Circuit Interruption

Theory and Techniques

Browne, Thomas E. Browne Jnr.

• Publisher: Routledge
• Year: 2019
• Print ISBN: 9780367839345
• eBook ISBN: 9781351460668
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Here-in one current, comprehensive source-is a wealth of both theoretical and practicalinformation on circuit interruption. Twenty-two authorities at the leading edge of researchand development provide a solid grasp of circuit breaker design and performance... and that's knowledge you can put to work immediately!arcuit Interruption surpasses other books in completeness and currency-includingcoverage of the sulfur hexafluoride puffer, the vacuum breaker, and the low-voltagemolded-case breakers, that are taking the place of many older types. In addition to thelatest theories and techniques, this major volume examines promising future trends.More than 400 clear illustrations help make the text easy to follow, and over 620 keyreferences point the way to the best places for continuing study.Today, the field of circuit interruption is so diverse that a thorough single source reallystands out. arcuit Interruption is that- source, the perfect reference for electrical, electronic,power, and design engineers; and researchers investigating circuit breaker design,interaction of breakers and power circuits, power transmission, power distribution,circuit interruption, electric contacts, and gaseous conduction. Moreover, this exceptionalbook serves as an excellent source for practicing power engineers as well as an invaluablesupplement to graduate-level engineering courses in circuit interruption, transmission,and distribution of power . . . and a supplement in professional seminars and society/association courses.

1

Introduction

Thomas E. Browne Jr. */Consultant to Westinghouse Research and Development Center, Pittsburgh, Pennsylvania

1.1History

1.2Mechanics

1.3Progress in Understanding

1.4The Future

References

1.1 History

The interruption of electric power circuits has always been an essential function, especially in cases of overloads or short circuits when immediate interruption of the current flow becomes necessary as a protective measure. In earliest times, circuits could be broken only by separation of contacts in air followed by drawing the resulting electric arc out to such a length that it could no longer be maintained. As the voltage and current capacity of power systems grew, however this means of interruption soon became inadequate and special devices called circuit breakers had to be developed. Circuit breakers continually developed more interrupting ability as the systems expanded, but by the early 1920s the shortcomings of available circuit breakers had become a limiting factor in the further growth of electric power systems. This situation led to a marked worldwide increase in research and development effort on circuit-interrupting devices which has continued to the present. This research and development has led to improved understanding of the basic principles, which has enabled circuit breaker development to keep pace with the requirements of our still expanding power systems.

The means for improving the interrupting ability of circuit breakers have been many and varied. The basic problem has been to control and quench, or extinguish, the high-power arc which necessarily occurs [1] at the separating contacts of a breaker when opening high-current circuits. The arc-quenching ambients have included air, compressed air, oil, strong magnetic fields, sulfur hexafluoride (SF₆), and a vacuum environment.

Compressed Air One of the earliest schemes [2] for “blowing out” the arc used air under pressure blown through the arc space between the opening contacts. Sometimes the air pressure is also used to open the contacts. For interrupting very high currents at moderate voltages cross-blast arrangements are used, forcibly lengthening the arc by blowing it against or around insulating splitters [3]. The more usual arrangement, suitable for higher voltages, confines the air blast by a nozzle, either conducting or insulating, so as to direct the flow axially through the arc space. Both single flow and double flow (blasting in opposite directions through two nozzles facing each other) are used, and multiple breaks are operated in series for the higher voltages. To increase the current-interrupting ability, in addition to larger nozzles, shunting resistors are used to reduce the amplitude and rate of rise of the circuit recovery voltage (see Chap. 2). In this case, auxiliary interrupters for the resistor current are required. Sometimes a succession of interrupters with increasing values of shunting resistance are employed to handle very large currents. Maximum ratings are also achieved by using very high air pressures, up to 50 or 80 atm, and opening of the contacts synchronously with the current has been tried [4] . This technology is now being superseded by that of SF₆ puffer breakers.

Oil Of many interrupting schemes tried in the period of active development before 1900, the use of mineral oil for arc quenching was included, but this medium did not come into general use until the early 1900s, when increasingly high transmission voltages demanded it. In the United States, Kelman [5] in California appears to have built the first commercial oil circuit breaker for high-voltage lines by simply immersing a switch, or a series of switches, in a steel tank filled with oil. These tank-type or bulk oil breakers have been used extensively in the United States for many years. Since the late 1920s these have been greatly improved by providing arc and pressure control structures, sometimes called explosion pots or deion grids, at the contact breaks [6]. These structures, of various forms, serve to limit the growth of the gas bubble surrounding the arc and to utilize the pressure generated by the gas formation to force a flow of oil and gas through the arc space.

Tank-type oil breakers were aided by improvement in the bushings needed to bring the current leads into the grounded metal tanks (see Chap. 8), increasing the ease with which relatively low-cost ring-type current transformers around the bushings could be provided as integral parts of the breakers.

