Patent Grant
11967471
The present application is based on PCT filing PCT/JP2019/033417, filed Aug. 27, 2019, the entire contents of which are incorporated herein by reference.
The present disclosure relates to a vacuum switch tube for a vacuum circuit breaker, which belongs among high voltage power distribution equipment, an electrical contact for the vacuum switch tube, and a manufacturing method for the electrical contact.
A vacuum circuit breaker installed in high voltage power distribution equipment is used to break a current in the event of a failure or an anomaly in the high voltage power distribution equipment. The vacuum circuit breaker includes a vacuum switch tube that has a function to break current. The vacuum switch tube has a structure in which a fixed electrode and a movable electrode are arranged coaxially so as to face each other in an insulated container kept in high vacuum.
When an overload current or a short-circuit current occurs in the power distribution equipment, the contact between the electrodes is instantly opened by moving the movable electrode away from the fixed electrode to break the current. However, since an arc is generated between the electrodes, the current is not broken instantly after the contact is opened. When an AC current is broken, the arc is weakened as the AC current decreases, and the break is established when the arc disappears. A phenomenon in which the current is momentarily broken before the AC current becomes zero is called current chopping.
During the current chopping, a large surge voltage called a switching surge occurs. If an apparatus connected to the power distribution equipment has capacitive or inductive nature, the apparatus connected thereto may be damaged by a large surge voltage that is to be generated. In order to lower the surge voltage, it is necessary to reduce a chopping current, which is the current flowing at the time when the current chopping occurs. Reduction of the chopping current can be achieved by sustaining the arc generated between the electrodes during the contact opening, nearly to the zero point of the AC current.
The sustainment of the arc depends on the number of particles present in the vacuum. Therefore, in order to sustain the arc, it is necessary to supply particles into the vacuum during the current chopping. There are two types of particles to be supplied: metal particles and thermions. As a conventional electrical contact material having low chopping current characteristics, a mixture of Ag, being a conductive component, with a high melting point metal, or with a carbide of a high melting point metal, such as WC, is selected. This is because electrode heating caused by the generated arc promotes evaporation of Ag being a conductive component and thermionic emission of the high melting point metal or the carbide of the high melting point metal, and thus the arc is sustained.
According to Richardson Dashman's equation, which shows a current density for the thermionic emission capacity, it is known that the thermionic emission capacity depends on the work function and the temperature of the material. In particular, the contribution of the temperature is large. Therefore, the high melting point metal and the carbide of the high melting point metal are widely used owing to their high melting points. From the above perspective, a vacuum switch tube using an Ag-WC electrical contact, which exhibits excellent low chopping current characteristics, has been developed and put into practical use.
In the conventional vacuum switch tube, stable low chopping current characteristics are obtained by adding, for example, Te or Se to an electrical contact material using Cu instead of Ag, as a conductive component, from a low-cost perspective (see, for example, Patent Document 1 and Patent Document 2). This is because Te and Se are low boiling point metals whose boiling points are very low among metals, and thus the electrode heating by arc irradiation causes a large amount of the low boiling point metals to evaporate, thereby making it possible to sustain the arc.
Although the low chopping current characteristics can be achieved by adding a low boiling point metal, the selective evaporation of the low boiling point metal, however, can also be regarded as material consumption of the electrical contact.
Therefore, as the number of times of opening and closing of the contact increases, the low boiling point metal is consumed, the amount of the metal vapor supplied to the space between the electrical contacts is decreased, and, as a result, the low chopping current characteristics are deteriorated. Therefore, in order to suppress the deterioration of the low chopping current characteristics, it is conceivable to increase the amount of the low boiling point metal to be added. However, if Te, which is the low boiling point metal, is added excessively to the electrical contact, Cu2Te, which is an intermetallic compound of Te and Cu, is generated, and thus the electrical contact becomes fragile. In view of this, by adding an appropriate amount of Mn, the mechanical strength of the electrical contact is secured (see, for example, Patent Document 3).
However, whereas the mechanical strength is secured, Mn and Cu forms solid solution with each other, and thus the conductivity of the electrical contact decreases, which causes a problem that the temperature of the vacuum switch tube to which the electrical contact is applied rises when the vacuum switch tube is energized.
