EP0388133A2

EP0388133A2 - Suspension insulator

Info

Publication number: EP0388133A2
Application number: EP90302638A
Authority: EP
Inventors: Takashi Imakoma; Seiichi Kondo; Fumio Kasaki
Original assignee: NGK Insulators Ltd
Current assignee: NGK Insulators Ltd
Priority date: 1989-03-17
Filing date: 1990-03-13
Publication date: 1990-09-19
Anticipated expiration: 2010-03-13
Also published as: EP0388133B1; JPH077612B2; EG19239A; JPH02244514A; EP0388133A3

Abstract

Disclosed is a suspension insulator having a shed (2) with several ribs (4) formed concentrically and integrally on the lower face and a head (3) integrally formed at the central portion on the upper face of the shed(2), with a metal cap (5) being fixed on the head (3) and a metal pin (7) being fixed in the cavity of the head (3). The insulator is designed to have dimensions in such a way that the following equations [1] to [3] are satisfied:
0.8 h < ℓ [1]
2.5≦ L/H≦ 4.5 [2]
D≦ L [3],
where, h is the height of the ribs (4); ℓ is the interval between the ribs (4); L is the creepage distance as measured along the surface of the shed (2), D is the diameter of the shed (2); and H is the connection pitch of a suspension insulator unit forming an insulator string.

Description

This invention relates to a suspension insulator which is used for supporting power cables on the supporting arms of steel towers and the like.
A suspension insulator of the type shown in Fig. 3 is conventionally known, in which the insulator body 1 comprises a shed 2 with several ribs 4 formed concentrically and integrally on the lower face and a head 3 integrally formed at the central portion on the upper face of the shed 2, wherein a metal cap 5 is fitted on the head 3 and fixed with a cement 6, and a metal pin 7 is fitted in the cavity of the head 3 and also fixed with a cement 6.
The conventional suspension insulator is designed to satisfy the following equation:
L/H = 2,
where, H is the connection pitch of a suspension insulator unit when connected into a string and L is the creepage distance as measured along the surface of the insulator body 1.
The connection pitch H is decided in accordance with JIS (Japanese Industrial Standard) corresponding to the power transmission voltage, whereby the creepage distance L is very limited. Accordingly, the conventional suspension insulator failed to have sufficient insulation resistance. In order to overcome the inconveniences of the conventional suspension insulator, a fog-type insulator has been proposed, in which the the ribs 4 have an increased height h, as shown in Fig. 4.
It is true that the fog-type suspension insulator can secure a longer creepage distance L (L/H = 3). However it suffers problems in that dirt such as sand easily collects between the high ribs 4, and that it is difficult to remove such dirt, because an inteval between adjacent ribs 4 is smaller than the height of the ribs 4, if such insulators are used in a desert area.
Because of the high ribs 4, a whole thickness of the shed 2 is increased and an interval between adjacent sheds 2 of the insulators connected is decreased. Therefore, there is a hazard of short-circuiting between the sheds 2.
Further, an aero-type suspension insulator has been known, in which the ribs 4 are omitted as shown in Fig. 5. This type of insulator enjoys an advantage that sand does not collect thereon, but the diameter of the shed 2 must be increased greatly in order to secure a sufficient creepage distance L, resulting in the increase in the size and weight of the insulator body.
It is an object of this invention to solve the problems inherent in the prior art suspension insulators as mentioned above and to provide a suspension insulator which hardly allows collection of dirt such as sand between the ribs formed on the lower face of the shed but allows easy washing off of the dirt collected.
It is another object of the invention to provide the suspension insulator which can secure longer creepage distance relative to the diameter of the shed.
The suspension insulator of this invention is designed to have dimensions in such a way that the following equations (1) to (3) are satisfied:
0.8 h < ℓ (1)
2.5 5 ≦ L/H≦ 4.5 (2)
and preferably D ≦ L (3),
where, h is the height of the ribs formed concentrically on the lower face of the insulator body; ℓ is an interval between the adjacent ribs; L is the creepage distance as measured along the surface of the shed; D is the diameter of the shed; and H is the connection pitch of a suspension insulator unit.
An embodiment of the invention is described by way of example with reference to the drawings, in which:-

Fig. 1 shows, in partially cutaway front view, an embodiment of a suspension insulator according to this invention;
Fig. 2 shows, in partially cutaway front view, the connected suspension insulators forming a string; and
Fig. 3 to 5 respectively show, in partially cutaway front views, different conventional suspension insulators.

