Polymer insulator

Abstract: A polymer insulator has a core member (2), an insulation overcoat member (3) arranged on an outer surface of the core member, and a securing metal fitting (4) fixed to an end portion of the core member in such a manner that the metal fitting contacts the insulation overcoat member. The polymer insulator further includes a seal portion (7) arranged at a boundary between the insulation overcoat member and the securing metal fitting, which is made of a sealing agent in which 80-250 parts by weight of ATH (Alumina trihydrate, Al 2 O 3 .3H 2 O) is included with respect to 100 parts by weight of a polymer component. Thereby, the polymer insulator has an improved tracking-erosion resistance performance.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polymer insulator having a core member, an insulation overcoat member arranged on an outer surface of the core member, and a securing metal fitting fixed to an end portion of the core member in such a manner that the end portion is contacted with the insulation overcoat member. 2. Description of Related Art Fig. 2 is a cross sectional view showing one embodiment of a known polymer insulator. In the embodiment shown in Fig. 2, a polymer insulator 1 comprises an FRP rod 2 as a core member, an insulation overcoat member 3 made of rubber such as silicone rubber which is arranged on an outer surface of the FRP rod 2, and securing metal fittings 4 which are secured and fixed to both ends of the FRP rod 2. The insulation overcoat member 3 comprises a sheath portion 5 and a plurality of sheds 6. In order to produce the polymer insulator mentioned above, the insulation overcoat member 3 is molded on the FRP rod 2, and then the securing metal fittings 4 are secured and fixed to both ends of the FRP rod 2. In this case, a seal portion 7 made of a sealing agent such as rubber of silicone system is arranged at a boundary between the insulation overcoat member 3 and the securing metal fittings 4, which is exposed to an external atmosphere, so as to prevent an inclusion of water or the like through the boundary. During a normal field test, the known polymer insulator having the construction mentioned above shows no problem on tracking-erosion properties of the insulation overcoat member 3 and the seal portion 7. However, if the known polymer insulator is used under a severe fouling condition, or, if the known polymer insulator is subjected to an acceleration damage test, there is a case such that an erosion occurs at the seal portion 7. Therefore, in order to improve reliability of the polymer insulator, it is necessary to improve tracking-erosion resistant properties much more. Moreover, the seal portion 7 is arranged at both ends of the polymer insulator 1, to which a high electric field is liable to be applied. Therefore, as shown.in Fig. 3, corona or dry-band-arc is liable to be generated at the seal portion 7, especially if the polymer insulator 1 is used under the severe fouling condition. Owing to this, in the seal portion 7, it is necessary to have excellent tracking-erosion resistant properties in addition to sealing properties which are same as those of the known polymer insulator. EP-A-617433 discloses an insulator similar to that of Fig.2. JP-A-8-41347 and JP-A-8-41348 describe silicone rubber containing aluminium trihydrater, optionally treated with silane coupling agent, as material of electrical insulators. SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the drawbacks mentioned above and to provide a polymer insulator which can improve tracking-erosion resistant properties by improving a seal portion. According to the invention, then is provided a polymer insulator as set out in claim 1. Alumina trihydrater (ATH) is Al 2 O 3 .3H 2 O. The present invention is achieved on the basis of the following finding obtained by various examinations. That is to say, in order to improve tracking-erosion resistant properties, it is effective to use a sealing agent, in which a predetermined amount of ATH preferably having a predetermined particle size and having a predetermined surface finishing is included, for the seal arranged between the insulation overcoat member and the securing metal fitting. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross sectional view showing an enlarged main portion of a polymer insulator according to the invention; Fig. 2 is a cross sectional view illustrating one embodiment of a known polymer insulator; and Fig. 3 is a cross sectional view for explaining an arc generation in the known polymer insulator. