In an enclosed-type zinc-oxide surge arrester with a grounded tank filled with an insulating medium, and a non-linear resistor assemblage which is connected at one end to a high voltage conductor and at the other end to the ground potential portion within the grounded tank, two or more ring-like shield means such as shield rings are disposed on the high voltage conductor side of the non-linear resistor assemblage at given distances. The use of the ring-like shield means makes more uniform the voltages shared by the respective non-linear resistors. The ring-like shield means may be formed with the configuration and diameter, and the number of them used being properly selected.

The invention relates to an enclosed-type zinc-oxide surge arrester without 
series gaps in which non-linear resistors containing zinc-oxide as the 
principal constituent are multilayered within a tank. 
General surge arresters have used SiC as the arrester elements. Recently, 
arrester elements with good non-linearity have been developed each of 
which is formed by adding a trace of materials such as Bi.sub.2 O.sub.3, 
CoO, MnO, Sb.sub.2 O.sub.3 and the like to zinc oxide as the principal 
constituent, mixing those materials, moulding those materials in a given 
shape, and then sintering the materials shaped at high temperature of 
1000.degree. C. or above. By using the arrester elements, a surge arrester 
without series gaps, called a gapless surge arrester, has been developed. 
In the gapless surge arrester, a plurality of arrester elements are stacked 
within a grounded tank filled with SF.sub.6 gas and the stacked arrester 
elements are connected at one end to a main circuit conductor and coupled 
at the other end with the ground potential portion. The dielectric 
constant of each non-linear resistor is high, e.g. 1000 to 2000 in the 
gapless surge arrester. Since those elements are influenced by the tank, 
the voltages shared by the respective arrester elements are not equal; the 
arrester elements on the main circuit conductor side have higher voltages 
applied thereto than the remaining ones. The uneven voltage distribution 
over the arrester elements is due to the stray capacity straying between 
the arrester elements and the grounded tank. The uneven potential 
distribution shortens the life time of the arrester elements subjected to 
the higher voltages shared. 
Accordingly, an object of the invention is to provide an enclosed-type 
zinc-oxide surge arrester with a grounded tank to make uniform the shared 
voltages constantly applied to the respective arrester elements. 
Another object of the invention is to provide an enclosed-type zinc-oxide 
surge arrester with a grounded tank, which has a control means capable of 
easily controlling a potential distribution over the arrester elements. 
An enclosed-type zinc-oxide surge arrester with a grounded tank according 
to the invention is provided with two or more ring-like shield means, for 
instance, shield rings, disposed on the main circuit conductor side of the 
arrester elements at given intervals, whereby the respective voltages 
shared by arrester elements are made uniform. The interval between the 
adjacent ring-like shield means is so selected as to form at least one 
equipotential surface extending to the arrester elements through both the 
ring-like shield means. The ring-like shield means may be formed by 
variously and properly changing the shape and diameter of the ring-like 
shield means, the number of the ring-like shield means used, or the like.

In FIG. 1 illustrating in partial cross section an enclosed-type zinc-oxide 
surge arrester, a grounded tank 2 hermetically filled with medium with 
good insulation such as SF.sub.6 gas is sealed by an insulating spacer 3 
at the opening disposed on the connection side. Disposed within the 
grounded tank is a non-linear resistor assemblage 1 comprising an 
assemblage of arrester elements of different resistance fixed at one end 
to the grounded tank 2 and at the other end to a high voltage conductor 4. 
The connecting portions at both ends of the non-linear resistor assemblage 
1 as viewed in the axial direction are enclosed by shield tubes 27 and 28 
for deconcentrating or unifying electric field distribution respectively. 
An umbrella-shaped shield means 26 is fixed on the high voltage side of the 
non-linear resistor assemblage 1, with the free end of the shield means 26 
on which a shield ring 20 as a ring-like member is integrally mounted. In 
this instance, the umbrella-shaped shield means 26 is shaped like a 
receptacle with a continuous side wall around its entire periphery. As 
another example, the means 26 may be comprised of a plurality of shield 
rings closely disposed. In this case, however, equipotential surfaces 
continuous to the equipotential surfaces of the non-linear resistor 
assemblage 1 through such shield rings are not formed and must be avoided, 
as shown in the figure. 
