Title of Invention	SUSPENSION INSULATOR.
Abstract	TITLE: SUSPENSION INSULATOR. A suspension insulator having a shed including an upper surface anhd an under surface has a pin fitting arranged at its center portion and projected fromthe under surface and a plurality of circular ribs arranged around the pin fitting in a concentric manner. In a first aspect of the invention, a resistance zone having a surface resistance (1cm x 1cm) of not greater than 4MQ and arranged ont he under surface at an outer peripheral portion of the resitance zone. In a second aspect of the invention, a resistance zone arranged on the under surface at an inner portion continuing from the pin fitting and existing between the pin fitting and an inner root portion of the rib.

Title of Invention

SUSPENSION INSULATOR.

Abstract

TITLE: SUSPENSION INSULATOR. A suspension insulator having a shed including an upper surface anhd an under surface has a pin fitting arranged at its center portion and projected fromthe under surface and a plurality of circular ribs arranged around the pin fitting in a concentric manner. In a first aspect of the invention, a resistance zone having a surface resistance (1cm x 1cm) of not greater than 4MQ and arranged ont he under surface at an outer peripheral portion of the resitance zone. In a second aspect of the invention, a resistance zone arranged on the under surface at an inner portion continuing from the pin fitting and existing between the pin fitting and an inner root portion of the rib.

