EP0403855B1 - Insulating spacer - Google Patents

Description

INSULATING SPACER BACKGROUND OF THE INVENTION Field of the Invention
• This invention relates to an insulating spacer which is disposed between two members, such as electrodes, differing in electric potential in an electric apparatus, equipment, etc., to maintain an insulation distance between the two members.
Description of the Prior Art
• Figure 1 shows a sectional view of an insulating spacer according to the prior art. In the figure, reference numbers 1 and 2 denote a higher- and a lower-potential electrode, as two members differing in electric potential, and reference number 3 denotes platelike insulating barriers disposed between the electrodes 1 and 2 to prevent flash over between the electrodes. Reference number 4 denotes unit spacers made of an insulating solid material and having a bar shape with a tetragonal cross section, three such unit spacers being disposed between the higher- and lower- potential electrodes 1 and 2 to maintain an insulation distance between the electrodes and to clamp each of the insulating barriers 3 between the unit spacers 4, thereby maintaining the barriers in position.
• The unit spacers 4 and insulating barriers 3 in practical use in an electric apparatus (not shown) or the like have dispersions of their dimensions or ruggedness of their surfaces. Therefore, it is not always the case that the space between the electrodes 1 and 2, in which the unit spacers 4 are disposed, is filled completely with the insulating solid material as shown in Figure 1. Namely, a gap 5 may in some cases be generated in the space, as for instance illustrated in Figure 2. The gap 5 is filled with an ambient insulating medium such as a gas (air, sulfur hexafluoride, etc.) and an insulating oil (neither shown). Assuming that the specific dielectric constant of the insulating medium in the gap 5 is ε₁, the specific dielectric constants of the insulating barriers 3 and the unit spacers 4 are equally ε₂, the length of the gap 5 generated in the space between the electrodes 1 and 2 is d₁, while the total dimension of solid insulator portions is d₂, and the potential difference between the electrodes 1 and 2 is V, then the electric field strength Eg in the gap 5 is $$\text{Eg =}\frac{\text{ε₂V}}{\text{ε₂d₁ + ε₁d₂}}$$

  

  
  On the other hand, the average electric field strength E₀ between the electrodes 1 and 2 is $$\text{E₀ =}\frac{\text{V}}{\text{d₁ + d₂}}$$

  

