Composite insulators

Abstract: A staggered shed type composite insulator includes a core member and a housing provided around the outer peripheral face of the core member and made of an insulating polymeric material, the housing comprising large-diameter shed portions, small-diameter shed portions and sheath portions arranged such that the large-diameter shed portions and the small-diameter shed portion are alternatively arranged via the respective sheath portions, wherein "1/p" is in a range from 4.3 to 5.0 in which "1" and "p" are a unit surface-leakage distance and an axial unit length of the housing, respectively.

Description: Background of the Invention (1) Field of the Invention The present invention relates to composite insulators. More particularly, the invention relates to a staggered shed type composite insulator which comprises a core member made of FRP or the like and a housing provided around the outer peripheral face of the core member and made of an insulating polymeric material and in which the housing is constituted by larger-diameter shed portions and small-diameter shed portions alternatingly arranged and separated by sheath portions. (2) Related Art Statement As conventional staggered shed type composite insulators, there are recited those shown in Figs. 1 to 3. Reference is also made to WO 91/06106. Fig. 1 shows a staggered shed type composite insulator in which a housing includes shed portions having the same large diameter and those having the same small diameter alternatingly arranged. Fig. 2 shows a staggered shed type composite insulator in which a housing includes large-diameter shed portions and small-diameter shed portions arranged such that two shed portions having the same small diameter are arranged between two adjacent large-diameter shed portions having the same diameter. Fig. 3 shows a staggered shed type composite insulator in which a housing includes large-diameter shed portions and small-diameter shed portions arranged such that two small-diameter shed portions having different diameters are arranged between two adjacent large-diameter shed portions having the same diameter. "Staggered shed type composite insulator" to which the present invention is applicable means a shed portion-provided composite insulators comprising a core member and a housing provided around the outer peripheral face of the core member and made of an insulating polymeric material, in which the housing comprises large-diameter shed portions, small-diameter shed portions and sheath portions arranged such that the large-diameter shed portions and the small-diameter shed portions are alternatingly arranged and separated by the sheath portions. The small-diameter shed portions may have the same diameter or different diameters. The large-diameter shed portions may also have the same diameter or different diameters. With respect to the staggered shed type composite insulators, it has not been studied what is an appropriate ratio (this ratio being referred hereinafter as "l/p") of a unit surface-leakage distance "l" between adjacent large-diameter shed tips to the axial pitch "p" of the large diameter sheds. It is believed that as the leakage distance increases, the pollution withstand voltage characteristic is accordingly improved. For this reason, with respect to the composite insulator in which the housing can be arbitrarily molded different from the conventional porcelain insulators, it is considered a desirable design from the standpoint of the pollution withstand voltage characteristic that the l/p value is increased as much as possible by prolonging the leakage distance as much as possible relative to a given length of the insulator, that is, by extending the shed portions radially as much as possible and decreasing a distance between adjacent shed portions as much as possible. Summary of the Invention However, increase of the l/p ratio increases the amount of rubber used for the housing. As a material for the housing, silicone rubber is now used, which is strongest against deterioration among organic materials, but silicone rubber is an expensive material. It is economically a problem that the pollution withstand voltage characteristic is not improved in proportion to increase in the l/p value, and the pollution withstand voltage characteristic does not match the increased amount of the silicone rubber. The problem becomes more serious if the pollution withstand voltage characteristic is reversely deteriorated even if the l/p value