GB2173355A - Suspension insulator - Google Patents

Abstract

A multi-shed suspension insulator comprises an insulator body 1, a metal cap 2 and a metal pin 4. The insulator body has at least two sheds 6,7, monolithic with the insulator body. To obtain the maximum withstand voltage with shortest insulator height, the ratio (L/P) of the surface leakage distance L (A to B) to the vertical shed pitch P is within the range of 5</=L/P</=9. In another embodiment (Fig. 2) the sheds 6,7 do not have skirts. In a further embodiment (Fig. 3) a small shed (8) is interposed between the others. Up to five sheds may be provided. <IMAGE>

Description

SPECIFICATION Suspension insulator The present invention relates to a suspension insulator used for insulating and supporting a power transmission conductor in an overhead power transmission line, and to an insulator string formed of such insulators.

A conventional suspension insulator for supporting a power transmission conductor in an overhead power transmission line, as illustrated in Fig. 6 of the accompanying drawings, comprises an insulator body 21, a metal cap 22 bonded to the upper end of the insulator body, and a metal pin 24 bonded to the lower side of the insulator body. Both of the metal cap 22 and metal pin 24 are bonded to the insulator body 21 by cement 23, and the insulator body 21 has one shed 25 projecting laterally from around its outer periphery. When supporting a high-voltage power transmission conductor, there is used an insulator string consisting of a large number of these suspension insulators connected in series, to obtain the desired withstand voltage.In the conventional suspension insulator illustrated in Fig. 6, however, the metal cap 22 which does not contribute to the withstand voltage occupies about 1/2 of the whole string length, and therefore when a large number of suspension insulators are linked in series, the resulting insulator string has a considerable length. As a result, the pylon or pole should be large to achieve the necessary clearance e.g. between the tower and the conductor supported by the insulator string, and the cost of constructing a power transmission line is consequently high.

The object of the present invention is to provide a multi-shed suspension insulator which can provide a high withstand voltage while the total length of the insulator string consisting of the insulators can be kept relatively small.

According to the present invention there is provided a suspension insulator having an insulator body, a metal cap bonded to the upper end of the insulator body, and a metal pin bonded to the lower side of the insulator body, said insulator body having at least two sheds projecting from the insulator body around its periphery and forming a monolithic structure with the body, wherein the ratio (L/P) of the surface leakage distance L of the insulator to the vertical shed pitch P of the insulator is within the range 5L/P'9.

Embodiments of the invention are described below by way of example with reference to the accompanying drawings, wherein: Figure 1 is a side view, partly in section, illustrating a first embodiment of the multi-shed insulator of the present invention; Figure 2 is a side view, partly in section, illustrating a second embodiment of the insulator of the present invention; Figure 3 is a side view, partly in section, illustrating a third embodiment of the insulator of the present invention; Figure 4 is a graph illustrating the relation between the value of L/P and the leakage distance per KV of withstand voltage of a insulator; Figure 5 is a graph illustrating the relation between the value of L/P and the height per KV of withstand voltage of a insulator; and Figure 6 is a side view, partly in section, illustrating the conventional insulator already discussed.

The insulators of Figs. 1 to 3 each have a ceramics insulator body 1, a metal cap 2 bonded to the upper end of the insulator body 1 by cement 3, and a metal pin 4 bonded to the lower side of the insulator body 1 by cement 5. The insulator body 1 is slender and long as illustrated in Fig. 1, and the metal pin 4 is inserted deep into a central recess of the insulator body 1.

Two vertically spaced or staged sheds 6 and 7 project from the insulator body 1 around its periphery and form a monolithic structure with the body 1. The number of sheds can be selected to be 2 or any suitable number greater than 2, e.g. 4, 5 and so forth, and it is preferable to form a small shed 8 between the sheds 6 and 7 as illustrated in Fig. 3.

These sheds 6,7 and 8 are designed so that the shed pitch P, i.e. in Figs. 1 to 3 the pitch between the upper shed 6 and the lower shed 7, and the surface leakage distance L (the shortest distance between the lower end A of the metal cap 2 and the lower end B of the insulator body 1 shown in Figs. 1 - 3 measured along the surfaces of the insulator body 1 and the sheds 6, 7 and 8) are in the ratio of L/P given by 5~L/P~9. The presence of the small shed 8 is ignored in calculating the shed pitch P.

The reason why the value of L/P is within the range of 5 to 9 is as follows. It has been found by experiment that the highest withstand voltage is obtained with the lowest insulator height in the case where L/P is 5 to 9, as illustrated by Fig. 4 which shows the relation between the value of LIP and the leakage distance per KV of the withstand voltage of a contaminated insulator and by Fig. 5 which shows the relation between the value of L/P and the height per KV of withstand voltage of a contaminated insulator. When L/P is less than 5, the leakage distance per KV of withstand voltage of the contaminated insulator is not so different from that within the range of 5 to 9 as shown by Fig. 4, but the height per KV of withstand voltage of the contaminated insulator is greatly increased as illustrated by Fig. 5, so that the pitch is long with respect to equal surface leakage distance.As a result, the length (height) of the insulator is long, and an insulator string obtained by linking the insulators has a great length and is expensive.