Disposed between both ends of the non-linear resistor assemblage 1 as 
viewed in its axial direction are two ring-like shields such as shield 
rings 21 and 22 further surrounding the assemblage 1. Those shield rings 
21 and 22 are fixed in a supporting manner to a high voltage conductor 4 
through connecting conductors 30 and 31, while being kept at substantially 
the same potential as that of the high voltage conductor 4. It is notable 
here that the features of the invention reside in the distances or 
intervals among shield rings 20, 21 and 22, and the arrangement and shape 
of the connecting members 30 and 31. To be more specific, those components 
are so arranged and shaped that the equipotential lines, as depicted by 
dotted lines in the figure, passing through the respective rings are 
continuous to the equipotential surfaces of the non-linear resistor 
assemblage 1. With this arrangement, at least two shield rings axially 
disposed between both ends of the non-linear resistor assemblage 1 are 
separately arranged at given intervals. With respect to the connecting 
conductors 30 and 31, the peripheral widths and the number of them are so 
selected as to satisfy the above-mentioned requirement to form the 
equipotential lines (parts of the equipotential surfaces) in the 
respective shield rings. Accordingly, those shield rings each may take the 
form of a rod, a belt, a lead wire or the like. In fixedly positioning the 
shield rings 21 and 22 by the connecting conductors 30 and 31, if the 
above-mentioned requirement cannot be satisfied, the connecting conductors 
30 and 31 are used at least as electrical connecting members and a 
dielectric member is additionally used for mechanically supporting them. 
It is preferable to fix the respective shield rings 21 and 22 to the high 
voltage conductor 4 in a supporting manner, in order to reform a potential 
distribution along the length of the non-linear resistor assemblage 1. 
Specifically, when the potentials of the shield rings 21 and 22 are set to 
be substantially equal to that of the high voltage conductor 4, the shield 
ring 22 perhaps is subjected to the most rigorous conditions of electrical 
field and insulation. The shield ring 22 is fixed to the same potential 
portion of the high voltage conductor 4, through the connecting conductor 
31. For this, there is no need for inserting an insulating supporting 
member between the shield ring 22 and the grounded tank 2 requiring 
electrical insulation. Therefore, it is possible to place the shield ring 
22 close to the grounded tank 2 by taking advantage of the excellent 
insulating characteristics of SF.sub.6 gas. This enables the shield ring 
22 to be located at the best position to adjust the potential distribution 
along the length of the non-linear resistor assemblage 1. In order to 
weaken the surface electric field of the shield ring 22, all that is 
necessary is to increase the diameter of the shield ring 22, i.e. the 
width thereof extending in the axial direction of the non-linear resistor 
assemblage 1. 
With such an arrangement, the voltages applied to the respective resistors 
of the non-linear resistor assemblage 1 are almost uniform, as seen from 
the potential distribution indicated by dots in FIG. 2. This may also be 
anticipated from the equipotential lines depicted by dotted lines in FIG. 
1. A straight line 11 shown in FIG. 2 indicates a uniform potential 
distribution straight line. In FIG. 2, the axis of ordinate represents a 
potential distribution by % while the axis of abscissa represents a height 
H from the ground potential side end of the non-linear resistor assemblage 
1, and the total height of the non-linear resistor assemblage 1 is 
represented by H.sub.o. 