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 is a cross sectional view showing an enlarged main portion of a polymer insulator according to the invention. The polymer insulator according to the invention shown in Fig. 1 has fundamentally the same construction as that of the known polymer insulator shown in Fig. 2. Therefore, in the embodiment shown in Fig. 1, reference numerals similar to those of Fig. 2 are denoted by the same reference numerals as those of Fig. 2, and the explanations thereof are omitted here. In the polymer insulator according to the invention shown in Fig. 1, the feature is that a seal portion 7 made of a sealing agent, in which 80-250 parts by weight of ATH is included with respect to 100 parts by weight of a polymer component, is arranged at a boundary between the insulation overcoat member 3 and the securing metal fitting 4, which is exposed to an external atmosphere. Here, a term "ATH" means alumina trihydrate (Al 2 O 3 .3H 2 O) Normally, ATH exists in a form of Al(OH) 3 . If a heat is applied, ATH changes in a form of Al 2 O 3 .3H 2 O. That is, the following reaction occurs by heating: 2Al(OH) 3 →Al 2 O 3 .3H 2 O). In this case, water is generated, and thus an applied heat is absorbed as a heat of evaporation of water. Therefore, it is known that, if a rubber includes ATH, it is possible to prevent a heat damage of the rubber including ATH. Any suitable polymer component may be used for the seal agent of the seal portion 7. All suitable polymer components of silicone type may be used, and the curing type is not limited. Among them, it is preferred to use poly-dimethyl-siloxane. Moreover, the particle size of ATH included in the sealing agent of the seal portion 7 is not particularly limited from a standpoint of improving tracking-erosion resistant properties. However, the particle size of ATH is preferably limited to 3 µm or more, more preferably 8µm or more from view points of acid resistant properties, sealing properties and water absorbing properties as clearly understood from the following examples. In addition, an upper limitation of the particle size of ATH is preferably 50µm. If the particle size of ATH is not less than 50 µm, the sealing agent may not be uniformly mixed and thus the seal portion 7 made of such a sealing agent may not have sufficient strength. Herein, particle size means average particle size. Hereinafter, actual examples will be explained. Experiment Sealing agents according to examples of the present invention some comparative examples , and a known example were prepared by setting conditions of ATH amount, ATH particle size, ATH surface finishing, and curing type as shown in the following Table 1. In Table 1, poly-dimethyl-siloxane was used as the polymer component. Moreover, the amount of ATH indicated is parts by weight of ATH with respect to 100 parts by weight of poly-dimethyl-siloxane. ATH surface finishing was effected by using silane coupling agent. Then, tracking-erosion resistant properties, acid resistant properties, and water absorbing properties of the thus prepared sealing agents were investigated, and also sealing properties of the polymer insulator using the thus prepared sealing agents for the seal portion was investigated. Hereinafter, the results of the investigations mentioned above were explained in this order. Name of sealing Name of sealing agent Amount of ATH (parts by weight) Particle size of ATH (µm) Surface finishing of ATH Curing type Known example 0 - - Condensation Comparative example 1 50 1 Yes Condensation Example of present invention 1 80 3 Yes Condensation Example 2 100 1 No Condensation Example 3 100 3 No Condensation Example of present invention 4 100 3 Yes Condensation Example of present invention 5 100 8 Yes Condensation Example of present invention 6 150 1 Yes Condensation Example of present invention 7 150 8 Yes Condensation Example of present invention 8 150 8 Yes Addition Example of present invention 9 200 8 Yes Condensation Example of present invention 10 250 8 Yes Condensation Comparative example 2 300 8 Yes Condensation (1) As to tracking-erosion resistant properties: A tracking-erosion resistant property test was performed as follows. At first, specimens of the sealing agents shown in Table 1 were prepared on the basis of IEC 587 test method. Then, a tracking test voltage of 4.5 kV was applied constantly to the thus prepared specimens, and it was confirmed whether or not the specimen achieved a standard of 6 hours according to IEC 587 test method in which the specimen was endured for 6 hours under such a voltage applying condition. For the specimens which did not achieve the standard of 6 hours, a time duration until a stop of the tracking- erosion resistant test was measured. The results were shown in the following Table 2. From the results shown in Table 2, it is understood that all the specimens according to the examples of the present invention in which 80 parts by weight or more of ATH is included in the sealing agent achieve the standard of 6 hours, while the specimen according to the known example in which no ATH is included in the sealing agent endures only for about 2 hours and the specimen according to the comparative example 1 in which 50 parts by weight of ATH is included in the sealing agent endures only for 3 hours. Name of sealing agent Name of sealing agent Time duration until stop of tracking-erosion resistant test based on IEC587 (hr) Known example 2.0 Comparative example 1 3.0 Example of present invention 1 > 6 Example 2 > 6 Example 3 > 6 Example of present invention 4 > 6 Example of present invention 5 > 6 Example of present invention 6 > 6 Example of present invention 7 > 6 Example of