Full Text	SUSPENSION INSULATOR BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a suspension insulator for high voltage transmission, which improves pollution withstand voltage characteristics. 2. Description of Related Art [0002] In the suspension insulator, if a salt component in seawater carried by wind and weather or a salt component in an exhaust gas from a factory is adhered to a surface of the suspension insulator and the suspension insulator becomes a wet state due to such a salt component, an insulation resistance of a surface of the suspension insulator is decreased. In this case, there is a drawback such that a flashover may occur due to reduction of pollution withstand voltage characteristics. Therefore, in the case of performing an insulation design of power trans- mission equipment, it is very important to take the pollution withstand voltage characteristics of the suspension insulator into consideration. [0003] Generally, it is known that the pollution withstand voltage characteristics can be improved by designing a longer creepage distance of the suspension insulator. Therefore, in the known suspension insulator, a method for designing a longer creepage distance by making a diameter of shed of the suspension insulator larger or by increasing the number of ribs arranged on an under surface of the suspension insulator or a depth between the ribs is adopted. However, in the method mentioned above, there is a drawback such that a dimension of the suspension insulator becomes necessarily larger. [0004] Moreover, except for a geometrical shape design mentioned above, there is known a method such that the pollution withstand voltage characteristics are recovered by arranging a conductive glaze on a surface of the insulator so as to flow a weak current thereon and by drying wet pollution substances adhered to the surface of the insulator by means of a heating effect due to the weak current. Further, a method for improving the pollution withstand voltage characteristics is also actually utilized wherein an electric field relaxation is performed by relaxing a potential inclination along the surface of the insulator and a discharge due to a partly and rapidly deviating electric field. This method mentioned above is very effective, but there arises a deterioration such that a surface of the conductive glaze is sometimes eroded by an environmental pollution or such that a surface resistance is sometimes increased. [0005] In order to solve the drawbacks mentioned above, the present applicant proposed a suspension insulator, in Japanese Patent Application No. 11-369186, such that a circular electrode is arranged on an under surface, to which a glaze is arranged, axially with respect to a pin fitting, or, such that a resistance zone having an electric resistance of 1 - 100 MO, (corresponding to a surface resistance of 4.3 - 430 MO) is arranged on the circular electrode and on a portion between the circular electrode and the pin fitting. The suspension insulator mentioned above shows a sufficient performance for the pollution withstand voltage characteristics that are necessary at that time. However, the suspension insulator mentioned above sometimes shows a poor performance for higher pollution withstand voltage characteristics and a higher discharge suppress performance that are recently required. Therefore, a suspension insulator having higher pollution withstand voltage characteristics and a higher discharge suppress performance is required. SUMMARY OF THE INVENTION [0006] An object of the present invention is to eliminate the drawbacks mentioned above and to provide a suspension insulator which can respectively perform an improvement of pollution withstand voltage characteristics and a prevention of a discharge and which can improve a voltage distribution along an insulator string. [0007] According to a first aspect of the invention, a suspension insulator having a shed including an upper surface and an under surface comprises: a pin fitting arranged at its center portion and projected from the under surface; a plurality of circular ribs arranged around the pin fitting in a concentric manner; a resistance zone having a surface resistance (1 cm x 1 cm) of not greater than 4MO and arranged on the under surface at an inner portion continuing from the pin fitting; and a conductive zone arranged on the under surface at an outer peripheral portion of the resistance zone. [0008] Moreover, according to a second aspect of the invention, a suspension insulator having a shed including an upper surface and an under surface comprises: a pin fitting arranged at its center portion and projected from the under surface; a plurality of circular ribs arranged around the pin fitting in a concentric manner; and a resistance zone arranged on the under surface at an inner portion continuing from the pin fitting and existing between the pin fitting and an inner root portion of the rib. [0009] As mentioned above, since the resistance zone having a predetermined resistance and the conductive zone are arranged in the first aspect of the invention, or, since the resistance zone is arranged at a predetermined position in the second aspect of the invention, a position to which a high electric power is concentrated can be moved outward with respect to the center of insulator, and thus it is possible to form a stable dry zone with a low power. As a result, it is also possible to improve a voltage distribution along the insulator string and thus it is also possible improve pollution withstand voltage characteristics and tc prevent a discharge. In the present invention, a term "a portion between the pin fitting and an inner root portion of the rib" means a portion between the pin fitting and a bottom of the target circular rib that is near to the pin fitting among a plurality of circular ribs. [0010] As a preferred embodiment of the invention, both in the first aspect of the invention in which the resistance zone and the conductive zone are arranged and in the second aspect of the invention in which the resistance zone is arranged at a predetermined position, the following embodiments are preferred since the effects of the invention can be further improved: a material of the resistance zone is a conductive glaze of ferric oxide series or a conductive glaze of tin oxide series; and a resistance zone as is the same as the resistance zone arranged on the under surface at an inner portion continuing from a cap fitting arranged on a head portion of the suspension insulator is arranged on an upper surface thereof, and a conductive zone as is the same as the conductive zone arranged on the under surface is arranged on the upper surface at an outer peripheral portion of the resistance zone. Moreover, in the first aspect of the invention, the following embodiments are preferred since the effects of the invention can be further improved: the conductive zone has a half or less surface resistance as compared with that of the resistance zone; the resistance zone is arranged at a portion between the pin fitting and an inner root portion of the rib; a material of the conduc- tive zone is a metal, a conductive glaze of ferric oxide series having a low resistance or a conductive glaze of tin oxide series having a low resistance, and a thickness of the glaze of the conductive zone is larger than that of the glaze of the resistance zone; and the conductive zone, whose thickness is larger than that of the resistance zone, is formed by arranging a recess to the under surface at a portion to which the conduc- tive zone is formed, and filling a conductive material such as the conductive glaze of ferric oxide series or tin oxide series in the thus formed recess. Furthermore, in the second aspect of the invention, the following embodiment is preferred since the effects of the invention can be improved: the resistance zone has a surface resistance (1 cm x 1 cm) of not greater than 4MO. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS [0011] Figs, la and lb are schematic views respectively showing one embodiment of a suspension insulator according to the first aspect of the invention; Fig. 2 is a partial cross sectional view illustrating another embodiment of the suspension insulator according to the first aspect of the invention; Figs. 3a and 3b are schematic views respectively depicting one embodiment of the suspension insulator according to the second aspect of the invention; Fig. 4 is a schematic view showing one example of the suspension insulator that is used when a theory for improving the pollution withstand voltage and suppressing the discharge is discussed; Fig. 5 is a graph illustrating a relation between a creepage distance from the pin fitting side and an electric power density; Fig. 6 is a flowchart depicting one example of calculation method that is used when a relation between a position of dry zone (resistance zone) and a maximum electric field is discussed; Fig. 7 is a graph showing a relation between the wet/dry step and a maximum creepage electric field; Fig. 8 is a schematic view illustration a relation between the creepage distance from the pin fitting side and an electric field in a creepage direction; Fig. 9 is a flowchart depicting one example of a calculation method for a voltage distribution along the insulator string; Fig. 10 is a graph showing a relation between a wet progress and the allotted voltage; Fig. 11 is a graph illustrating a relation between an electric power at which the pollution 2one starts to be dry and the number of insulators with high allotted voltage or allotted voltage; Figs. 12a and 12b are schematic views respectively depicting a drying state during the pollution test in the case that the insulator string is constructed by the suspension insulators according to the invention (Fig. 12a) and by the known suspension insulator (Fig. 12b); Fig. 13 is a schematic view showing an end position of the resistance zone when it is discussed; Fig. 14 is a schematic view illustrating a surface temperature distribution of the known suspension insulator in which the circular conductive zone and the dry zone (resistance zone) are not arranged; and Fig. 15 is a schematic view depicting a surface temperature distribution of the suspension insulator according to the invention in which the circular conductive zone and the dry zone (resistance zone) are arranged. DESCRIPTION OF THE PREFERRED EMBODIMENT [0012] Figs, la and lb are schematic views respectively showing one embodiment of a suspension insulator according to the first aspect of the invention. Fig. la shows a partial cross sectional view that is viewed from a front side, and Fig. lb illustrates a schematic view that is viewed from an under side. In Figs, la and lb, a suspension insulator 1 accord- ing to the invention has a construction mentioned below. A number of concentric circular ribs 13 are integrally formed to an inner under surface of a shed portion 12 of an insulator body 11. Here, a first circular rib 13-1 and a second circular rib 13-2 are arranged from a near side of a pin fitting 12. In this embodiment, a projection portion 13A that is nearest to the pin fitting 12 is not the circular rib, but a so-called circular hump. A hallow head portion 14 is integrally formed at a center upper portion of the shed portion 12. A generally known insulation glaze is arranged on an overall surface of the insulator body 11 so as to form an insulation glaze zone 15 as an insulation surface. [0013] Sand zones 16 are respectively arranged on an outer side surface and an inner side surface of the head portion 14. A cap fitting 22 of a ground side and the pin fitting 21 of a power supply side are respectively fixed to an outer surface and an inner surface of the head portion 14 of the insulation body 11 via Portland cement. Therefore, the cap fitting 22 and the pin fitting 21 are positioned at both ends of the insulator body 11 respectively. A depression portion 22a for fastening the pin fitting 21 of the other suspension insulator 1 arranged just above is formed in an upper portion of the cap fitting 22. An under portion of the pin fitting 21 is fastened to the depression portion 22a of the other suspension insulator 1 arranged just beneath. In this manner, a number of the suspension insulators 1 according to the invention are connected with each other in series, and this state is called as a string. [0014] Features of the first aspect of the suspension insulator 1 according to the invention lie in the constructions such that a resistance zone 31 having a surface resistance (1 cm x 1 cm) of not greater than 4 MO is arranged on an under surface at an inner portion continuing from the pin fitting 21 and such that a circular conductive zone 32 is arranged on the under surface at an outer peripheral portion of the resistance zone 31. In this case, it is preferred that the circular conductive zone 32 has a half or less surface resistance as compared with that of the resistance zone 31. Moreover, it is preferred that a material of the conductive zone 32 is a metal, a conductive glaze of ferric oxide J f series having a low resistance or a conductive glaze of tin oxide series having a low resistance, and a thickness of the glaze of the conductive zone is larger than that of the glaze of the resistance zone 31. Further, it is preferred that the resistance zone 31 is arranged at a portion between the pin fitting 21 and the inner root portion of the first circular rib 13-1 or between the pin fitting 21 and an inner root portion of the second circular rib 13-2. In the embodiment shown in Fig. 1, the resistance zone 31 is arranged from the pin fitting 21 to the inner root portion of the first circular rib 13-1. Furthermore, it i$ preferred that a material of the resistance zone 31 is a conductive glaze of ferric oxide series or a conductive glaze of thin oxide series. Moreover, it is preferred that a resistance zone as is the same as the resistance zone 31 arranged on the under surface at an inner portion continuing from the cap fitting 22 arranged on the head portion 14 of the suspension insulator 1 is arranged on an upper surface of the suspension insulator 1, and a conductive zone as is the same as the conductive zone 32 arranged on the under surface is arranged on the upper surface at an outer peripheral portion of the resistance zone 31. [0015] Fig. 2 is a partial cross sectional view showing another embodiment of the suspension insulator according to the first aspect of the invention. In the embodiment shown in Fig. 2, portions similar to those of Figs, la and lb are denoted by the same reference numerals as those of Figs, la and