GB1598951A - Insulator - Google Patents

Abstract

The insulating body (1) is notable for being largely free of condensation. At least on its surface, it consists of an insulating material made of synthetic resin. Such an insulating body (1) may consist of a porous insulating material having an inhomogeneous density distribution, the core (3) continuously merging into the dense outer skin (2) made of the same insulating material. The insulating material of such insulating bodies may, instead of the pores, also be provided with hollow bodies, it being possible for the pores or hollow bodies to be filled with an insulating gas or a quenching gas. <IMAGE>

Description

(54) "INSULATOR" (71) We, SEMENS AKTrENGFSELL- SCHAFT, a German company of Berlin and Munich, Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an insulator.

A considerable number of the operational disturbances affecting electrical equipment installed in rooms where outdoor conditions prevail are attributable to overstressing of the insulator surface. The expression "indoor installations" means installations of the type installed in rooms where outdoor conditions prevail. The moisture-induced phenomena which lead to flashovers on insulator surfaces are essentially extraneous-layer phenomena. The two components of the extraneous layer, namely dust and moisture, penetrate into the indoor installations because these installations normally have virtually unavoidable air gaps, as a result of which the air has to be changed approximately three times per hour. The dust collecting on the surface of the insulator is generally moistened by condensation of the water vapour present in the surrounding air.The degree of moistening is at its greatest when the surface of the insulator becomes covered with dew.

Dew formation always occurs when humid air from outside enters a cold indoor installation and when the surface temperature of the insulator is below the dew point of the inflowing air. The layer of air adjoining the cold surface of the insulator, the relative humidity level increasing with decreasing air temperature until, with the onset of over-saturation, water condenses on the surface of the insulator. Thus, in January 1968, a whole number of operational disturbances to indoor electrical equipment occurred in Germany and the adjoining countries in consequence of a sudden change from cold to humid weather, producing flashovers on insulators of all kinds. Brackets, bushings, switch rockers, terminal boxes and transducers were all involved.

In that case, the disturbances were caused by a so called extraneous-layer flashover which occurred when a soiling layer and a considerable covering of moisture were present on the surface of the insulator. The flashover was normally over in a matter of seconds.

In the event of soiling and persistent, but fairly minimal moistening, so-called tracking short circuits occur. Under the influence of a high electrical field, the surface of the insulator undergoes erosion (otherwise known as tracking). Experience has shown that the gradual erosion of part of the insulator surface culminates in a flashover only ofter several days when several such current bridges have joined together. This phenomenon is clearly distinguished by its relative slowness and the progressive destruction of the insulator from the extraneous layer flashover produced by fairly rapid and intense dewing.

In order to avoid disturbances caused by dew formation to indoor electrical equipment, the insulators have to be oversized, the necessary increase in the minimum leakage paths necessitating the use of insulators with so-called skirts or ribs. Although this approach is adopted with insulators in the form of brackets or bushings, it does involve considerable extra financial outlay by comparison with the smooth insulators which may be used in dry indoor installations. However, this approach would be extremely expensive to adopt for many of the other insulators referred to above, such as switch rockers, transducers, etc.

For this reason, indoor installations are frequently heated so that the surface temperature of the insulator is always kept above the dew point of the surrounding air. Heating involves high operating costs in the case of continuous operation and the use of sophisticated control systems for intermittent operation. In many cases, no auxiliary voltage is available for heating at the critical moment.

Another way of preventing dew formation is to dry the replacement air which inevitably penetrates. However, unreasonable investment costs are involved in this case, too, taking into account the fact that, normally, the air is changed three times per hour.

Even the relativity elaborate hermetic encapsulation of an indoor installation with dry indoor air gives rise to difficulties on account of the cooling problems involved. In addition, the fact that the installation has occasionally to be opened is a fundamental obstacle to this approach.

Accordingly, it is desirable to provide an insulator in which the susceptibility to dewing is largely eliminated.

According to the present invention, there is provided an insulator having at least one surface layer of porous insulating material.

