US3792188A

US3792188A - Conductive particle trap for high-power, gas-insulated transmission system

Info

Publication number: US3792188A
Application number: US00282366A
Authority: US
Inventors: J Cronin
Original assignee: ITE Imperial Corp
Current assignee: ABB Inc USA
Priority date: 1972-08-21
Filing date: 1972-08-21
Publication date: 1974-02-12
Anticipated expiration: 1991-02-12
Also published as: CA972837A; CH558585A

Abstract

A method of placing a low-frequency, high voltage, gas-filled power transmission system into service by applying a conditioning voltage to the system prior to application of system voltage.

Description

United States Patent n91 Cronin Feb. 12,1974

[ 1 CONDUCTIVE PARTICLE TRAP FOR HIGH-POWER, GAS-INSULATED TRANSMISSION SYSTEM [75] Inventor:

[73] Assignee: I.T.E. Imperial Corporation,

Philadelphia, Pa.

[22] Filed: Aug. 21, 1972 [21] Appl. No.: 282,366

John C. Cronin, Greensburg, Pa.

[52] US. Cl 174/28, 174/14 R, 317/3, 317/262 R [51] Int. Cl. H011) 9/06 [58] Field of Search... 317/3; 174/14 R, 28; 55/146, 55/154 [56] References Cited UNITED STATES PATENTS 3,515,939 6/1970 Trump 317/3 3,433,883 3/1969 Hahne 174/28 FOREIGN PATENTS OR APPLICATIONS 1,133,270 11/1968 Great Britain 174/28 Primary Examiner-William M. Shoop, Jr. Assistant Examiner-Harry E. Moose, Jr. Attorney, Agent, or Firm-Ostro1enk, Faber, Gerb &

Soffen [57] ABSTRACT A method of placing a low-frequency, high voltage, gas-filled power transmission system into service by applying a conditioning voltage to the system prior to application of system voltage.

4 Claims, 3 Drawing Figures CONDUCTIVE PARTICLE TRAP FOR HIGH-POWER, GAS-llNSUlLATED TRANSMISSION SYSTEM BACKGROUND OF THE INVENTION This invention relates to high-power, gas-insulated transmission systems, and more specifically relates to conductive particle traps for trapping conductive or semiconductive particles in a gas-filled transmission system which is operated at from 50 to 60 Hz. and at voltages in excess of 60,000 volts.

In gas-insulated systems, it is essential to prevent conducting or semiconducting particles from entering the gas space in between the system conductors where an appreciable electric field exists. Thus, it can be demonstrated that very severe reductions in dielectric strength occur where conductive particles are present in a gas space.

Several techniques have been proposed in the past to control the movement of these particles.

By way of example, US. Pat. No. 3,515,939 to Trump discloses the use of an essentially zero-field region within the gas space which acts to trap conducting particles. Thus, a perforated screen is placed within the grounded outer housing of the gas-insulated transmission line. Conducting and semiconducting particles, which are contaminants in the gas space, pass through the holes of the screen and are unable to acquire enough energy in the low-field region defined between the screen and the outer housing to move back out through the screen.

Other methods proposed for the control of these particles within the gas space employed the use of an adhesive or tacky material on the conductor surfaces in order physically to hold particles which deposit on the coating. This type arrangement is shown in US. Pat. No. 3,553,410.

In accordance with the present invention, a novel contamination control structure is provided which uses a low but not necessarily zero dielectric fields at the outer ground electrode. Thus, in the Trump US. Pat. No. 3,515,939, the attempt is made to produce an essentially zero-field region. in accordance with the present invention, however, a low-field region which contains insufficient energy to cause conductive or semiconductive particles to move out of the field, against the force of gravity, is provided by corrugating the outer housing.

When using an outer corrugated housing the electric field at the base or largest diameter portion of the corrugations will be much lower than at the surface of a smooth cylindrical enclosure with the same inner diam eter as the smallest diameter portion of the corrugations. In any specific example, the reduction in field intensity will be a function of the system dimensions and in particular will depend upon the depth of the corrugations. In a preferred embodiment of the invention, the corrugation depth is about one-tenth the outer diameter of the corrugated housing so that only about onetenth of the dielectric stress exists at the inside surface of the corrugations, as compared to the stress which would exist at the surface of an equivalent smooth cylindrical enclosure of constant diameter. By varying the depth and shape of the corrugations, even greater reductions in stress can be achieved. However, no attempt is made to produce a field free region as in US. Pat. No. 3,515,939.

Corrugated housings per se have been used in the past in connection with electrical coaxial conductors for high-frequency transmission lines. Thus, US. Pat. No. 3,433,883, shows a cable having a corrugated outer housing. This cable, however, is for the transmission of electric power at radio frequencies and relatively low voltages, as compared to the low frequency and high voltages used with the present invention and the corrugations were used solely to allow the cable to flex if necessary. By contrast, the invention is used in relatively low-frequency power transmission systems where this low frequency is the conventional utility power frequency of from 50 to Hz. Moreover, the transmission system of the invention is intended for transmission of extremely high voltages (above 60,000 volts) which require the use of atmospheres such as SP or gas mixtures including SF It is only when the voltages exceed, for example, 60,000 volts, that conductive and semiconductive particles become a serious problem and jeopardize the dielectric integrity of the gas. Thus, the use of corrugated outer conductors in an air dielectric radio frequency transmission system is solely for cable flexibility, and is unrelated to a particle trapping function. This particle trapping function is realized for the first time with the new combination of the invention of a corrugated outer housing in a gas-filled, power-transmission system which is operative at low frequency, and at power line voltages in excess of 60,000 volts.

