GB2123852A - Electrode contacts for high current circuit interruption - Google Patents

Abstract

Electrode contact materials exhibiting a high degree of arc stability, particularly at low arc currents, are employed in electrical circuit interruption devices, particularly vacuum interrupters. Solidified mixtures of mutually insoluble metals provide high arc stability, thus permitting arc interruption without the generation of excessively high voltage transients. Mixtures of copper and 1-92 wt.% vanadium have been found to be particularly effective for such purposes. Furthermore, the use of directionally solidified composite materials provides improved heat conduction and low resistance path characteristics. Other pairs of metal components disclosed are:- Cu and Bi Cu and Li Cu and Pb 63-99% Al and Ti Al and 1-82% Sb 80-99% Al and Cr 1-98% Al and Be Al and Bi me

Description

SPECIFICATION Electrode contacts for high current circuit interruption The present invention relates to electrical contacts and electrical contact materials for use in high current vacuum interruption devices. The present invention is also particularly directed to directionally solidified materials for use in such contacts.

In electrical power transmission systems, it frequently becomes necessary or desirable to interrupt the current flow. This function is often served by contacts which are separated in an evacuated housing. This configuration takes advantage of the desirable dielectric properties of vacuum conditions. However, in vacuum as in other interrupting media, current flow does not cease immediately upon separation of the contacts. Rather, contact parting initiates an arc which in the particular case of vacuum interrupters is maintained by a hot plasma formed by localized heating, vaporization and ionization of contact metal. In an AC power system, contact parting generally is random in time with respect to the sinusoidal variation of system current and voltage.Upon formation of the arc, system current continued to flow during most of the remainder of the half cycle of current in which the arc is initiated. As the arc current approaches a sinusoidal zero, the processes which sustain the discharge tend to become unstable causing the arc to extinguish suddenly. At this point the magnitude of the current may typically lie in the range of from a fraction of an ampere to a few tens of amperes depending upon contact material as well as a variety of other conditions.

Because this transition to a current zero condition occurs in such a short time (as low as approximately 10-8 secs.) parasitic inductive effects can give rise to high voltage transients, particularly if the transition to zero current occurs from a relatively high current value. As is well known in the electrical arts, the voltage transient generated is governed by the relation E=-L dl/dt.

Thus, it is seen that a rapid transition to current zero conditions readily gives rise to understandably high levels of voltage transients.

Accordingly, it is desirable to produce plasma arcs exhibiting a high degree of arc stability. That is to say, in alternating current systems, it is generally desirable to create conditions that favor the continuation of the plasma arc for as long a time as possible, but not longer than the occurrence of the next current zero point following contact separation. In this way, the transition to a current zero value following arc extinction occurs from a relatively low level of arc current. This minimizes the generation of undesirably high levels of voltage transients.

The enhancement of arc stability at low levels of arc current is an important consideration in the selection of a contact material for use in vacuum circuit interruption devices. Usually, high arc stability is achieved by choosing metals exhibiting low boiling points. Such a choice represents a significant compromise in that, as a class such low boiling point metals also exhibit relatively poor dielectric strength in vacuum conditions.

Other experimenters in the field have observed that certain alloys exhibit properties superior to either of their constituents with respect to arc stability. For example, copper, in a particular circuit, exhibits a current chopping level of approximately 5 amps, while tungsten, by itself, exhibits a current chopping level of 10 to 20 amps under similar conditions. However, it has been found that a copper tungsten alloy may exhibit a current chopping value less than other such values. That is, certain alloys exhibit qualities superior to either of their constituents with regard to arc stability at the low current levels of interest herein.

In many circuit interrupter designs, the electrical contacts are mounted upon conductive base members. Such construction is often required by economic considerations since it is generally undesirable to fabricate the entire electrode assembly from the more expensive contact material. Furthermore, the base members serve as convenient heat sinks and thermal conductors for removing heat from the electrical contact. Accordingly, a highly desirable property of any electrical contact material is that it exhibit a high degree of thermal conductivity, particularly in a direction from the electrode contact surface to the body of the base member or electrode support.Furthermore, in addition to the existence of a thermal path, it is also highly desirable to provide a path of low electrical resistance between the electrical contact surface and the body of the base member upon which the contact is mounted. However, in this regard it should be noted that the addition of one metal to another for which solubility of one component in the other is a few percent or more reduces their electrical conductivity. The heat conductive properties and a low level of energy dissipation in the bulk contact material are particularly important in vacuum because heat generated at the contact interface is dissipated principally by conduction through the electrode supports. For contacts operating in air or other atmosphere, cooling is additionally provided by convection at the lateral contact surfaces.Other differences in the operation of contacts in air as compared with operation in vacuum also exists. In particular, the choice of contact material for use in air is severely limited by the problem of oxidation and consequent high contact resistance. This is especially true of contact materials employing copper. Thus, in general, the selection of materials for use in vacuum devices is significantly different than the selection for use in devices in which the contacts are exposed to air.