is increased. Based on this knowledge, the present inventors discovered that the l/p value needs to be appropriately set from the standpoint of the pollution withstand voltage with respect to the staggered shed type composite insulator. Further, the present inventors proceeded with their study on the shape of the staggered shed type composite insulator from the standpoint of the pollution withstand voltage. As a result, it was clarified that there is some relationship between a straight distance from the tip of a large-diameter shed portion to a small-diameter shed portion and the pollution withstand voltage, and that the straight distance from the tip of the large-diameter shed portion to the small-diameter shed portion needs be optimized from the standpoint of the pollution withstand voltage. Furthermore, it was clarified during the course of studying the shape of the staggered shed type composite insulator that the pollution withstand voltage is related to the ratio between the radial projection length "a" of the large-diameter shed portions and the axial pitch "p", and that the ratio between the radial projection length "a" of the large-diameter shed portions and the axial unit length "p" of the housing needs to be optimized from the standpoint of the pollution withstand voltage. Furthermore, it was also clarified that the relationship between the pollution withstand voltage and the l/p value of the staggered shed type insulator, that between the pollution withstand voltage and the straight distance from the tip of the large-diameter shed portion to the tip of the small-diameter shed portion, and that between the pollution withstand voltage and the ratio of the axial unit length "p" of the large-diameter shed portion to the radial projection length "a" of the large-diameter shed portion are related to one another from the standpoint of the pollution withstand voltage. That is, with respect to the composite insulators having silicone rubber housings with shed portions staggered, the present inventors made experiments and study in which pollution tests were actually conducted to examine (1) the relationship between the l/p value and the pollution withstand voltage, (2) the relationship between the pollution withstand voltage and the straight distance "c" from the tip of the large-diameter shed portion to that of the small-diameter shed portion, and (3) the relationship between the pollution withstand voltage and the ratio "p/a". As a result, it was clarified that the pollution withstand voltage characteristic is not improved in one-way direction with respect to the l/p value, but the pollution withstand voltage has a local maximum at an appropriate l/p value, and decreases as the l/p goes beyond this value so that the pollution withstand voltage exhibits a parabola-shaped characteristic. It was also clarified that by affording this appropriate l/p value to the housing, the pollution withstand voltage characteristic is exhibited to the maximum to remove the necessity to excessively increase the volume of the rubbery material and solve the above problems. It was also clarified that the above is true with respect to the relationship between the pollution withstand voltage and the straight distance "c" from the tip of the large-diameter shed portion to that of the small-diameter shed portion and the relationship between the pollution withstand voltage and the ratio "p/a". The staggered shed type composite insulator according to the present invention is set out in claim 1. These and other features and advantages of the invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations or changes of the same could be easily made by the skilled person in the art to which the invention pertains. Brief Description of the drawings For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 shows one form of the staggered shed type composite insulator discussed above; Fig. 2 shows another form of the staggered shed type composite insulator discussed above; Fig. 3 shows a further form of the staggered shed type composite insulator discussed above; Fig. 4 is a still further embodiment of the staggered shed type composite insulator in which various dimensions are shown according to the present invention; Fig. 5 is a graph showing experimental results upon the relationship between the "l/p" value and the pollution withstand voltage; Fig. 6 is a graph showing experimental results upon the relationship between the straight distance "c" from the tip of the large-diameter