lb, and the explanations thereof are omitted here. In the embodiment shown in Fig. 2, features different from the embodi- ment shown in Figs, la and lb are that the conductive zone 32, whose glaze thickness is larger than that of the resistance zone 31, is formed by arranging a recess 33 having a circular cross section to the under surface of the insulator body 11 at a portion to which the circular conductive zone 32 is formed, and filling a conductive material constructing the conductive zone 32 such as the conductive glaze of ferric oxide series or tin oxide series in the thus formed recess 33, Moreover, a shape of the suspension insulator 1 is different from the embodiment shown in Figs, la and lb, and the suspension insulator 1 shown in Fig. 2 has a thin and long rib shape. In the suspension insulator 1 shown in Fig. 2 does not have the circular hump 13A shown in Figs, la and lb. [0016] If the conductive zone 32 is formed as mentioned above, it is possible to further prevent a stress concentration and a concentration of electric stresses to the conductive zone 32, and thus it is a preferred embodiment. Moreover, in an anti-pollution type insulator such as the suspension insulator 1 shown in Fig. 2 having a thin and long rib shape, it is possible to move a position of the dry zone form near the pin fitting to the outer ribs as is clearly understood from the following embodiments. Therefore, it is possible to further improve the effects of the invention such that a wide dry zone can be formed as compared with the suspension insulator 1 shown in Figs, la and lb. [0017] Figs. 3a and 3b are schematic views respectively showing one embodiment of a suspension insulator according to the second aspect of the invention. Fig. 3a shows a partial cross sectional view that is viewed from a front side, and Fig. 3b illustrates a schematic view that is viewed from an under side. In the embodiment shown in Figs. 3a and 3b, portions similar to those of Figs, la and lb are denoted by the same reference numerals as those of Figs, la and lb, and the explanations thereof are omitted here. In the embodiment shown in Figs. 3a and 2b, features different from the embodiment shown in Figs, la and lb are that only the resistance zone 31 is arranged at a portion between the pin fitting 21 and the inner root portion of the circular rib and the conductive zone 32 is not arranged, i.e. the resistance zone 31 is arranged from the pin fitting 21 and the inner root portion of the first circular rib 13-1 or from the pin fitting 21 and the inner root portion of the second circular rib 13-2. In the embodiment shown in Figs. 3a and 2b, the resistance zone 31 is arranged from the pin fitting 21 and the inner root portion of the first circular rib 13-1. [0018] In this case, it is preferred that a surface resistance (1 cm x 1 cm ) of the resistance zone 31 is not greater than 4 MO. Moreover, as is the same as the second aspect of the invention, it is preferred that a material of the resistance zone 31 is a conductive glaze of ferric oxide series or a conductive glaze of tin oxide series. Further, it is preferred that a resistance zone as is the same as the resistance zone 31 continuing from the cap fitting 22 arranged on the head portion 14 of the suspension insulator 1 is arranged on an upper surface thereof, and a conductive zone is arranged on the upper surface at an outer peripheral portion of the resistance zone. [0019] In the first aspect of the invention and the second aspect of the invention mentioned above, since the resistance zone 31 having a predetermined surface resistance and the circular conductive zone 32 are arranged in the first aspect of the invention, and, since the resistance zone 31 is arranged at a predetermined position in the second aspect of the invention, it is possible to move a zone having a high electric power density, that is apt to be positioned near the pin fitting 21 in the known suspension insulators, toward an outer circular ribs, and thus it is possible to form a stable dry zone with a low electric power. As a result, it is also possible to improve a voltage distribution along the insulator string and thus it is possible to improve pollution withstand voltage characteristics and to prevent a discharge. [0020] Then, in the suspension insulator according to the invention having the construction mentioned above, a theory for improving the pollution withstand voltage and suppressing the discharge is discussed. [0021] At first, an electric power density distribution with respect to a single suspension insulator is calculated under a condition such that a surface resistance of the pollution zone is constant. As a suspension insulator used for this calculation, use is made of a known suspension insulator (normal one) in which the resistance zone and the circular conductive zone are not arranged with respect to a base suspension insulator having a diameter of 250 mm and a shape shown in Fig. 4, and a suspension insulator according to the invention in which the resistance zone 31 is arranged from the pin fitting 21 to a rib bottom El positioned at the inner root portion of the first circular rib 13-1 by using the conductive glaze and the conductive zone 32 is arranged at the rib bottom El with respect to the base suspension insulator mentioned above. Calculation conditions are as follows: (1) A surface resistance (1 cm x 1 cm) of the pollution zone : 14.7 MO (resistance per single insulator : 10 MO); (2) A surface resistance (1 cm x 1 cm) of the conductive glaze : 0.5 MO; (3) The electric power density distribution is calculated from an integrated resistance of the pollution zone and the conductive glaze; and (4) A resistance of the conductive glaze between the pin fitting and the circular conductive zone : about 100 kO. [0022] The calculation result is shown in Fig. 5 by a relation between a creepage distance from the pin fitting side where a position of the pin fitting is 0 and an electric power density. From the result shown in Fig. 5, it is understood that, in the known normal suspension insulator in which the resistance zone and the circular conductive zone are not arranged, an electric power density is high at a portion around the pin fitting and a dry zone 41 is formed around the pin fitting 21, so that a discharge occurs around the pin fitting 21. On the other hand, it is understood that, in the suspension insulator according to the invention in which the circular conductive zone 32 is arranged at the rib bottom E1 and the conductive glaze is arranged from the pin fitting 21 to the rib bottom El as the resistance zone 31, an electric power density is decreased at a portion near the pin fitting 21 and thus a portion of the dry zone 41 is moved outward with respect to the circular conductive zone 32. [0023] Then, a relation between a position of the dry zone and a maximum electric field that occur in the suspension insulator is discussed. In the suspension insulator having the construction shown in Fig. 4, it is assumed that the dry zone 41 having a constant width of 40 mm in the creepage direction (radius direction) is formed, at a portion between the cement 18 and the circular hump 13A, at the rib bottom El that is positioned at the inner root portion of the first circular rib 13-1, at the first circular rib 13-1, at a rib bottom E2 that is positioned at the inner root portion of the second circular rib 13-2, or at the second circular rib 13-2. Then, maximum creepage electric fields are calculated per the number of repeating the wet/dry steps. Moreover, in the case that the dry zone 41 is arranged at the first circular rib 13-1 and the second circular rib 13-2, a first circular rib (estimation) and a second circular rib (estimation) that are estimated after a step 4, are also calculated. In this case, the reason, such that the estimation values in the case that the dry zone 41 is arranged at the first circular rib and the second circular rib are calculated, is as follows. That is to say, since a porcelain volume of respective circular ribs 13-1 and 13-2 is small, a temperature of the overall circular rib is increased at constant. There- fore, an effect of reducing an electric field is expected because the overall rib is dried up. The calculation is performed according to a calculation flowchart shown in Fig. 6 under a condition such that a surface resistance of the pollution zone is a constant conductivity at an initial value and is in proportion to the electric power density after the next step. The result of. the calculation is shown in Fig. 7. In Fig. 7, as an electric field at which a discharge occurs, use is made of a value of 5 kV/cm since this value is known in this technical field. [0024] From the result shown in Fig. 7, the followings are understood: An incrcase of the maximum elecric field is rapid in the case of a normal state in which a portion near the cement of the pin fitting side is dried; An increase of the maximum electric field becomes slow when the dry zone is formed outward with respect to the pin fitting; and An increase of the electric field becomes particularly slow in the case that the dry zone is formed at the rib. It is supposed that, since a radius difference between an inner surface and an outer surface is small at the rib portion and since a porcelain volume of the circular rib is small, a temperature of the rib is increased before reaching to the electric field at which a discharge occurs, and thus it is possible to prevent such a discharge. [0025] Then, a relation between a forming position of the dry zone 41 and a variation of electric field is summarized on the basis of the calculation result shown in Fig. 7 and is compared. That is to say, from the result shown in Fig. 7, a relation between a creepage distance from the pin fitting 21 and an electric field in a creepage direction is obtained for respective steps with respect to the suspension insulator in which the dry zone 41 of 40 mm is arranged at the first circular rib 13-1 and the suspension insulator in which the dry zone 41 is arranged around the pin fitting 21 i.e. at a portion between the cement and the circular hump 13A. The result is shown in Fig. 8. In the case that the dry zone 41 of 40 mm is arranged at the first circular rib 13-1, a radius of the inner root portion of the first circular rib 13-1 is 62.1 mm and a radius of a position 40 mm outward (in a creepage direction) from the inner root portion is 74.9 mm. Therefore, a radius ratio between an inner portion and an outer portion is 1.21. On the other hand, in the case that the dry zone 41 of 40 mm is arranged around the pin fitting 21, a radius of the cement portion is 23.4 mm and a radius of a position 40 mm outward (in a creepage direction) from the cement portion is 45.3 mm. Therefore, a radius ratio between an inner portion and an outer portion is 1.94. [0026] Then, characteristics of the insulator string in which a number of suspension insulators are connected in series are investigated. As a factor determines voltage distribution, there is an easily drying property of the dry zone. Allotted voltage in the insulator string is highest at a power supply end and becomes smaller at a center portion. These allotted voltages at the power supply end and at the center portion are different by about 3 times. In order to. simulate this situation easily, it is assumed that an initial resistance of the single suspension insulator is varied in a range of 10 - 30 MO, equidistantly. If a wet/dry operation proceeds by one step from this initial state, it is assumed that the resistance of respective insulators is decreased by 0.7 times. Since a current flowing through the insulator string is increased, power losses of some insulators with high allotted voltage may become greater than the predetermined value. In this case, resistance values of these insulators are increased 1.4 times. The assumptions for calculation are as follows: (1) 200 kV is applied to the insulator string having 20 insulators in series; (2) In order to calculate easily, a calculation is performed under an assumption such that a resistance of the insulator is not varied in accordance with a temperature variation; and (3) Since an impedance of the insulator does not become greater than 50MO due to a capacitance of the insulator, an upper limit of the resistance value is set by arranging a resistor of 50MO in parallel. [0027] A calculation result of a relation between a wet progress and a allotted voltage is shown in Fig. 10, and a calculation result of a relation between an electric power at which the pollution zone starts to be dry and the number of insulators with high allotted voltage is shown in Fig. 11. The wet progress in Fig. 10 shows a numerical value corre- sponding to a time progress. From the result shown in Fig. 10, it is understood that, in an initial state, a voltage load difference is 3 times, but, according to a wet progress, some insulators show high allotted voltage and other insulators show very low allotted voltage. From the result shown in Fig. 11, it is understood that, if the electric power at which the pollution zone starts to be dry becomes lower, the number of the insulators with high allotted voltage is increased and the maximum allotted voltage becomes low. Therefore, it is understood that, the insulator, which can be dried up with a low electric power, shows an excellent voltage distribution along an insulator string. [0028] Then, by performing a pollution test for the insulator string having 20 insulators in series, a drying condition of the suspension insulator is investigated with respect to the suspension insulator according to the invention in which the circular conductive zone and the resistance zone are arranged and the known suspension insulator in which the circular conductive zone and the resistance zone are not arranged. The results are shown in Figs. 12a and 12b. Fig. 12a shows a drying condition of the suspension insulator according to the invention, and Fig. 12b illustrates a drying condition of the known suspension insulator. In Figs. 12a and 12b, a portion colored by red shows a drying state. From the result shown in Fig. 12a, it is understood that, in the insulator string using the suspension insulator according to the invention, 12 suspension insulators among 20 show a drying state at near the second rib, from the result shown in Fig. 12b, it is understood that, in the insulator string using the known suspension insulators, 5 suspension insulators among 20 show a drying state at a portion between the pin fitting and the rib bottom El, and a ratio of the dried up suspension insulator is 25%. [0029] From the