Accordingly, efforts by those in the past to form mixtures of metal components for electrode contacts have generally not met with complete success, primarily because of the undesirable increase in the electrical resistivity of materials exhibiting mutual solubility. It thus appears desirable that contact materials exist as a matrix of distinct materials in elemental form. However, in certain matrices, higher boiling point metals have been particularly found to be desirable in that they characteristically exhibit higher dielectric strength in vacuum. Furthermore, the stability of vacuum arcs is known to be enhanced by the boundaries at the electrode surface between mutually insoluble components. Thus, it has been found in the past that, for example, a tungsten matrix which has been impregnated with copper has a greater arc stability than either tungsten or copper used in pure form.

Additionally, it is pointed out herein that experimenters in other fields have in the past employed the technique of directional solidification of materials in a number of different processes and materials. However, these processes have not generally been employed for the production of vacuum contact materials.

However, K. Chrost and S. Wojciechowski, from the Institude of Transformers, Electrical Machines and Apparatus at the Technical University of Lodz, Lodz, Poland, have disclosed the use of directionally solidified Ag-Cu mixtures for electrical contacts in a paper titled "Properties of Directionally Solidified Silver-Copper Contact Materials" in Proceedings of the 9th International Conference on Electric Contact Phenomena, pgs. 259-264, 1978. Additionally, other experimenters have proposed the construction of fibrous material manufactured from extruded wire. For example, H.H. Kocher and D. Stöckel have disclosed the use of such heterogeneous fiber composite contact materials in a paper titled "Material Transformer of Composite Contact Materials". However, no directional solidification is discussed.Neither is there discussed any compositions comprising copper in vanadium. In a paper titled "Behavior at Closing or Opening of Silver-Metallic-Oxide Materials in Electric Power Engineering" by M. Poniatowski K. -H. Schröder and E. P. Shulz in Proceedings of the 8th International Conference on Electric Contact Phenomena, pgs. 353-358, 1976, an extruded silver/silver oxide material having SnO2 inclusions of a fibrous structure vertically oriented to the contact surface is shown.A similar silver/silver oxide compound is disclosed in a paper titled "Replacement of Silver/Cadmium Oxide by Silver/Tin Oxide in Low Voltage Switching Devices" by Manfred Poniatowski, Ernst-Dieter Schulz and Axel Wirths in Proceedings of the 8th International Conference on Electric Contact Phenomena, pgs. 359-364, 1976. However, none of these papers are directed to the directional solidification of material such as copper vanadium and other such materials.

Furthermore, the use of copper and vanadium mixtures have not been previously employed in the manufacture of electrical contact materials.

However, U.S. Patents No. 3,514,557, issued to John W. Ranheim, teaches the desirability of employing a nonrefractory metal such as copper in a matrix of vanadium boride (VB2), a refractory compound. No mention is made of directional, dendritic or fibrous structuring of the material and the teachings of Ranheim appear to be solely directed to cases in which metaloids such as boron or silicon are employed to produce a compound, rather than elemental, refractory material.

Additionally, it should also be pointed out that in several situations of interest, it is also desirable to physically transfer the arc from its present position to other positions or other electrodes.

Again, such transition phenomena have not been achieved in the past through the use of directional solidification processes or materials as applied to the manufacture of electrical contacts.

Summary of the invention In accordance with a preferred embodiment of the present invention, an electrical contact for use in high current electrical circuit interruption devices comprises a block of conductive material having at least one contact face; the material comprises a directionally solidified mixture of at least two essentially mutually insoluble metal components. In a preferred embodiment of the present invention, the electrode contact material comprises a mixture of copper and vanadium with between about 1% and about 92% by weight of vanadium. Such electrical contacts are employed in two different ways in electrode assemblies. In one embodiment of the present invention, the direction of solidification is generally parallel to the contact surface on which the arc is struck.

