Abstract:
An indicating sectional two-part fuse is provided having a high-current current-limiting section, and a low-current expulsion section, mechanically connected together, and in electrical series, the low-current section utilizing a standard-type fuse-link, which may be replaced. The high-current current-limiting section may contain one or more current-limiting type fuse elements, supported by an interrupter rod within an enclosed casing filled with an arc-extinguishing granular material, and the low-current expulsion section of the fusible device may comprise an open-ended fuse tube, through which the standard fuse-link may extend, with the cable of the fuse-link extending out of the open end of the fuse tube.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present patent application is a continuation of patent application Ser. No. 633,189, filed Nov. 19, 1975, which, in turn, was a continuation of patent application Ser. No. 519,724, filed Oct. 31, 1974, which, in turn, was a continuation of patent application Ser. No. 67,183, filed Aug. 26, 1970, all abandoned. 
    
    
     Patent application Ser. No. 622,279, filed Oct. 10, 1975 by Frank L. Cameron, and assigned to the assignee of the present application, may be pertinent to the present invention. 
     BACKGROUND OF THE INVENTION 
     In recent years, in electrical systems, the use of capacitor units for phase correction has become more and more common place. Capacitors are usually mounted in banks, and have any convenient, or desired kvar. ratings. It is very desirable to have a current-limiting fuse in series with each capacitor to prevent case rupture in the event of breakdown, and to prevent damage to the entire capacitor bank. Preferably, the fuse should be capable of disconnecting itself from the circuit, thereby eliminating tracking, which can cause oscilatory current which can damage other capacitors in the bank. 
     In one well-known type of capacitor-bank construction, an indicating fuse is mounted on the aluminum bus above each capacitor unit. Connection to the capacitor units is made by a flexible pigtail. The pigtail is passed through a coil-spring fuse-element ejector and indicator mounted up on the capacitor-unit terminal. The spring is held in a curved position when the fuse is intact. When the fuse blows, the spring retracts to a horizontal position, pulling the pigtail, or fuse link from the fuseholder. Normally, the strain link of prior art fuses, that is, the link which resists the force of the coil spring is in tension, requiring that the link be firmly anchored at one end to a conductor rod, and be anchored at the other end to the pigtail of the fuse. An improved construction of such type is set forth in U.S. Pat. No. 3,259,719, issued July 5, 1966, to Robert T. Innis and George A. Rusnak, and assigned to the assignee of the instant application. 
     U.S. Pat. No. 3,467,934, issued Sept. 16, 1969 to Robert T. Innis and George E. Mercier, teaches a two-part indicating fuse adaptable for capacitor-unit protection, and including a flexible fuse casing. It is contempated in this patent that when fuse operation occurs, there will result a breakaway action between the two fuse casing sections of the device to thereby permit a visible indication of fuse operation. 
     SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, there is provided a dual current-limiting type of capacitor fuse incorporating two series sections, one being a high-current section, rendering a current-limiting action, and the other being a low-current section, rendering an expulsive action during its operation. Preferably, the two fuse sections are attached mechanically together, and the low-current section utilizes a standard fuse-link, which is readily replaceable. The low-current section employs an openended expulsion fuse-tube, through the open end of which extends the fuse-link cable. 
     During high-current or fault-current operation, the high-current element serves a current-limiting function, to thereby restrict the current flow to a safe magnitude until the fuse can isolate the faulted unit. During relatively low-current interruption, the fuse-link, within the expulsion unit, fuses, and provides an observable indicated condition of the fuse device. 
     It is, accordingly, a general object of the present invention to provide an improved dual sectionalized fuse, which utilizes a current-limiting section, and also an expulsion section, the latter using the universal-type fuse-link, readily obtainable in the open market. 
     It is a further object of my invention to provide an improved fuse with changeable minimum-melt fuse characteristics by reducing the standard link size at will for a particular application. 
     Another object of the present invention is to provide an improved indicating-type fuse for a capacitor unit, which will protect against gas generation internally within the capacitor casing, and which is of an improved construction. 
     Still a further object of the present invention is the provision of a two-part sectionalized fuse having a high-current section and a low-current section, the high-current section functioning to provide a current-limiting action, and the low-current section operating to use replaceable standard-type fuse-links. 