The present disclosure is made to solve the problem described above and an object is to provide an electrical contact in which a low boiling point metal is added and both mechanical strength and conductivity are secured at the same time without using Mn.
An electrical contact according to the present disclosure includes a base material made of Cu, particles of a high melting point substance dispersed in the base material, the particles being made of at least one of a high melting point metal or a carbide of the high melting point metal, and Te and Ti dispersed in the base material, wherein, the Te of 3.5 to 14.5 mass % is added where the total is 100 mass %, and Ti/Te is 0.12 to 0.38, and at least part of the Te and at least part of the Ti form an intermetallic compound of the Te and the Ti.
A vacuum switch tube according to the present disclosure comprises an electrical contact that includes, a base material made of Cu, particles of a high melting point substance dispersed in the base material, the particles being made of at least one of a high melting point metal or a carbide of the high melting point metal, and Te and Ti dispersed in the base material, wherein, the Te of 3.5 to 14.5 mass % is added where the total is 100 mass %, and Ti/Te is 0.12 to 0.38, and at least part of the Te and at least part of the Ti form an intermetallic compound of the Te and the Ti.
According to the electrical contact of the present disclosure, regarding the composition of Ti and Te, Te is 3.5 to 14.5 mass %, and Ti/Te is 0.12 to 0.38. As a result, both the mechanical strength and the conductivity can be secured without adding Mn.
According to the vacuum switch tube with the electrical contact of the present disclosure, regarding the composition of Ti and Te, Te has 3.5 to 14.5 mass %, and Ti/Te is 0.12 to 0.38. As a result, both the mechanical strength and the conductivity can be secured without adding Mn.
According to the manufacturing method for the electrical contact of the present disclosure, regarding the mixing ratio of a Ti powder and a Te powder, the Te powder of 3.5 to 14.5 mass % is mixed, and Ti/Te is 0.12 to 0.38. As a result, it is possible to manufacture an electrical contact in which the mechanical strength and the conductivity are secured.
A bellows 12 is attached to the movable electrode rod 7. The bellows 12 allows the movable electrode 9 to move in the axial direction while keeping the inside of the breaking chamber 2 vacuum and airtight. The movement of the movable electrode 9 in the axial direction causes the movable electrode 9 to come into contact with or separate away from the fixed electrode 8. On the top of the bellows 12, a metal arc shield for bellows 13 is provided. The arc shield for bellows 13 prevents arc steam from adhering to the bellows 12. Further, a metal arc shield 14 for the insulated container is provided in the breaking chamber 2 so as to cover the fixed electrode 8 and the movable electrode 9. The arc shield 14 for the insulated container prevents arc steam from adhering to the inner wall of the insulated container 3.
Generally, the fixed electrode 8 and the movable electrode 9 as well as the fixed electrical contact 10 and the movable electrical contact 11 have a disc shape. In the following description, it is assumed that the electrical contacts in the present embodiment are disc shaped.
First, a manufacturing method for the electrical contact according to the present embodiment will be described. The electrical contact of the present embodiment is manufactured through the following steps, a step of mixing raw material powders and pressing the mixed powder using a predetermined press die to form a molded body, a step of calcining the molded body to obtain a sintered body, a step of infiltrating Cu into the obtained sintered body to obtain an infiltrated body, and a step of machining the obtained infiltrated body into a predetermined shape to obtain the electrical contact. Hereinafter, the manufacturing method for the electrical contact according to the present embodiment will be described in detail.
In the step of mixing raw material powders and pressing the mixed powder using a predetermined press die to form a molded body, a Cu powder as a base material for a conductive component, a Cr powder as a high melting point metal to be an arc resistant component, and a Ti powder and a Te powder as low boiling point metals for sustaining the arc are mixed, and then the resulting mixed powder is pressed and formed into a shape using a pressing machine, thereby obtaining a Cu—Cr—Ti—Te molded body. When the mass of the mixed powder is 100 mass % (hereinafter referred to as wt %), the powders are mixed such that the mass of the Ti powder is 20 to 80 wt %, the mass of the Te powder is 3.5 to 14.5 wt %, and the mass of the Cu powder and the Cr powder is the rest. In this case, Ti/Te is 0.12 to 0.38 in mass ratio. The experiments conducted to determine the value for each of the above substances will be described later.