results mentioned above, the followings are understood: (1) Since a radius difference between an inner portion and an outer portion is small at the rib portion, an electric field distribution in a creepage direction becomes relatively constant, and thus it is possible to suppress a discharge occurrence; (2) Since a porcelain volume of the rib portion is small, a dry zone can be formed by a low electric power, and thus it is possible to improve a voltage distribution along a string; and (3) From the results mentioned in (1) and (2), a pollution withstand voltage is improved, and thus it is possible to obtain a suspension insulator which can suppress a discharge. [0030] Hereinafter, actual experiments will be explained. Experiment 1 (as to positions of circular conductive zone and resistance zone) In order to obtain a preferred embodiment about the positions of the circular conductive zone and the resistance zone, a suspension insulator was prepared, in which the circular conductive zone (electrode) was arranged at a position among positions (1)-(7) shown in Fig. 13 and the resistance zone was arranged from the pin fitting to the one of the positions (1)-(7) mentioned above. The thus prepared suspension insulators were connected with each other in five series to obtain the insulator string. Then, with respect to the thus obtained insulator string, clean fog tests based on JEC-0201 "AC Voltage Insulation Tests" defined as a standard of the Japanese Electrotechnical Committee were performed under a condition such as a salt deposit density of 0.25 mg/cm2 and a test voltage of 55 kV so as to investigate and estimate discharge condition. The results are shown in the following Table 1. [0031] [0032] From the results shown in Table 1, it was understood that, the suspension insulator, in which the circular conductive zone (electrode) was arranged at near the rib bottom £1 positioned at the inner root portion of the first circular rib 1-3-1 and the resistance zone was arranged from the pin fitting to the rib bottom El, was most effective. Moreover, it was understood that, the suspension insulator, in which the circular conductive zone was arranged at the rib bottom E2 between the first circular rib 13-1 and the second circular rib 13-2, was effective. Further, it was understood that, the suspension insulator, in which the circular conductive zone was arranged at respective ribs, was improved as compared with the known suspension insulator having no conductive zone and resistance zone. [0033] Experiment 2 (as to resistance value of resistance zone) In order to obtain a most preferred zone of a resistance value of the resistance zone, the known suspension insulator in which the circular conductive zone and the resistance zone were not arranged, the comparative suspension insulator in which the circular conductive zone was arranged at the rib bottom El positioned at the inner root portion of the first circular rib 13-1 and the circular conductive zone was electrically connected to the pin fitting, and the suspension insulator according to the invention in which the circular conductive zone was arranged at the rib bottom El positioned at the inner root portion of the first circular rib 13-1 and the resistance zone having a predetermined surface resistance (1 cm x 1 cm) was arranged at a portion between the pin fitting and the rib bottom El, were prepared respectively. [0034] Then, the thus prepared suspension insulators were connected in series in such a manner that 7 units were connected in the known suspension insulator and 5 units were connected both in the comparative suspension insulator and the suspension insulator according to the invention to obtain the insulator strings. Then, with respect to the thus obtained insulator strings, clean fog tests were performed in the same manner as that of experiment 1 under a condition such that a salt deposit density of 0.25 mg/cm2 and a test voltage of 55 kV so as to investigate and estimate discharge occurrence and withstand voltages. The result of the known suspension insulator is shown in the following Table 2, and the results of the comparative suspension insulator and the suspension insulator according to the invention are shown in the following Table 3. In Table 2 and Table 3, the discharge occurrence is shown in such a manner that, when the discharge occurs 2 times among 5 times trials, an indication is 2/5, and the withstand voltage is shown in such a manner that, when 7 times endure among 8 times trials, an indication is 7/8. [0035] [0036] [0037] From the results shown in Table 2 and Table 3, it was under- stood that a surface resistance (1 cm x 1 cm) should be not greater than 4 MO in the present invention. Moreover, from the results of the clean fog tests, it was understood that the comparative suspension insulator, in which the circular conductive zone was electrically connected to the pin fitting, was effective. However, in cold switch on condition in which AC voltage is suddenly applied under polluted and wet condition or in condition in which sea water is sprayed, it is necessary to use a creepage distance between the circular conductive zone and the pin fitting. In the polluted and wet condition, a resistance between the circular conductive zone and the pin fitting becomes a few kO - a few 10 kO. In the suspension insulator according to the invention, the resistance between the circular conductive zone and the pin fitting can be higher resistance such as 50 kO - 1 MO, and thus it is possible to use a creepage distance at this portion effectively. [0038] In the embodiments mentioned above, the explanations are made to the suspension insulator in which the circular conductive zone and the resistance zone are arranged on the under surface. However, the same effects as is the same as the embodiments mentioned above can be obtained, if only the resistance zone having a surface resistance of not greater than 4 MO is arranged on the under surface, and further if the resistance zone as is the same as the resistance zone i.e. the resistance zone having a surface resistance (1 cm x 1 cm) arranged on the under surface at an inner portion continuing from the cap fitting arranged on the head portion of the suspension insulator is arranged on an upper surface thereof, and the conductive zone as is the same as the conductive zone arranged on the under surface is arranged on the upper surface at an outer peripheral portion of the resistance zone. [0039] Experi_me_nt_3,(as to effect of rib shape) In order to investigate an effect of a rib shape, the suspension insulator (CA-845) having a normal rib shape shown in Figs, la and lb and the suspension insulator (CA-894) having a thin and long rib shape shown in Fig. 2 were prepared. Shed shape parameters of respective suspension insulators are shown in the following Table 4. In addition, with respect to the respective suspension insulators, a normal suspension insulators in which the circular conductive zone and the resistance 2one are not arranged and the suspension insulator according to the invention in which the circular conductive zone is arranged at the rib bottom positioned at the inner root portion of the first circular rib 13-1 and the resistance zone having a predetermined surface resistance is arranged at a portion between the pin fitting and the rib bottom, were prepared. In the calcula- tion of CA-894, since the circular hump 13A is calculated as the first circular rib, but the situation is substantially same as that of CA-845. [0040] With respect to the thus prepared suspension insulators, a pollution withstand voltage (kV/unit) was measured according to the clean fog test, when a salt deposit density (=SDD, mg/cm2) is 0.25 or 1.0 and the non-soluble material deposit density (=NSDD, mg/cm2) is 0.1 or 0.2 respectively. The pollution withstand voltage was measured by measuring an overall pollution withstand voltage of the insulator string using the thus prepared suspension insulators according to the invention connected 5 units with each other in series, and calculating an average value per one unit from the thus measured overall pollution withstand voltage value. The result is shown in the following Table 5. [0041] [0043] From the result shown in Table 5, it was understood that CA-894 having a thin and long rib shape can improve the pollution withstand voltage effectively as compared with CA-845 having a normal rib shape. Moreover, even in the CA-845 having a normal rib shape, the effects of the invention were detected slightly. Both in CA-894 and CA-845, it was understood that the discharge occurrence of the suspension insulator according to the invention was very small as compared with the normal suspension insulator. [0044] Experiment 4 (as to dry zone) In order to investigate a state of the dry zone according to the invention, with respect to the suspension insulator according to the invention having a thin and long rib shape in which the circular conductive zone and the resistance zone were arranged as shown in Fig. 2 and the suspension insulator with same shed profile in which the circular conductive zone and the resistance zone were not arranged, a surface temperature distribution at a pollution wet state was measured according to the clean fog test. A surface temperature distribution of the normal suspension insulator is shown in Fig. 14, and a surface temperature distribution of the suspension insulator according to the invention is shown in Fig. 15. If the results of Figs. 14 and 15 were compared, it was understood that the dry zone having a short width was formed at a portion near the pin in the normal suspension insulator, and, on the other hand, a temperature of the rib was increased gradually in a constant manner and a width of the dry zone was widened in the suspension insulator according to the invention. [0045] As mentioned above in detail, according to the invention, since the resistance zone having a predetermined resistance and the conductive zone are arranged in the first aspect of the invention, or, since the resistance zone is arranged at a predetermined position in the second aspect of the invention, a position to which a high electric power is concentrated can be moved outward with respect to the center of insulator, and thus it is possible to form a stable dry zone with a low electric power. As a result, it is also possible to improve a voltage distribution along the insulator string and thus it is possible to improve pollution withstand voltage characteristics and to prevent a discharge. 1. A suspension insulator having a shed with an upper surface and an under surface comprising: a pin fitting arranged at its center portion and projected from the under surface; a plurality of circular ribs arranged around the pin fitting in a concentric manner; a resistance zone having a surface resistance (1 cm x 1 cm) of not greater than 4 MO and arranged on the under surface at an inner portion continuing from the pin fitting; and a conductive zone arranged on the under surface at an outer peripheral portion of the resistance zone. 2. The suspension insulator according to claim 1, wherein the conductive zone has a half or less surface resistance as compared with that of the resistance zone. 3. The suspension insulator according to claim 1, wherein the resistance zone is arranged at a portion between tho pin fitting and an inner root portion of the rib. 4. The suspension insulator according to claim 1, wherein a material of the resistance zone is a conductive glaze of ferric oxide series or a conductive glaze of tin oxide series. 5. The suspension insulator according to claim 1, wherein a material of the conductive zone is a metal, a conductive glaze of ferric oxide series having a low resistance or a conductive glaze of tin oxide series having a low resistance, and a thickness of the glaze of the, conductive zone is larger than that of the glaze of the resistance zone. 6. The suspension insulator according to claim 5, wherein the conductive zone, whose thickness is larger than that of the resistance zone, is formed by arranging a recess to the under surface at a portion to which the conductive zone is formed, and filling a conductive material such as the conductive glaze of ferric oxide series or tin oxide series in the thus formed recess. 7. The suspension insulator according to claim 1, wherein a resistance zone as is the same as the resistance zone arranged on the under surface at an inner portion continuing from a cap fitting arranged on a head portion of the suspension insulator is arranged on an upper surface thereof, and a conductive zone as is the same as the conductive zone arranged on the under surface is arranged on the upper surface at an outer peripheral portion of the resistance zone. 8. A suspension insulator having a shed with an upper surface and an under surface comprising: a pin fitting arranged at its center portion and projected from the under surface; a plurality of circular ribs arranged around the pin fitting in a concentric manner; and a resistance zone arranged on the under surface at an inner portion continuing from the pin fitting and existing between the pin fitting and an inner root portion of the rib. 9. The suspension insulator according to claim 8, wherein the resistance zone has a surface resistance (1 cm x 1 cm) of not greater than 4MO. 10. The suspension insulator according to claim 8, wherein a material of the resistance zone is a conductive glaze of ferric oxide series or a conductive glaze of tin oxide series. 11. The suspension insulator according to claim 8, wherein a resistance zone as is the same as the resistance zone continuing from the cap fitting arranged on the head portion of the suspension insulator is arranged on an upper surface thereof, and a conductive zone is arranged on the upper surface at an outer peripheral of the resistance zone. A suspension insulator having a shed including an upper surface and an under surface has a pin fitting arranged at its center portion and projected from the under surface and a plurality of circular ribs arranged around the pin fitting in a concentric manner. In a first aspect of the invention, a resistance zone having a surface resistance (1 cm x 1 cm) of not greater than 4 MO and arranged on the under surface at an inner portion continuing from the pin fitting and a conductive zone arranged on the under surface at an outer peripheral portion of the resistance zone. In a second aspect of the invention, a resistance zone arranged on the under surface at an inner portion continuing from the pin fitting and existing between the pin fitting and an inner root portion of the rib.

