CA1175889A - Grading means for high voltage metal enclosed gas insulated surge arresters - Google Patents

Abstract

GRADING MEANS FOR HIGH VOLTAGE METAL ENCLOSED GAS
INSULATED SURGE ARRESTERS
ABSTRACT OF THE DISCLOSURE
Capacitance grading is provided within gas insulated lightning arresters containing stacked zinc oxide varistors by means of a grading ring electrically connected to the line terminal or, for arresters of the higher voltage ratings, by means of a plurality of telescoping external electrostatic shields. The shields are arranged so that the degree of overlap between sequential shields decreases from the line end to the ground end of the varistor stacks. The capacitance grading is provided by the degree of overlap between the sequential shields and the ratio of the radii of the overlapping shields.

Description

G.RADI~G MEANS FOR HIGH VOLTA~E ~ETAL ENCLOSED GAS
.
` _ _ ` ` INSUL~TED SURGF ARRESTERS _ _ Background oE the InveIltion This invention relates to high voltacJ~ arrester devices contained within gas insulated metal enclosu:res such as described within U.S. Patents 3,767,973, Osmundsen et al, issued October 23/ 1973, 3,fl42,318, Nitta, issued October 15, 197~ and German Patent 888,132.
When a plurality of zinc oxide type varistor disks are arranged in a stack configuration and eIectrically connected in series, the capacitive properties of the disks creates the combination o both series capacitance circuits along the stack and parallel capacitance circuits between the individual varistors in the stack and ground. When a high voltage is applied to the line end of the stack, the electric fieId becomes adversely distorted resulting in a nonunion distribution of voltage across the stack from the line end to the ground end thereof. A disproportionate share of the applied voltage appearing across the 2Q varistors closest to the line end of the stack could cause severe damage to these varistors. This is a particularly severe probIem for gas insulated, metal enclosed surge arresters because of the adverse influence of the enhanced capacitance to grollnd caused by the presence of the metal enclosure.
~1~

