US 3733521 A
A voltage grading arrangement for lightning arresters particularly adapted for use in substations in which the conductors and the lightning arresters are enclosed within grounded metallic enclosures, the enclosures being filled with an "electronegative" gas such as sulphur hexafluoride (SF6) under pressure. The lightning arrester elements are enclosed in a porcelain housing and grading elements such as capacitors or resistors or both are disposed on the exterior of the porcelain housing and within the surrounding metal enclosure.
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United States Patent 1 Kalb [111 3,733,521 1 May 15, 1973  LIGHTNING ARRESTER  Inventor: John W. Kalb, Medina, Ohio  Assignee: The Ohio Brass Company, Mansfield, Ohio 22 Filed: Sept. 27, 1971  Appl.No.: 183,812
 U.S. Cl ..3l7/70, 315/36  Int. Cl. ..H02h 9/06  Field of Search ..3l5/36; 317/70  References Cited UNITED STATES PATENTS 3,099,770 7/1963 Sorrow et al. ..315/36 2,298,114 10/1942 Estorff ..317/70 3,414,759 12/1968 Connell et al. ..317/7() X Primary Examiner-James D. Trammell Attorney-Wm. Crighton Sessions et al.
[ 5 7] ABSTRACT A voltage grading arrangement for lightning arresters particularly adapted for use in substations in which the conductors and the lightning arresters are enclosed within grounded metallic enclosures, the enclosures being filled with an electronegative gas such as sulphur hexafluoride (SF under pressure. The lightning arrester elements are enclosed in a porcelain housing and grading elements such as capacitors or resistors or both are disposed on the exterior of the porcelain housing and within the surrounding metal enclosure.
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BACKGROUND OF THE INVENTION This invention relates to lightning arresters and more particularly to voltage grading for lightning arrester assemblies for substations in which the conductors and lightning arresters are disposed in grounded metal enclosures that are filled with a pressurized electronegative gas such as sulphur hexafluoride (SP although the invention is also useful in other environments.
Present day lightning arresters embody a series of spark gaps utilized with either series connected valve blocks as in the Connell and Putt U.S. Pat. No. 3,414,759 or without series valve blocks as disclosed in my co-pending application Ser. No. 1,186, filed Jan. 7, 1970. In order to secure consistency and uniformity in sparkover voltage and to distribute the power line and surge voltages among the several gaps various types of voltage grading circuits are employed. By the use of such circuits, the voltages across the arrester gaps can be distributed as desired among the gaps. The voltages would otherwise vary substantially because in high voltage installations the arresters may be quite tall and the capacitance effects due to ground plane and surrounding structures differ at different points along the lengths of the arresters.
Voltage grading, as is known in the art, may be arranged to distribute the voltage substantially evenly across the gaps or to arrange for more of the voltage to be applied to certain of the gaps then others and thus to secure a cascading effect in sparkover and a lower sparkover voltage. As examples of patents illustrating grading systems see the Connell US. Pat. No. 3,496,409 and the Osterhus U.S. Pat. No. 3,377,503. In known systems, the components such as resistors, capacitors and auxiliary gaps that make up the grading system are conventionally enclosed within an elongated tubular insulating housing, usually composed of porcelain, along with the other components of the arrester such as the gap elements, coils and valve blocks. The housing is gas-tight and the interior of the housing is conventionally filled with nitrogen gas.
In conventional lightning arrester installations such known grading schemes operate with a reasonable degree of effectiveness, although reliability and consistency of operation may be impaired and the voltage distribution within the arrester changed by contamination of the exterior of the insulating housing or by other changes in the environment in which the arrester operates. Conventional voltage grading, however, is not satisfactory in substations embodying gas insulated systems (hereinafter sometimes referred to as ST systems, although other gases may be employed) yet such SF substations are becoming of substantial economic importance in urban areas for the reason that SF substations occupy only a small fraction of the space required by conventional substations and hence may be enclosed in buildings or placed in urban locations where the cost of the land would otherwise make the installation of a substation prohibitively expensive.
The difficulty with lightning arresters in SE; substations arises from the fact that it is desirable to enclose the lightning arresters for such substations in metal enclosures which contain SF 8 gas under pressure, the enclosures being at ground potential. Since the enclosures are located a comparatively small radial distance from the porcelain housings of the arresters, (for example, in a typical installation the porcelain housings may have a diameter of 12 inches while the metal enclosures have a diameter of 30 inches) the internal components of the arresters, including the grading elements, are subjected to a very high radial field. The intemal nitrogen atmosphere within the porcelain housing is severely overstressed by this high field and corona discharge within the housing to an unacceptable level results. The durability and reliability of the arresters and the consistency of sparkover voltage may also be impaired.