The efficiency of the corrugation as a particle trap in the present invention may be appreciated by considering that the ability of particles to move under the influence of an electric field is a function of particle size and shape, particle density, gas density and the intensity of the electric field. Therefore, to move a particle which rests at the base of the corrugations will require .a higher voltage on the conductor than for a particle of similar size on the surface of a smooth enclosure. In the preferred embodiment of the invention, this ratio will be at least 10 to 1. Therefore, the gas-insulated transmission system will have its reliability improved since particles which will migrate in the bottom of the corrugations have a very low probability, essentially zero, of moving upwardly and into the high-stress regions.

In accordance with another aspect of the invention, before a system is put into service, a conditioning voltage may first be applied to allow most particles capable of moving to migrate into the corrugations. The initial conditioning voltage is preferably obtained from a low energy or high impedance source such as a d-c test set and acts to move conductive particles created during installation or maintenance into the corrugation bottoms. By using a high impedance or low energy source for the conditioning voltage, flashover during the conditioning period will not cause permanent damage. The system voltage may then be applied to the line after this initial purging action.

The use of the corrugated housing of the invention has certain physical advantages that will be apparent. Thus, the conductive enclosure imparts mechanical flexibility to the system, which can be advantageous when the system is to be laid on a relatively irregular support surface, such as the bottom of a ditch, or a seabed, or the like.

Another advantage is that the corrugations are formed in the enclosure during its fabrication, to eliminate the need for adding additional components, such as perforated shields or adhesive surfaces during assembly of the bus system.

A major advantage of the invention is that the corrugations produce particle traps which run the entire length of the transmission system. This is in contrast to discrete, longitudinally spaced traps along the length of the transmission line, where the probability of quickly trapping particles after their generation is relatively low. Thus, particles which are generated during operation of the transmission line due, for example, to sliding contacts between the insulator supports and bus will be immediately trapped with the present invention, since they do not have to migrate along the length system until they reach a trap, as in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal cross-sectional drawing of a bus constructed in accordance with the present invention.

FIG. 2 is a cross-sectional view of FIG. 1 taken across the section line 22 in FIG. 1.

FIG. 3 illustrates an embodiment of the invention, in which the bottom of the corrugations is formed with a relatively sharp angle to lower the field strength at the bottom of the corrugations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2, the invention is illustrated as applied to a transmission line which connects a relatively low-frequency generating station 10, shown as a 50 to 60 Hz. source, which may have an output voltage in excess of 60,000 volts and, for example, 230,000 volts. The transmission line 11 of the invention then connects this source to a suitable load circuit, schematically illustrated as the load 12.

Obviously, the system may be multi-phase but only one of the phase conductors is shown in FIG. 1. Thus, the conductor of FIG. 1 consists of a central conductor 13 which is enclosed by a corrugated grounded conductive housing 14. Conductor 13 will be sized in accordance with the current which must be carried by the transmission line, and is conventionally supported within the corrugated housing 14 by a plurality of longitudinally spaced insulators, one of which is shown as insulator disk 15. The interior of the enclosure is then filled with an electro-negative gas, for example, sulfur hexafluoride, at a pressure of 3 to 15 atmospheres. Suitable gas mixtures such as mixtures of SF 6 and nitrogen may be used to reduce gas liquification problems.

In one embodiment of the invention, the outer diameter D of the corrugations may be 12 inches while the inner diameter of the corrugations D may be 10.75 inches. The distance between the peaks of adjacent corrugations, shown in FIG. 1, as distance D may typically be 3.5 inches. The diameter of the conductor 13 may typically be 4.5 inches.

The system is then capable of transmitting low frequency power of from 50 to 60 Hz. at a voltage of 230 KV, by virtue of the careful control of the construction of insulators 15 and by virtue of the dielectric properties of the insulation gas such as the pressurized sulfur hexafluoride gas within housing 14.

In accordance with the invention, the corrugated housing in a low-frequency, high-voltage, gas-filled power transmission system, causes a plurality of continuous particle traps to be defined along the length of the system by virtue of the low-field intensity at the outer diameter regions of the internal corrugated surfaces as compared to the field intensity of regions within housing 14 which are closer to conductor 13. Thus, at the bottom of the corrugations (it being noted that housing 14 will be horizontally disposed), the electric field at surface 14a will be one-tenth the field at some point intermediate the housing 14 and conductor 13. Moreover, this relatively low-field region will not be able to impart sufficient energy to contamination particles which are conductive or semiconductive and which are formed or exist within the housing 14, so that such particles will be trapped by gravity at the bottoms of the corrugation regions.