Full Text

SUSPENSION INSULATOR
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a suspension insulator for high
voltage transmission, which improves pollution withstand voltage
characteristics.
2. Description of Related Art
[0002] In the suspension insulator, if a salt component in seawater
carried by wind and weather or a salt component in an exhaust gas from
a factory is adhered to a surface of the suspension insulator and the
suspension insulator becomes a wet state due to such a salt component,
an insulation resistance of a surface of the suspension insulator is
decreased. In this case, there is a drawback such that a flashover may
occur due to reduction of pollution withstand voltage characteristics.
Therefore, in the case of performing an insulation design of power trans-
mission equipment, it is very important to take the pollution withstand
voltage characteristics of the suspension insulator into consideration.
[0003] Generally, it is known that the pollution withstand voltage
characteristics can be improved by designing a longer creepage distance
of the suspension insulator. Therefore, in the known suspension
insulator, a method for designing a longer creepage distance by making a
diameter of shed of the suspension insulator larger or by increasing the
number of ribs arranged on an under surface of the suspension insulator
or a depth between the ribs is adopted. However, in the method
mentioned above, there is a drawback such that a dimension of the
suspension insulator becomes necessarily larger.
[0004] Moreover, except for a geometrical shape design mentioned
above, there is known a method such that the pollution withstand voltage
characteristics are recovered by arranging a conductive glaze on a surface
of the insulator so as to flow a weak current thereon and by drying wet
pollution substances adhered to the surface of the insulator by means of a
heating effect due to the weak current. Further, a method for improving
the pollution withstand voltage characteristics is also actually utilized
wherein an electric field relaxation is performed by relaxing a potential
inclination along the surface of the insulator and a discharge due to a
partly and rapidly deviating electric field. This method mentioned
above is very effective, but there arises a deterioration such that a
surface of the conductive glaze is sometimes eroded by an environmental
pollution or such that a surface resistance is sometimes increased.
[0005] In order to solve the drawbacks mentioned above, the present
applicant proposed a suspension insulator, in Japanese Patent Application
No. 11-369186, such that a circular electrode is arranged on an under
surface, to which a glaze is arranged, axially with respect to a pin fitting,
or, such that a resistance zone having an electric resistance of 1 - 100 MO,
(corresponding to a surface resistance of 4.3 - 430 MO) is arranged on
the circular electrode and on a portion between the circular electrode and
the pin fitting. The suspension insulator mentioned above shows a
sufficient performance for the pollution withstand voltage characteristics
that are necessary at that time. However, the suspension insulator
mentioned above sometimes shows a poor performance for higher
pollution withstand voltage characteristics and a higher discharge
suppress performance that are recently required. Therefore, a suspension
insulator having higher pollution withstand voltage characteristics and a
higher discharge suppress performance is required.
SUMMARY OF THE INVENTION
[0006] An object of the present invention is to eliminate the drawbacks
mentioned above and to provide a suspension insulator which can
respectively perform an improvement of pollution withstand voltage
characteristics and a prevention of a discharge and which can improve a
voltage distribution along an insulator string.
[0007] According to a first aspect of the invention, a suspension
insulator having a shed including an upper surface and an under surface
comprises: a pin fitting arranged at its center portion and projected from
the under surface; a plurality of circular ribs arranged around the pin
fitting in a concentric manner; a resistance zone having a surface
resistance (1 cm x 1 cm) of not greater than 4MO and arranged on the
under surface at an inner portion continuing from the pin fitting; and a
conductive zone arranged on the under surface at an outer peripheral
portion of the resistance zone.
[0008] Moreover, according to a second aspect of the invention, a
suspension insulator having a shed including an upper surface and an
under surface comprises: a pin fitting arranged at its center portion and
projected from the under surface; a plurality of circular ribs arranged
around the pin fitting in a concentric manner; and a resistance zone
arranged on the under surface at an inner portion continuing from the pin
fitting and existing between the pin fitting and an inner root portion of
the rib.
[0009] As mentioned above, since the resistance zone having a
predetermined resistance and the conductive zone are arranged in the
first aspect of the invention, or, since the resistance zone is arranged at a
predetermined position in the second aspect of the invention, a position
to which a high electric power is concentrated can be moved outward
with respect to the center of insulator, and thus it is possible to form a
stable dry zone with a low power. As a result, it is also possible to
improve a voltage distribution along the insulator string and thus it is
also possible improve pollution withstand voltage characteristics and tc
prevent a discharge. In the present invention, a term "a portion between
the pin fitting and an inner root portion of the rib" means a portion
between the pin fitting and a bottom of the target circular rib that is near
to the pin fitting among a plurality of circular ribs.
[0010] As a preferred embodiment of the invention, both in the first
aspect of the invention in which the resistance zone and the conductive
zone are arranged and in the second aspect of the invention in which the
resistance zone is arranged at a predetermined position, the following
embodiments are preferred since the effects of the invention can be
further improved: a material of the resistance zone is a conductive glaze
of ferric oxide series or a conductive glaze of tin oxide series; and a
resistance zone as is the same as the resistance zone arranged on the
under surface at an inner portion continuing from a cap fitting arranged
on a head portion of the suspension insulator is arranged on an upper
surface thereof, and a conductive zone as is the same as the conductive
zone arranged on the under surface is arranged on the upper surface at an
outer peripheral portion of the resistance zone. Moreover, in the first
aspect of the invention, the following embodiments are preferred since
the effects of the invention can be further improved: the conductive zone
has a half or less surface resistance as compared with that of the
resistance zone; the resistance zone is arranged at a portion between the
pin fitting and an inner root portion of the rib; a material of the conduc-
tive zone is a metal, a conductive glaze of ferric oxide series having a
low resistance or a conductive glaze of tin oxide series having a low
resistance, and a thickness of the glaze of the conductive zone is larger
than that of the glaze of the resistance zone; and the conductive zone,
whose thickness is larger than that of the resistance zone, is formed by
arranging a recess to the under surface at a portion to which the conduc-
tive zone is formed, and filling a conductive material such as the
conductive glaze of ferric oxide series or tin oxide series in the thus
formed recess. Furthermore, in the second aspect of the invention, the
following embodiment is preferred since the effects of the invention can
be improved: the resistance zone has a surface resistance (1 cm x 1 cm)
of not greater than 4MO.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0011] Figs, la and lb are schematic views respectively showing one
embodiment of a suspension insulator according to the first aspect of the
invention;
Fig. 2 is a partial cross sectional view illustrating another
embodiment of the suspension insulator according to the first aspect of
the invention;
Figs. 3a and 3b are schematic views respectively depicting one
embodiment of the suspension insulator according to the second aspect
of the invention;
Fig. 4 is a schematic view showing one example of the
suspension insulator that is used when a theory for improving the
pollution withstand voltage and suppressing the discharge is discussed;
Fig. 5 is a graph illustrating a relation between a creepage
distance from the pin fitting side and an electric power density;
Fig. 6 is a flowchart depicting one example of calculation
method that is used when a relation between a position of dry zone
(resistance zone) and a maximum electric field is discussed;
Fig. 7 is a graph showing a relation between the wet/dry step
and a maximum creepage electric field;
Fig. 8 is a schematic view illustration a relation between the
creepage distance from the pin fitting side and an electric field in a
creepage direction;
Fig. 9 is a flowchart depicting one example of a calculation
method for a voltage distribution along the insulator string;
Fig. 10 is a graph showing a relation between a wet progress
and the allotted voltage;
Fig. 11 is a graph illustrating a relation between an electric
power at which the pollution 2one starts to be dry and the number of
insulators with high allotted voltage or allotted voltage;
Figs. 12a and 12b are schematic views respectively depicting a
drying state during the pollution test in the case that the insulator string
is constructed by the suspension insulators according to the invention
(Fig. 12a) and by the known suspension insulator (Fig. 12b);
Fig. 13 is a schematic view showing an end position of the
resistance zone when it is discussed;
Fig. 14 is a schematic view illustrating a surface temperature
distribution of the known suspension insulator in which the circular
conductive zone and the dry zone (resistance zone) are not arranged; and
Fig. 15 is a schematic view depicting a surface temperature
distribution of the suspension insulator according to the invention in
which the circular conductive zone and the dry zone (resistance zone) are
arranged.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0012] Figs, la and lb are schematic views respectively showing one
embodiment of a suspension insulator according to the first aspect of the
invention. Fig. la shows a partial cross sectional view that is viewed
from a front side, and Fig. lb illustrates a schematic view that is viewed
from an under side. In Figs, la and lb, a suspension insulator 1 accord-
ing to the invention has a construction mentioned below. A number of
concentric circular ribs 13 are integrally formed to an inner under surface
of a shed portion 12 of an insulator body 11. Here, a first circular rib
13-1 and a second circular rib 13-2 are arranged from a near side of a pin
fitting 12. In this embodiment, a projection portion 13A that is nearest
to the pin fitting 12 is not the circular rib, but a so-called circular hump.
A hallow head portion 14 is integrally formed at a center upper portion
of the shed portion 12. A generally known insulation glaze is arranged
on an overall surface of the insulator body 11 so as to form an insulation
glaze zone 15 as an insulation surface.
[0013] Sand zones 16 are respectively arranged on an outer side
surface and an inner side surface of the head portion 14. A cap fitting
22 of a ground side and the pin fitting 21 of a power supply side are
respectively fixed to an outer surface and an inner surface of the head
portion 14 of the insulation body 11 via Portland cement. Therefore,
the cap fitting 22 and the pin fitting 21 are positioned at both ends of the
insulator body 11 respectively. A depression portion 22a for fastening
the pin fitting 21 of the other suspension insulator 1 arranged just above
is formed in an upper portion of the cap fitting 22. An under portion of
the pin fitting 21 is fastened to the depression portion 22a of the other
suspension insulator 1 arranged just beneath. In this manner, a number
of the suspension insulators 1 according to the invention are connected
with each other in series, and this state is called as a string.
[0014] Features of the first aspect of the suspension insulator 1
according to the invention lie in the constructions such that a resistance
zone 31 having a surface resistance (1 cm x 1 cm) of not greater than
4 MO is arranged on an under surface at an inner portion continuing
from the pin fitting 21 and such that a circular conductive zone 32 is
arranged on the under surface at an outer peripheral portion of the
resistance zone 31. In this case, it is preferred that the circular
conductive zone 32 has a half or less surface resistance as compared with
that of the resistance zone 31. Moreover, it is preferred that a material
of the conductive zone 32 is a metal, a conductive glaze of ferric oxide J f
series having a low resistance or a conductive glaze of tin oxide series
having a low resistance, and a thickness of the glaze of the conductive
zone is larger than that of the glaze of the resistance zone 31. Further,
it is preferred that the resistance zone 31 is arranged at a portion
between the pin fitting 21 and the inner root portion of the first circular
rib 13-1 or between the pin fitting 21 and an inner root portion of the
second circular rib 13-2. In the embodiment shown in Fig. 1, the
resistance zone 31 is arranged from the pin fitting 21 to the inner root
portion of the first circular rib 13-1. Furthermore, it i$ preferred that a
material of the resistance zone 31 is a conductive glaze of ferric oxide
series or a conductive glaze of thin oxide series. Moreover, it is
preferred that a resistance zone as is the same as the resistance zone 31
arranged on the under surface at an inner portion continuing from the cap
fitting 22 arranged on the head portion 14 of the suspension insulator 1
is arranged on an upper surface of the suspension insulator 1, and a
conductive zone as is the same as the conductive zone 32 arranged on the
under surface is arranged on the upper surface at an outer peripheral
portion of the resistance zone 31.
[0015] Fig. 2 is a partial cross sectional view showing another
embodiment of the suspension insulator according to the first aspect of
the invention. In the embodiment shown in Fig. 2, portions similar to
those of Figs, la and lb are denoted by the same reference numerals as
those of Figs, la and lb, and the explanations thereof are omitted here.
In the embodiment shown in Fig. 2, features different from the embodi-
ment shown in Figs, la and lb are that the conductive zone 32, whose
glaze thickness is larger than that of the resistance zone 31, is formed by
arranging a recess 33 having a circular cross section to the under surface
of the insulator body 11 at a portion to which the circular conductive
zone 32 is formed, and filling a conductive material constructing the
conductive zone 32 such as the conductive glaze of ferric oxide series or
tin oxide series in the thus formed recess 33, Moreover, a shape of the
suspension insulator 1 is different from the embodiment shown in
Figs, la and lb, and the suspension insulator 1 shown in Fig. 2 has a thin
and long rib shape. In the suspension insulator 1 shown in Fig. 2 does
not have the circular hump 13A shown in Figs, la and lb.
[0016] If the conductive zone 32 is formed as mentioned above, it is
possible to further prevent a stress concentration and a concentration of
electric stresses to the conductive zone 32, and thus it is a preferred
embodiment. Moreover, in an anti-pollution type insulator such as the
suspension insulator 1 shown in Fig. 2 having a thin and long rib shape,
it is possible to move a position of the dry zone form near the pin fitting
to the outer ribs as is clearly understood from the following embodiments.
Therefore, it is possible to further improve the effects of the invention
such that a wide dry zone can be formed as compared with the
suspension insulator 1 shown in Figs, la and lb.
[0017] Figs. 3a and 3b are schematic views respectively showing one
embodiment of a suspension insulator according to the second aspect of
the invention. Fig. 3a shows a partial cross sectional view that is
viewed from a front side, and Fig. 3b illustrates a schematic view that is
viewed from an under side. In the embodiment shown in Figs. 3a and
3b, portions similar to those of Figs, la and lb are denoted by the same
reference numerals as those of Figs, la and lb, and the explanations
thereof are omitted here. In the embodiment shown in Figs. 3a and 2b,
features different from the embodiment shown in Figs, la and lb are that
only the resistance zone 31 is arranged at a portion between the pin
fitting 21 and the inner root portion of the circular rib and the conductive
zone 32 is not arranged, i.e. the resistance zone 31 is arranged from the
pin fitting 21 and the inner root portion of the first circular rib 13-1 or
from the pin fitting 21 and the inner root portion of the second circular
rib 13-2. In the embodiment shown in Figs. 3a and 2b, the resistance
zone 31 is arranged from the pin fitting 21 and the inner root portion of
the first circular rib 13-1.
[0018] In this case, it is preferred that a surface resistance (1 cm x
1 cm ) of the resistance zone 31 is not greater than 4 MO. Moreover, as
is the same as the second aspect of the invention, it is preferred that a
material of the resistance zone 31 is a conductive glaze of ferric oxide
series or a conductive glaze of tin oxide series. Further, it is preferred
that a resistance zone as is the same as the resistance zone 31 continuing
from the cap fitting 22 arranged on the head portion 14 of the suspension
insulator 1 is arranged on an upper surface thereof, and a conductive
zone is arranged on the upper surface at an outer peripheral portion of
the resistance zone.
[0019] In the first aspect of the invention and the second aspect of the
invention mentioned above, since the resistance zone 31 having a
predetermined surface resistance and the circular conductive zone 32 are
arranged in the first aspect of the invention, and, since the resistance
zone 31 is arranged at a predetermined position in the second aspect of
the invention, it is possible to move a zone having a high electric power
density, that is apt to be positioned near the pin fitting 21 in the known
suspension insulators, toward an outer circular ribs, and thus it is
possible to form a stable dry zone with a low electric power. As a
result, it is also possible to improve a voltage distribution along the
insulator string and thus it is possible to improve pollution withstand
voltage characteristics and to prevent a discharge.
[0020] Then, in the suspension insulator according to the invention
having the construction mentioned above, a theory for improving the
pollution withstand voltage and suppressing the discharge is discussed.
[0021] At first, an electric power density distribution with respect to a
single suspension insulator is calculated under a condition such that a
surface resistance of the pollution zone is constant. As a suspension
insulator used for this calculation, use is made of a known suspension
insulator (normal one) in which the resistance zone and the circular
conductive zone are not arranged with respect to a base suspension
insulator having a diameter of 250 mm and a shape shown in Fig. 4, and
a suspension insulator according to the invention in which the resistance
zone 31 is arranged from the pin fitting 21 to a rib bottom El positioned
at the inner root portion of the first circular rib 13-1 by using the
conductive glaze and the conductive zone 32 is arranged at the rib