present invention 8 > 6 Example of present invention 9 > 6 Example of present invention 10 > 6 Comparative example 2 > 6 (2) As to acid resistant properties: An acid resistant property test was performed in such a manner that the sealing agents each having a constant amount according to the examples 1-10, the comparative examples 1-2, and the known example were immersed into nitric acid solution having a concentration of 1 normal for 100 hours and weight decrease rate of the sealing agents were measured. The results were shown in the following Table 3. Normally, when particles other than the polymer component are existent in the sealing agent, the weight decrease becomes larger after the acid resistant property test mentioned above. Therefore, it is estimated that the sealing agent has no problem if it has the same weight decrease rate as that of the known example. From the results shown in Table 3, it is understood that the examples 2 and 3 have a large weight decrease rate. This is because ATH used in the examples 2 and 3 is not subjected to the surface finishing and thus ATH is eluted. Moreover, it is understood that, if use is made of ATH to which the surface finishing is effected, the specimen has the same weight decrease rate as that of the known example and indicates an excellent acid resistant property. Name of sealing agent Name of sealing agent Weight decrease rate after acid immersion (%) Known example 5 Comparative example 1 5 Example of present invention 1 5 Example 2 30 Example 3 25 Example of present invention 4 5 Example of present invention 5 5 Example of present invention 6 5 Example of present invention 7 5 Example of present invention 8 5 Example of present invention 9 7 Example of present invention 10 8 Comparative example 2 10 (3) As to water absorbing property: A water absorbing property test was performed in such a manner that the sealing agents according to the examples 1-10. the comparative examples 1-2, and the known examples were immersed in an ion-exchanged water and water absorbing properties were judged on the basis of the weight increase rate and a variation of volume resistivity of the sealing agent. In addition, for reference, the same water absorbing property test was performed for a silicone rubber constituting the insulation overcoat member. The results were shown in the following Table 4. From the results shown in Table 4, it is understood that the examples 2 and 3 have a large water absorbing amount. This is because ATH used in the examples 2 and 3 is not subjected to the surface finishing. Moreover, it is understood that the examples 2 and 3 have a large reduction rate of volume resistivity after water absorbing. This is also because ATH is not subjected to the surface finishing and thus a conduction path is liable to be generated after water absorbing. If such a sealing agent is used for an actual product, a performance of the seal portion becomes inferior as compared with a rubber constituting the insulation overcoat member. Therefore, an arc generation due to electric discharge is concentrated on the seal portion, and thus an erosion of the seal portion is liable to be generated. Accordingly, it is preferred to use ATH to which the surface finishing using silane coupling agent is effected. Moreover, in the case that the surface finished ATH is used, if an amount of ATH is increased in excess as shown in the comparative example 2, a water absorbing amount is increased and a volume resistivity us decreased as compared with that of rubber constituting the insulation overcoat member. Therefore, it is necessary to set an amount of ATH in the sealing agent up to 250 parts by weight with respect to 100 parts by weight of the polymer component. Name of sealing agent Weight increase rate after ion-exchanged water immersion for 300 hours (%) Variation of volume resistivity upper value: before immersion lower value: after immersion (Ω·cm) Known example 0.1 1 × 10 15 1 × 10 14 Comparative example 1 0.2 1 × 10 15 1 × 10 14 Example of present invention 1 0.3 5 × 10 14 1 × 10 14 Example 2 0.9 1 × 10 14 1 × 109 Example 3 0.8 1 × 10 14 1 × 10 10 Example of present invention 4 0.3 5 × 10 14 1 × 10 14 Example of present invention 5 0.3 5 × 10 14 1 × 10 14 Example of present invention 6 0.3 5 × 10 14 1 × 10 14 Example of present invention 7 0.3 5 × 10 14 1 × 10 14 Example of present invention 8 0.3 5 × 10 14 1 × 10 14 Example of present invention 9 0.4 1 × 10 14 1 × 10 13 Example of present invention 10 0.6 5 × 10 13 1 × 10 12 Comparative example 2 0.6 1 × 10 13 5 × 10" Rubber of overcoat member 0.3 1 × 10 14 1 × 10 14 (4) As to sealing properties: A sealing property test was performed as follows on the basis of IEC 1109 test method. At first, polymer insulators, in which sealing agents according to the examples 1-10, the comparative examples 1-2, and the known example were used for the exposed seal portion between the insulation overcoat member and the securing metal fitting, were prepared. Then, the thus prepared polymer insulator was boiled in an NaCl aqueous solution having a concentration of 0.1 % for 100 hours and then immersed into