shed portion to that of the small-diameter shed portion and the pollution withstand voltage; and Fig. 7 is a graph showing the relationship between the ratio "p/a" between the axial unit length "p" and the radial projection length "a" of the large-diameter shed portion and the pollution withstand voltage. Detailed description of the Invention In the following, pollution withstand voltage experiments actually conducted by the present inventors with respect to the staggered shed type composite insulators shown in Fig. 4 will be explained. The experiments were conducted by a clean fog method. The experiments were conducted with respect to the staggered shed type composite insulators having dimensions shown in Table 1, in which silicone rubber was used as a housing-forming material. Pollution withstand voltage characteristic obtained in the above experiments is totally shown in Table 1, and Figs. 5 to 7 show the relationship between the l/p value and the pollution withstand voltage, the relationship between the straight distance "c" from the tip of the large-diameter shed portion to that of the small-diameter shed portion and the pollution withstand voltage, and the relationship between the radial projection length "a" of the large-diameter shed portion and the axial unit length or pitch "p" of the housing large-diameter shed portions. In Figs. 5 to 7, "resistive voltage" of the ordinate means "withstand voltage per unit effective length (for example, l m)", that is, "50 % F.O.V. (flashover voltage) per unit effective length", which is a value obtained by dividing the average flashover voltage by the axial length of the insulator. Since the pollution withstand voltage is proportional to the 50 % F.O.V value, the former can be known from the latter. The pollution withstand voltages shown in Table 1 and Figures 5 to 7 are not absolute values, but relative values. Pollution withstand voltage characteristic of variously shaped shed portions of composite insulators Pitch p mm Radial projection lengths of shed portions a+b mm Leakage distance per each pitch l/p mm p/a Tip-to-tip distance between sheds mm ESDD, mg/cm 2 0.03 0.5 Pollution withstand voltage (Relative value) 50 70+50 5.32 0.71 32 90 93 55 70+50 4.93 0.79 34 95 99 60 70+50 4.60 0.86 36 100 102 70 70+50 4.09 1.00 40 98 95 75 70+50 3.88 1.07 42.5 97 92 80 70+50 3.70 1.14 45 94 88 85 70+50 3.54 1.21 47 91 85 (1) Relationship between "l/p" value and the pollution withstand voltage (See Table 1 and Fig. 5) The optimum "l/p" value is not always identical over all conditions, but slightly changes depending upon the degree of pollution. The polluted degree can be represented by an equivalent salt deposit density (abbreviated as ESDD). In the above experiments, the polluted degree was set at 0.03 to 0.5 mg/cm 2 , which range almost covers almost all possible cases in actually used states. This is the reason why the above range was employed in the experiments. The other experiments discussed below were also conducted in this range. From the experimental results, it was clarified that the resistive voltage (V/H: y) could be approximated by either of the following equations. (1) Relationship between "l/p" (x) and resistive voltage (V/H: y) i) In case of ESDD = 0.03 mg/cm 2 y = - 11.532 x 2 + 101.28 x - 122.86 = - 11.532 (x - 4.39) 2 + 99.513 ii) In case of ESDD = 0.5 mg/cm 2 y = - 14.024 x 2 + 129.62 x - 199.2 = - 14.024 (x - 4.62) 2 + 100.311 Therefore, it was clarified that in case of ESDD = 0.03 mg/cm 2 , the pollution withstand voltage was the maximum at l/p = 4.39, whereas in case of ESDD = 0.5 mg/cm 2 , it was the maximum at l/p = 4.62. The "l/p" value which gives the maximum resistive voltage in the case of ESDD = 0.03 mg/cm 2 differs from that giving the maximum resistive voltage in case of ESDD= 0.5 mg/cm 2 by only 0.23. Furthermore, if an intermediate value l/p = 4.51 between the above two figures is taken (4.51 being a value calculated by substituting x = l/p in the above expression, such is true in the following), the pollution withstand voltage was reduced to only 99.9 % of its maximum value at the polluted degree of 0.03 mg/cm 2 , whereas the pollution withstand voltage was reduced to only 99.8% of its maximum value at 0.5 mg/cm 2 . Therefore, it was clarified that both were not substantially different from their respective maximum values. When the l/p was set at 5.0 in the case of 0.03 mg/cm 2 , the pollution withstand voltage was reduced to only 95.6 % of its maximum value, whereas when