bottom El with respect to the base suspension insulator mentioned above.
Calculation conditions are as follows:
(1) A surface resistance (1 cm x 1 cm) of the pollution zone : 14.7 MO
(resistance per single insulator : 10 MO);
(2) A surface resistance (1 cm x 1 cm) of the conductive glaze : 0.5 MO;
(3) The electric power density distribution is calculated from an
integrated resistance of the pollution zone and the conductive
glaze; and
(4) A resistance of the conductive glaze between the pin fitting and the
circular conductive zone : about 100 kO.
[0022] The calculation result is shown in Fig. 5 by a relation between
a creepage distance from the pin fitting side where a position of the pin
fitting is 0 and an electric power density. From the result shown in
Fig. 5, it is understood that, in the known normal suspension insulator in
which the resistance zone and the circular conductive zone are not
arranged, an electric power density is high at a portion around the pin
fitting and a dry zone 41 is formed around the pin fitting 21, so that a
discharge occurs around the pin fitting 21. On the other hand, it is
understood that, in the suspension insulator according to the invention in
which the circular conductive zone 32 is arranged at the rib bottom E1
and the conductive glaze is arranged from the pin fitting 21 to the rib
bottom El as the resistance zone 31, an electric power density is
decreased at a portion near the pin fitting 21 and thus a portion of the
dry zone 41 is moved outward with respect to the circular conductive
zone 32.
[0023] Then, a relation between a position of the dry zone and a
maximum electric field that occur in the suspension insulator is
discussed. In the suspension insulator having the construction shown in
Fig. 4, it is assumed that the dry zone 41 having a constant width of 40
mm in the creepage direction (radius direction) is formed, at a portion
between the cement 18 and the circular hump 13A, at the rib bottom El
that is positioned at the inner root portion of the first circular rib 13-1, at
the first circular rib 13-1, at a rib bottom E2 that is positioned at the inner
root portion of the second circular rib 13-2, or at the second circular rib
13-2. Then, maximum creepage electric fields are calculated per the
number of repeating the wet/dry steps. Moreover, in the case that the
dry zone 41 is arranged at the first circular rib 13-1 and the second
circular rib 13-2, a first circular rib (estimation) and a second circular
rib (estimation) that are estimated after a step 4, are also calculated.
In this case, the reason, such that the estimation values in the case that
the dry zone 41 is arranged at the first circular rib and the second
circular rib are calculated, is as follows. That is to say, since a
porcelain volume of respective circular ribs 13-1 and 13-2 is small, a
temperature of the overall circular rib is increased at constant. There-
fore, an effect of reducing an electric field is expected because the
overall rib is dried up. The calculation is performed according to a
calculation flowchart shown in Fig. 6 under a condition such that a
surface resistance of the pollution zone is a constant conductivity at an
initial value and is in proportion to the electric power density after the
next step. The result of. the calculation is shown in Fig. 7. In Fig. 7,
as an electric field at which a discharge occurs, use is made of a value of
5 kV/cm since this value is known in this technical field.
[0024] From the result shown in Fig. 7, the followings are understood:
An incrcase of the maximum elecric field is rapid in the case
of a normal state in which a portion near the cement of the pin fitting
side is dried;
An increase of the maximum electric field becomes slow when
the dry zone is formed outward with respect to the pin fitting; and
An increase of the electric field becomes particularly slow in
the case that the dry zone is formed at the rib. It is supposed that, since
a radius difference between an inner surface and an outer surface is small
at the rib portion and since a porcelain volume of the circular rib is small,
a temperature of the rib is increased before reaching to the electric field
at which a discharge occurs, and thus it is possible to prevent such a
discharge.
[0025] Then, a relation between a forming position of the dry zone 41
and a variation of electric field is summarized on the basis of the
calculation result shown in Fig. 7 and is compared. That is to say, from
the result shown in Fig. 7, a relation between a creepage distance from
the pin fitting 21 and an electric field in a creepage direction is obtained
for respective steps with respect to the suspension insulator in which the
dry zone 41 of 40 mm is arranged at the first circular rib 13-1 and the
suspension insulator in which the dry zone 41 is arranged around the pin
fitting 21 i.e. at a portion between the cement and the circular hump 13A.
The result is shown in Fig. 8. In the case that the dry zone 41 of 40 mm
is arranged at the first circular rib 13-1, a radius of the inner root portion
of the first circular rib 13-1 is 62.1 mm and a radius of a position 40 mm
outward (in a creepage direction) from the inner root portion is 74.9 mm.
Therefore, a radius ratio between an inner portion and an outer portion is
1.21. On the other hand, in the case that the dry zone 41 of 40 mm is
arranged around the pin fitting 21, a radius of the cement portion is
23.4 mm and a radius of a position 40 mm outward (in a creepage
direction) from the cement portion is 45.3 mm. Therefore, a radius
ratio between an inner portion and an outer portion is 1.94.
[0026] Then, characteristics of the insulator string in which a number
of suspension insulators are connected in series are investigated. As a
factor determines voltage distribution, there is an easily drying property
of the dry zone. Allotted voltage in the insulator string is highest at a
power supply end and becomes smaller at a center portion. These
allotted voltages at the power supply end and at the center portion are
different by about 3 times. In order to. simulate this situation easily, it
is assumed that an initial resistance of the single suspension insulator is
varied in a range of 10 - 30 MO, equidistantly. If a wet/dry operation
proceeds by one step from this initial state, it is assumed that the
resistance of respective insulators is decreased by 0.7 times. Since a
current flowing through the insulator string is increased, power losses of
some insulators with high allotted voltage may become greater than the
predetermined value. In this case, resistance values of these insulators
are increased 1.4 times. The assumptions for calculation are as follows:
(1) 200 kV is applied to the insulator string having 20 insulators in series;
(2) In order to calculate easily, a calculation is performed under an
assumption such that a resistance of the insulator is not varied in
accordance with a temperature variation; and
(3) Since an impedance of the insulator does not become greater than
50MO due to a capacitance of the insulator, an upper limit of the
resistance value is set by arranging a resistor of 50MO in parallel.
[0027] A calculation result of a relation between a wet progress and a
allotted voltage is shown in Fig. 10, and a calculation result of a relation
between an electric power at which the pollution zone starts to be dry
and the number of insulators with high allotted voltage is shown in
Fig. 11. The wet progress in Fig. 10 shows a numerical value corre-
sponding to a time progress. From the result shown in Fig. 10, it is
understood that, in an initial state, a voltage load difference is 3 times,
but, according to a wet progress, some insulators show high allotted
voltage and other insulators show very low allotted voltage. From the
result shown in Fig. 11, it is understood that, if the electric power at
which the pollution zone starts to be dry becomes lower, the number of
the insulators with high allotted voltage is increased and the maximum
allotted voltage becomes low. Therefore, it is understood that, the
insulator, which can be dried up with a low electric power, shows an
excellent voltage distribution along an insulator string.
[0028] Then, by performing a pollution test for the insulator string
having 20 insulators in series, a drying condition of the suspension
insulator is investigated with respect to the suspension insulator according
to the invention in which the circular conductive zone and the resistance
zone are arranged and the known suspension insulator in which the
circular conductive zone and the resistance zone are not arranged.
The results are shown in Figs. 12a and 12b. Fig. 12a shows a drying
condition of the suspension insulator according to the invention, and
Fig. 12b illustrates a drying condition of the known suspension insulator.
In Figs. 12a and 12b, a portion colored by red shows a drying state.
From the result shown in Fig. 12a, it is understood that, in the insulator
string using the suspension insulator according to the invention, 12
suspension insulators among 20 show a drying state at near the second rib,
from the result shown in Fig. 12b, it is understood that, in the insulator
string using the known suspension insulators, 5 suspension insulators
among 20 show a drying state at a portion between the pin fitting and the
rib bottom El, and a ratio of the dried up suspension insulator is 25%.
[0029] From the results mentioned above, the followings are
understood:
(1) Since a radius difference between an inner portion and an outer
portion is small at the rib portion, an electric field distribution in a
creepage direction becomes relatively constant, and thus it is
possible to suppress a discharge occurrence;
(2) Since a porcelain volume of the rib portion is small, a dry zone can
be formed by a low electric power, and thus it is possible to
improve a voltage distribution along a string; and
(3) From the results mentioned in (1) and (2), a pollution withstand
voltage is improved, and thus it is possible to obtain a suspension
insulator which can suppress a discharge.
[0030] Hereinafter, actual experiments will be explained.
Experiment 1 (as to positions of circular conductive zone and resistance
zone)
In order to obtain a preferred embodiment about the positions
of the circular conductive zone and the resistance zone, a suspension
insulator was prepared, in which the circular conductive zone (electrode)
was arranged at a position among positions (1)-(7) shown in Fig. 13 and
the resistance zone was arranged from the pin fitting to the one of the
positions (1)-(7) mentioned above. The thus prepared suspension
insulators were connected with each other in five series to obtain the
insulator string. Then, with respect to the thus obtained insulator string,
clean fog tests based on JEC-0201 "AC Voltage Insulation Tests" defined
as a standard of the Japanese Electrotechnical Committee were performed
under a condition such as a salt deposit density of 0.25 mg/cm2 and a test
voltage of 55 kV so as to investigate and estimate discharge condition.
The results are shown in the following Table 1.
[0031]
[0032] From the results shown in Table 1, it was understood that, the
suspension insulator, in which the circular conductive zone (electrode)
was arranged at near the rib bottom £1 positioned at the inner root
portion of the first circular rib 1-3-1 and the resistance zone was arranged
from the pin fitting to the rib bottom El, was most effective. Moreover,
it was understood that, the suspension insulator, in which the circular
conductive zone was arranged at the rib bottom E2 between the first
circular rib 13-1 and the second circular rib 13-2, was effective. Further,
it was understood that, the suspension insulator, in which the circular
conductive zone was arranged at respective ribs, was improved as
compared with the known suspension insulator having no conductive
zone and resistance zone.
[0033]
Experiment 2 (as to resistance value of resistance zone)
In order to obtain a most preferred zone of a resistance value
of the resistance zone, the known suspension insulator in which the
circular conductive zone and the resistance zone were not arranged, the
comparative suspension insulator in which the circular conductive zone
was arranged at the rib bottom El positioned at the inner root portion of
the first circular rib 13-1 and the circular conductive zone was
electrically connected to the pin fitting, and the suspension insulator
according to the invention in which the circular conductive zone was
arranged at the rib bottom El positioned at the inner root portion of the
first circular rib 13-1 and the resistance zone having a predetermined
surface resistance (1 cm x 1 cm) was arranged at a portion between the
pin fitting and the rib bottom El, were prepared respectively.
[0034] Then, the thus prepared suspension insulators were connected
in series in such a manner that 7 units were connected in the known
suspension insulator and 5 units were connected both in the comparative
suspension insulator and the suspension insulator according to the
invention to obtain the insulator strings. Then, with respect to the thus
obtained insulator strings, clean fog tests were performed in the same
manner as that of experiment 1 under a condition such that a salt deposit
density of 0.25 mg/cm2 and a test voltage of 55 kV so as to investigate
and estimate discharge occurrence and withstand voltages. The result
of the known suspension insulator is shown in the following Table 2, and
the results of the comparative suspension insulator and the suspension
insulator according to the invention are shown in the following Table 3.
In Table 2 and Table 3, the discharge occurrence is shown in such a
manner that, when the discharge occurs 2 times among 5 times trials, an
indication is 2/5, and the withstand voltage is shown in such a manner
that, when 7 times endure among 8 times trials, an indication is 7/8.
[0035]
[0036]
[0037] From the results shown in Table 2 and Table 3, it was under-
stood that a surface resistance (1 cm x 1 cm) should be not greater than
4 MO in the present invention. Moreover, from the results of the clean
fog tests, it was understood that the comparative suspension insulator, in
which the circular conductive zone was electrically connected to the pin
fitting, was effective. However, in cold switch on condition in which
AC voltage is suddenly applied under polluted and wet condition or in
condition in which sea water is sprayed, it is necessary to use a creepage
distance between the circular conductive zone and the pin fitting.
In the polluted and wet condition, a resistance between the circular
conductive zone and the pin fitting becomes a few kO - a few 10 kO.
In the suspension insulator according to the invention, the resistance
between the circular conductive zone and the pin fitting can be higher
resistance such as 50 kO - 1 MO, and thus it is possible to use a creepage
distance at this portion effectively.
[0038] In the embodiments mentioned above, the explanations are
made to the suspension insulator in which the circular conductive zone
and the resistance zone are arranged on the under surface. However, the
same effects as is the same as the embodiments mentioned above can be
obtained, if only the resistance zone having a surface resistance of not
greater than 4 MO is arranged on the under surface, and further if the
resistance zone as is the same as the resistance zone i.e. the resistance
zone having a surface resistance (1 cm x 1 cm) arranged on the under
surface at an inner portion continuing from the cap fitting arranged on
the head portion of the suspension insulator is arranged on an upper
surface thereof, and the conductive zone as is the same as the conductive
zone arranged on the under surface is arranged on the upper surface at an
outer peripheral portion of the resistance zone.
[0039]
Experi_me_nt_3,(as to effect of rib shape)
In order to investigate an effect of a rib shape, the suspension
insulator (CA-845) having a normal rib shape shown in Figs, la and lb and
the suspension insulator (CA-894) having a thin and long rib shape shown
in Fig. 2 were prepared. Shed shape parameters of respective suspension
insulators are shown in the following Table 4. In addition, with respect
to the respective suspension insulators, a normal suspension insulators in
which the circular conductive zone and the resistance 2one are not
arranged and the suspension insulator according to the invention in
which the circular conductive zone is arranged at the rib bottom positioned
at the inner root portion of the first circular rib 13-1 and the resistance
zone having a predetermined surface resistance is arranged at a portion
between the pin fitting and the rib bottom, were prepared. In the calcula-
tion of CA-894, since the circular hump 13A is calculated as the first
circular rib, but the situation is substantially same as that of CA-845.
[0040] With respect to the thus prepared suspension insulators, a
pollution withstand voltage (kV/unit) was measured according to the
clean fog test, when a salt deposit density (=SDD, mg/cm2) is 0.25 or 1.0
and the non-soluble material deposit density (=NSDD, mg/cm2) is 0.1 or
0.2 respectively. The pollution withstand voltage was measured by
measuring an overall pollution withstand voltage of the insulator string
using the thus prepared suspension insulators according to the invention
connected 5 units with each other in series, and calculating an average
value per one unit from the thus measured overall pollution withstand
voltage value. The result is shown in the following Table 5.
[0041]
[0043] From the result shown in Table 5, it was understood that CA-894
having a thin and long rib shape can improve the pollution withstand
voltage effectively as compared with CA-845 having a normal rib shape.
Moreover, even in the CA-845 having a normal rib shape, the effects of
the invention were detected slightly. Both in CA-894 and CA-845, it
was understood that the discharge occurrence of the suspension insulator
according to the invention was very small as compared with the normal
suspension insulator.
[0044]
Experiment 4 (as to dry zone)
In order to investigate a state of the dry zone according to the
invention, with respect to the suspension insulator according to the
invention having a thin and long rib shape in which the circular
conductive zone and the resistance zone were arranged as shown in
Fig. 2 and the suspension insulator with same shed profile in which the
circular conductive zone and the resistance zone were not arranged, a
surface temperature distribution at a pollution wet state was measured
according to the clean fog test. A surface temperature distribution of
the normal suspension insulator is shown in Fig. 14, and a surface
temperature distribution of the suspension insulator according to the
invention is shown in Fig. 15. If the results of Figs. 14 and 15 were
compared, it was understood that the dry zone having a short width was
formed at a portion near the pin in the normal suspension insulator, and,
on the other hand, a temperature of the rib was increased gradually in a
constant manner and a width of the dry zone was widened in the
suspension insulator according to the invention.
[0045] As mentioned above in detail, according to the invention, since
the resistance zone having a predetermined resistance and the conductive
zone are arranged in the first aspect of the invention, or, since the
resistance zone is arranged at a predetermined position in the second
aspect of the invention, a position to which a high electric power is
concentrated can be moved outward with respect to the center of
insulator, and thus it is possible to form a stable dry zone with a low
electric power. As a result, it is also possible to improve a voltage
distribution along the insulator string and thus it is possible to improve
pollution withstand voltage characteristics and to prevent a discharge.