The embodiment shown in FIG. 1 has other additional features. The first 
feature is that the diameters of the shield rings 21 and 22 are larger, 
compared to that of the shield ring 20. This feature is advantageous for 
the shield ring 22 to be disposed close to the ground potential portion, 
in the insulating point of view. In particular, the shield ring 22 is 
located closer to the ground potential side of the non-linear resistor 
assemblage 1, with respect to the midpoint of the axial length of the 
non-linear resistor assemblage 1. Accordingly, the electric field easing 
action by the diameter of it and the excellent insulation of the SF.sub.6 
gas and the like may be utilized. The second feature is that the diameter 
of the shield ring 20 is selected to be smaller than those of the 
remaining shield rings. This feature is effective in improving the 
potential distribution on the high potential side of the non-linear 
resistor assemblage 1. The shield ring 22 with a large diameter is 
effective in improving the potential distribution on the ground potential 
side of the non-linear resistor assemblage 1. The third feature is that, 
when the number of the shield rings located between both ends of the 
non-linear resistor assemblage 1 in the axial direction thereof is three 
or more, the distances from the high voltage side end of the assemblage 1 
to the first and second shield rings are smaller than the distance between 
the second and third shield rings 21 and 22. It was experimentally 
confirmed that the third feature well reforms the potential distribution 
on the ground potential side of the non-linear resistor assemblage 1. In 
the figure, the shield rings 20, 21 and 22 are illustrated with the 
continuous or solid cross section; however, each shield ring may be a 
hollow ring with a circle, a C-shape or an elliptical shape in cross 
section. The shield ring 20 may be replaced by a skirt-like ring shield 
formed by folding the free end of the umbrella-like shield means 26. An 
example of the surge arrester using the skirt-like ring shield is 
illustrated in FIG. 3. The example in the figure is provided with a 
cap-like shield 40 covering the high voltage side portion of the 
non-linear resistor assemblage 1 and slightly extending at the lower end 
beyond the high voltage end of the non-linear resistor assemblage 1. 
Ring-like shields 41, 42, 43 with C-shaped cross sections surround the 
outer periphery of the non-linear resistor assemblage 1 at given distances 
thereamong. The cap-like shield 40, and the ring-like shields 41, 42 and 
43 are separated with such distances as to form the equipotential surfaces 
continuously extending to the equipotential surfaces of the non-linear 
resistor assemblage 1 through those shields 41, 42 and 43. Those shields 
40 to 43 are coupled with each other by a rod-like connecting conductor 
44. The cap-like shield 40 is fixed to a high voltage conductor 4 or a 
connecting portion 45. The high voltage conductor 4 is electrically 
connected with the connecting portion 45 through a conductor 46. Such a 
cap-like shield 40 has the actions of the umbrella-like shield means 26, 
the shield ring 20, and the shield tube 27 shown in FIG. 1. The potential 
distribution along the length of the non-linear resistor assemblage 1 may 
be improved to such a degree as in the case of FIG. 1. 
If a plurality of shield rings are provided between the end surface of the 
high voltage side of the non-linear resistor assemblage 1 and the end of 
the ground side thereof, the lower ends of the shield ring 20 in FIG. 1 
and the tubular shield 40 in FIG. 3 may be located closer to the high 
voltage conductor 4 than to the end surface of the high voltage side of 
the non-linear resistor assemblage 1. 
FIG. 4 shows a schematic diagram of another embodiment of the enclosed-type 
surge arrester according to the invention. This embodiment is the same as 
that of FIG. 1 in that the non-linear resistor assemblage 1 is connected 
at the one end to the high voltage conductor 4 and coupled at the other 
end with the ground potential which is equal to that of the grounded tank 
2. The major difference of the embodiment from that in FIG. 1 resides in 
that three shield rings 20, 21 and 22 with substantially the same 
diameters are provided around the outer periphery of the non-linear 
element assemblage 1 for the purpose of controlling the potential 
distribution. 
The shield rings 20, 21 and 22 are held at given distances in the axial 
direction of the non-linear resistor assemblage 1, while being kept at 
substantially the same potential as that of a high voltage conductor 4 
(not shown). 
FIG. 5 shows the potential distribution curves, where the axis of ordinate 
represents a potential distribution by % while the axis of the abscissa 
represents the height H of the non-linear resistor assemblage 1 from its 
ground potential end, and the total height of the non-linear resistor 
assemblage 1 is represented by H.sub.o. The potential distribution in the 
surge arrester thus constructed considerably approximates to a uniform 
distribution straight line 11, as indicated by a potential distribution 
curve 23 shown in FIG. 5. The uneven coefficient of the potential 
distribution is approximately 1.1 or less, with respect to 1.0 for the 
uniformity thereof. 