a fuchsine solution. After that, sealing properties were judged on the basis of whether or not dyes were intruded into an inside of the securing metal fitting. The results were shown in the following Table 5. Normally, the sealing property is thought to be better if such an intrusion of dyes is not detected. From the results shown in Table 5, it is understood that the example 2 shows an intrusion of dyes. The reasons are as follows. That is, since ATH used in the example 2 has a small particle size such as 1 µm and is not subjected to the surface finishing, an elongation of the seal portion is little and the seal portion does not endure such a boiling stress. Moreover, the examples 3 and 4 use ATH having a particle size of 3 µm. In the example 3 in which ATH is not subjected to the surface finishing, the sealing property is a permissible lower level. However, in the example of present invention 4 in which ATH is subjected to the surface finishing, since affinity between ATH and rubber is improved and thus an elongation and an adhesion strength of the seal portion are also improved, the seal portion show a sufficient sealing property. Further, in the example of present invention 6 in which 150 parts by weight of ATH having a particle size of 1 µm , since ATH is subjected to the surface finishing, its sealing property barely maintains a permissible lower level. Furthermore, in the case that ATH having a particle size of 8 µm is used as shown in the examples of present invention 5, 7-10, a sufficient elongation of the seal portion can be achieved if an amount of ATH is large, and thus a sufficient sealing property can be obtained. Contrary to this, as shown in the comparative example 2 in which ATH having the same particle size of 8 µm is used, if 300 parts by weight of ATH is added, an amount of rubber becomes too small, and thus a sufficient elongation can be achieved, thereby decreasing its sealing property. Name of sealing agent Dye intrusion into metal fitting Known example no-intrusion Comparative example 1 no-intrusion Example of present invention 1 no-intrusion Example 2 intrusion Example 3 a little intrusion Example of present invention 4 no-intrusion Example of present invention 5 no-intrusion Example of present invention 6 a little intrusion Example of present invention 7 no-intrusion Example of present invention 8 no-intrusion Example of present invention 9 no-intrusion Example of present invention 10 no-intrusion Comparative example 2 intrusion (5) Summary of the results: The following Table 6 summarizes the tracking-erosion resistant properties, the acid resistant properties, and the water absorbing properties of the sealing agents, and also the sealing properties of the polymer insulator using the sealing agents for the seal portion. From the results shown in Table 6, it is understood that it is necessary to use a sealing agent in which 80-250 parts by weight of ATH is included with respect to 100 parts by weight of the polymer component, if mainly taking into consideration of the tracking-erosion resistant properties. In addition, it is understood that it is preferred to use ATH having a particle size of 3 µm or more, more preferably about 8 µm, and that it is preferred to use ATH to which the surface finishing using silane coupling agents is performed, if taking into consideration of the another properties other than the tracking-erosion resistant properties for reference. As clearly understood form the above explanations, according to the invention, the sealing agent, in which 80-250 parts by weight of ATH having preferably a predetermined particle size, to which a predetermined surface finishing is performed, is used for the seal portion arranged at a boundary between the insulation overcoat member and the securing metal fitting of the polymer insulator, which is exposed to an external atmosphere. Therefore, the polymer insulator according to the invention has an improved tracking-erosion resistant property.

Claims: A polymer insulator having a core member (2), an insulation overcoat member (4) arranged on an outer surface of said core member, a securing metal fitting (3) fixed to an end portion of said core member in such a manner that said metal fitting (3) contacts said insulation overcoat member (4), and a sealing body (7) arranged at a boundary between said insulation overcoat member (4) and said securing metal fitting (3), said sealing body (7) being made of a sealing agent comprising a polymer component, characterised in that said sealing agent contains 80 - 250 parts by weight, per 100 parts by weight of said polymer component, of alumina trihydrate particles having a surface finishing of one or more silane coupling agents. A polymer insulator according to claim 1, wherein said polymer component is poly-dimethyl-siloxane. A polymer insulator according to claim 1 or 2, wherein the particle size of said alumina trihydrate is 3 µm or more.

Inventors: KONDOU TAKANORI, KOSHINO YUKIHIRO, KONDOU, TAKANORI, KOSHINO, YUKIHIRO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1998
Main classification: H01B17
IPC codes: C08L83, C08K9, H01B17