Patent Publication Number: US-6714115-B1

Title: Self-recovering current limiting device with liquid metal

Description:
BACKGROUND 
     The present invention relates to a self-recovering current-limiting device with liquid metal, including electrodes made of solid metal for the connection to an electric circuit to be protected and a plurality of compression spaces which are partially filled with liquid metal. 
     Soviet Union Patent Publication SU 922 911 A describes a self-recovering current-limiting device containing two electrodes made of solid metal which are separated by first insulating bodies which are designed as a pressure-resistant insulating housing. Inside the insulating housing, compression spaces are formed by insulating intermediate walls and second insulating bodies which are arranged therebetween and designed as ring-shaped sealing disks, the compression spaces being partially filled with liquid metal and arranged one behind the other and interconnected via connecting channels of the intermediate walls, the connecting channels being filled with liquid metal and arranged off-center. Thus, in normal operation, a continuous, inner conductive connection exists between the electrodes via the liquid metal. In the current-limiting event, the liquid metal is displaced from the connecting channels as a result of the high current density. In this manner, the electrical connection of the electrodes via the liquid metal is interrupted, resulting in the limiting of the short-circuit current. Subsequent to clearing or eliminating the short circuit, the connecting channels refill with liquid metal whereupon the current-limiting device is operational again. In German Patent Application DE 40 12 385 A1, a current-limiting device having only one compression space is described and vacuum, protective gas, or an insulating liquid are mentioned as the medium above the liquid level. It is known from German Patent Application DE 26 52 506 A1 to use gallium alloys, in particular GaInSn alloys as liquid metal in contact devices. Known, for example, from Japanese Patent Abstract JP 40 4312737 A are, current-limiting devices in which a tubular current-limiting chamber filled with liquid metal is in communication with a gas-filled, spring-loaded cylinder-piston device for absorbing the sudden pressure increase developing in the current-limiting event due to the vaporization of liquid metal. 
     To enable the current-limiting devices mentioned at the outset to be used for different cases of application, they have to be differently designed to the effect that they do not operate in response to an overloading of more or less short duration, depending on the case of application. Thus, for example, in conjunction with generators, current-limiting devices must not operate in response to a short-time overload current which is 2 . . . 6 times the nominal current or, in conjunction with motors of poor iron quality or transformers, in response to a short-time overload current which is 6 . . . 18 times the nominal current, but have to operate only in response to a current which in comparison is higher. Until now, therefore, there has been the disadvantageous requirement for the manufacturers to offer a considerable number of current-limiting devices to permit a suitable selection with respect to the conditions on the user side. 
     U. S. Pat. No. 4,429,295 describes a self-recovering device current limiting device containing hollow cylindrical electrodes made of solid metal, two compression spaces which are completely filled with liquid metal, and an intermediate wall which separates the compression spaces and which features connecting channels. The electrodes, together with in each case one inner piston made of insulating material, constitute a cylinder-piston device for taking up the evaporating pressure in the current-limiting event against a restoring means in the form of an inert gas or a spring means. British Patent GB 1 209 020 discloses a self-recovering current-limiting device in which a fixed electrode and a movable electrode are conductively connected via a reservoir which is completely filled with liquid metal and via a connecting channel. In the current-limiting event, the movable electrode is moved by the pressure of vaporizing liquid metal against a gaseous restoring means, it being possible for a plunger which is linked to the movable electrode and protrudes outward to be connected to an actuator for an indicating means or a circuit-breaker. After the current-limiting event has ended, the condensing liquid metal causes the movable electrode to return to the original position together with the plunger. The above described current-limiting devices are not suitable for an adjustment to a desired nominal current factor. Soviet Union Patent Publication SU 1 529 303 A describes an electric switching device with liquid metal, both of whose electrodes of solid metal, together with an insulating intermediate piece, constitute a receptacle whose inner surface is provided with sections of different diameter. By actuating the piston of a bellows filled with liquid metal, the liquid level inside the receptacle rises or decreases, as a result of which an electrical connection or disconnection takes place between the electrodes. The above described switching device is not suitable for limiting an overcurrent. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a current-limiting device which can be adapted to a desired current-limiting behavior, i.e., in particular with respect to the response, or minimum trip, current, in a manner which is easy to handle and reliably reproducible. 