SUMMARY A general object of the present invention therefore is to provide lightning arresters embodying voltage grading that is substantially unaffected by the external environment in which the lightning arrester is disposed. A more specific object is the provision of a voltage grading system or arrangement for lightning arresters particularly adapted for use in connection with gas insulated systems in which the lightning arresters are enclosed in a housing filled with a dielectric gas such as SP Further objects and advantages of the invention will become apparent from the following description of the preferred forms thereof.
Briefly, the above and other objects and advantages of the invention are attained by disposing the grading elements on the exterior of the porcelain housing for the arrester and, in the case of an arrester designed for service in gas insulated substations, within the pressurized and grounded enclosure surrounding the arrester proper. With this arrangement the internal elements of the arrester are substantially unaffected by ordinary environmental changes and are not subjected to the extremely high radial voltage stresses to which they would otherwise be subjected in SF systems. While in such installations the grading elements on the exterior of the housing are subjected to a very intense radial electrical field, this does not result in corona or other electrical disturbances because the elements are in an atmosphere of pressurized SP or other similar electronegative gas which has the ability to suppress corona.
The external grading can be used in conjunction with internal grading of conventional nature if desired, or can be used without any internal grading. For example, it is contemplated that standard commercial lightning arresters with the desired internal grading can be adapted for use in gas insulated substations simply by enclosing them in porcelain housings without weather sheds and disposing the grading components on the exterior of the housings. The grading can be accomplished by capacitors or resistors or combinations of capacitors and resistors disposed on the exterior of the porcelain housing. If the grading involves resistance, the resistance can be provided by a conducting film such as a film composed of a conductive glaze on the outside of the porcelain housing. It is preferable that longitudinally spaced, circumferentially extending conducting bands or other conductive means be provided along the exterior surface of the porcelain housing in order to distribute electrical charges circumferentially about the outer surface of the housing.
BRIEF DESCRIPTION OF THE DRAWINGS view showing the present invention as adapted to a lightning arrester for service in a gas insulated substation;
FIG. 2 is a diagrammatic view illustrating simple grading circuits that may be employed in the apparatus; and
FIG. 3 is a fragmentary diagram illustrating a modification of the external grading circuit.
DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawings, a lightning arrester indicated in general at is shown in conjunction with an enclosed, gas insulated conductor 11 that leads to a transformer or other equipment that is to be protected from high voltage surges resulting from lightning strokes, switching surges and the like. The conductor l1 conventionally is tubular and is supported within a gas-tight, conductive enclosure in the form of a metal tube 12, the conductor being centered with re spect to the tube by insulators 13. The conductor 11 is connected to the lightning arrester 10 by a conductor 14 and the lightning arrester is disposed within a gastight enclosure 15 that is attached in leakproof relation to the tube 12 by any convenient fitting 16. The enclosure 15, tube 12 and fitting 16 may all be made of aluminum. The interior of the tube 12 and the interior of the enclosure 15 are in open communication with each other, and like other elements of the enclosure for conductors in the substation, the tube 12 and enclosure 15 are filled with an appropriate electro-negative gas such as SP under pressure. The entire enclosing system, including the tube 12 and the enclosure 15, is grounded. Various dielectric gases can be employed such as halogenated hydrocarbons and sulphur hexafluoride. These gases are electro-negative. Their dielectric strength increases generally (but not necessarily linearly) with pressure and they have the ability to absorb large numbers of free electrons and to suppress corona discharge to a much greater extent than nitrogen, the gas that is ordinarily used in the housings of lightning arresters. The gas that is considered to be the most suitable at the present time is sulphur hexafluoride at a pressure of about 22 pounds per square inch gage.
The lightning arrester 10 may take any conventional or appropriate form for the duty to which the arrester is subjected. In FIG. 1 the internal arrester components indicated in general at 20 are made in accordance with my aforesaid application Ser. No. 1,186, to which reference is made for a complete description. Briefly, the arrester elements include parallel-connected coil and non-linear resistor assemblies 21 and packs 22 of gap units in which the spark gaps themselves are disposed. The gap units provide serially connected spark gaps and the assemblies 21 are also connected in series with the spark gaps. Conventional grading components may or may not be included with the gap units depending on the design and service requirements. The number of assemblies 21 and gap packs 22 depends upon the system voltage and the design of the arrester elements but, as an example, for a 345 kV substation about 15 gap packs and associated coil assemblies in a stack about 10 feet high would be required to provide a reasonable sparkover voltage of about 540 kV for such a system.