1. A suspension insulator having a shed with an upper
surface and an under surface comprising:
a pin fitting arranged at its center portion and projected from the
under surface;
a plurality of circular ribs arranged around the pin fitting in a
concentric manner;
a resistance zone having a surface resistance (1 cm x 1 cm) of not
greater than 4 MO and arranged on the under surface at an inner portion
continuing from the pin fitting; and
a conductive zone arranged on the under surface at an outer
peripheral portion of the resistance zone.
2. The suspension insulator according to claim 1, wherein the
conductive zone has a half or less surface resistance as compared with
that of the resistance zone.
3. The suspension insulator according to claim 1, wherein the
resistance zone is arranged at a portion between tho pin fitting and an
inner root portion of the rib.
4. The suspension insulator according to claim 1, wherein a
material of the resistance zone is a conductive glaze of ferric oxide
series or a conductive glaze of tin oxide series.
5. The suspension insulator according to claim 1, wherein a
material of the conductive zone is a metal, a conductive glaze of ferric
oxide series having a low resistance or a conductive glaze of tin oxide
series having a low resistance, and a thickness of the glaze of the,
conductive zone is larger than that of the glaze of the resistance zone.
6. The suspension insulator according to claim 5, wherein the
conductive zone, whose thickness is larger than that of the resistance
zone, is formed by arranging a recess to the under surface at a portion to
which the conductive zone is formed, and filling a conductive material
such as the conductive glaze of ferric oxide series or tin oxide series in
the thus formed recess.
7. The suspension insulator according to claim 1, wherein a
resistance zone as is the same as the resistance zone arranged on the
under surface at an inner portion continuing from a cap fitting arranged
on a head portion of the suspension insulator is arranged on an upper
surface thereof, and a conductive zone as is the same as the conductive
zone arranged on the under surface is arranged on the upper surface at an
outer peripheral portion of the resistance zone.

