BISMUTH GLASS COATED VARISTOR

Abstract

 The present disclosure relates to a method of applying a glass coating to a varistor 2. The method comprises providing a glass powder comprising bismuth, adhering particles of the glass powder to a side 4 of the varistor by means of a dry electrostatic technique, and heating the adhered glass particles at an elevated temperature during a time period sufficient to melt said glass particles to form a continuous glass coating on the varistor side.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method of applying a glass coating to a varistor.

BACKGROUND

[0002] Metal oxide (MO) varistor blocks are used in e.g. surge arrestors to protect electrical equipment from potentially harmful voltage surges. The MO-varistor blocks require a passivating, gas-tight glass layer on the rim of the cylindrical varistor body.

[0003] Glass powder is applied as an aqueous slurry by wet spray and a preheating of the MO-blocks in order to vaporise the slurry water. This application technology gives a low material yield of e.g. 30%. The rest of the material is scrapped as hazardous waste since it contains lead from the lead glass. Application by wet spray requires preparation of a slurry which is time consuming, and when it is prepared it is time sensitive. The slurry contains glass powder, deionized water, binder and dispersion agent.

[0004] US 3,959,543 discloses application of a glass anti-flashover collar having a thickness of about 3-5 mils (76-127 micrometers, µm) to a varistor disk of a surge arrester. A slurry of glass particles is applied to the outside of the disc, after which the disc is fired at a temperature of between 640-650°C for 30 minutes to fuse the glass particles to each other.

[0005] To reduce the temperature needed to form the anti-flashover coating, an organic polymer, optionally with a filler material, may be used instead of a glass coating. However, such an organic coating has to be thicker than a glass coating in order to act as an oxygen and flashover barrier.

[0006] US 4,559,167 discloses a metal oxide varistor which is coated with an organic polymer and a filler material such as glass particles, as an oxygen barrier. The organic polymer is mixed with the filler material and is applied to the side face of the uncoated active part of the varistor. The varistor is then heated to 120-160°C to cure the polymer forming a 0.5 mm thick coating. A problem with the organic polymer coating is that it is not completely gas tight but allows oxygen to leave the metal oxide varistor, thus deteriorating the properties of the varistor.

SUMMARY

[0007] It is an objective of the present invention to provide an improved way of providing a continuous glass coating to an outer side of a non-linear resistance element, herein called a varistor, to function as a flashover protection and/or gas (particularly oxygen) barrier.

[0008] The inventors have surprisingly realised that it is possible to adhere bismuth glass particles to a semiconductor such as a varistor by means of a dry electrostatic technique where glass particles are provided with an electrostatic charge and sprayed onto the varistor side where they adhere. Typically, the semiconductor is connected to ground to improve the adhering.

[0009] According to an aspect of the present invention, there is provided a method of applying a glass coating to a varistor. The method comprises providing a glass powder comprising bismuth, typically bismuth oxide, adhering particles of the glass powder to a side of the varistor by means of a dry electrostatic technique, and heating the adhered glass particles at an elevated temperature during a time period sufficient to melt said glass particles to form a continuous glass coating on the varistor side.

[0010] According to another aspect of the present invention, there is provided a varistor wherein a side of the varistor is coated with a continuous glass layer comprising bismuth, typically bismuth oxide.

[0011] According to another aspect of the present invention, there is provided a surge arrestor comprising a plurality of varistors of the present disclosure stacked on top of each other.

[0012] By means of using the glass comprising bismuth, the dry electrostatic technique may be used. Also, the temperature for melting the adhered glass particles to a continuous glass coating/layer may be reduced. Further, lead-free glass may be used, replacing lead with bismuth, reducing environmental lead pollution. The dry electrostatic technique may provide a more even thickness of the continuous glass coating, allowing the use of less glass while still ensuring a desired minimum thickness of the continuous glass coating. Also, glass particles which did not adhere to the semiconductor can be reused easily and do not have to be discarded, further reducing the amount of glass required as well as the impact on the environment.

[0013] The bismuth is typically in the form of bismuth oxide, e.g. Bi₂O₃, but may additionally or alternatively be in form of other bismuth containing compound(s).

