US3627905A - High-voltage electrical insulator having a predetermined surface conductance - Google Patents

High-voltage electrical insulator having a predetermined surface conductance Download PDF

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Publication number
US3627905A
US3627905A US883249A US3627905DA US3627905A US 3627905 A US3627905 A US 3627905A US 883249 A US883249 A US 883249A US 3627905D A US3627905D A US 3627905DA US 3627905 A US3627905 A US 3627905A
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insulator
conductance
voltage
discharges
voltage electrical
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US883249A
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Marcus William Astle-Fletcher
Leslie John Giles
Joseph Kenneth Hill
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British Railways Board
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British Railways Board
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B17/00—Insulators or insulating bodies characterised by their form
    • H01B17/50—Insulators or insulating bodies characterised by their form with surfaces specially treated for preserving insulating properties, e.g. for protection against moisture, dirt, or the like

Definitions

  • Askin AuomeySommers & Young ABSTRACT A high-voltage electrical insulator of synthetic plastics material having a predetermined electrical conductance to inhibit discharges of the order of 7 milliamperes and less but to permit larger current discharges.
  • Preferably said conductance is provided by surface conductance and preferably it is discharges of 5 milliamperes and less which are inhibited.
  • HIGH-VOLTAGE ELECTRICAL INSULATOR HAVING A PREDE'I'ERMINED SURFACE CONDUCTANCE This invention relates to high voltage electrical insulators suitable for outdoor use. Such insulators as widely used today are of glazed porcelain and are perfectly satisfactory in operation. However, recent interest has been directed to the use of plastics materials for high-voltage insulators, for example, glass-fiber reinforced plastics materials, which have certain advantages over glazed porcelain.
  • the invention is based on the discovery that if surface electrical discharges of the order of 7 milliamperes and less are inhibited there is no significant tracking; hence higher magnitude surface electrical discharges can be permitted. Accordingly the invention provides an insulator of synthetic plastics material having a continuous coating on its outer surface to produce a surface such that the plastics insulator has a predetermined electrical conductance so as to inhibit discharges of the order of 7 milliamperes and less.
  • said electrical conductance is such as to inhibit discharges of milliamperes and less but to permit higher current discharges.
  • V/x is the voltage stress (volts per centimeter) at which the surface breaks down; this means that surface discharge cannot take place at a lower stress
  • E is the voltage across the insulator
  • L is the minimum distance along the surface of the insulator between its ends
  • i is the minimum leakage current through the pollution at which discharge is to be permitted
  • i is the current that normally flows by reason of the surface conductance of the insulator when the insulator is dry.
  • the critical value for War is l7.5 kv./cm
  • E is 35 kv.
  • L is 1 meter
  • p is greater than 5 ma.
  • the surface conductance provided in the insulator will have to provide a leakage current 3 of about A. and the conductance would be about .000285p. mho.
  • the surface conductance of the insulator increases (i.e. the resistivity decreases) with increasing voltage stress, for example, by using known materials in the coating whose ohmic resistance decreases with an increase in voltage stress, then the steady state leakage can be reduced.
  • the surface resistivity for a voltage stress E/L is 10 times greater than that at the breakdown stress V/x then the steady state current leakage through the dry insulator can be reduced say from I00 to l0p.A.
  • the surface conductance of the insulator may be provided by a coating of flexibilized resin intimately mixed with antimony doped tin oxide powder, the coating being cured on the insulator.
  • a high-voltage insulator comprising a synthetic plastic bod and a continuous coatin of a conductive material on the su ace thereof, said coating avmg a thickness and berng of a material to inhibit discharge of electrical currents therethrough below about 7 mA while passing currents above said 7 mA.
  • V/x is the voltage stress (volts per centimeter) at which the surface breaks down; this means that surface discharge cannot take place at a lower stress
  • E is the voltage across the insulator
  • L is the minimum distance along the insulator between its ends
  • i is the minimum current of the permitted discharge and is greater than 7 mA
  • the surface conductance is calculated from the value of i,

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  • Insulators (AREA)

Abstract

A high-voltage electrical insulator of synthetic plastics material having a predetermined electrical conductance to inhibit discharges of the order of 7 milliamperes and less but to permit larger current discharges. Preferably said conductance is provided by surface conductance and preferably it is discharges of 5 milliamperes and less which are inhibited.

