The Vacuum Interrupter

Theory, Design, and Application

Paul G. Slade

• Publisher: CRC Press
• Year: 2020
• Print ISBN:
  9780367531720
  9780367275051
• Edition: 2
• eBook ISBN:
  9781000170085
• Topic: Technology & Engineering
• Subtopic: Electromagnetism
• Index: Technology & Engineering

Title: The Vacuum Interrupter: Theory, Design, and ApplicationShelving guide: Electrical Engineering

Dr. Paul Slade draws from his nearly six decades of active experience to develop this second edition of The Vacuum Interrupter: Theory, Design, and Application. This book begins by discussing the design requirements for high voltage vacuum interrupters and then the contact requirements to interrupt the vacuum arc. It then continues by describing the various applications in which the vacuum interrupter is generally utilized.

Part 1 of this book begins with a detailed review of the vacuum breakdown process. It continues by covering the steps necessary for the design and the manufacture of a successful vacuum interrupter. The vacuum arc is then discussed, including how it is affected as a function of current. An overview of the development and use of practical contact materials, along with their advantages and disadvantages, follows. Contact designs that are introduced to control the high current vacuum arc are also analyzed.

Part 2, on application, begins with a discussion of the arc interruption process for low current and high current vacuum arcs. It examines the voltage escalation phenomenon that can occur when interrupting inductive circuits. The occurrence of contact welding for closed contacts subjected to the passage of high currents, and for contacts when closing on high currents, is explored. The general requirements for the successful manufacture and testing of vacuum circuit breakers is then presented. The general application of vacuum interrupters to switch load currents, especially when applied to capacitor circuits, is also given. The interruption of high short circuit currents is presented along with the expected performance of the two major contact designs.

Owing to the ever-increasing need for environmentally friendly circuit protection devices, the development and application of the vacuum interrupter will only increase in the future. At present the vacuum circuit breaker is the technology of choice for distribution circuits (5kV to 40.5kV). It is increasingly being applied to transmission circuits (72.5kV to 242kV). In the future, its application for protecting high voltage DC networks is assured.

Audience

• This is a practical source book for engineers and scientists interested in studying the development and application of the vacuum interrupter
• Research scientists in industry and universities
• Graduate students beginning their study of vacuum interrupter phenomena
• Design engineers applying vacuum interrupters in vacuum switches, vacuum contactors, vacuum circuit breakers, and vacuum contactors
• It provides a unique and comprehensive review of all aspects of vacuum interrupter technology for those new to the subject and for those who wish to obtain a deeper understanding of its science and application
• Scientists and engineers, who are beginning their research into vacuum breakdown and aspects of the vacuum arc, will find the extensive bibliography and phenomenological descriptions to be a useful introduction

Part 1

Vacuum Interrupter Theory and Design

1 High Voltage Vacuum Interrupter Design

We cannot pretend to offer proofs. Proof is an idol before whom the pure mathematician tortures himself. In Physics we are generally content to sacrifice before the lesser shrine of plausibility.

Arthur Eddington

1.1 Introduction

There are two major criteria to consider when designing a vacuum interrupter for high voltage:

1. 1. The ability to support high voltage across the vacuum interrupter when the contacts are open
2. 2. The ability to interrupt ac and or dc current over a wide range of circuit conditions

These two criteria are of course linked especially in the design for circuit interruption (this will be discussed in Chapters 3 and 4 in this volume). In this chapter, the high voltage design will be discussed without reference to interrupting current. What drives the high voltage design? The simple answer is that once a vacuum interrupter in an electrical circuit opens its contacts, it is expected to permanently withstand the voltage imposed across the contacts by that circuit. For the vacuum interrupter designer, however, the answer is somewhat more complicated. For a design to be successful, the vacuum interrupter, when placed in an operating mechanism, must pass a series of voltage tests. Committees around the world consisting of equipment users and equipment manufacturers have set the criteria for these high voltage tests. These committees have determined the voltage levels by consensus decisions over the last century. In this chapter I shall consider only the values given by the IEC (International Electro-Technical Commission), IEEE (American Institute of Electrical and Electronic Engineers) and the GB-DL (Chinese National Standard [GB] and Chinese Department of Electrical Power Standard [DL], which are similar to the Russian (Gost) standards) [1 –4]. These now represent the major standards to which most countries conform even though some countries maintain their own individual variations [5]. These voltage standards therefore give vacuum interrupter designers a set of guidelines on which they must base their designs.

There are two major high voltage tests that are used to ensure that an open vacuum interrupter has a high probability of satisfactorily withstanding any voltage that may appear across it when installed in an ac circuit. The first is the ability to withstand an ac voltage for one minute across the open vacuum interrupter. The value of this withstand voltage is considerably higher than the maximum operating voltage in any given ac circuit. The withstand voltage level, set early in the last century by the American Institute of Electrical Engineers is 2.25 times the rated circuit voltage plus 2000 volts. Table 1.1 gives the present standards, which now show a considerable variation from this simple formula. This ac withstand requirement is very conservative [2 –6] ranging from greater than three times the system voltage for lower circuit voltages to two times for higher circuit voltages. Garzon [1] speculates that the higher one minute withstand voltage values were originally adopted in lieu of switching surge tests.

TABLE 1.1
Voltage Ratings According To IEEE, IEC, and GB/DL Standards

The second test is used to simulate an overvoltage resulting from a lightning strike on the circuit. The voltage pulse has a rise time of 1.2µs to its peak value and a decay time to half this peak value in 50µs; see Figure 1.1. The 1.2µs value is defined as 1.67 times the time interval t_r that is the time for the pulse to go from 30% to 90% of its final peak value. The 50µs time begins where the straight line joining the 30% and the 90% voltage values intersects with the time axis and ends at the time where the voltage has declined to its 50% level. This Basic Impulse Level (BIL) or Lightning Impulse Withstand Voltage (LIWV) actually reflects the insulation coordination practices used in the design of electrical systems. Table 1.1 gives the peak voltages for the BIL tests. For some IEEE voltages two BIL levels are given. The lower one is for applications on a grounded wye distribution system equipped with surge arresters [2]. The IEC standards specify two values for all voltage classes up to 36kV [3]. In practice the vacuum interrupter designer uses the higher value at each voltage. In contrast with the conservative ac withstand voltage test, the BIL test only defines a limit for the switching system and defines the level of system coordination required when higher voltage pulses are impressed on a circuit by lightning strikes.

FIGURE 1.1 The basic impulse level (BIL) or lightning impulse withstand value (LIWV) voltage wave shape.

In the IEEE standards for outdoor circuit breakers another impulse test is required. This is known as the “chopped wave” test [1, 2]. Here the initial voltage pulse has the same shape as the BIL voltage but is chopped to zero 2µs or 3µs after t = 0 in Figure 1.1. This test reflects the coordination with a surge arrestor and has a peak voltage value higher than that for the BIL test, see also Table 1.1.

1.2 The External Design

1.2.1 Electrical Breakdown in Gas

There are two components to the...

Table of contents

• Cover
• Half-Title
• Title
• Copyright
• Dedication
• Contents
• Preface and Acknowledgments for the First Edition
• Preface and Acknowledgments for the Second Edition
• Author
• Introduction
• PART 1 Vacuum Interrupter Theory and Design
• PART 2 Vacuum Interrupter Application
• Author Index
• Subject Index