  
  In general, an insulating solid material (inclusive of one which is impregnated with an insulating medium) in most cases has a higher specific dielectric constant than that of an insulating medium. By way of example, here, a case where ε₁ = 1 and ε₂ = 3 will be dealt with. When d₁/d₂ is varied, the ratio Eg/E₀ calculated from the equations (1) and (2) and taken as field concentration factor is varied as represented by the graph shown in Figure 3. When the length d₁ of the gap 5 is small, the field concentration factor is 3 at maximum, that is, the field strength Eg in the gap 5 reaches 3 times the average field strength E₀. Thus, the portion of the gap 5 is exposed to very severe conditions on an insulation basis and, if Eg exceeds the dielectric strength of the insulating medium in that portion, a partial discharge might result. Therefore, careful consideration should be given to the field strength Eg in the gap 5 in designing the electric apparatus. While the above description has been based on the case of ε₂/ε₁ = 3, the field concentration factor Eg/E₀ will be further greater where the ratio ε₂/ε₁ is more than 3, so that special care should be taken of selection of the combination of the insulating medium with the solid insulating material.
• The conventional insulating spacers, constructed as above, have had the possibility of a gap being generated to cause a local concentration of electric field on the gap. The conventional insulating spacers have therefore been limited in selection of the insulating solid material, constituting the insulating spacers, and the material for the insulating medium surrounding the spacers. In some cases, it has been necessary to take such countermeasure as enlarging the distance between the electrodes or the like members to lower the average field strength therebetween.
SUMMARY OF THE INVENTION
• This invention contemplates overcoming the above-mentioned drawbacks of the prior art.
• It is accordingly an object of this invention to provide an insulating spacer which does not cause a local concentration of electric field.
• In order to attain the above object, an insulating spacer according to this invention comprises the features of claim 1.
• The above and other objects and novel features of this invention will be more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, which are for illustration only and are not intended for limiting the scope of this invention.
BRIEF DESCRIPTION OF THE DRAWINGS
• Figures 1 and 2 are each a sectional view of an insulating spacer according to the prior art;
• Figure 3 is a graph representing the field concentration factor in the condition of Figure 2;
• Figure 4 is a sectional view of an insulating spacer according to one embodiment of this invention;
• Figure 5 is a sectional view of an insulating spacer according to another embodiment of this invention; and
• Figure 6 is a sectional view of an insulating spacer according a further embodiment of this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Figure 4 shows a sectional view of an insulating spacer according to one embodiment of this invention. In the figure, reference numbers 1 to 3 denote higher- and lower-potential electrodes and an insulating barrier, respectively, similar to those shown in Figure 1. Reference characters 6A and 6B each denote a unit spacer formed of an insulating solid material such as a fibrous material, a ceramic, a resin, etc. The unit spacers each have a hollow shape, namely, a tubular shape with tetragonal cross section, comprising a hollow portion 7 and a peripheral portion 8 surrounding the hollow portion 7. The hollow portion 7 is filled with an ambient insulating medium, for instance, a gas or an insulating oil (neither shown). In the same manner as in Figure 1, three unit spacers are disposed between the higher- and lower- potential electrodes 1 and 2 to maintain an insulation distance between the two electrodes and to clamp each insulating barrier 3 between the unit spacers 6A and 6B. The width W₁ of the unit spacer 6A is smaller than the width W₂ of the unit spacer 6B, and the width of the hollow portion 7 of the unit spacer 6A is smaller accordingly. The three unit spacers 6A and 6B are so disposed that side walls of the peripheral portions 8 thereof are stacked in zigzag and not aligned on a straight line, in the vertical direction in the figure.
• Assuming a rectilinear path extending downward in the figure from an arbitrary point in the higher-potential electrode 1, then the assumed path cannot be filled up with the insulating solid material only, and inevitably involves at least one interval in which an insulating medium such as a gas, an insulating oil, etc., is present. Therefore, even if a narrow gap (not shown) is newly generated at a point on the rectilinear path assumed, a local concentration of electric field would never occur at that point. Referring to the rectilinear path represented by dash-and-dot line A, for instance, about one-third of the distance between the higher- and lower- potential electrodes 1 and 2 is constituted of the hollow portion 7. With d₁/d₂ = 0.5 in Figure 3, therefore, the field concentration factor for this case is about 1.8. Namely, the presence of the hollow portion 7 prevents the ratio d₁/d₂ from being reduced to a value approximate to 0, and, accordingly, the field concentration factor is moderated as compared with those in the prior art. A construction in which the central unit spacer 6B is smaller than the other unit spacers 6A, contrary to the figure, also has the same effect.
• Figure 5 shows a sectional view of an insulating spacer according to another embodiment of this invention, illustrating the case of using three kinds of unit spacers 6A, 6B and 6C differing in width. In this case, a further reduction in the field concentration factor is achievable, as compares with the case shown in Figure 4.
• Figure 6 shows a sectional view of an insulating spacer according to a further embodiment of this invention, illustrating the case of using one kind of unit spacers 6A. With the unit spacers 6A alternately staggered, horizontally in the figure, the same effect as in Figure 4 is produced.
• As has been described hereinabove, according to this invention the unit spacers are each made in a hollow form and are so stacked that the side walls of the peripheral portions of the unit spacers are not aligned on a plane. The construction precludes the possibility that the space between two members such as electrodes may be filled, for the most part, with insulating solid material while a narrow gap may be present at the remaining minor part of the space. According to this invention, therefore, there is obtained the effect of preventing a local concentration of electric field.

Claims (4)

1. An insulating spacer for holding an insulation distance between opposed surfaces of two members differing in electric potential, comprising:
      a plurality of unit spacers (6A,6B), characterised in that each unit spacer has a hollow tubular shape with a peripheral portion (8) of an insulating solid material and a hollow portion (7) surrounded by the peripheral portion (8), said unit spacers (6A,6B) being stacked between the opposed surfaces of the two members, whereby side walls of the peripheral portions (8) of the unit spacers are not aligned on a plane over the entire distance between the opposed surfaces.
2. The insulating spacer as set forth in claim 1, wherein the plurality of unit spacers (6A) comprise one kind of unit spacers of the same size.
3. The insulating spacer as set forth in claim 1, wherein the plurality of unit spacers (6A,6B) comprise a plurality of kinds of unit spacers of different sizes.
4. The insulating spacer as set forth in claim 1, wherein the plurality of unit spacers (6A,6B) comprise unit spacers of more than one kind.

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