The surface layer can cover for example a core which imparts a high degree of mechanical stability. However, the insulator may consist entirely of an insulating material of low thermal capacity, or it may have a central cavity. In the context ofthe invention, alow thermal capacity is a thermal capacity below that of conventtional insulating materials, such as porcelain or solid plastic materials. Accordingly, only a very small quantity of heat is required for heating the surface of the insulator. This quantity of heat is taken from the warmer ambient air adjoining the surface. However, since this quantity of heat is only small, the layer of air adjoining the surface of the insulator is not cooled to any significant extent, so that the removal of heat is not accompanied by a significant drop in temperature in this surface layer.For this reason, the temperature generally does not fall below the dew point. If there is a considerable difference in temperature between the surface of the insulator and the warm air flowing into the indoor installation, the temperature can fall below the dew point in this special case. As a result, the water vapour present in the air is condensed on the surface of the insulator. By virtue of the limited thermal capacity of the insulator, however, the heat of condensation released by the condensation of a microscopic water layer is all that is needed in this case to bring the surface temperature of the insulator up to the ambient temperature. However, this microscopic layer of water of condensation is unable to bring about any significant reduction in the insulating properties of the surface of the insulator.As a result, extraneous layer flashover which requires a fairly significant water layer, is completely ruled out, and tracking short circuits, which require a lighter but more persistent covering of dew, is highly improbable.

Substantial freedom from dew formation may also be obtained by providing the insulator with at least one surface layer of an insulating material of low thermal conductivity. In the context of the invention, low thermal conductivity is a thermal conductivity which is below that of conventional insulating materials, such as porcelain or solid plastic materials. This sub- stantial freedom from dew formation is further improved if the surface of the insulator has both a low thermal conductivity and a low thermal capacity. The low thermal conductivity provides for the low-inertia adaption of the surface of the insulator to the surrounding warmer layers of air, because heat losses from the heated surface by thermal conduction into the colder interior of the insulator are largely precluded.Accordingly, the surface of the insulator can attain the temperature of the ambient air without any significant or persistent cooling of the adjoining air layers and consequent condensation of water.

The present invention also provides an insulator including at the surface thereof at least one layer of porous insulating material, the layer having thereon a thin impervious outer skin with a low thermal time constant.

The outer skin of this insulator is intended to prevent excessive soiling. By strengthening the outer skin, it is possible to increase the mechanical load capacity of the insulator. By virtue of its minimal thickness, this outer skin has a very low thermal capacity. An outer skin of the type in question assumes the temperature of the surrounding warmer air very quickly and with only slight cooling of the ambient air.

If dew formation should occur, it will be over very quickly and will only be microscopic. The deposited layer of water will begin to evaporate immediately afterwards so that only brief, very minimal dew formation, if any, will occur on the surface of the insulator.

The insulating material of an insulator of low thermal capacity and/or low thermal conductivity may be at least partly porous, whereby the insulator will have a particularly low thermal conductivity and thermal capacity.

The surface temperature of such a porous insulator, which quickly adapts itself by virtue of its low thermal capacity,cannot be "aftercooled" sufficiently quickly from the interior on account of its poor thermal conductivity: By virtue of the unusually low thermal capacity of a porous insulator, the aquisition of the temperature of the ambient air takes place more quickly than in conventional insulators despite the absence or only minimal presence of dew formation, so that the danger of tracking short circuits is also ruled out.

When the porous insulating material is a porous plastics material produced from a liquid starting material, the insulating material can be made porous by the addition to the liquid starting material of a blowing agent having insulating and/or quenching properties. Frigen for example may be used as the blowing agent. Pores can also be produced by stirring a gas having insulating and/or quenching properties into the liquid starting material. Filling the pores with gases having insulating and/or quenching properties improves the insulating capacity of the insulators and may contribute towards the quenching of any arcs formed.

In one advantageous embodiment of the insulator, the pores are produced by mixing hollow bodies into the liquid starting material.

The hollow bodies may be spherical in shape and may be filled with a gas having insulating and/or quenching properties. The hollow bodies, preferably hollow balls, may consist for example of glass, a ceramic material or a synthetic resin. They act as a filler substitute Apart from their substantial freedom from dew formation, such insulators has lower s1lrii age and notch stresses than the usual insulator; tended by, for example, quartz powder. These insulators have relatively thin, but impervious, outer skin which, by virtue of its low thermal time constant, is able to attain the temperature of the in-coming warmer ambient air with hardly any distortion, so that the surface remains free from dew.