the l/p = 5.0 at 0.5 mg/cm 2 , the pollution withstand voltage was reduced to only 98.0% of its maximum value. Therefore, it is clarified that if the polluted degree is set in the above range, the reduction percentage is only not more than 4.4 % irrespective of how the ESDD varies, not more than 4.4 % being substantially different from the maximum value. It is clarified that the l/p value which gives a value almost substantially not different from the maximum value in the polluted degree range of 0.03 to 0.5 mg/cm 2 in the actual use condition may be set in any point within the narrow range of 4.65 ±0.35. Thus, it becomes apparent that a composite insulator which always gives nearly the maximum pollution-resisting voltage characteristic over substantially all the actual use condition can be obtained. (2) Relationship between the shed tip-to-tip distance "c" (x) and the resistive voltage (VH: y) (See Table 1 and Fig. 6) i) In case of ESDD = 0.03 mg/cm 2 y = - 0.1532 x 2 + 12.05 x - 137.68 - 0.1532 (x - 39.3) 2 + 99.27 ii) In case of ESDD = 0.5 mg/cm 2 y = - 0.1387 x 2 + 10.154 x - 87.394 = - 0.1387 (x - 36.60) 2 + 98.45 Therefore, it became apparent that the appropriate value of the shed tip-to-tip distance differs depending upon the polluted degree. In case of ESDD = 0.03 mg/cm 2 , the pollution withstand voltage was the maximum at the shed tip-to-tip distance = 39.3 mm, whereas in case of ESDD = 0.5 mg/cm 2 , it was the maximum at the shed tip-to-tip distance = 36.6 mm. The shed tip-to-tip distance which gives the maximum resistive voltage in the case of ESDD = 0.03 mg/cm 2 differs from that giving the maximum resistive voltage in case of ESDD= 0.5 mg/cm 2 by only 2.7 mm. Furthermore, if an intermediate value of 38.0 mm between the above two distances is taken as the shed tip-to-tip distance (38.0 mm being a value calculated by substituting x = l/p in the above expression, such is true in the following), the pollution withstand voltage was reduced to only 99.7 % of its maximum value at the polluted degree of 0.03 mg/cm 2 , whereas the pollution withstand voltage was reduced to only 99.7% of its maximum value at 0.5 mg/cm 2 . Therefore, it was clarified that both were not substantially different from their respective maximum values. Further, if ESDD = 32 mm at 0.03 mg/cm 2 , the pollution withstand voltage was reduced to only 91.6% of its maximum value, whereas if ESDD = 32 mm at 0.5 mg/cm 2 , the pollution withstand voltage was reduced to only 97% of its maximum value. Therefore, it is clarified that if the shed tip-to-tip distance is set in a range of 32-40 mm, the reduction percentage of the pollution withstand voltage is only not more than 8.4 % irrespective of how the ESDD varies, not more than 8.4 % being substantially different from the maximum value. It is clarified that only by setting the shed tip-to-tip distance at any point within the narrow range of 36 ± 4mm, a composite insulator which always gives nearly the maximum pollution-resisting voltage characteristic over substantially all the actual use condition can be obtained. (3) Relationship between the ratio "p/a" (x) of the axial unit length "p" to the radial projection length "a" of the large-diameter shed portion of the housing and the resistive voltage (VH: y) (See Table 1 and Fig. 7) i) In case of ESDD = 0.03 mg/cm 2 y = - 140.82 x 2 + 270.29 x - 30.453 = - 140.82 (x - 0.960) 2 + 99.25 ii) In case of ESDD = 0.5 mg/cm 2 y = - 135.36 x 2 + 236.95 x - 4.9976 = - 135.36 (x - 0.875) 2 + 98.70 Therefore, it became apparent that the appropriate value of the ratio "p/a" of the axial unit length "p" to the radial projection length "a" of the large-diameter shed portion of the housing differs depending upon the polluted degree. In case of ESDD = 0.03 mg/cm 2 , the pollution withstand voltage was the maximum at the "p/a" = 0.960, whereas in case of ESDD = 0.5 mg/cm 2 , it was the maximum at the "p/a"= 0.875. The ratio "p/a" of the axial unit length "p" to the radial projection length "a" of the large-diameter shed portion of the housing which gives the maximum resistive voltage in the case of ESDD = 0.03 mg/cm 2 differs from that giving the maximum resistive voltage in case of ESDD= 0.5 mg/cm 2 by only 0.085. Furthermore, if "p/a" = 0. 