    
    
     Further objects and advantages will readily become apparent upon reading the following specification, taken in conjunction with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates, in perspective, a conventional capacitor bank incorporating an indicating-type of fuse according to the present invention; 
     FIG. 2 illustrates, in perspective, a typical capacitor unit utilized in the conventional bank of FIG. 1; 
     FIG. 3 illustrates more clearly, in side elevation, the improved fuse structure of the present invention, with the dotted lines indicating the observable blown condition of the fuse; 
     FIG. 4 illustrates, in vertical plan, the electrical diagram of a fuse connection to the central bus; 
     FIG. 4A illustrates diagrammatically the electrical components involved between the bank terminals; 
     FIG. 5 illustrates, to an enlarged scale, the improved protective fusible device of the present invention; 
     FIG. 6 illustrates the removable expulsion-type fuse-link holder of the improved fusible device of the present invention; and, 
     FIG. 7 illustrates a conventional-type fuse-link with its flexible cable, which may be readily replaced within the expulsion-type fuse-holder of FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Open structural-type capacitor banks, commonly referred to as &#34;stack type&#34; equipments, are the most economical method of obtaining large blocks of kilovars at voltages from 2400 volts up to the highest transmission voltages. Capacitor units are mounted and interconnected at the factory into a structural frame or &#34;stacking unit&#34;. Large banks are assembled at field locations by bolting insulators and stacking units on top of each other and interconnecting them. Selection of the capacitor unit voltage and kvar. and the stacking unit size are dependent upon the system voltage, bank kvar. and manner of connection. Capacitor units rated 25, 50, 100 or 150 kvar., and from 2400 to 20,000 volts are arranged in series groups to match the system voltage. Sufficient units are connected in parallel in each series group to provide the required total bank kvar. 
     FIG. 1 illustrates a typical capacitor bank 1 comprising a plurality of capacitor units 2 having fuse connections 3 to the bus 4, with an individual fuse connection, such as illustrated in FIG. 3 of the drawings. The electrical connections are more readily apparent from an inspection of FIGS. 4 and 4A of the drawings. FIG. 2 illustrates a typical two-terminal capacitor unit 2, with terminal bushings 5, 6, and the dotted lines 8 in FIG. 3 illustrate the position of the fuse 3 indicating a blown condition, with the fuse pigtail 10 hanging downwardly, and the capacitor unit 2 completely isolated and disconnected from the central energized bus 4, as well as indicating for ease of maintenance. 
     The protective fusible device 3, in accordance with the present invention, as illustrated more clearly in FIGS. 3 and 5, comprises a dual-sectional fuse construction having a first current-limiting high-current section 12 and a second low-current expulsion unit 13, the latter accommodating standard fuse links 14. Generally, the internal construction of the high-current current-limiting section 12 is described in U.S. Pat. No. 3,259,719, issued July 5, 1966 to R. T. Innis and George A. Rusnak, and assigned to the assignee of the instant application. As shown, the high-current section 12 utilizes an interrupter rod 16 composed of steatite, or other suitable insulating material, and having one or more spiral grooves 17 extending the length thereof. The space or volume 18 inside the tube 19, between the wall of the tube 19 and the interrupter rod 16, is filled with a suitable material 21, such, for example, as white sand. The helical grooves 17 may contain main fuse wires 23, composed of silver, for example, or other conducting material, which run the length of the grooves 17. It will be understood that the main fuse wires 23 may be a single wire, or several strands of wire twisted together, depending upon the desired current-carrying capacity of the fuse device 3. 
     As shown, the casing 19 has secured to its upper and lower ends, as by staking pins 24, a pair of conducting caps or ferrules 26, 27, the upper cap 26 having a clamping stud portion 29, which extends through the bus 4, and is secured thereto by a mounting nut 31. 
     The lower cap, or ferrule 27 of the high-current current-limiting section 12 contains a mounting-stud portion 32, which is threaded externally, as at 32a, and accommodates the mounting sleeve 34, which is secured, as by a threaded connection 35, to an insulating expulsion-tube or fuse-holder 36, which may be fibre-lined at 37, as shown. The low-current expulsion-tube 36 utilizes a standard-type fuse-link 14, having a button head 14a, which is replaceable. As shown in FIG. 3, the flexible fuse-link cable 10 extends through the open lower end 36a of the expulsion fuse-tube 36, and it is electrically connected to the capacitor terminal 6, as shown in FIG. 3. 