This embodiment is particularly desirable in those situations in which arc motions or transfer is to be accomplished. Additionally, the electrode contact of the present invention may be employed in an electrode assembly in which the direction of solidification is generally perpendicular to the surface on which the plasma arc is struck. It is this latter embodiment which is most effective for thermal transfer and electrical resistance characteristics.

The present invention also embraces the use of directionally solidified contact materials in vacuum interrupters. In particular, a preferable form of contact comprises a multicomponent mixture formed using at least two mutually insoluble metal components. Furthermore, a material comprising a mixture of copper and vanadium may also be employed as an electrical contact, with or without directional solidification.

Accordingly, it is an object of the present invention to provide an electrical contact exhibiting high arc stability at low arc current levels.

It is a further object of the present invention to provide directionally solidified electrical contacts and materials therefor which enhance thermal and electrical contact properties, especially in vacuo.

Additionally, it is an object of the present invention to provide directionally solidified electrode contacts and materials therefore which enhance the transfer of the plasma arc from one position to another.

It is also an object of the present invention to provide electrical contacts and contact material, especially for vacuum circuit interrupters, comprising a mixture of copper and vanadium, not only as a mixture but also in a directionally solidified form.

Lastly, it is an object of the present invention to provide electrode assemblies employing the electrical contacts and materials of the present invention.

Description of the figures The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:: Figure 1 is a graph of current versus time illustrating a current interruption cycle; Figure 2 is a graph similar to Figure 1 more particularly illustrating several low current arc stability conditions; Figure 3 is a cross-sectional side elevation view of an electrode assembly embodying electrode material of the present invention; Figure 4 is a plan view of one of the electrode assemblies shown in Figure 3; Figure 5 is a cross-sectional side elevation view of an embodiment of the present invention further illustrating the use of directionally solidified electrode contact material; Figure 6 is a cross-sectional side elevation view of an electrode assembly similar to Figure 5 illustrating a different orientation for the directionally solidified material; Figure 7 is a photomicrograph in longitudinal section of a copper vanadium electrode contact material; and Figure 8 is similar to Figure 7 except that a cross-sectional view of the material is shown.

Detailed description of the invention The graph shown in Figure 1 illustrates one complete cycle of an electrically interrupted current waveform. In the graph shown, the curve exhibits a periodicity of T. For the case of conventional U.S. line current frequencies T=1 6.6 milliseconds. In order to electrically interrupt a circuit carrying the described sinusoidal waveform, it is conventional practice to initiate separation of electrical circuit interrupter contacts at a random time, say, t1. However, in almost all instances a plasma arc develops between the separated electrical contacts thus creating a continuing current path through the ionized plasma material which chiefly comprises vaporized metallic material from the surface of the electrical contacts.The current continues to rise to a maximum value and then proceeds to fall off sinusoidally until such time as the current reaches a range of value during the current suddenly and unpredictably drops to zero at a time t2. At the critical current value, the current falls off very rapidly. As discussed above, this rapid rate of current change is responsible for the generation of large voltage transients.

Accordingly, it is therefore generally desirable that the threshoid current level from which this rapid transition occurs be as low as possible. The timing and current value associated with this current fall off is a consequence of the lack of arc stability at low current levels. This sudden extinction of current is termed current chopping. Chopping currents are primarily determined by the electrode contact material employed. Under ideal conditions, the arc would extinguish at a time t3 corresponding to the point T/2 second subsequent to the previous zero crossing.

The phenomenon of current chopping is explored in greater detail in Figure 2 which shows an enlargement of a portion of the graph illustrated in Figure 1 immediately preceding the times3. In particular, there are shown three possible arc extinction transitions at times t2a, t2b, and t2C. If current chopping occurs at time t2a the arc is described as being relatively unstable.

However, if current chopping occurs at time t2C, then the arc is described as being relatively stable.

In general, high arc stability at these low current levels is a desirable feature of electrical circuit interruption devices and is, in particular, a primary advantage of the present invention.