l 17S8~9 5D-5696 The purpose of this invention is to provide a means for grading the capacitances occurring along the varistor stack in quch a manner as to cause the voltage distribu-tion to become more nearly linear.
Summar~ of the Invention The invention comprises arrangements for compensating for the adverse effects of abnormally high capacitance ~o ground in metal enclosed, yas insulated surge arresters. For arresters of lower voltage rating, the use of a simple ring extending partially down the arrester stack and connected to the line end b~ a predetermined plurality of support members is adequate. For higher voltage arresters a~
more comple~ arrangement oE a plurali-ty of telescoping cylinc~rical capacitox shields with appropriate elec-trical connection to the arrester stack are provided. One embodiment comprises a plurality of concentrically arranged cylindrical capacitor elements of a stepped diameter configuration with the large diameter portion of each element over-lapping a preceding element. The radius and degree of overlap of each capacitor element in the stack is carefully tailoredto provide the desired shieIding and capacitance grading from the line end to the ground end of the stack.
Brief Description of the ~rawings Figure la is a fron-t perspective view in partial section of a metal enclosed arrester without shieIding;
Figure lb is a schematic representation of the capacitance network of Figure la;
Figure lc is a graphic representation of the voltage distribution for Figure la;
Figure 2a is a front perspective view in partial section of an arrangement of a graded surge arrester using a grading ring;
Figure 2b is a schematic representation of the L :~758~ SD-5~96 capacitance network of Figure 2a;
Figure 2c is a graphic representation oE the voltage distribution for Figure 2a;
Figure 3 is a front perspective view in partial section of a graded surge arrester using concentric cylindrical shieIds;
Figure 4 is a side sectional view of the concentric cylindrical shielded embodiment of Figure 3;
Figure 5 i5 an electrical schematic depicting the capacitances of Figure 4; and Figure 6 is a graphic representation o~ the voltage distribution is the arres-ter of Figure ~.
D _ o~ the 'Preerred Embo'dimen-t Figure la shows an arrester sys-tem 10 eonsisting of a me-tal container 11 sealed at the top by means of a top flange 12 and at the bottom by means of a bottom flange 13 and containing a filling of sulfur hexafluoride insulating gas (SF6). A plurality of zinc oxide varistor disks 14 each with metal electrodes 15 are arranged in a stack such'that each zinc oxide varistor disk 14 in the stack is eIectrically connected in series with each other disk. The zinc oxide varistor disks 14 are in turn contained within a porcelain housing 16 which is open at either end in order to permit the transfer of insulating SF6 gas to within the vicin~ty of zinc oxide varistor disks 140 At normal operating ~oltage on a 60 Hz source, the disk current is primarily capacitive. The stack of disks 14 in Fig. la between line and ground is represented in the circuit of Figure lb as a series stack of capacitors r each of value C . The capacitors Cg represent the stray capacitance of each disk 14 to ground. When an AC voltage is applied to the line end of such a network, capacitive current must flow in the direction depicted by arrows.
It is readily seen that more current must flow through capacitors Cs at the line end than at the ground end 1 ~5~8~ 5~-5696 causing the voltage across the disks 14 at the line end to be greater than across those disks 14 near ground.
This effect is shown in Figure lc where the voltage distribution along the stack of varistors 14 is qualitively shown at A. B represents t,he ideal uniform volta~e distribution that would occur lf there were no ground capacitance (C =0). The voltage per disk at any point in the stack of disks 14 is defined by the slope of curvie A. Near the line end of the stack of disks 14, this slope is considerably ~reater than the slope of curve B. The resul-t i9 such that if uncorrec~ed, cll.sks 14 near the line end will support a disproportionate share o~ the total voltage and will correspondingly exhibit a higher wakts loss and a decreased electrical and thermal stability.
It i~ therefore apparent -that -the disproportiona-te voltage is caused by the currents flowing in the ground capactitances C shown in the circuit o Figure lb. One feasible solution for the lower voltage rated arresters is to provide a grading ring 6 as shown in Figure 2. The arrester system 10 of Figure 2a is similar to that of Figure la except for the provision of grading ring 6 which is both supported by and electrically connected to the line end of assembly 10 by means of support members 7. The purpose of grading ring 6 and support members 7 is to reduce the capacitance to ground of disks 14 in the line end of the stack which is depicted as Cgl, Cg", and Cg"' in Figure 2b and to provide additional capacitance Cr', CR", CR"' from line to a few of the disks 14 near the line end, particularly disks 14 close to grading ring 6 and supports 7. Figure 2b shows that the currents depicted by arrows flowing into upper ground capacitances Cg', Cg", Cg"' are partially supplied from the line by currents flowing in capacitors CRI~ CR", CR"'. The currents through disks 14 near the line end of the stack are thereby reduced so that l ~75~3~ .3 SD-5696 the voltage distribution along -the stack of di~ks 14 now has the conEigura-tion shown at C in Figure 2c. The ideal voltage distribution, with no grouncl capacitance, is shown at D for comparison purposes. ~he desired capacitance is achieved by adjusting Ihe diameter and depth o~ ring 6 as well as by varying the number of support members 7. It is anticipated that adequate shieIding, by means of grading ring 6, is practical for arresters used on system voltages up to 345 KV. For higher voltage arresters, however, the required diameter and depth for grading ring 6 becomes quite large so that a corresponding large and expensive container 11 must:
be used.
'rO overcome the problems involved with the higher voltage systems, an arrester system 10 containing a multiplicity of stac]ced series connected housings 16a-16d is shown in Figure 3. ~ series o.E telescoped concentric cyclindrical shiels 17-20 are arranged in a predetermined manner and eIectrically connected to the junction points (7-9) between housings 16a-16d such that the capacitance between cylindrical shieldsl7-20 forces a uniform voltage distribution between housings 16a-16d. The number of housings 16a-16d is selected such that th~ voltage rating for each housing 16a-16d is low and the voltage dis~ribution within each housiny 16a-16d is relatively uniform. It is anticipated that an upper limit for the voltage rating for each housing 16a-16d is in the order of lOOKV. A total of four housings 16a-16d, as shown, would be employed in the design of a 396 KV arrester for use on a 550 KV system.
Arrester system 10 also contains metal container 11 sealed at the top by means of top flange 12 and at the bottom by means of bottom flange 13 and containing a filling of insulating SF6 gas as described for the .35 arrester system of Figure la.