Description

United States Patent Appl. No. Filed Patented Assignee HIGH-VOLTAGE ELECTRICAL INSULATOR HAVING A PREDETERMINED SURFACE CONDUCTANCE 3 Claims, No Drawings U.S. Cl .1 174/140 C, 174/209 lnt.Cl ..l-l0lb 17/42, HOlb 17/64 Field 01' Search 174/137. 1, 137.2, 138 C, 140, 140.1, 140C, 141 C, 209, 210, 211
[56] References Cited UNITED STATES PATENTS 1,661,823 3/1928 Hawley 174/140 CX 1,735,560 11/1929 Austin l74/140C 2,947,801 8/1960 Doolittle 174/140 CX 3,243,505 3/1966 Clark 174/140 C 3,368,026 2/1968 Vince 174/140 C 3,416,033 12/1968 Hoover et a1. ..1.74/140(.1 UX FOREIGN PATENTS 1,021,081 2/1966 Great Britain 174/21 1 1,068,219 5/1967 Great Britain.... 174/140 C Primary Examiner Laramie E. Askin AuomeySommers & Young ABSTRACT: A high-voltage electrical insulator of synthetic plastics material having a predetermined electrical conductance to inhibit discharges of the order of 7 milliamperes and less but to permit larger current discharges. Preferably said conductance is provided by surface conductance and preferably it is discharges of 5 milliamperes and less which are inhibited.
HIGH-VOLTAGE ELECTRICAL INSULATOR HAVING A PREDE'I'ERMINED SURFACE CONDUCTANCE This invention relates to high voltage electrical insulators suitable for outdoor use. Such insulators as widely used today are of glazed porcelain and are perfectly satisfactory in operation. However, recent interest has been directed to the use of plastics materials for high-voltage insulators, for example, glass-fiber reinforced plastics materials, which have certain advantages over glazed porcelain.
The use of high-voltage plastics insulators outdoors is at the present time seriously restricted by the phenomena known as tracking, i.e, carbonization of the surface of the insulator by surface electrical discharges, which results in a permanent degradation of the insulating properties of the insulator. It is the object of this invention to prevent such tracking.
The invention is based on the discovery that if surface electrical discharges of the order of 7 milliamperes and less are inhibited there is no significant tracking; hence higher magnitude surface electrical discharges can be permitted. Accordingly the invention provides an insulator of synthetic plastics material having a continuous coating on its outer surface to produce a surface such that the plastics insulator has a predetermined electrical conductance so as to inhibit discharges of the order of 7 milliamperes and less.
Preferably said electrical conductance is such as to inhibit discharges of milliamperes and less but to permit higher current discharges.
Surface electric discharge of an insulator is caused by the rapid drying-out of wet pollution on its surface by the leakage current flowing through the pollution. In the case where, in accordance with the invention, surface conductance is provided on the insulator it can. be shown that surface discharges will not occur unless the current through the pollution exceeds a limiting value according to the following expression:
V E 5 3T3 i,
where:
V/x is the voltage stress (volts per centimeter) at which the surface breaks down; this means that surface discharge cannot take place at a lower stress,
E is the voltage across the insulator,
L is the minimum distance along the surface of the insulator between its ends,
i, is the minimum leakage current through the pollution at which discharge is to be permitted,
i, is the current that normally flows by reason of the surface conductance of the insulator when the insulator is dry.
Since in accordance with the invention the value of i, will be given by a value of the order of 7 milliamperes the surface conductance can be determined by applying Ohms law (K==l/E) to the calculated value of g.
If for example the critical value for War is l7.5 kv./cm, E is 35 kv., (peak value of 25 kv. AC supply) L is 1 meter and p is greater than 5 ma., then the surface conductance provided in the insulator will have to provide a leakage current 3 of about A. and the conductance would be about .000285p. mho.
If the surface conductance of the insulator increases (i.e. the resistivity decreases) with increasing voltage stress, for example, by using known materials in the coating whose ohmic resistance decreases with an increase in voltage stress, then the steady state leakage can be reduced. Thus if the surface resistivity for a voltage stress E/L is 10 times greater than that at the breakdown stress V/x then the steady state current leakage through the dry insulator can be reduced say from I00 to l0p.A. The surface conductance of the insulator may be provided by a coating of flexibilized resin intimately mixed with antimony doped tin oxide powder, the coating being cured on the insulator.
We claim:
1. A high-voltage insulator comprising a synthetic plastic bod and a continuous coatin of a conductive material on the su ace thereof, said coating avmg a thickness and berng of a material to inhibit discharge of electrical currents therethrough below about 7 mA while passing currents above said 7 mA.
2. An insulator in accordance with claim 1 wherein i is calculated from the following formula:
where:
V/x is the voltage stress (volts per centimeter) at which the surface breaks down; this means that surface discharge cannot take place at a lower stress,
E is the voltage across the insulator,
L is the minimum distance along the insulator between its ends,
i, is the minimum current of the permitted discharge and is greater than 7 mA,
1', is the current that normally flows through the surface of the insulator when the insulator is dry, and
the surface conductance is calculated from the value of i,
using Ohms law.
3. An insulator in accordance with claim 1 wherein the surface electrical conductance increases with increasing voltage stress.
a a a a a i