Figure 3 illustrates another advantage obtainable with the directionally solidified electrode contact material of the present invention. In particular, conductive electrode base members 12 have embedded therein annular electrode contact rings 10. More particularly, there is shown arc 13 struck between contacts 1 0. The electrode assembly structure shown in Figure 3 is similar to configurations currently employed in conventional designs for vacuum circuit interrupter devices. In particular, in such devices it is generally desirable to promote the transfer of the arc from between electrode contacts 10 to conductive base member 12. In accordance with the present invention, annular electrode contact 10 is fashioned from composite materials which have been directionally solidified in a radial direction as suggested by the horizontal hatching of contact 10.In particular such solidification facilitates the transfer of arc 13 from between contacts 10 to between the larger, more widely separated, current-carrying structures 12. Structure 1 2 is more particularly illustrated in Figure 4 in which it is seen that it essentially comprises a disk-shaped structure having annular electrode contact 10 affixed thereto. Furthermore, conductive base member 1 2 preferably includes slots or spiral indentations 14 to promote arc rotation. Such construction is conventional for electrical vacuum circuit interrupter designs.

Figures 5 and 6 illustrate other electrode assemblies which may be employed with the directionally solidified materials of the present invention. In particular, in Figure 5 conductive base 1 6 functions not only as a current conductive element but also as a thermal heat sink. However, electrode contact 20 provides the principal surface on which the arc is struck. It is contact 20 which determines the degree of arc stability in the interrupter. In particular, as suggested by the vertical hatchings, contact 20 exhibits directional solidification in a direction which is substantially perpendicular to the surface upon which the arc is struck. Accordingly, the embodiment of the present invention illustrated in Figure 5 offers three significant advantages over the electrode contacts which are conventionally employed.First, directional solidification in the direction shown provides increased thermal conductivity between arc surface 21 and conductive base member 16. Secondly, the directional solidification in the direction shown provides an increased electrical conductivity between arcing surface 21 and base member 1 6.

This arises out of the presence of numerous conductive channels in contact 20. Thirdly, the directional solidification provided in contact 20 preserves arc stability by maintaining the boundary structure between elemental metals.

Furthermore, the electrode material exhibits excellent dielectric strength. Accordingly, Figure 5 illustrates a preferred embodiment of the present invention.

In those applications in which arc transfer or motion is desired, a directionally solidified material may be employed to facilitate this transfer. Such a material and configuration is illustrated in Figure 6 in which contact 30 is employed. Contact 30 exhibits directional solidification in a direction substantially parallel to arcing surface 21. Since arcs tend to move more readily along the direction of the elongated component, this directional solidification is thus useful in controlling the movement of the arc at the electrode contact surfaces, that is the arcing surfaces. This aspect of the present invention is also illustrated in Figure 3 above in which it is generally desirable to provide an outward radial motion to the plasma arc.

In general, the electrode material of the present invention comprises a mixture of at least two mutually insoluble metal components which have been directionally solidified. A material which is found to be particularly useful is a mixture of copper and vanadium in which the vanadium is present in an amount of between 1% to about 92% by weight. The copper-vanadium directionally solidified material of the present invention is more particularly described below and illustrated in Figures 7 and 8. Other mutually insoluble materials which may be directionally solidified in accordance with the present invention include aluminum and antimony, silver and copper, copper and bismuth, copper and lead, copper and lithium, aluminum and beryllium, silver and nickel and aluminum, aluminum and chromium, aluminum and titanium and aluminum and vanadium.Table I below provides a nonexclusive list of materials and compositional ranges employable in the present invention.

Table I Range Materials {By weight percent) Ag-Ni 1%98% Ni Al-Be 1 %98% Al Alibi (Any compositional range) Al-Cr 8099% Al Al-Sb 1%--82% Sb Al-Ti 63%99% Al Cu-Bi (Any compositional range) Cu-Li (Any compositional range) Cu-Pb (Any compositional range) Cu-V 19/0--92% V The process of directional solidification is itself well known in the metallurgical arts. However, it has not heretofore been used for the purpose of enhancing arc motion in vacuum contact materials. Furthermore, it has not heretofore been applied to the problem associated with arc stability at low arc current levels.In the directional solidification process, an ingot of the desired mixture is passed through a hot zone within which the material is locally melted and resolidifies as the ingot is passed through the hot zone in a gradual process. The directionalization of the resultant structure appears to rest on the affinity of like atoms for one another during resolidification. For the purposes of the present invention, the electrode material for use in the directional solidification process comprises two or more components, one of which is essentially insoluble in the remaining constituents, this constituent preferably being a metal of relatively high electrical conductivity. Again, copper and vanadium in particular, have been found to exhibit the desired insolubilities, boiling points and electrical conductivities.Furthermore, they have been found to be highly amendable to directional solidification processes. However, it has also been found by the instant inventor that an electrode contact comprising copper and vanadium, even in undirectionally solidified conditions, exhibits superior arc stability characteristics.