1 ~7~8~ 5D-5696 Four porcelain housings 16a-16d are stackecl within metal container 11, one above khe other, and each housing 16a 16d contains a plurali-ty of zinc oxide disks 14 with metal electrodes l'i arranged in a stack such thak each individual disk 14 is electrically connected in series with each other disk 14. Porcelain housing 16a-16d are terminated at each end by metal flanged fittings 30 to facilitate boltiny them together and provision is made for venting to the surrounding SF6 atmosphere to permit transfer of the insulating SF6 gas within container 11 to within the vicinity of zinc oxide di~k~
In order -to provide capacitive grading along the stack of housings 16a-16dl Eirst, second, third, and fourth cylindrical shiels 17-20 are employed in the ~ollowing manner. ~ach of the cylindrical shiels 18-20 consists of a linear portion, such as 20a, and a bell~shaped portion, such as 20b. The lowermost cylindrical shieId 17 i9 of a single diameter only, eclual to that of the linear portions of shields 18 to 20. Each cylindrical shield 17-20 in concentrically arranged around porcelain housings 16a-16d such that fourth cylindrical shield 20 overlaps third cylindrical shield 19 to a greater extent than third cylindrical shield 19 overlaps second cylindrical shield 1~, and second cylindrical shieId 18 overlaps first cylindrical shieId 17 to a lesser extent than third cylindrical shieId 19 overlaps second cylindrical shield 18. Electrical line connnection is brought into top flange 12 by means of top conduit 21 and the stack becomes connected to ground by means of bottom conduit 22. Shields 17-20 are electrically connected to adjacent flanges 30 by means of conductive supports 1-~ respectively. A
capacitive distribution of voltage is developed between beIl portion 20b of fourth cylindrical shield 20 and linear portion lga of third cylindrical shild 19, 1 17c58~9 5D-5696 between bell portion 19b of third cyli:ndrical shield 19 and linear portion 18a of second cylindric.al shield 18 and beIl portion 18b of second cylindrical shield 18 and the first cylindrical shield 17.
Figure 4 illustrates the relationship between the distance of overlap 11 between first cylindrical shieId 17 and second cylindrical shield 18, the distance of overlap 12 between second shield 18 and third sh.ield 19 and the distance of overlap 13 between fourth shield 20 and third shield :L9. The rad:ius o:E l:inea:r portions 17a-20a of shields 17-20 is designated :R~
and the radius of bell shape portions 18b-20b of shieLd 18-20 is designa~ed RB. The radius o:E ~rounded m~tal container 11 is designated ~G~ The capacitance between any two concentric cylinders of overlapping l.ength 1 in centimeters, radius RA and radius ~ in centimeters, is given by the expression;

C ~ 11.2.7 1) picofarads ~ A) This expression allows both the intershield capacitances to be determined as well as the capacitance from each cylindrcial shield 17-20 to the grounded metal container 11 .
For the embodiments depicted in Figures 3 and 4, shields 17-20 are described as comprising metal cylin-ders. Cylindrical shields17-20 are used for capci-tive purposes by providing capacitive charging currents in a manner to be described below and do not carry power current at any time. Shields 17 20 can, therefor, be made`very thin and can even comprise a metal foil wound on an insulating tube such as paper or fiber board .
The schematic diagram representing the relationship between the capacitances existing within systems 10 of Figures 3 and 4 is shown in Figure 5 as follows. The 1 1758~ 3 5D-56g6 capacitance to ground for first cylindrical shield 17, second cylindrical shield L8, -th:ird cylindrical shield 19 and fourth cylindrical shield 20 iS represented by Cl, C2r C3 and C4, respectively. The intershield 5 capacitance existing between second cylindrical shield 18 and first cylindrical shield 17 is given by C21, between third cylindrical shield 19 and second cylindrical shield 18 is given by C and between bourth cylindrical shield 20 and third cylindrical shield 19 by C43. The 10 primary function oE shield 17-20 is to provide capacitive ~rading such that the voltage from line 21 to ground will divide equally among housinc3s 16a-16d. For the voltage to divide equal:Ly in the schematic diagram of Figure 5, the voltages across Cl, C21, C32, and C~3 15 must all e~ual one quarter ol~ the vol-tage rom line terminal 21 to grolmd. ~n order to provide -the required voltage division, tha capacitance relation~hips mus~
be as follows:
~1) C21 C