The arrester components are enclosed within a gastight tubular insulating member such as the porcelain housing 23. It is to be noted that the housing 23 is not provided with weather sheds, these being unnecessary since the arrester is protected from the environment by the enclosure 15. The housing is closed by conductive end plates such as cast aluminum plates 24 and 25 which are secured in sealed relationship by conventional seals to the ends of the housing 23 and the top and bottom coil assemblies 21 are electrically connected to the end plates 24 and 25 respectively by conventional means. The interior of the completed enclosure is filled with dry nitrogen gas at normal atmosphefic pressure in accordance with known practices. It is not desirable to use SF in the interior of the arrester housings because the dielectric strength of SP is too high to permit proper operation of conventional gaps at the desired sparkover voltage. The top plate 24 is connected to the conductor 11 by the conductor 14; the entire assembly is disposed within the metal enclosure 15 with the bottom plate 25 supported by an aluminum spacer 26 that rests on the bottom 27 of the enclosure and centers the arrester with respect to the enclosure. The bottom plate 25 is electrically connected to the enclosure 15 through the spacer 26 and the enclosure 15 is grounded in any conventional manner as well as being connected at its upper end to the grounded tubular enclosure 12.
With this construction it will be evident that with any reasonable relationship between the diameter of the porcelain housing 23 and the metal enclosure 15, which is at ground potential, there will be a very high radial field between the internal parts of the arrester that are at high potential and'the grounded enclosure 15. Under such conditions the nitrogen gas within the housing 23 would be overstressed and there would be severe corona problems. The result would be that the reliability and consistency of operation of the arrester would be seriously impaired and unacceptable levels of radio noise might be generated.
According to the present invention these difficulties are eliminated by disposing grading elements on the exterior of the porcelain tube 23 so that the environment that the internal parts of the arrester see" is a potential that gradually decreases from the line voltage at the top of the arrester to ground voltage adjacent the bottom of the arrester. In the present embodiment the grading is accomplished by capacitors disposed on the exterior of the porcelain housing 23. A series of spaced, preferably annular conductive bands 28 are disposed on the exterior of the porcelain housing 23. The bands are thin and may be made of copper foil adhesively secured to the exterior of the housing. The bands furnish convenient connections for the grading elements such as the capacitors 29, and also distribute the voltage circumferentially around the housing. Adjacent bands are connected by capacitors 29. The uppermost band 28 is connected to the member 24 as by a coating 30 of conductive paint, and the lowermost band is connected to the member 25 by a similar coating 31 of conductive paint. Instead of capacitors, resistors or combinations of resistors and capacitors can be employed if desired. The effect is to distribute the voltage between line and ground throughout the length of the housing 23 and thus the internal parts of the arrester are not subject to undue radial stresses. While the components on the exterior of the porcelain housing are subjected to a very strong radial field, this does not create any problem in the pressurized SF atmosphere within the enclosure since that gas has the capability of suppressing corona and has high dielectric strength.
If it is desired to provide substantially uniform voltage distribution along the housing, greater capacitance is required near the top of housing 23 than near the bottom as indicated by the progressively decreasing number of capacitors 29 shown in FIG. 1 from the top to the bottom of the housing. The following table gives an example of appropriate values of capacitance for an arrester as described above with the arrester elements enclosed in a porcelain housing 23 about feet, six inches tall, having a diameter of about 12 inches within a metal enclosure having a diameter of 30 inches, the enclosure being filled with SF at a pressure of 22 p.s.i.g. The exterior of the housing 23 is divided into twenty segments by the bands 28. For each segment, the capacitance to the grounded enclosure 15 is about 12 pico farads (pf). The table indicates the capacitance, calculated to the nearest 5O pf, required to give substantially uniform voltage distribution along the housing. In the table the capacitance values are given for the capacitors adjacent each segment of the housing starting with the top or line potential end and progressing to the bottom of ground end of the housing, Segment No. l being between the first and second bands 28, Segment No. 2 being between the second and third bands 28, and so on.
Segment No. Capacitance Segment No. Capacitance (P (PO 1 2300 l l 700 2 2 100 12 600 3 I900 13 500 4 1700 14 400 5 1550 15 300 6 1300 16 250 7 l l 50 l 7 200 8 1000 l 8 200 9 900 I9 150 10 800 150 As noted above, this arrangement can be used either with or without internal grading and if internal grading is employed the grading can be arranged to cause one gap or group of gaps in the arrester to fire first in manners known to those skilled in the art. The external grading can be arranged to provide substantially equal voltage drops between successive bands 28 or, if desired, the external grading can be arranged to cause greater voltage to be imposed upon one gap or group of gaps thereby causing that gap or group of gaps to fire first upon subjecting the arrester to voltage in excess of its spark-over voltage.