When a powder such as a Cr particle that is relatively hard and is not deformed plastically becomes finer, the specific surface area of the powder is larger, which makes it difficult to densify the powder in the case of press molding because there are many voids near the contact points between the powders. If the particle diameter is minute, the press molding pressure for obtaining a molded body having a desired density becomes too high, and cracks may occur during the press molding. Therefore, it is desirable that the average particle diameter of the Cr powder is larger than 0.1 μm. In addition, however, when the average particle diameter of the Cr powder is large, variation occurs at the time of breaking, and thus the low chopping current characteristics may become unstable. Therefore, it is desirable that the average particle diameter of the Cr powder should be 120 μm or smaller.
For the average particle diameter of the raw material powders, for example, the average particle diameter in the particle size distribution measured by a laser-diffraction-type particle-size distribution measurement device is adopted.
In the step of calcining the molded body to obtain the sintered body, the Cu—Cr—Ti—Te molded body is sintered at 500 to 950 degrees Celsius under hydrogen atmosphere or under vacuum of 1×10−5 Pa or lower. The sintering temperature should be at least 30 degrees Celsius lower than 988 degrees Celsius, which is the boiling point of Te.
In the step of obtaining the infiltrated body by infiltrating Cu into the sintered body, a Cu disk or a Cu square plate with a size equal to or smaller than the sintered body is placed directly under the sintered body under hydrogen atmosphere or under vacuum of 1×10−5 Pa or lower, and then the infiltration is performed at a temperature of 1083 degrees Celsius, which is the melting point of Cu, or higher, and not higher than 1140 degrees Celsius. When the temperature during the infiltration is 1140 degrees Celsius or higher, the following problems may occur: Te sublimation, which starts when saturated vapor pressure among the low boiling point metals existing in the sintered body becomes high, causes the sintered body to expand, and thus a dense electrical contact cannot be obtained; or Cu in the sintered body melts, and thus the shape of the sintered body collapses.
As for the arrangement, the Cu disk or the Cu square plate may be under the sintered body and vice versa. Alternatively, the sintered body may be sandwiched between two of the Cu disks from top and bottom. In the step of machining the infiltrated body into a desired shape to obtain an electrical contact, the contact material is ground until it has a thickness and a diameter required in design as the fixed electrical contact or the movable electrical contact for the vacuum switch tube. Finally, an electrical contact can be obtained by applying taper processing or surface polishing to the end portion.
In the step of machining the infiltrated body into a predetermined shape to obtain an electrical contact, the contact material is ground until it has a thickness and a diameter required in design as the fixed electrical contact or the movable electrical contact for the vacuum switch tube. Finally, an electrical contact can be obtained by applying taper processing or surface polishing to the end portion. Next, Working Examples and Comparative Examples will be given and described in more detail.
In Working Example 1, a Cu powder with an average particle diameter of 10 μm, a Cr powder with an average particle diameter of 40 μm, a Te powder with an average particle diameter 40 μm, and a Ti powder with an average particle diameter of 30 μm were mixed for 30 minutes or longer using a ball mill or a V-type powder mixer or the like to make a uniformly mixed powder. The mixed powder was placed in a steel die having an inner diameter φ of 23 mm and was compression molded at a pressure of 20 to 100 MPa using a hydraulic press machine to form a molded body having a thickness of 5 mm.
