishing contact between electrode 95 and third electrode 
116. When this embodiment of the invention is installed as shown in FIG. 
7, it is contemplated that the insertion of fuse 125 onto the line will 
minimize commutating duty on switch 122 and facilitate arc interruption. 
Operation of fuse 125 will serve to divert current into parallel 
current-suppressive impedance 126. 
As in the embodiment of FIGS. 4 and 5, contact 100 and electrode 95 
together function as a movable electrode member of opposite polarity to 
electrode 96. When the movable electrode is separated from fixed electrode 
96, arcing occurs in the intervening gap. Given the wide separation 
between electrodes 95 and 96, substantially all the arcing occurs between 
electrode 96 and contact 100. Also as in FIGS. 4 and 5, the electrodes 95 
and 96, and contact 100, function as a relatively movable electrode pair. 
Another embodiment of a current interrupter according to the invention is 
shown in FIGS. 8 and 9. In this embodiment, interrupter 127 includes a 
pair of substantially ring-shaped electrode members 128 and 129 disposed 
in a housing 130. Electrodes 128 and 129 are concentric with axis 131 of 
the housing, as is shown most clearly in FIG. 9. Both electrodes include a 
portion which protrudes from the housing for interconnection with a power 
line. Each electrode has a ring-shaped contact head. Together the heads 
form concentric circles around axis 131. Head portion 132 of electrode 128 
has a smaller radius and is inside head portion 133 of electrode 129. A 
substantially ring-shaped contact member 134 is supported for movement in 
housing 130. The support includes arms 135 slidably engaging the central 
shaft 136 of the housing. A biased latch 139 is provided, as in the first 
embodiment. Contact 134 is shown in its closed position in FIG. 8, in 
which it contacts both electrodes 128 and 129 to provide a conductive path 
therebetween. Spring means 137 hold the contact in its closed position. 
FIG. 9 shows contact 134 in its fully opened position, separated from both 
electrodes 128 and 129. 
Extending between electrodes 128 and 129, below head portions 132 and 133 
and contact 134, is a chamber 138 in housing 130. Chamber 138 is 
substantially annular in shape, tapering downward to a single column in 
the lower portion of the housing. A passage 140 between chamber 138 and 
the interior of the housing is provided between the head portions 132 and 
133 of the electrodes, with the passage being substantially ring-shaped. 
Contact member 134 blocks and closes passage 140 when in its closed 
position. As in the previous embodiments, housing 130, including chamber 
138, is filled with a suitable dielectric fluid such as oil or liquid 
SF.sub.6. Piston 141 provides means in the housing for rapidly increasing 
the pressure of the liquid in chamber 138. A chemical propellant 142 
between piston 141 and cap 144 serves to drive the piston against the 
liquid. The detonating signal is carried on wire 146. Resilient foam 
members 147 absorb the initial pulse pressure as in previous embodiments. 
In operation, the interrupter shown in FIGS. 8 and 9 is installed on line 
in a power distribution system with a parellel current-suppressive 
impedance as in the previous embodiments. Apparatus (not shown) monitors 
line current and when a rapid rise in current indicates a fault, the 
opening method of the invention is initiated. The monitoring apparatus 
detonates propellant 142 using line 146, causing piston 138 to move 
upwardly as indicated by arrow 148. Such movement causes a rapid increase 
in pressure in the chamber which drives contact member 134 from its closed 
position against the force of springs 137. The flow of liquid proceeds 
through passage 140, as indicated by arrows 149. Arcing occurs in between 
both head portions 132 and 133, and contact 134. Liquid escaping from 
chamber 138 produces a transverse flow across the ring-shaped arcing gaps 
separating contact 134 from the electrodes, producing a large voltage drop 
between the arcing electrodes to divert the current into the parallel 
impedance. As in the embodiments of FIGS. 4-7, foam members 147 are 
compressed by the pressure of the liquid entering the housing. Latch 139 
prevents a reclosure of the contact following separation. 
The embodiment shown in FIGS. 8 and 9 provides large contact surfaces which 
allow for high continuous current carrying capability. Nevertheless, all 
points of the arcing gap are subjected to strong dielectric liquid 
cross-flow from the annular fluid chamber. Replacement of the explosive 
charge 142 and replacing and resetting contact 130 permit re-use of this 
embodiment of the interrupter. 
The current interrupter and method of this invention provides improved 
voltage drop characteristics for use in diverting fault currents into a 
parallel current-suppressive impedance. The use of a fluid column to drive 
the electrodes apart, or move a bridging contact, provides an extremely 
efficient and fast-acting actuating mechanism. Furthermore, the resultant 
cross-flow of dielectric fluid across the arcing gap helps cool the 
electrodes and increase the voltage drop between the arcing electrodes. 
The use of a chemical propellant provides extremely high driving forces 
for the movable piston and can separate the movable contact from the 
electrodes in less than one millisecond. 
Other embodiments of current interrupters are possible within the scope of 
the invention. The electrodes may assume other shapes, for example. 
Likewise, the chamber containing the column of dielectric fluid can assume 
different shapes to accommodate alternative locations for the driving 
piston. Dielectric gases such as air can be used to transmit the opening 
force to drive the electrodes apart. The addition of further contact 
electrodes, such as the third electrode shown in FIG. 6, is also possible. 
A current interrupter and method has been provided for a fault current 
limiter which is both fast-acting and causes a large voltage drop between 
the arcing electrodes. The invention employs chemical propellants to open 
movable contacts. In addition, a strong transverse flow of dielectric 
fluid is provided across the arcing gap of the interrupter.