The potential distribution formed when a single tube is used for those 
shield rings, is as illustrated by a curve 24 in FIG. 5. As seen from the 
curve 24, the voltage apportionment is severe for the non-linear resistors 
at the midportion of the non-linear resistor assemblage 1. The connecting 
conductor 30 in FIG. 4 may be an impedance means such as a capacitor or a 
resistor. If such an impedance means is used for the connecting conductor 
30, the shield rings 21 and 22 have potentials different from that of the 
shield ring 20. Accordingly, use of the impedance means is effective for 
finely adjusting the equipotential surface. The favorable features which 
are brought about when the impedance means is used, are similar to those 
of the surge arrester shown in FIG. 3. In either case, since the shield 
ring 22 disposed closest to the ground potential side has no relation with 
the midpotential component of the non-linear resistor assemblage 1, it may 
be located at a position suitable mainly for improving the potential 
distribution by effectively using the insulating characteristics of 
SF.sub.6 gas. 
Other embodiments of the surge arrester according to the invention are 
illustrated in FIGS. 6 to 10 and may attain the effects similar to those 
by the surge arrester in FIG. 4. In those figures, the supporting member 
for each shield ring is not illustrated. 
The embodiment in FIG. 6 uses a metal base 25 extending in the axial 
direction which is positioned at the end of the ground potential side of 
the non-linear resistor assemblage 1. The metal base 25 cooperates with 
the shield rings 20, 21 and 22 to expand a range of the potential 
distribution adjustment thereby to improve the potential distribution at 
the end of the ground potential side. 
The embodiment in FIG. 7 employs two shield rings. In this embodiment, the 
uniformity of the potential distribution is deteriorated, when compared 
with the embodiment in FIG. 4, but a more preferable potential 
distribution may be obtained as compared with the curve 24 in FIG. 5. 
In the embodiment shown in FIG. 8, an umbrella-like shield 26 is provided 
between the uppermost shield ring 20 and the high voltage conductor 4, and 
the shields 20, 21 and 22 have the diameters which are substantially equal 
to each other. The embodiment with such a construction may attain the 
effects like those of FIG. 1. Further, the potential distribution on the 
high voltage conductor 4 side of the non-linear resistor assemblage 1 can 
be improved. 
In the embodiment shown in FIG. 9, the cross sectional areas of the shield 
rings are larger as they are closer to the ground potential side of the 
non-linear resistor assemblage 1. This embodiment may ease the electric 
field strength at the edge of the shield ring 22. As a result, the shield 
ring 22 may be located at the best position to improve the potential 
distribution on the ground potential side of the non-linear resistor 
assemblage 1. 
FIG. 10 shows a further embodiment of the surge arrester according to the 
invention. The feature of this embodiment is in that the diameters of the 
shield rings are larger as it approaches the ground potential side of the 
non-linear resistor assemblage 1. With this construction, a potential 
distribution between the high voltage conductor 4 side of the non-linear 
resistor assemblage 1 and the ground potential side is improved so as to 
become more desirable, compared with the surge arrester having no shield 
to improve the potential distribution of the non-linear resistor 
assemblage 1. The use of this embodiment enables the potential 
distribution curve in FIG. 5 to approximate to the even distribution curve 
11. 
The shield rings 20, 21 and 22 in the respective embodiments may take the 
form of a ring. However, if they each exhibit a ring-like shape for the 
electric field, they may be divided into a plurality of segments along the 
periphery thereof. 
As a modification of the embodiments, auxiliary shields may additionally be 
provided near the ring-like shield members represented by the shield 
rings. 
As described above, in the scheme involved in the present invention, a 
plurality of ring-like shields surround the outer periphery of the axial 
portion of the non-linear resistor assemblage 1 which is defined between 
both ends thereof, and are disposed therealong at given intervals. The 
ring-like members are electrically connected with each other by a non-ring 
shaped connecting conductor whereby the potential of the ring-like shields 
are substantially equal to that of a high voltage conductor. Therefore, 
the equipotential surface continuous to the equipotential surface of the 
non-linear resistor assemblage 1 through a space between the adjacent 
ring-like shields may be formed thereby the make substantially uniform the 
voltages applied to and born by the non-linear resistors of the assemblage 
1. Additionally, the shields used to make uniform the shared voltage may 
be formed with the shape of a ring or the resemblance to it, with the 
result that the manufacturing of the surge arrester is easy and the 
construction thereof is simple.