The obtained molded body was sintered at 900 degrees Celsius for two hours under hydrogen atmosphere to obtain a sintered body. Then, the sintered body was placed on a Cu disk with a thickness of about 2 mm and a diameter φ of 20 mm and the infiltration was performed at 1110 degrees Celsius for two hours under hydrogen atmosphere to obtain an electrical contact of Working Example 1. When the infiltrant Cu was not melted, the temperature was raised in steps by 10 degrees Celsius and the infiltration treatment was performed again. The temperature at which Cu melts during the infiltration treatment is defined as an infiltration temperature. The composition of the electrical contact obtained in Working Example 1 is shown in the table in
In Working Examples 2 to 13, electrical contacts were fabricated in the same steps as in Working Example 1 and the effect of Ti concentration was verified. Note that the composition ratio in the electrical contact was changed by adjusting the mass ratio of each powder when mixed powders were made. In Working Examples 2 to 4, with Te concentration fixed at 3.5 wt %, the compositions of the electrical contacts are shown in the table in
In Comparative Examples 1 to 8, electrical contacts were fabricated in the same steps as in Working Example 1 and the effect of Ti concentration was verified. Note that the mass ratio of each powder was adjusted to change the composition ratios of the electrical contacts when mixed powders were made. Te concentrations were fixed as follows: at 3.5 wt % in Comparative Examples 1 and 2; at 2.5 wt % in Comparative Example 3; at 9 wt % in Comparative Examples 4 and 5; at 14.5 wt % in Comparative Example 6 and 7; and at 15.5 wt % in Comparative Example 8. The compositions of the electrical contacts in Comparative Examples 1 to 3 are shown in
In the present embodiment, Working Examples 1 to 13 and Comparative Examples 1 to 8 were evaluated from the perspectives of manufacturability, low chopping current characteristics, and breaking characteristics. In addition, whether their conductivities are satisfactory or not was verified. Description will be made in detail in due order.
First, a description will be made on the manufacturability. The manufacturability is evaluated in terms of whether electrical contacts can be manufactured. There are two evaluation points for the manufacturability of electrical contacts. The first evaluation point is whether the density ratio of an electrical contact exceeds 95%, which is a usable density ratio as an electrical contact. In the present embodiment, the density ratio is defined as the ratio of the density of a prototype electrical contact to the theoretical density calculated from the compounding composition. If the density ratio is 95% or lower, there may be voids in the electrical contact. In the brazing process during the vacuum switch tube assembly, these voids in the electrical contact absorb the brazing material by capillarity, thereby possibly increasing the frequency of brazing defects. Therefore, in practice, a density ratio exceeding 95% is desirable, and an electrical contact whose density ratio does not reach 95% is judged to be defective.
The second evaluation point is that the electrical contact fabricated by the infiltration treatment has a mechanical strength to an extent to allow the electrical contact to be cut out into a predetermined shape by machining. An electrical contact with less mechanical strength is regarded as a good electrical contact since the weldability is fine. However, if the mechanical strength is excessively low, cracks occur during machining, thereby making the machining difficult. Therefore, a mechanical strength at least at a level not to cause cracking during the machining is required. In the present embodiment, the case where no crack occurred during machining was judged as a pass, and the case where cracks occurred was judged as a failure.
Next, the evaluation of the low chopping current characteristics and the breaking characteristics will be described. The evaluation of the low chopping current characteristics and the breaking characteristics is based on the results of chopping current tests and breaking current tests using, as test contacts, the electrical contacts obtained in each of Working Examples and Comparative Examples. The test contacts with a thickness of 3 mm and a diameter φ of 20 mm were fabricated by machining the electrical contacts with a thickness 5 mm and a diameter φ of 23 mm obtained in each of Working Examples and Comparative Examples. Further, taper processing for making an about 15 degrees taper angle with respect to the surface of each test contact was performed in the end portion from the edge up to 2 mm inside thereof. Two test contacts after the taper processing were fabricated, and a vacuum switch tube for evaluation was assembled using one of them for the fixed contact and the other for the movable contact. Using the vacuum switch tubes for evaluation prepared according to each of Working Examples and Comparative Examples, the chopping current tests and the breaking current tests were conducted to evaluate the low chopping current characteristics and the breaking characteristics.
In the chopping current tests, a circuit was prepared, in which a 20Ω resistor and a vacuum switch tube for evaluation were connected in series, and a 10 A current was caused to flow the circuit using an AC 200 V power supply. When the contacts of the vacuum switch tube for evaluation were opened from their closed state, the current just before the arc current becomes zero was measured. The measured current was defined as chopping current. The chopping current tests were conducted 1000 times using the same vacuum switch tube for evaluation, and the average value of the measured chopping current values was registered as the chopping current value of each of Working Examples and Comparative Examples. Note that the chopping current value must be 1 A or lower in order to avoid the damage to a connected electric apparatus due to the increase in surge voltage that occurs at the time of breaking. In these tests, if the chopping current value was 1 A or lower, the test was passed.