The amenability of the copper vanadium mixture for directional solidification is more particularly illustrated in Figures 7 and 8, both of which illustrate scanning electron micrographs of copper-vanadium. Figure 7 illustrates a longitudinal section of a directionally solidified copper-vanadium mixture. Figure 8 is identical to Figure 7 except that a cross-sectional view of the copper-vanadium mixture is illustrated. In the view shown, the copper shows as the lighter of the two materials present. Each scanning electron micrograph shown in Figures 7 and 8 has been prepared from the same specimen and the view is of surfaces which have been metallographically polished. Furthermore, in Figure 7 there are also shown a number of white patches which are, in fact, artifacts of the imaging process. Similar patches can also be seen to a lesser extent in Figure 8.Again, these are artifacts of the imaging process. For each micrograph, a dimension scale of 1 50 microns is provided for comparison. In Figures 7 and 8 it is apparent that the copper is present in the form of a plurality of columnar striations extending from one surface to the other in a vanadium matrix. It is thus seen that the phenomena of directional solidification extends throughout the entire material and is not merely a surface phenomenon. Accordingly, in addition to having boundaries between constituent materials at the electrode surface for stabilizing the arc, the directionally solidified materials of the present invention are made with a further property that fibers or dendritic columns of one component such as copper are joined along the contact axis of symmetry thus promoting electrical and thermal conduction from the contact surface.The material employed in Figure 7 and 8 is for a 50% vanadium and 50% copper (by weight) mixture.

Furthermore, the micrographs in Figures 7 and 8 were taken of surfaces which are essentially perpendicular to one another.

Accordingly, from the above it may be appreciated that the electrode material of the present invention enhances arc stability at low current levels, provides increased thermal conductance between arcing surface and the conductive base member, and, additionally, provides increased electrical conductivity in the same direction. Furthermore, it should also be appreciated that the present invention permits the use of directionally solidified electrode contact materials in a number of electrical circuit interruption devices in which arc transfer may be readily facilitated through the judicious choice of solidification direction. Lastly, it is also seen that the present invention significantly enhances the performance capabilities of vacuum arc circuit interrupters.

While the invention has been described in detail herein, in accord with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. Accordingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.

Claims (11)

Claims

1. An electrical contact for use in vacuum interruption devices comprising: a block of conductive material having at least one contact face, said material comprising a directionally solidified mixture selected from the group consisting of the following pairs of mutually-insoluble metal components: copper and vanadium with between about 1% and about 92% vanadium, copper and bismuth, copper and lithium, copper and lead, aluminum and titanium with between about 63% and about 99% aluminum, aluminum and antimony with between about 1% and about 82% antimony, aluminum and chromium with between about 80% and about 99% aluminum, aluminum and beryllium with between about 1% and about 98% aluminum, aluminum and bismuth, and silver and nickel with between about 1% and about 98% nickel, said percents being weight percents.

2. The electrode contact of claim 1 in which said electrode comprises a circular disk.

3. The electrode contact of claim 1 in which said electrode comprises an annular ring.

4. The electrode contact of claim 1 in which said electrode comprises a flat slab.

5. An electrode assembly comprising: a conductive base member; and the electrode of claim 1 affixed to a surface of said base member.

6. The assembly of claim 5 in which directional solidification is generally at right angles to said surface.

7. The assembly of claim 5 in which said directional solidification is substantially parallel to said surface.

8. An electrode contact for use in high current electrical circuit interruption devices comprising: a solidified mixture of copper and vanadium, said vanadium comprising between about 1% and about 92% by weight of said electrode contacts.

9. A material, particularly usable in an electrical contact, comprising a solidified mixture of copper and vanadium, said vanadium comprising between about 1% and about 92% by weight.

10. A material as claimed in claim 9 and substantially as hereinbefore described with reference to the foregoing examples.

11. A contact as claimed in claim 1 and substantially as hereinbefore described with reference to and as illustrated in the drawings.