The embodiment of this invention will be described below referring to Figs. 1 and 2.
An insulator body 1 comprises a shed 2 with a plurality of ribs 4 formed concentrically and integrally on the lower face and a head 3 integrally formed at the central portion on the upper face of the shed 2, wherein a metal cap 5 is fitted on the head 3 and fixed witha cement 6, and a metal pin 7 is fitted in the cavity of the head 3 and also fixed with a cement 6.
A zinc sleeve 8 is disposed to surround the metal pin 7 for preventing electrolytic corrosion of the latter member. Now, sizing of each portion will be described below.
Provided that the shed 2 of the present suspension insulator has a diameter of D: a rib height of h;an interval between adjacent ribs of ℓ; a creepage distance as measured along the surface of the main body 1 of L (i. e., the distance as measured from the metal cap 5 to the metal pin 7 along the surface of the shed); and a connection pitch of H when connected to form an insulator string as shown in Fig. 2, these dimensions are determined in such a way that they may satisfy the following equations (1) to (3):
0. 8 h < ℓ (1)
2.5 ≦ L/H≦ 4.5 (2)
D ≦ L (3)
The relationship between the factors as defined in the equation (1) allows the spaces defined between the ribs 4 to have smaller depth. Therefore a whole height of the shed A can be reduced. Thus, when the present suspension insulator is used in a desert area, dust coming into such spaces hardly collects at the recessed faces between the ribs, and even if some amount of the dust should be accumulated on such faces, it can easily be washed off.
Moreover, the creepage distance L is 2.5 to 4.5 times as long as the connection pitch H according to the equation (2). Therefore, the total number of the insulators as the insulator string can be reduced as compared with the case when the conventional aero-type suspension insulators are used. In such case, the creepage distance of the insulator string of the invention is set the same as the conventional one. Accordingly, the design of the steel tower can be scaled down.
Further, as shown in Fig. 2, a larger interval B can be secured between the sheds 2 as compared with the case when the conventional fog-type insulators are used, even if the connection pitch is identical with the conventional one, and thus possible short-circuiting between the sheds can also be prevented. In the case that the ratio L/H is over 4.5, it is impossible to produce the insulator itself.
Furthermore, the creepage distance L is larger than the diameter of the shed D. As a result, the size of the insulator can be smaller than that of the conventional aero-type insulator.

Now, comparison between the performances of the present and three types of prior art insulators will be made as summarized in the following Tables 1 and 2. Table 1 shows difference in the lengths of insulator strings comprising the respective insulators when the creepage distance L is 4600 mm and the mechanical strength is 120 KN.

Table 1

Type of suspension insulator	Required number of insulator (unit)	Length of insulator string (mm)	Percentage (%)
1. Conventional	16	2336	100
2. Fog-type	11	1606	69
3. Aero-type	14	2044	88
4. Present invention	10	1460	63

As apparent from Table 1, the insulator of this invention enables use of smaller number of insulator units when the insulator string is formed as compared with the case when any of the conventional insulators are used. Table 1 also shows percentages of the lengths of three types insulator strings as compared with the length of the conventional insulator. The length of the insulator string of this invention can be greatly shortend.

Table 2 shows difference in the characteristic values of insulator strings each comprising 10 units.

Table 2

Type of suspension insulator	Creepage distance (mm)	Lightning impulse flashover voltage (KV)	Wet flashover voltage (KV)
1. Conventional	2920 (100)	930 (100)	335 (100)
2. Fog-type	4320 (148)	950 (102)	295 (88)
3. Aero-type	3400 (116)	660 (70)	335 (100)
4. Present invention	4670 (160)	950 (102)	335 (100)
(Note) The numerical values in parentheses respectively show percentages (%) of the characteristic values of the respective insulators provided that those of the conventional suspension insulators' are 100 %.

As apparent from Table 2, according to the present invention, creepage distance can be increased without lowering the lightning impulse flashover voltage and wet flashover voltage.
As has been described above, this invention not only can reduce collection of dust on the lower faces of the shed even when used in a desert area and facilitate washing off of such dirt if collected between the ribs, but also allows to secure sufficient creepage insulation distance for the shed without increasing the diameter thereof.

Claims

1. A suspension insulator having an insulator body (1) with a shed (2), a pair of connection members (5,7) on respectively the upper and lower sides of the shed (2) and a plurality of ribs (4) formed on the lower shed (2), characterized in that said insulator has dimensions such that the following equations [1] and [2] are satisfied.
0.8 h < ℓ [1]
2.5 ≦ L/H ≦ L/5 [2],
where h is the height of the ribs (4) from the lower face of the shed (2) ; ℓ is the interval between adjacent ribs (4) ; L is the creepage distance along the surface of the shed (2) from one connection member(7) to the other connection member(5); and H is the connection pitch of the suspension insulator when forming an insulator string.

2. The suspension insulator according to Claim 1, wherein the creepage distance and the shed diameter D are such that the relation D ≦ L is satisfied.

3. The suspension insulator according to Claim 1 or Claim 2, wherein the ribs (4) are disposed concentrically about the connection member (7) on the lower side of the shed (2).

4. The suspension insulator according to any one of claims 1, 2 and 3, wherein the connection member (7) on the lower side of the shed has a metal sleeve 8 to inhibit electrolytic corrosion.