Particularly suitable insulating materials are duroplasts because an insulator made of such a material has a high thermal and mechanical stability. By virtue of their favourable electrical properties, particularly their high tracking resistance, it is of advantage to use epoxy resins or silicone resins as the insulating material. If the insulators are to be exposed to mechanical loads which homogeneous foam insulators are unable to withstand, it is sufficient to provide a conventional insulator of high mechanical strength with a coating of foamed plastic in order to guarantee the required freedom from dew formation. In addition, the probability of diffusion breakdown is reduced in the case of foam plastic insulators.In conventional insulators, such breakdowns occur, in the absence of dew formation, simply as a result of diffusion of the water present in the ambient air into the insulator (which is free from water after production), the formation accumulations of water in the insulating material giving rise to the creation of current paths below the surface of the insulator as a result of which parts of the surface can flake off so that the insulating properties of the insulators deteriorate. In the case of foam plastic insulators, the water vapour which has diffused into the insulators can be safely stored in the bubbles of the foam, the gas bubbles even being electrically stengthened.

It is of advantage for the insulator to be in the form of an integral foam body. Integral foam or structured foam bodies are characterised by an irregular distribution of density such that the foam core continuously merges with an im- pervious outer skin of the same plastic material.

As a result, an insulator of this type has a thin smooth impervious outer skin of low thermal capacity which, in addition to its freedom from dew formation, improves the mechanical and electrical properties of the insulator.

It is of advantage for an insulator to have an alternating sequence of sections of different thermal conductivity and thermal capacity in the direction of the electrical potential gradient. The surface zones of higher thermal conductivity but lower thermal capacity attain the temperature of the warmer ambient air more quickly than the surface zones of low thermal conductivity.As a result, the first-mentioned surface zones can become covered with dew to an extent which, although minimal by comparison with conventional insulators, is nevertheless fairly significant by comparison with the surface zones of low thermal conduct ivi.y. When thermal equilibrium with the ambient air has been attained, the deposition film of water begins to evaporate from the more heavily dew-covered zones of higher thermal conductivity, whilst the condensation of water vapour from the air, albeit minimal, persists on the surface zone of higher thermal conductivity. As a result, the surface zone of high thermal conductivity, but low thermal capacity, dry off quickly so that the surface zones of lower thermal conductivity are separated after a short time by dry zones.Accordingly, this "series connection" of surface zones of different thermal conductivity means that, with the onset of dew formation, the insulation capacity is primarily attributable to the surface zones of low thermal conductivity which then are still barely covered with dew, whereas these lightly dew-covered surface zones are subsequently supported in their insulation capacity by the initially more heavily dew covered, but meanwhile dried, surface zones of higher thermal conductivity. Accordingly, an insulator of the type in question always has dry surface zones which guarantee its insulation capacity in the presence of humid air. In the case of ribbed insulators, preferably in post form, the ribbed section may serve as a surface zone of high thermal conductivity and low thermal capacity.

The ribs are thin and have relatively large surfaces by comparison with their volume. By virtue of their thinness, the ribs are substantially or completely non-porous where the insulator has an impervious, non-porous outer skin, and accordingly represent surface zones of high thermal conductivity and low thermal capacity. By contrast, the surface zones situated between the ribs have low thermal conductivity so that little, if any, dew is formed on these surface zones in the initial phase of the penetration of humid air. The temperature of the ribs is quickly assimilated at the expense of some dew formation by virtue of their comparatively high thermal conductivity and low thermal capacity.After the equality of temperature between the ribs and the ambient air has- been quickly reached, the layer ofmoisture deposited begins to evaporate so that the ribs dried off whilst the equalisation of temperature takes place more slowly on the surface zones situated between the ribs, resulting in more persistent, but only very minimal, dew formation. By this time, however, the ribs have dried off again so that the insulating strength is now temporarily a consequence of the fact that the ribs are dry.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 shows a smooth cylindrical insulator; and Figure 2 shows a ribbed insulator in the form of a post insulator.