8. A suspension insulator having a shed with an upper
surface and an under surface comprising:
a pin fitting arranged at its center portion and projected from the
under surface;
a plurality of circular ribs arranged around the pin fitting in a
concentric manner; and
a resistance zone arranged on the under surface at an inner portion
continuing from the pin fitting and existing between the pin fitting and
an inner root portion of the rib.
9. The suspension insulator according to claim 8, wherein the
resistance zone has a surface resistance (1 cm x 1 cm) of not greater than
4MO.
10. The suspension insulator according to claim 8, wherein a
material of the resistance zone is a conductive glaze of ferric oxide
series or a conductive glaze of tin oxide series.
11. The suspension insulator according to claim 8, wherein a
resistance zone as is the same as the resistance zone continuing from the
cap fitting arranged on the head portion of the suspension insulator is
arranged on an upper surface thereof, and a conductive zone is arranged
on the upper surface at an outer peripheral of the resistance zone.
A suspension insulator having a shed including an upper
surface and an under surface has a pin fitting arranged at its center
portion and projected from the under surface and a plurality of circular
ribs arranged around the pin fitting in a concentric manner. In a first
aspect of the invention, a resistance zone having a surface resistance
(1 cm x 1 cm) of not greater than 4 MO and arranged on the under
surface at an inner portion continuing from the pin fitting and a
conductive zone arranged on the under surface at an outer peripheral
portion of the resistance zone. In a second aspect of the invention, a
resistance zone arranged on the under surface at an inner portion
continuing from the pin fitting and existing between the pin fitting and
an inner root portion of the rib.

Documents:

475-cal-2001-granted-abstract.pdf

475-cal-2001-granted-claims.pdf

475-cal-2001-granted-correspondence.pdf

475-cal-2001-granted-description (complete).pdf

475-cal-2001-granted-drawings.pdf

475-cal-2001-granted-form 1.pdf

475-cal-2001-granted-form 18.pdf

475-cal-2001-granted-form 2.pdf

475-cal-2001-granted-form 26.pdf

475-cal-2001-granted-form 3.pdf

475-cal-2001-granted-form 5.pdf

475-cal-2001-granted-letter patent.pdf

475-cal-2001-granted-priority document.pdf

475-cal-2001-granted-reply to examination report.pdf

475-cal-2001-granted-specification.pdf

475-cal-2001-granted-translated copy of priority document.pdf

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Patent Number

218702

Indian Patent Application Number

475/CAL/2001

PG Journal Number

15/2008

Publication Date

11-Apr-2008

Grant Date

09-Apr-2008

Date of Filing

27-Aug-2001

Name of Patentee

NGK INSULATORS, LTD.

Applicant Address

2-56 SUDA-CHO, MIZUHO-KU, NAGOYA CITY, AICHI PREF, JAPAN.

Inventors:

#	Inventor's Name	Inventor's Address
1	IRIE TAKASHI	C/O. NGK INSULATORS LTD. 2-56 SUDA-CHO, MIZUHO-KU, NAGOYA CITY, AICHI PREF, JAPAN.
2	OGAWA SHIGERU	-DO-
3	SATO KENJI	-DO-
4	AKIZUKI MASAHIRO	-DO-

PCT International Classification Number

H01B 17/02

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2000-257,294	2000-08-28	Japan
2	2001-211,068	2001-07-11	Japan