In Europe, the trend after World War II has been toward “minimum oil” construction in which the arc control structures are surrounded by closely fitting oil-filled casings of insulating material. These live-tank breakers require no bushings but in use must be provided with separately insulated and so relatively expensive current transformers.

In the United States, oil circuit breakers have been used up to voltages of only 345 kV. At higher voltages compressed air has been used, but SF₆ gas is now becoming the medium of choice.

Water It should be mentioned that pure water, usually with a nonionizing antifreeze material added, has had some use in Europe [7,8] as a nonflammable substitute for oil. The arc-produced gases, steam and hydrogen, are as effective as the vapor and hydrogen from oil in quenching the arc, but insulation problems have limited the use of this medium and at present no breakers are being built that use this technique.

Solid Materials Gas-blast generation by ablation of an arc-enclosing solid material such as hard fiber has been used extensively in load-break switches and power fuses. Boric acid is especially effective as the gas source in “expulsion” fuses [9,10], but this medium has not found use in power circuit breakers.

Magnetic Fields Another arc control means, usually limited to relatively low-voltage service, has been the use of magnetic blowout structures. Either by arrangement of leads or by series-connected coils, magnetic fields transverse to the arc serve to lengthen the arc and usually to force it into a quenching chamber. This scheme has been employed under oil but generally is a feature of air-break devices, from low-voltage contactors to breakers for circuit voltages up to 15 kV (see Chap. 11).

An early Westinghouse development was the air deion breaker, which utilized in the quenching chamber the minimum reignition voltage of multiple series-connected short arcs magnetically rotated at very high speed between copper plates. It was found, however, that air-break breakers for voltages up to 15 kV or so could be built more cheaply by magnetically driving the arc into labyrinths of insulating plates. These plates, or sometimes fins, are of ceramic or other heat-resisting materials, and various arrangements are used to lengthen the arc and force it into close contact with the plate surfaces. Arrangements of insulated metal plates without the arc-rotating feature are used in some low-voltage breakers (Chap. 14).

Gas Blast (SF₆) Early in 1950 it was demonstrated (see Chap. 9) that the electronegative gas sulfur hexafluoride, previously known for its superior dielectric properties [11], had extraordinary ability to interrupt ac arcs. In its first application to a power circuit breaker, the required gas flow for arc quenching was produced by thermal expansion of arc-heated gas. The “puffer” principle, piston-driven flow energized by the opening mechanism, was also used in early load-break switches. However, for the highest breaker interrupting ratings, early practices followed that of compressed-air designs in which separate compressors and storage tanks were provided to furnish the gas blast. Since an elevated gas pressure of a few atmospheres on the discharge side of the interrupting nozzles was also needed to maintain dielectric strength comparable to that of oil, these were called two-pressure breakers. Both dead-tank and live-tank breakers of this type were built and are in use. In recent years further development of puffer or single-pressure designs (Chap. 10) has achieved the highest needed ratings and these devices are now becoming the standard for the industry.

Vacuum (See Chaps. 12 and 13). The conceptual advantages of switching in a vacuum environment, the absence of a medium to break down dielectrically or to sustain arcing, and other practical advantages have long attracted experimenters. Some partial success in power circuit interruption in vacuum was achieved as early as 1926 [12] , but performance was not consistent. By 1962 vacuum and metal purification technologies had advanced to the point that reliable interruption of high currents in vacuum could be demonstrated [13] . This required extensive development, particularly of contact materials and forms. Vacuum interrupters are now used successfully at medium voltages in breakers, reclosers, and enclosed switchgear, and also in lower-voltage contactor applications. Recent research and development [14,15] shows promise for ultrahigh current interruption by these breakers and also for possible use at transmission voltages.

Direct-Current Breakers The interruption of direct or continuous currents presents a special problem in that the current must be forced to zero and the magnetically stored energy must be dissipated. At voltages up to a few kilovolts air-break breakers with strong series magnetic fields and insulated metal or nonmetallic arc “chutes” are employed to lengthen and cool the arc, thus raising its sustaining voltage above ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Preface
• Table of Contents
• Contributor
• 1 Introduction
• 2 Electrical and System Aspects
• 3 Circuit Breaker Application
• 4 Nature of the Electric Arc
• 5 Physical Theory of the Arc in a Gas Blast
• 6 Calculation of Arc-Circuit Interaction
• 7 Postare Dielectric Recovery in a Blast Arc
• 8 Dielectric Properties of Circuit Breakers
• 9 SF6 Breaker Research and Development
• 10 Single-Pressure SF6 Circuit Breakers
• 11 Magnetic Air Circuit Breakers
• 12 Interruption in Vacuum
• 13 Vacuum Circuit Breaker Application and Surge Protection
• 14 Molded-Case Low-Voltage Circuit Breakers
• 15 Electric Contact Phenomena
• 16 The Mechanical Operation of Power Circuit Breakers Rated Over 15kV
• 17 Interruption Testing
• Appendix: Basic Concepts of Gaseous Conduction
• Index