A cylindrical insulator 1 is shown half cut away in Figure 1. An insulator of this type may be used for example as a post insulator. The insulator 1 is made of integral foam. It has an impervious outer skin 2 and a foam core 3 the latter having an irregular distrubution of density. This irregular distribution of density arises out of the fact that the size of the pores increases continuously towards the interior of the insulator. The impervious and substantially nonporous outer skin 2 has a thickness of approximately half a millimetre. Fixing sockets 8 and 9 are inserted into recesses 6 and 7 extending axially into the end faces of the insulator, their function being to hold the insulator or the voltage-carrying element.

The insulator 1 may consist of for example foamed epoxy resin produced by the use of air, insulating distrubution of density may be determined by the control of temperature in the mould during hardening of the plastic material.

By virtue of its porous structure, an insulator of this type has both an extremely low thermal capacity and also a very low thermal conductivity. By virtue of its minimal thickness, the impervious outer skin has a low thermal capacity so that, on encountering humid air, it is able to adapt itself to the temperature of the ambient air with low thermal inertia without removing large quantities of heat therefrom. In this way, the layers of air near the surface are not significantly cooled so that their oversaturation, followed by dew formation, is largely eliminated. The heat loss from the heated outer skin 2 is to the inner parts of the insulator 1 is extremely small by virtue of the low thermal conductivity and low thermal capacity of the foam mass.By virtue of its comparatively high thermal conductivity, the outer skin 2 does not have any significant temperature gradient over its layer thickness.

Hiterto, it has not been possible to use cylindrical insulators (i.e. insulators without ribs or skirts) in indoor installations exposed to the danger of dew formation although, from the point of view of manufacture and price, they are considerably more favourable than insulators provided with ribs or skirts.

It is also possible for the insulators not to have an impervious surface layer 2 providing the freedom from dew formation is maintained.

In this case, however, mechanical stability is lower than in the case of integral foam insullators.

Any susceptibility towards dew formation may also be largely eliminated in the same way from other insulators, for example switch rockers, transducers or insulating plates of the type used for ballast.

Where an insulator of the type in question has to satisfy increased mechanical requirements, it is also possible to eliminate susceptibility to dew formation by a laminar structure.

In this case, a non-porous insulating element made of a conventional material may be provided with a coating of low thermal conductivity and low thermal capacity. For example, a solid plastic or porcelain element may be provided over its entire surface with a covering of foam plastic.

Another rotationally symmetrical insulator 10 suitable for use as a post insulator is shown half cut away in Figure 2. This insulator 10 is also made of an integral foam. At its surface it has an impervious outer skin 12. This impervious outer skin adjoins a foam core 13 of irregular density and pore distribution. The end faces 14 and 15 of the insulator 10 are provided with cylindrical recesses 16 and 17 which are inserted fixing sockets 18 and 19 for assembling the insulator 10 and the voltage-carrying elements. In contrast to the insulator shown in Figure 1, the insulator shown in Figure 2 has ribs or skirts 20. The ribs are thin and substantially non-porous so that they represent surface zones of low thermal capacity, but compara- tively high thermal conductivity.Accordingly, in the presence of heat, they quickly assume the temperature of the ambient air possibly with at the most some slight dew formation.

The circular surface zones of the stem lying between the ribs have a lower thermal conductivity so that they remain dry, at least during the initial phase of the penetration of warm air, and guarantee maintenance of the insulation capacity. By the time more warm air has penetrated, the large-surface ribs or skirts 20, because they have quickly assumed the temperature of the ambient air, with the result that the layer of dew present on them soon begins to evaporate, whereas the surface zones of the stem have still not reached full temperature equalisation with the ambient air and, for this reason, may still be lightly covered by dew.

Accordingly, a number of dry surface zones is available for maintaining the insulation capacity in the event of warm air penetrating into an insulator of this type, i.e. an insulator provided with ribs or skirts.