     The present invention provides a self-recovering current-limiting device. The device includes a first and a second electrode for connection to an electric circuit to be protected, each of the first and second electrodes being made of a respective solid metal. A plurality of pressure-resistant insulating bodies and a plurality of insulating intermediate walls supported by the plurality of insulating bodies are provided. The plurality of insulating intermediate walls define a plurality of connecting channels therein and the plurality of insulating intermediate walls and the plurality of pressure-resistant insulating bodies at least partially define a plurality of compression spaces. The plurality of compression spaces are disposed one behind the other between the first and second electrodes and are at least partially filled with a liquid metal, a level of the liquid metal above the connecting channels being changeable using an adjusting device. 
     By an adjustment of the adjusting device, a change in volume of the liquid metal inside the compression spaces is effected, changing the filling level above the connecting channels. It was found that the magnitude of the response current increases with increasing filling level of the liquid metal above the connecting channels, provided that the remaining conditions are identical. In this manner, it is achieved for an individual device to be adjustable to a required nominal current factor of a plurality of possible nominal current factors on the user side. The current-limiting device is intended to operate in response to the nominal current, which the installation to be protected is designed for, multiplied by the nominal current factor. 
     In a first embodiment of the present invention at least one of the compression spaces is at least partially designed as a cylinder-piston device whose piston and, consequently, the filling level can be adjusted by an adjustable and arrestable adjusting device. Advantageous in this context is the conductive connection of the piston to the adjacent electrode, for example, via welded-on flexible conductors so that the piston acts like an inner electrode. 
     In a second embodiment of the present invention at least one of the compression spaces is connected to a reservoir which is filled with liquid metal, the reservoir being operatively connected to an adjusting device which can be adjusted and arrested from outside. Preferably, one of the electrodes is in communication with the reservoir. The reservoir is advantageously constituted by a cylinder-piston device or of a bellows. 
     The adjusting device is expediently connected to a plunger and/or can be arrested by way of arresting or clamping means and/or is provided with an adjusting scale, in this case advantageously in conjunction with a pointer. 
     If a plurality of current-limiting devices are combined to form a multipole device, then it is recommendable for the adjusting devices of all poles to be connected for jointly adjusting the same nominal current factor of the poles, for example, via a bridge. 
     GaInSn alloys as the liquid metal to be used are easy to handle because of their physiological harmlessness. An alloy of 660 parts by weight of gallium, 205 parts by weight of indium, and 135 parts by weight of tin is liquid from 10° C. to 2000° C. at normal pressure and possesses sufficient electrical conductivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Further details and advantages of the present invention ensue from the following exemplary embodiments which will be explained on the basis of Figures. 
     FIGS. 1 a, b  shows a longitudinal section of a first embodiment of the current-limiting device according to the present invention; 
     FIGS. 2 a, b  shows a longitudinal section of a second embodiment of the current-limiting device according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the embodiment of the present invention according to FIG.  1  and FIG. 1 b , current-limiting device  1  is enclosed by a pressure-resistant first insulating body in the form of a molded housing  10 . Molded housing  10  is constituted by a half shell  101  on the left-hand side and a half-shell  102  on the right-hand side. Inside molded housing  10  are located a left-hand electrode  121  and a right-hand electrode  122 . Electrodes  121  and  122  are composed of massive copper and protrude through molded housing  10  via an outer connecting conductor  123 , respectively. Starting at left-hand electrode  121 , pressure-resistant second insulating bodies in the form of ring-shaped sealing disks  3  and insulating intermediate walls  41  through  44 , which are provided with connecting channels  5 , are alternately arranged in molded housing  10 . 
     A cylinder-piston device  13  is located between intermediate wall  44  situated furthest on the right and right-hand electrode  122 . Cylinder-piston device  13  is essentially constituted by a pressure-resistant insulating cylinder  131  and a piston  132  which is guided therein in a direction perpendicular to intermediate walls  41  through  44 . Cylinder  131  is supported in molded housing  10  in the same way as sealing disks  3  and intermediate walls  31  through  44 , provision being made for means, which are not shown, for frictionally connecting these elements, for example, continuous clamping bolts along the two lines  8  and, preferably, for sealing rings between these elements. Piston  132 , which is designed as an open hollow cylinder, is sealingly guided in cylinder  131  for which purpose a sealing ring  133  is provided at the lateral surface of piston  132 . Plunger  134  of piston  132  protrudes through right-hand electrode  122  and half shell  102 . Piston  132  moves against the force of a restoring spring  135  which is braced between piston  132  and right-hand electrode  122 . Restoring spring  135  acts upon piston  132  in the direction of intermediate wall  44 . Piston  132 , which is also composed of copper, is conductively connected to right-hand electrode  122  via a flexible copper Litz wires  136  which are welded-on at both ends so that piston  132  acts like an electrode. 