Note that the trap exists completely along the length of the transmission system when a corrugated housing 14 is used, so that particles need not have any appreciable longitudinal migration before being trapped.

It will be further observed that the power transmission system will be relatively flexible by virtue of the corrugated outer housing 14, as compared to a rigid system which would employ a cylindrical outer housing having a constant diameter.

In placing this system in operation, it may be preferable to initially operate the system at a voltage lower than rated voltage before applying load to the transmission line. This will initially trap conductive and semiconductive contaminants which were produced during the installation of the line. Thus, an initial voltage above 60,000 volts is applied to the line before connection to a load circuit, causing the trapping of most conductive particles in the low-field regionsin the corrugations. The rated voltage, for example, 230,000 volts may thereafter be applied and the load circuits connected with the gas region within the transmission line substantially free of conductive particle contaminants created during installation.

In FIG. 1, the corrugations in housing 14 are formed so that the corrugations are generally sinusoidal in cross-section. FIG. 3 shows a second embodiment of the invention, wherein the corrugations 20 are formed with generally rounded inner diameter sections 21 and relatively sharp outer diameter sections 22. Thus, the area within outer diameter sections 22 will have a lower field intensity than they would if the corrugations were sinusoidally shaped and, therefore, serve as improved particle trapping regions. Note that the corrugation shape could also be V shaped in cross-section or could have any other desired section for producing a desired low-field intensity at the corrugation bottom.

Although the present invention has been described in connection with a preferred embodiment thereof, many variations and modifications will now become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

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

1. The method of placing a low-frequency, high voltage, gas-filled power transmission system into service, said power transmission system including a horizontally disposed elongated central conductor surrounded by and insulated from an outer corrugated housing, with the space between the said central conductor and said corrugated housing being filled by an insulation gas; said method comprising the steps of initially applying a conditioning voltage between said outer housing and said central conductor from a relatively low energy conditioning source for a length of time sufficient to cause substantially all conducting and semiconducting particles which contaminate said gas to move to bottom regions of said corrugated housing and thereafter connecting system voltage to said power transmission system wherein the energy of said system is substantially greater than the energy of said conditioning source.

2. The method of placing a low-frequency, high voltage, gas-filled power transmission system into service, said power transmission system including a horizontally disposed elongated central conductor surrounded by and insulated from an outer housing which has a conductive particle trap means therein, with the space between the said central conductor and said housing being filled by an insulation gas; said method comprising the steps of initially applying a conditioning voltage between said outer housing and said central conductor from a relatively low energy conditioning source for a length of time sufficient to cause substantially all conducting and semiconducting particles which contaminate said gas to move to said conductive particle trap means of said housing and thereafter connecting system voltage to said power transmission system wherein the energy of said system is substantially greater than the energy of said conditioning source.

3. The method of placing a low-frequency, high voltage, gas-filled power transmission system into service, said power transmission system including a horizontally disposed elongated central conductor surrounded by and insulated from an outer corrugated housing, with the space between the said central conductor and said corrugated housing being filled by an insulation gas; said method comprising the steps of initially applying a relatively low voltage between said outer housing and said central conductor for a length of time sufficient to cause substantially all conducting and semiconducting particles which contaminate said gas to move to bottom regions of said corrugated housing and thereafter connecting system voltage to said power transmission system.

4. The method of placing 'a low-frequency, high voltage, gas-filled power transmission system into service, said power transmission system including a horizontally disposed elongated central conductor surrounded by and insulated from an outer housing which has a conductive particle trap means therein, with the space between the said central conductor and said housing being filled by an insulation gas; said method comprising the steps of initially applying a relatively low voltage between said outer housing and said central conductor for a length of time sufficient to cause substantially all conducting and semiconducting particles which contaminate said gas to move to said conductive particle trap means of said housing and thereafter connecting system voltage to said power transmission system.

Claims

1. The method of placing a low-frequency, high voltage, gasfilled power transmission system into service, said power transmission system including a horizontally disposed elongated central conductor surrounded by and insulated from an outer corrugated housing, with the space between the said central conductor and said corrugated housing being filled by an insulation gas; said method comprising the steps of initially applying a conditioning voltage between said outer housing and said central conductor from a relatively low energy conditioning source for a length of time sufficient to cause substantially all conducting and semiconducting particles which contaminate said gas to move to bottom regions of said corrugated housing and thereafter connecting system voltage to said power transmission system wherein the energy of said system is substantially greater than the energy of said conditioning source.

4. The method of placing a low-frequency, high voltage, gas-filled power transmission system into service, said power transmission system including a horizontally disposed elongated central conductor surrounded by and insulated from an outer housing which has a conductive particle trap means therein, with the space between the said central conductor and said housing being filled by an insulation gas; said method comprising the steps of initially applying a relatively low voltage between said outer housing and said central conductor for a length of time sufficient to cause substantially all conducting and semiconducting particles which contaminate said gas to move to said conductive particle trap means of said housing and thereafter connecting system voltage to said power transmission system.