As already mentioned, an insulator becomes covered with dew as a result of the fact that, on the penetration of warm moist air, the surface temperature of the insulator is below the dew point of the warm air. The surface temperature of the insulator of the invention quickly attain the temperature of the ambient air by the transfer of only small amounts of heat. In other words, the invention provides insulators having a low thermal time constant. The thermal time constant of an insulator is determined by the thermal properties of its constituent material and by its configuration. If the thermal time constant of the insulator is known, it is possible by suitably dimentioning the walls of the indoor installation to contribute towards the prevention of dew formation on the insulator when installed in the indoor installation. This may be done by ensuring that the walls of the indoor installation have a sufficiently high thermal capacity. Thus, when the heated moist air impinges on the walls of the indoor installation, condensation thereon of the entrained water vapour results in partial drying of the air and hence in a reduction in the dew point temperature of the air flowing into the indoor installation through the ventilation slots. In addition, the inflowing air undergoes a reduction in temperature on the cold walls so that the walls of the indoor installation act as "heat buffers". Accordingly, the temperature of the inflowing air follows the increase in temperature of the external air with some delay.

Accordingly, it becomes easier for the surface of the insulator to follow the increase in temperature, so that the susceptibility to dew formation of the insulators can be further reduced by adapting the thermal properties of the indoor installation to the thermal properties of the insulators used.

WHAT WE CLAIM IS: 1. An insulator including at least one surface layer of porous insulating material.

2. An insulator including at the surface thereof at least one layer of porous insulating material, the layer having thereon a thin impervious outer skin with a lower thermal time constant.

3. An insulator as claimed in Claim 1 or 2, the porous insulating material being a porous plastics material produced form a liquid starting material, the pores having been produced by adding a blowing agent having insulating and/or quenching properties to the liquid starting material.

4. An insulator as claimed in Claim 1 or 2, the porous insulating material being a porous plastics material produced form a liquid starting material, the pores having been produced by stirring a gas having insulating and/or quenching properties into the liquid starting material.

5. An insulator as claimed in Claim 1 or 2, the porous insulating materfial being a porous plastics material produced from a liquid starting material, the pores having been produced by mixing hollow bodies into the liquid starting material.

6. An insulator as claimed in Claim 5, wherein the hollow bodies are spherical in shape.

7. An insulator as claimed in Claim 5 or 6, wherein the hollow bodies are filled with a gas having insulating and/or quenching properties.

8. An insulator as claimed in any of Claims 1 to 7, being a post insulator.

9. An insulator substantially as hereinbefore described with reference to, and as shown in, Figure 1 of Figure 2 of the accompanying drawings.

10. An electrical installation including an insulator as claimed in any of Claims 1 to 9.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. and hence in a reduction in the dew point temperature of the air flowing into the indoor installation through the ventilation slots. In addition, the inflowing air undergoes a reduction in temperature on the cold walls so that the walls of the indoor installation act as "heat buffers". Accordingly, the temperature of the inflowing air follows the increase in temperature of the external air with some delay. Accordingly, it becomes easier for the surface of the insulator to follow the increase in temperature, so that the susceptibility to dew formation of the insulators can be further reduced by adapting the thermal properties of the indoor installation to the thermal properties of the insulators used. WHAT WE CLAIM IS:

1. An insulator including at least one surface layer of porous insulating material.

2. An insulator including at the surface thereof at least one layer of porous insulating material, the layer having thereon a thin impervious outer skin with a lower thermal time constant.

3. An insulator as claimed in Claim 1 or 2, the porous insulating material being a porous plastics material produced form a liquid starting material, the pores having been produced by adding a blowing agent having insulating and/or quenching properties to the liquid starting material.

4. An insulator as claimed in Claim 1 or 2, the porous insulating material being a porous plastics material produced form a liquid starting material, the pores having been produced by stirring a gas having insulating and/or quenching properties into the liquid starting material.

5. An insulator as claimed in Claim 1 or 2, the porous insulating materfial being a porous plastics material produced from a liquid starting material, the pores having been produced by mixing hollow bodies into the liquid starting material.

6. An insulator as claimed in Claim 5, wherein the hollow bodies are spherical in shape.

7. An insulator as claimed in Claim 5 or 6, wherein the hollow bodies are filled with a gas having insulating and/or quenching properties.

8. An insulator as claimed in any of Claims 1 to 7, being a post insulator.

9. An insulator substantially as hereinbefore described with reference to, and as shown in, Figure 1 of Figure 2 of the accompanying drawings.

10. An electrical installation including an insulator as claimed in any of Claims 1 to 9.