875 (0.875 being a value calculated by substituting x = l/p in the above expression, such is true in the following), the pollution withstand voltage was reduced to only 99 % of its maximum value at the polluted degree of 0.03 mg/cm 2 , whereas the pollution withstand voltage was its maximum value for this "p/a" value at 0.5 mg/cm 2 . Therefore, it was clarified that both were not substantially different from their respective maximum values. Further, if "p/a" = 0.75 at the polluted degree of 0.03 mg/cm 2 , the pollution withstand voltage was reduced to only 93.8 % of its maximum value, whereas if the "p/a" = 0.75 or 1.0 at 0.5 mg/cm 2 , the pollution withstand voltage was reduced to only 97.4 % of its maximum value. Therefore, it is clarified that if the "p/a" value is set in a range of 0.75 to 1.0, the reduction percentage of the pollution withstand voltage is only not more than 6.2 % at the maximum irrespective of how the ESDD varies, not more than 6.2 % being substantially different from their maximum values. The reasons why the shape of the housing of the composite insulator gives the maximum pollution-resisting voltage so long as the shape falls in a range are considered as follows: The accurate reasons why each of (1) the appropriate "l/p" value, (2) the appropriate shed tip-to-tip distance "c" and (3) the ratio "p/a" of the axial unit length "p" to the radial projection length of the large-diameter shed portion of the housing gives the maximum pollution withstand voltage characteristic must await future detailed investigations. However, judging from the experiments and study of the present inventors, their knowledge is as follows: (1) The pollution withstand voltage ordinarily increases linearly with increase in the leakage distance. (2) However, if the distance between the adjacent shed portions becomes too short, another factor needs to be considered. That is, spark is likely to occur between the tips of these shed portions when voltage applied in the polluted state. If spark bridges the tips of the shed portions, electric short cut occurs there, so that even if the unit surface-leakage distance is long radially inwardly from there, no effect cannot be expected. This becomes more conspicuous as the shed tip-to-tip distance decreases. In summary, if the above (1) and (2) are superimposed upon each other, the pollution withstand voltage takes its maximum value somewhere with respect to the changes in the "l/p" value, the shed tip-to-tip distance "c" and/or the ratio "p/a" of the axial unit length to the radial projection length "a" of the large-diameter shed portion of the housing. According to the staggered shed type composite insulator of the present invention, the following effects can be obtained. (1) The most excellent pollution withstand voltage characteristic can be obtained by appropriately selecting the changes in the "l/p" value, the shed tip-to-tip distance "c" and/or the ratio "p/a" of the axial unit length to the radial projection length "a" of the large-diameter shed portion of the housing. (2) Since the "l/p" value can be set in an appropriate range, it is unnecessary to excessively increase the use amount of the rubber for the housing so as to obtain the maximum pollution withstand voltage. That is, the composite insulator having a so-called cost performance can be obtained.

Claims: A staggered shed type composite insulator comprising a core member and a housing provided around the outer peripheral face of the core member and made of an insulating polymeric material, said housing comprising large-diameter shed portions, small-diameter shed portions and sheath portions arranged such that the large-diameter shed portions and the small-diameter shed portion alternate in the axial longitudinal direction of the insulator and are separated by the sheath portions, wherein the composite insulator has two or more of the following features (1), (2) and (3): (1) the ratio "l/p" is in a range from 4.3 to 5.0 in which "l" is the surface leakage distance, in a longitudinal axial cross-section of the insulator, between the radially outermost tips of adjacent large diameter sheds and "p" is the axial pitch of the large diameter sheds; (2) the straight-line distance from the radially outermost tip of a large-diameter shed portion to that of the next adjacent small-diameter shed portion is 32 to 40 mm; and (3) the ratio "p/a" is in the range 0.75 to 1.0 in which "p" is as in (1) above and "a" is the radial projection length of the large-diameter shed portions from the sheath portions.

Inventors: FUKAMI YUKITERU, KONDO KUNIAKI, MORIYA TSUTOMU, FUKAMI, YUKITERU, KONDO, KUNIAKI, MORIYA, TSUTOMU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1997
Main classification: H01B17
IPC codes: H01B17