(2) C32 =

(3) C43 = 3C3 + 2C2 ~ Cl Appropriate dimensions for electrostatic shields 17-20 and for metal container 11 of Figures 3 and 4 are RA, RB, RG, 11, 12, and 13. A preliminary design for a 396 KV rated arrester for use in a 550KV AC system, as one example, required the following dimensions.
E~eight of porcelain housing 16 . . . . . . .42"
Ouside diameter of porcelain housing 16 . . . . .10"
Radius of lower portions (17, 18a-20a) RA . . .11l' Radius of upper portions (18b-20b) RB . . .13"
Radius of metal container 11 RG . . . . . . . .24"
Overlapping length (I7-18) 11 . ~ ....... 7.25"
Overlapping length (18-19) 12 . . . 20.5"
Overlapping length (I9-20) 13 . . . . . . . . . 39.25"

L 17r788(3 g The dimensions given are for providing equal distribution of volta~e be-tween housings 16a-16d. The distribution of volta~e between dlsks :L4 wlthin any housing 16a-16d must be separately de-termined. Each housing 16a-16d in Figures 3 and 4 is completely enclosecl by one of the cylindrical shields17-20 connected such that there is no stray capacitance to ground.
Since the shieId arrangement is identical for each housing 16a-16d the voltage distribution within each housing 16a-16d will be the same.
In -the prac-tical case where each housing 16a-16d is ra-ted at about lOOKV and with reasonable dimellsions for RA, ~ , and RG, the voltage dis-tribution within housings 16a-16d will approximate tha-t shown at C in Figure 2C. The distribut:Loll of volta~e across all ~our housings 16a-16d in series is depic-ted in Figure 6.
The actual voltage distribution is shown as a solid curve E. The ideal voltage distribution with no ground capacitance is shown at F in dotted lines for comparison. The voltage distribution in the absence o~ any capacitance shielding is shown a~ G.

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A surge arrester assembly having graded capacitance comprising:
a plurality of zinc oxide varistor discs arranged in a stack provided with a line connection at one end and a ground connection at an opposite end;
a metal housing coextensive with said varistor stack; and a succession of capacitor shields encompassing said varistor stack, each said shield electrically connected to a point in the portion of said stack encompassed thereby, some of said shields having a large diameter portion and a small diameter portion with the large diameter portion of one shield in spaced overlapping relation with the small diameter portion of a successive shield to provide intershield capacitance at the regions of overlap.

2. The system of claim 1 wherein said capacitance shield cylinders have a greater intershield capacitance at said line end than at said ground end of said housing.

3. The arrester assembly of claim 1 wherein the capacitor shields at said line end overlap each other to a greater extent than said capacitor shields at said ground end of said stack in order to provide a greater inter-shield capacitance at said line end than said ground end of said stack.

4. The arrester assembly of claim 1 wherein said capacitance shields are selected from the group consisting of metals and metal-covered insulators.

5. The arrester assembly of claim 1 further including an insulating housing coextensive with said varistor stack and intermediate said stack and said metal housing.

6. The arrester assembly of claim 1 further including a filling on an insulating gas selected from the group consisting of SF6 and N2.

7. The arrester assembly defined in claim 1 or 3 wherein said shields having large and small diameter portions are oriented with their small diameter portions directed toward the line end of said stack and the one of said shields at the ground end of said stack being a straight cylinder having a diameter conforming to the small diameter portions of the other shields and being partially overlapped by the large diameter portion of the adjacent one of said shields.