Claims (2)

  1. 2. An insulator in accordance with claim 1 wherein ig is calculated from the following formula: where: V/x is the voltage stress (volts per centimeter) at which the surface breaks down; this means that surface discharge cannot take place at a lower stress, E is the voltage across the insulator, L is the minimum distance along the insulator between its ends, ip is the minimum current of the permitted discharge and is greater than 7 mA, ig is the current that normally flows through the surface of the insulator when the insulator is dry, and the surface conductance is calculated from the value of ig using Ohms law.
  2. 3. An insulator in accordance with claim 1 wherein the surface electrical conductance increases with increasing voltage stress.
US883249A 1969-12-08 1969-12-08 High-voltage electrical insulator having a predetermined surface conductance Expired - Lifetime US3627905A (en)

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US88324969A 1969-12-08 1969-12-08

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4216000A (en) * 1977-04-18 1980-08-05 Air Pollution Systems, Inc. Resistive anode for corona discharge devices
US4563544A (en) * 1983-04-29 1986-01-07 Ceraver, S.A. Electrical insulator offering reduced sensitivity to pollution
US20050185356A1 (en) * 2004-02-23 2005-08-25 Phoenix Contact Gmbh And Co. Kg Overvoltage protection element and ignition element for an overvoltage protection element
CN105719779A (en) * 2016-04-29 2016-06-29 南方电网科学研究院有限责任公司 Support composite insulator and calculation method of surface conductivity of support composite insulator
CN105810373A (en) * 2016-04-29 2016-07-27 南方电网科学研究院有限责任公司 Porcelain sleeve for arc extinguishing chamber of circuit breaker and calculation method for surface conductivity of porcelain sleeve

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1661823A (en) * 1921-06-04 1928-03-06 Locke Insulator Corp Insulator
US1735560A (en) * 1924-08-11 1929-11-12 Ohio Brass Co Insulator for withstanding fog conditions
US2947801A (en) * 1957-05-02 1960-08-02 Fred B Doolittle Contamination and moisture resistant insulator
GB1021081A (en) * 1962-07-27 1966-02-23 Steatite & Porcelain Products High-tension electric insulators
US3243505A (en) * 1963-02-25 1966-03-29 Steatite & Porcelain Prod Ltd Insulator having semi-conductive layers to increase the capacitance thereof
GB1068219A (en) * 1964-12-07 1967-05-10 Doulton & Co Ltd Improvements in stabilised insulators
US3368026A (en) * 1965-11-02 1968-02-06 Doulton & Co Ltd Electrical insulator having improved surface electrical stress distribution
US3416033A (en) * 1966-04-22 1968-12-10 Granger Associates Tension link for use with electrostatically charged objects

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1661823A (en) * 1921-06-04 1928-03-06 Locke Insulator Corp Insulator
US1735560A (en) * 1924-08-11 1929-11-12 Ohio Brass Co Insulator for withstanding fog conditions
US2947801A (en) * 1957-05-02 1960-08-02 Fred B Doolittle Contamination and moisture resistant insulator
GB1021081A (en) * 1962-07-27 1966-02-23 Steatite & Porcelain Products High-tension electric insulators
US3243505A (en) * 1963-02-25 1966-03-29 Steatite & Porcelain Prod Ltd Insulator having semi-conductive layers to increase the capacitance thereof
GB1068219A (en) * 1964-12-07 1967-05-10 Doulton & Co Ltd Improvements in stabilised insulators
US3368026A (en) * 1965-11-02 1968-02-06 Doulton & Co Ltd Electrical insulator having improved surface electrical stress distribution
US3416033A (en) * 1966-04-22 1968-12-10 Granger Associates Tension link for use with electrostatically charged objects

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4216000A (en) * 1977-04-18 1980-08-05 Air Pollution Systems, Inc. Resistive anode for corona discharge devices
US4563544A (en) * 1983-04-29 1986-01-07 Ceraver, S.A. Electrical insulator offering reduced sensitivity to pollution
US20050185356A1 (en) * 2004-02-23 2005-08-25 Phoenix Contact Gmbh And Co. Kg Overvoltage protection element and ignition element for an overvoltage protection element
US7817395B2 (en) * 2004-02-23 2010-10-19 Phoenix Contact Gmbh & Co. Kg Overvoltage protection element and ignition element for an overvoltage protection element
CN105719779A (en) * 2016-04-29 2016-06-29 南方电网科学研究院有限责任公司 Support composite insulator and calculation method of surface conductivity of support composite insulator
CN105810373A (en) * 2016-04-29 2016-07-27 南方电网科学研究院有限责任公司 Porcelain sleeve for arc extinguishing chamber of circuit breaker and calculation method for surface conductivity of porcelain sleeve

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