     First compression spaces  61  having a constant volume are formed by sealing disks  3 , left-hand electrode  121  and intermediate walls  41  through  44 . A second compression space  62  having a variable volume is formed by intermediate wall  44  situated furthest on the right, cylinder  131  and the piston. All compression spaces  61  and  62  are partially filled with a liquid metal  7 , for example, a GaInSn alloy. Liquid level  71  thereof is always situated above connecting channels  5  so that normally, a continuous conductive connection exists between electrodes  121  and  122  via liquid metal  7 , piston  132  and copper Litz wires  136 , the conductive connection being limited or interrupted only when the response current is exceeded. Located above liquid level  7  is a vacuum or a protective gas whose pressure was adjusted by the manufacturer. 
     When moving plunger  134 , the resulting change in volume of compression space  62  causes liquid level  71  of the liquid metal  7 , which is distributing itself, to change in all compression spaces  61 ,  62 . In the leftmost or rightmost position of piston  132 , liquid level  71  occupies the maximum height Hmax (FIG. 1 a ) or the minimum height Hmin (FIG. 1 b ) above connecting channels  5 , respectively, under the assumption of the depicted horizontal position of use of current-limiting device  1 . The height of liquid level  71  above connecting channels  5  influences the magnitude of the response current of current-limiting device  1 . In the direction of its free end, plunger  134  is provided with snap-in grooves  141  which cooperate in a locking manner with a pointer  143  which is guided in right-hand half shell  102  and acted upon by a locating spring  142 . Snap-in grooves  141  are associated with values of an adjusting scale  144 . These values correspond to the values of the nominal current factors which can be adjusted via adjusting device  14  which is constituted by adjusting means  141  through  144 . In the example, the nominal current factors “ 6 ” (FIG. 1 b ), “ 12 ”, and “ 14 ” (FIG. 1 a ) can be adjusted. 
     In the now following description and representation of current-limiting device  2  according to the embodiment of the present invention shown in FIG. 2 a  and FIG. 2 b , only the significant differences from the embodiment shown in FIG. 1 a  and FIG. 1 b  will be pointed out, the same reference symbols being used for identical elements. 
     Left-hand electrode  221 , sealing disks  3 , intermediate walls  41  through  44  as well as right-hand electrode  222  form compression spaces  61  having a constant volume. Current-limiting device  2  is enclosed in an insulating and force-locking manner by a molded housing  20  composed of a left-hand and a right-hand half shell  201  and  202 , respectively. Via a connecting port  25 , compression spaces  61  are in communication with a reservoir  26  which is preferably completely filled with liquid metal  7 . Reservoir  26  is designed as a (for example, metallic) bellows which is sealingly joined to the outside of right-hand electrode  222  in the region of an admission aperture  203  in right-hand half shell  202 . The free end face of reservoir  26  is connected to a plunger  264  which is movably guided in a stationary bearing  245 . When plunger  264  is moved to its rightmost position, reservoir  26  takes up its largest volume, involving a taking in of liquid metal  7  from compression spaces  61  as a result of which liquid level  71  occupies the smallest adjustable height Hmin above connecting channels  5  (FIG. 2 a ). When plunger  264  is moved to its leftmost position, reservoir  26  takes up its smallest volume, involving a delivery of liquid metal  7  into compression spaces  61  as a result of which liquid level  71  occupies the largest adjustable height Hmax above connecting channels  5  (FIG. 2 b ). For a defined adjustment, plunger  264  is provided with an adjusting scale  244 . Using adjusting device  24  which is composed of adjusting means  244  through  246 , it is possible to adjust liquid level  71  to any arbitrary height between Hmin and Hmax and thus, to adjust in a continuous manner, the nominal current factor of current-limiting device  2 . 
     The present invention is not limited to the specific embodiments described above but is intended to be defined in scope by the appended claims. Thus, current-limiting device  2  can be modified to the effect that a cylinder-piston device is provided for reservoir  26  in lieu of a bellows. A further possible embodiment consists in that, when using a plurality of current-limiting devices  1  or  2  arranged in parallel to form a multipole device, plungers  134  of cylinder-piston devices  13  or plungers  264  of reservoirs  26  are rigidly connected to each other, only one adjusting device  14  or  24  being required here for all poles of the multipole current-limiting device. Moreover, it is possible for current-limiting device  1  to be modified in such a manner that copper Litz wires  136  are directly connected to right-hand connecting conductor  123  while right-hand electrode  122  is omitted.