The arrangement has important advantages from an operational standpoint since the environment in which the arrester operates in effect remains constant. There is no chance of contamination of the exterior of the porcelain housing and contamination of the exterior of the enclosure 15 does not affect the operation of the arrester since this enclosure is at ground potential at all times. Because of the constant environment in which the arrester elements operate, consistency of operation and reliability in service are readily obtainable.
FIG. 2 is an electrical diagram showing a simple internal grading scheme used in conjunction with the arrester elements and external grading of FIG. 1. In this embodiment of the invention the conductor 11 and its tubular housing 12, the enclosure 15, the porcelain housing 23 and the arrester components are the same as in FIG. 1 and the same reference characters have been applied to corresponding parts. The arrester components are shown diagrammatically. Each assembly 21 is made up of a coil 35 and a non-linear resistor 36 in the form of a valve block and each gap pack 22 comprises a plurality of gaps 37.
The internal grading is accomplished by parallelconnected capacitors 39 and resistors 40 connected across the gaps 37. The grading on the exterior of the housing is provided by capacitors 29 as previously described.
Other intemal grading schemes may be employed such as that shown in the aforesaid Connell US. Pat. No. 3,496,409. Also, as shown in FIG. 3, the external grading may include not only capacitance as provided by capacitors 29, for example, but also resistance as diagrammatically indicated at 41.
From the foregoing it will be evident that the invention provides an improved grading system for lightning arresters that is particularly adapted for use in gasinsulated substations. The external grading employed in the present system insures that the internal elements of the lightning arresters are substantially unaffected by ordinary changes in the environment and thus consistency and reliability of operation of the arresters is improved.
I. A lightning arrester assembly for a gas-insulated electrical system in which the conductors are enclosed within grounded metallic enclosing members containing gas, said assembly embodying a lightning arrester comprising a stack of gap elements providing a plurality of serially connected spark gaps, an elongated sealed housing composed of insulating material surrounding said gap elements, said housing having atone end thereof a first conductive member that is at line potential when the arrester is in service and a grounded conductive member at the other end thereof, a grounded conductive enclosure surrounding and spaced from said tubular housing, an electro-negative dielectric gas filling the space between the exterior of said tubular housing and the interior of said conductive enclosure and voltage grading means on the exterior of said insulating housing extending between said conductive members and disposed within said conductive enclosure and the gas therein.
2. A lightning arrester assembly according to claim 1 wherein the gas within said conductive enclosure is under super atmospheric pressure.
3. A lightning arrester assembly according to claim 2 wherein said gas is sulphur hexafluoride.
4. A lightning arrester assembly according to claim 1 having connecting means providing open communication between the interior of the enclosure and the interior of one of said enclosing members.
5. A lightning arrester assembly according to claim 4 in which said connecting means also constitutes an electrical connection between said enclosure and said one of said enclosing members.
6. A lightning arrester assembly according to claim 5 having an electrical connection extending through said connecting means from one of said conductors to said first conductive member.
7. A lightning arrester assembly according to claim 1 in which said insulating housing is a tubular porcelain member.
8. A lightning arrester assembly according to claim 1 in which the grading means on the exterior of the housing comprises a series of annular conductive bands spaced along the exterior of the insulating housing and grading elements interconnecting at least some of said bands.
9. A lightning arrester assembly according to claim 8 wherein the grading elements include capacitors disposed on the exterior of the housing.
10. A lighting arresting assembly according to claim 8 wherein the grading elements comprise capacitors and resistance elements disposed on the exterior of the housing.
11. A lightning arrester assembly accordingto claim lating housing between said conductive members.
13. A lightning arrester according to claim 12 in which said insulating housing is a tubular porcelain member.
14. A lightning arrester according to claim 12 in which the grading means comprises a series of annular conductive bands spaced along the exterior of the insulating housing and grading elements interconnecting at least some of said bands.
15. A lightning arrester according to claim 14 wherein the grading elements include capacitors disposed on the exterior of the housing.
16. A lightning arrester according to claim 14 wherein the grading elements comprise capacitors and resistance elements disposed on the exterior of the housing.
17. A lightning arrester according to claim 12 also having grading means within said housing.
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