In the break tests, a circuit was prepared, in which a thyristor and a vacuum switch tube for evaluation were connected in series, and an energizing current generated using the discharge from a capacitor bank was caused to flow the circuit with the contacts of the vacuum switch tube for evaluation closed. Then, when the contacts of the vacuum switch tube for evaluation were opened, the pass/fail of the test was judged based on whether or not the current can be normally broken. The capacitor bank was charged by an external power source. The break tests were conducted by incrementing the energizing current by 1 kA, starting from 2 kA, and when the break test was finished at 4 kA, the pass/fail of the break test was judged. The pass of the break test means that neither recurrence nor continuation of the arc occurs when the contacts of the vacuum switch tube for evaluation are opened.
Next, the evaluation of the conductivity of the fabricated electrical contacts will be described. Since the electrical contacts are current-carrying members, high conductivity is required. For the standard of the electrical conductivity, the international annealed copper standard (IACS), in which the volume resistivity of internationally-adopted annealed standard copper: 1.7241×10−2 μΩm is defined as 100% IACS, is used. In the present embodiment, the conductivity of an Ag-WC contact which is widely used as a low surge contact was set as the criterion, and if the conductivity was higher than 20% IACS, the test was passed.
The compositional distribution of the internal structure of the Ti—Te intermetallic-compound particles 17 was measured using the compositional analysis function, enabled by wavelength-dispersive X-ray spectroscopy or energy-dispersive X-ray spectroscopy, attached to the SEM. From the measurement result, it was found that the ratio of the number of atoms of Ti and Te was 1:2 or 3:4.
In order to maintain the mechanical strength that enables the machining of the electrical contact, Ti was added in an amount that would not lead to formation of Cu2Te, which is a cause of the fragility of the electrical contact. In order to make the Ti—Te intermetallic-compound particles 17, the ratio of the number of atoms of Ti:Te=1:2 or 3:4 is desired, and then Ti/Te by the ratio of the number of atoms should be 0.5 or higher. Therefore, it can be said that Ti/Te by mass ratio should be 0.17 or higher. In the following, description will be made using mass ratio.
The results of Working Examples 1 to 13 and Comparative Examples 1 to 8 are described in due order. First, Working Examples 1 to 4 and Comparative Examples 1 to 2 will be described referring to
From the above results, it was found that the value of Ti/Te in the formation of the TiTe2 intermetallic compound or the Ti3Te4 intermetallic compound needs to be at least 0.12 or more, instead of at least 0.17 or more. From the Ti—Te state diagram in
In a cross-sectional analysis of Comparative Example 1 with Ti/Te of 0.09 or lower, a large number of Cu2Te particles, which is a cause of fragility, was detected. It is considered that this is because the amount of Ti for making the Ti—Te intermetallic compound is insufficient. In Working Examples 1 to 4, satisfactory results were able to be obtained also in the current chopping tests and the break tests. In addition, in Working Examples 1 to 4, the Ti component dissolved in Cu was cross-sectionally analyzed, and it was confirmed that the Ti concentration thereof was 1 wt % or lower, and the conductivity was 20% IACS or higher. Therefore, it can be said that the electrical contacts of Working Examples 1 to 4 according to the present embodiment have improved conductivities, and thus can suppress the heat generation when energized.
Next, using
The second reason is the increase in infiltration temperature due to the addition of Ti.
In Comparative Example 2, in order to sufficiently melt the infiltrant Cu with a raised melting point, the process was performed at a heating temperature of 1150 degrees Celsius. However, since also the Cu that was contained in the molded body melted, so that it was difficult to maintain the shape of the electrical contact. In addition, Te exists as an intermetallic compound with Ti, so that the melting point of Te is raised. However, the saturated vapor pressure of Te is very low, and therefore, if it is infiltrated at high temperature, part of Te may evaporate. Since evaporation of Te leads to a decrease in the density of the molded body, it was judged that the manufacture by processing it at 1150 degrees Celsius or higher is impossible. From the above, it was found that the addition of Ti in an amount that does not form Cu2Te, which is a cause of fragility, is effective, whereas the addition of Ti more than necessary impairs the manufacturability of the electrical contact.