In general, when- the surface leakage distance of an insulator is increased, the withstand voltage under contamination per one insulator becomes high. However, when the surface leakage distance is merely-increased so that L/P exceeds- 9, the shed pitch P becomes relatively short and an arc is liable to generate between sheds. As a result, the withstand voltage efficiency of the insulator under contamination deteriorates, and the surface leakage distance per KV of withstand voltage of a contaminated insulator is long as illustrated in Fig. 4.

The configuration of each shed may be that of an ordinary shed having a large number of skirts projecting downwardly from its lower surface as illustrated in Fig. 1 or may be that of aerodynamic type sheds as illustrated in Figs. 2 and 3.

The illustrated insulators thus constructed are the same as the conventional suspension insulator in that the lower end of the metal pin 4 of an upper insulator is engaged with the metal cap 1 of a lower insulator body, so that a large number of suspension insulators are linked in series for insulating and supporting a power transmission conductor. However, in the insulator of the present invention, the proportion of the whole height of the insulator occupied by the metal cap 2 while the proportion occupied by the insulating insulator body 1 which contributes effectively to insulation is large, while the value of L/P is within the range of 5 to 9 which as shown above is the most beneficial range for the withstand voltage.Therefore, insulators of the present invention can show an improvement over the conventional insulator of about 15-20% in the withstand voltage value over the whole length of the resulting insulator string. In other words, when insulators of the present invention are used, the length of an insulator string necessary to obtain-the same withstand voltage value as that obtained by the use of the conventional insulators, can be reduced by 15-20% compared with the length of the insulator string formed of the- conventional insulators.

For example, in the multi-shed suspension insulator of the present invention illustrated in Fig.

1, when the height H is 235 mm, the shed diameter D is 254 mm and the shed pitch P is 85 mm, the surface leakage distance L is 635 mm, and L/P is 7.47. Therefore, the leakage distance per KV of withstand voltage of the insulator under contamination is 23 mm/KV as illustrated in Fig. 4. Accordingly, when using this multi-shed suspension insulator for a 400 KV power transmission line, the designed withstand voltage under contamination is 400X(1 /V3)x 1.1 (safety factor)=254 KV and hence the required surface leakage distance of an insulator string is 23X254=5,842 mm.

The number of insulators needed is thus 5,842/635=10 insulators, and therefore the whole length of the insulator string is 235X 10=2,350 mm. In contrast, with the conventional standard type suspension insulator illustrated in Fig. 6, the height H is 146 mm, the shed diameter D is 254 mm, the surface leakage distance L is 292 mm, and the leakage distance per KV of withstand voltage of the insulator under contamination is 21 mm/KV under the same contamination condition as in the case of the insulator of Fig. 1. Therefore, when the number of insulators and the total length of an insulator string necessary for supporting a 400 KV power transmission line are calculated by the same method as described above, 19 insulators are required and the total length of the insulator string is 2,774 mm.

This illustrates that, with insulators according to the present invention the total length of an insulator string giving the same withstand voltage as that obtained by the use of the conventional standard type suspension insulator, can be shortened to for example 85% compared with the total length of the insulator string formed of the conventional insulators, i.e. the length of the insulator string can be shortened by about 15% based on the length of the string formed of conventional insulators. Furthermore, the-weight of the insulator string formed by the conventional insulators is 99 kg, but that of the insulator string formed of the illustrated insulators of the present invention is 88 kg. This means that the weight saving by the use of the insulator of the present invention is in this case about 11%.

As is apparent from the above explanation, the present invention can provide a high withstand voltage without use of an insulator string having a large total length, so that pylon size can be reduced even in the case of insulating and supporting a high voltage power transmission line, and the construction cost of a power transmission line can be reduced.

Claims (5)

1. A suspension insulator having an insulator body, a metal cap bonded to the upper end of the insulator body, and a metal pin bonded to the lower side of the insulator body, said insulator body having at least two sheds projecting from the insulator body around its periphery and forming a monolithic structure with the body, wherein the ratio (L/P) of the surface leakage distance L of the insulator to the vertical shed pitch P of the insulator is within the range 5-'L/P-'9.

2. A suspension insulator according to claim 1 having two sheds.

3. A suspension insulator according to clam 1 or claim 2 having a small shed on the body between the said two sheds of pitch P.

4. A multi-shed- suspension insulator substantially as herein described with reference to and as shown in Fig. 1, Fig. 2 or Fig. 3 of the accompanying drawings.

5. An insulator string formed of a plurality of insulators according to any one of the preceding claims.