[0014] It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

[0015] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of "first", "second" etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Embodiments will be described, by way of example, with reference to the accompanying drawings, in which:

Fig 1 is a schematic front view of an embodiment of a surge arrestor, in accordance with the present invention.

Fig 2 is a schematic flow chart of an embodiment of a method of the present invention.

DETAILED DESCRIPTION

[0017] Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown. However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

[0018] Figure 1 schematically illustrates an embodiment of a surge arrestor 1, in accordance with the present invention. The surge arrestor 1 comprises a plurality of non-linear resistance elements 2, herein called varistors, in the form of discs stacked on top of each other. The varistors 2 may e.g. be metal oxide (MO) varistors, for instance having zink oxide as a principal constituent. The varistor 1 is configured to protect electrical equipment against voltage or current surges by allowing a current to pass through the discs, in a direction perpendicular to the plane of the discs, when the voltage between the two electrical conductors 3a and 3b connected to the surge arrestor exceeds a predetermined threshold. The voltage rating for each varistor disc 2 may e.g. be at least 1, 2 or 3 kilovolts (kV), and as high as 10 kV or even at least 10 kV.

[0019] The one or more varistor discs may thus form a cylinder, e.g. a circular cylinder, of the surge arrestor 1. To prevent flashover and/or to prevent depletion of especially oxygen from the varistors 2, the lateral area (i.e. the sides 4) of the varistor(s) is coated with a continuous glass coating formed by glass particles which have been fused together under elevated temperature, in accordance with the present invention.

[0020] In accordance with the present invention, a dry electrostatic application technique is used instead of the conventional wet spray technique to adhere glass particles to the varistor side 4. The electrostatic technique makes it possible to adhere the glass particles to the side 4 by applying a charge to the dry particles and spray them onto the surface of the varistor side when the varistor 2 is connected to ground. The dry glass powder which is sprayed but does not adhere to the varistor side may be gathered automatically and recycled for future use.

[0021] No water, binder or dispersion agent is needed.

[0022] Since the material yield is by means of the recycling much higher (e.g. as high as above 90% or 95%), the technique facilitates the use of more expensive lead free glass. This also improves the working condition for the operators and at the same time gives benefits to the environment. Additionally, the electrostatic equipment is easy to clean and maintain in comparison to the wet spray equipment.

[0023] Figure 2 is a schematic flow chart of an embodiment of the method for applying a glass coating to a varistor 2, of the present invention. A glass powder comprising bismuth oxide is provided S₁. Then, particles of the glass powder are adhered S₂ to a side 4 of the varistor 2 by means of a dry electrostatic technique. Then, the adhered S₂ glass particles are heated at an elevated temperature during a time period sufficient to melt the glass particles to form a continuous glass coating on the varistor side 4.

[0024] In some embodiments of the present invention, the glass powder comprises at least 40%, e.g. at least 50%, by weight of bismuth oxide. The high amount of bismuth lowers the melting point of the glass particles and allows for treating the varistor 2 (having glass particles adhered to its side 4) at a lower temperature and/or during a shorter period of time in order to fuse the particles with each other to form a continuous glass coating. The use of high amounts of bismuth also allows for a reduction or elimination of lead in the glass.

[0025] In some embodiments of the present invention, the elevated temperature, at which the adhered glass particles are heated S₃, is below 600°C, e.g. below 550°C. The presence of bismuth in the glass lowers the melting point of the glass and allows for a lower elevated temperature to be used, thus reducing the risk of damaging the varistor with the elevated temperature treatment.

[0026] The time period during which the adhered glass particles are heated may be up to 60 minutes, or even up to 90 minutes. However, in order to speed up the coating process and reduce strain on the varistor, the time period is preferably shorter. In some embodiments of the present invention, the time period, during which the adhered glass particles are heated S₃, is less than 20 minutes, e.g. at most 15 minutes. The heating time period is then typically followed by a time period of cooling down the coated varistor. The presence of bismuth in the glass lowers the melting point of the glass and allows for heating S₅ at the elevated temperature during a shorter period of time, thus reducing the risk of damaging the varistor with the elevated temperature treatment.