Next, Comparative Example 3 will be described. Comparative Example 3 has about the same Ti/Te as Working Example 3, while having Te concentration of 2.5 wt %. In Comparative Example 3, the infiltrated body has a density ratio of 95% or higher, and the Ti—Te intermetallic compound was confirmed to exist from the cross-sectional analysis. The result on the evaluation of the low chopping current characteristics obtained by using the vacuum switch tube for evaluation showed that the chopping current was greater than 1 A to fail the test. It is considered that this is because Te, which is a low boiling point metal for securing low surge characteristics, is insufficient. Therefore, it is considered that Te concentration of 3.5 wt % or higher is required to obtain the stable low chopping current characteristics.
From the Ti and Te state diagram shown in
Next, the results of Working Examples 5 to 13 and Comparative Examples 4 to 8 will be described. As shown in
In Working Examples 5 to 9, the infiltrated bodies with a density ratio of 95% or higher were able to be fabricated, and the machining was able to be performed without a problem. Also, the conductivity is 20% ACS, and both the low chopping current characteristics and the breaking characteristics were satisfactory. In Comparative Example 4, in which Ti/Te is lower than 0.09, the infiltrant Cu melted at 1110 degrees Celsius, whereas a large number of Cu2Te particles, which is a cause of fragility, was detected in the cross-sectional analysis and also cracks occurred during the machining. In Comparative Example 5, the infiltration was not sufficient at an infiltration temperature up to 1130 degrees Celsius. The cause of this is believed to be similar to that in Comparative Example 2. To summarize the results shown in
Next, the results of Working Examples 10 to 13 and Comparative Examples 6 to 8 will be described. As shown in
Next, in order to examine the content of Cr, which is an arc resistant component, Working Examples 14 to 16 and Comparative Examples 9 to 10 were tested. The table in
In Working Examples 14 to 16, the Ti—Te intermetallic compound was formed and the machining was able to be performed. In addition, in the evaluation of the low chopping current characteristics and the breaking characteristics, satisfactory results were obtained in common. There was a correlation between the Cr amount and the conductivity, in which the smaller the Cr amount is, the higher the conductivity tends to be. In
In
Next, Working Examples 17 to 19 and Comparative Examples 11 to 12 were tested to examine the practical range of a Cr particle diameter. The table in
In Working Example 17, in which the Cr particle diameter is 1 μm, the press molding was able to be performed. The press molding was able to be performed also in Working Examples 18 to 19. In Comparative Example 11, in which the Cr particle diameter is 0.1 μm, cracks occurred on a side of the molded body. It is considered that this is because the pressure at the time of molding becomes high in order to obtain a predetermined density of the molded body. In Comparative Example 12, in which the Cr particle diameter is 150 μm, the break tests were failed occasionally. It is considered that this is because if the particle diameter of the particles of Cr, which is a high melting point substance, is 150 μm or larger, the contact surface no longer has an even structure, and, as a result, the generated arc stays on the Cr particles of the high melting point substance. To summarize the results shown in
Further, Working Examples 20 to 22 were tested with the arc resistant component changed, for example, to W, Cr3C2, and WC, in which Cr3C2 is Cr carbide and WC is W carbide.
As shown in the results of
In the present embodiment, the electrical contact that can maintain high conductivity was fabricated by using Ti, instead of Mn, as an additive for maintaining the mechanical strength. From the test results of Working Examples and Comparative Examples above, it can be said that by adjusting Te concentration to 3.5 to 14.5 wt % and Ti/Te to 0.12 to 0.38, part of Ti forms TiTe2 or Ti3Te4. As a result, Cu2Te, which is a cause of fragility, is not formed, and thus the mechanical strength can be maintained. In addition, the electrical contact according to the present embodiment can satisfy the low chopping current characteristics and the breaking characteristics. That is, it is possible to fabricate an electrical contact that satisfies the breaking performance, the low surge characteristic ability, and the energizing performance for a large current.
The Cr concentration and the particle diameter can change depending on other conditions and the like. Therefore, as long as the Cr particle diameter is in a range that produces the effects described in the disclosure, the range of the Cr particle diameter is not limited to the range larger than 0.1 μm and 120 μm or smaller, obtained from Working Examples 1 to 19 and Comparative Examples 1 to 12.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/033417 | 8/27/2019 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2021/038706 | 3/4/2021 | WO | A |
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| Number | Date | Country | |
|---|---|---|---|
| 20220254577 A1 | Aug 2022 | US |