     There is a need for insuring that an operated or blown fuse 3 gives a positive observable indication when a piece of electrical equipment has partially, or completely failed. For instance, if a capacitor-unit 2 has partially, or completely failed, the fuse should operate to protect against gas generation internally inside the capacitor casing 2a, which could otherwise rupture the capacitor case. The fuse should also disconnect, and indicate a blown condition of the fuse, so the failed unit 2 will be completely disconnected from the circuit L1, L2, and a maintenance man can readily observe and locate the failed capacitor unit 2, and thereby provide for replacement scheduling. 
     FIG. 3 shows the basic construction of an inexpensive current-limiting indicating fuse 3, with a replaceable low-current standard-type fuse-link element 14. The advantage of using this separate high-and-low-current fuse construction 3 is that the low-current element 14 is a standard fuse-link, and it is replaceable, if this element is damaged due to a partially-failed unit 2, parallel-unit failure, bus flashover, or shipping damage. The fuse construction 3 of the present invention also has a very good disconnecting and observable indicating feature, since the low-current element 13 has the fuse-link cable 10 objected through the open and 36a of the fuse-tube 36, either on high-or low-current operation. 
     It will be obvious that during heavy fault current interruption, the high-current interrupting section 12 provides a current-limiting feature, which restricts the current flow to a safe magnitude, as the fuse 3 is isolating the faulted unit. During this same high-current interruption, as will be obvious, the low-current section 13 will also fuse, and eject the fuse-link cable 10 out of the open end 36a of the expulsion-tube section 36. Thus, during such high-current interruption, which occurs at high speed, the danger of case rupture and damage to adjacent units 2 is eliminated. 
     Under fault conditions, the fusible elements 23 inside the high-current unit 12 melt and/or vaporize, and the fused metal, within the current-limiting section, condenses in the sand 21 surrounding the assembly 16. The ensuing arc is confined in the spiral grooves 17 in the interrupter bar 16 by the sand 21. This restriction of the arc produces a high arc voltage, which opposes the system voltage, and limits the fault current to a safe value. The energy generated in the current-limiting section 12 is absorbed by the quartz sand 21. 
     During low-current operation, the fusible section, constituting a part of the replaceable fuse-link 14, fuses and the gases, which are generated within the fibre-lined expulsion tube 36, eject the fuse-link cable 10 out of the open end 36a of the expulsion-tube 36 to an indicating position 8, as shown more readily in FIG. 3 of the drawings. The high-current section 12 is not affected during such low-current operation. 
     It is to be observed that should the low-current section 13 become damaged, or otherwise unsuitable for use, the entire low-current unit 13 may be readily unscrewed from the mounting stud 32 of the high-current unit 12, and an entire new low-current unit 13 may be utilized. During normal operation, however, only the universal cable-type fuse-link 14 will need to be replaced, as well known by those skilled in the art. Thus, the improved fusible protective device 3 of the present invention utilizes a standard replaceable fuse-link 14 during normal operation. 
     For a particular application, by way of example, the individual capacitor units 2 each had a 150 kvar. rating, the voltage of the line was 20 K.V., and the normal operating current of the individual capacitor unit 2 was 7.5 amperes. With regard to the fusible device 1, there were 3 smooth silver wires, each of 0.02 inches diameter, laid together side-by-side in a single groove 17 formed on the interrupter rod 16 of the high-current unit 12. The fuse-link rating of the low-current unit was 12 amperes. 
     Although the invention has been described with particular relation to a capacitor unit 2, where it has particular suitability; nevertheless, it is to be understood that the improved fusible protective device 3 of the present invention could be used with other electrical equipment, other than capacitor units, such as a transformer, instrument, etc., as will be obvious. 
     To assist in the ejection of the fuse-link cable 10, a spring device 15, secured to the capacitor terminal 6, may be employed to assist in such ejection action. However, for certain application, it may not be necessary to use the spring device 15, and the generation of gases internally within the expulsion tube 36 may alone be relied upon to effect ejection and an indicating ejection position of the fuse-link cable 10. 
     Although there has been illustrated and described a specific structure, it is to be clearly understood that the same was merely for the purpose of illustration, and that changes and modifications may readily be made therein by those skilled in the art, without departing from the spirit and scope of the invention.