[0027] In some embodiments of the present invention, the glass particles (of the glass powder) have a particle size distribution with a mass-median-diameter, D₅₀, within the range of 4-10, e.g. 5-8 such as 7-8, 5-7 or 5-6, micrometers. This particle size allows for suitable coating with an electrostatic coating equipment while also allowing the formation of a uniform glass coating after a relatively short time period of heating S₃.

[0028] By means of the electrostatic adhering S₂ of the glass particles, a more uniform glass coating may be obtained, compared with the wet spraying techniques. A more uniform thickness of the glass coating implies that less glass may be needed to form the coating while still ensuring a suitable minimum thickness of the glass coating to prevent flashover and/or oxygen depletion.

[0029] In some embodiments of the present invention, the glass coating has an average thickness of less than 150 micrometers, preferably less than 100 micrometers, e.g. less than 80, less than 60 or less than 50 micrometers.

[0030] Additionally or alternatively, in some embodiments of the present invention, the glass coating has a thickness throughout the coating which is less than 100 micrometers, e.g. less than 80, less than 60 or less than 50 micrometers. That the coating has a thickness throughout the coating which is less than a specified thickness implies that the coating at no point of its extension over the varistor side 4 exceeds the specified thickness.

[0031] Additionally or alternatively, in some embodiments of the present invention, the glass coating has a thickness which varies less than 50 micrometers, e.g. less than 40, 30, 20 or 10 micrometers, throughout the coating. That the coating thickness varies less than a specified amount throughout the coating implies that the difference between the largest thickness of the coating and the smallest thickness of the coating, at any points of the extension of the coating over the varistor side 4, is less than the specified amount.

[0032] The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.

Claims

1. A method of applying a glass coating to a varistor (2), the method comprising:

providing (S₁) a glass powder comprising bismuth;

adhering (S₂) particles of the glass powder to a side (4) of the varistor (2) by means of a dry electrostatic technique; and

heating (S₃) the adhered (S₂) glass particles at an elevated temperature during a time period sufficient to melt said glass particles to form a continuous glass coating on the varistor side (4).

2. The method of claim 1, wherein the glass powder comprises at least 40%, e.g. at least 50%, by weight of bismuth oxide.

3. The method of claim 1 or 2, wherein the elevated temperature is below 600°C, e.g. below 550°C.

4. The method of any preceding claim, wherein the time period is less than 20 minutes, e.g. at most 15 minutes.

5. The method of any preceding claim, wherein the glass particles have a particle size distribution with a mass-median-diameter, D₅₀, within the range of 4-10 micrometers.

6. The method of any preceding claim, wherein the glass coating has an average thickness of less than 100 micrometers, e.g. less than 80, less than 60 or less than 50 micrometers.

7. The method of any preceding claim, wherein the glass coating has a thickness throughout the coating which is less than 100 micrometers, e.g. less than 80, less than 60 or less than 50 micrometers.

8. The method of any preceding claim, wherein the glass coating has a thickness which varies less than 50 micrometers, e.g. less than 40, 30, 20 or 10 micrometers, throughout the coating.

9. A varistor (2) wherein a side (4) of the varistor is coated with a continuous glass layer comprising bismuth.

10. The varistor of claim 9, wherein the glass coating comprises at least 40%, e.g. at least 50%, by weight of bismuth oxide.

11. The varistor of claim 9 or 10, wherein the glass coating has an average thickness of less than 100 micrometers, e.g. less than 75, less than 60 or less than 50 micrometers.

12. The varistor of any claim 9-11, wherein the glass coating has a thickness throughout the coating which is less than 100 micrometers, e.g. less than 75, less than 60 or less than 50 micrometers.

13. The varistor of any claim 9-12, wherein the glass coating has a thickness which varies less than 50 micrometers, e.g. less than 40, 30, 20 or 10 micrometers, throughout the coating.

14. The varistor of any claim 9-13, wherein the varistor (2) has a voltage rating of at least 1 kilovolt, e.g. at least 2 or at least 3 kilovolts.

15. A surge arrestor (1) comprising a plurality of varistors (2) of any claim 9-14 stacked on top of each other.

Drawing