The Materials, Design, and Manufacture of the Vacuum Interrupter

Introduction

The modern development of the vacuum interrupter began in the 1950s. There has been continuous development of its performance and application since that time. Figure 3.1 illustrates my own experience from 1966 to 2002 while working for the Westinghouse Electric Corporation and its successor the Eaton Corporation in the vacuum interrupter business. In 1966, we see that in order to achieve a 12.5kA interrupting ability in a 15kV circuit, the vacuum interrupter had to have a diameter of at least 182mm. Since 1966 there has been continuous development in vacuum evacuation technology, vacuum interrupter materials and computer design software. There has also been improved understanding of vacuum arc physics and high voltage, vacuum breakdown phenomena. The result is that the vacuum interrupter’s size has steadily decreased until years later the same 15kV, 12.5kA performance can be achieved in a vacuum interrupter with a diameter of only 50mm.

The internal components of a typical vacuum interrupter are shown in Figure 3.2. The contacts are housed within an evacuated envelope in which the ambient gas pressure is between 10~2 - 10~4 Pa. When the contacts touch, the vacuum interrupter is in the closed condition, its natural state. The contacts open by withdrawing the contact attached to the moving terminal from the contact attached to the fixed terminal. The moving terminal is attached to a bellows; consequently, the vacuum is maintained inside the vacuum interrupter. When the contacts separate arcing is established within the interrupter. As I have discussed in Chapter 2 in this volume, this arc burns in the metal vapor evaporated from the contact surfaces. The metal vapor continually leaves the intercontact region and condenses on the contact surfaces and the surrounding metal vapor shield. The latter is isolated from one or from both contacts and serves to protect the insulating envelope from excessive metal vapor deposition. At current zero, contact vapor production ceases and the original vacuum condition is rapidly approached. The dielectric strength of the interrupter also increases, and the circuit is interrupted. The vacuum arc interruption process and the high voltage recovery of the contact gap will be discussed in Chapter 4 in this volume. As I have discussed in Chapter 1 in this volume, the contacts in the open position isolate the circuit voltage internally by the intercontact gap and externally by the insulating envelope.

In this chapter I will first present the development of vacuum interrupter contact materials: their properties will be discussed, as well as their strengths and weaknesses for particular applications. I will then discuss the contact structures that have been developed to control the high-current, columnar vacuum arc (see Section 2.4). Finally, I will present a general description of vacuum interrupter design, the components that are incorporated in it and outline the manufacturing techniques that have evolved.

The Westinghouse (now the Eaton) experience in the reduction of the vacuum interrupter’s diameter from 1968 for the 15kV, 12.5kA function

FIGURE 3.1 The Westinghouse (now the Eaton) experience in the reduction of the vacuum interrupter’s diameter from 1968 for the 15kV, 12.5kA function.

The cross-section of a vacuum interrupter

FIGURE 3.2 The cross-section of a vacuum interrupter.

Vacuum Interrupter Contact Materials

Introduction

The vacuum environment offers definite advantages to the developer of contact materials for use in the vacuum interrupter since there is no ambient gas to contaminate the contact surfaces. Thus, mixtures of materials that cannot be contemplated for application in gaseous environments such as air or SF6 can be considered. Also, changes in the contact surface after arcing are only affected by the interaction of the contact materials themselves, and not by complex oxides that can form, for example, in air [1]. Thus, once the contact surface has stabilized, the contact resistance will be steady and consistent throughout the vacuum interrupter’s life. The lack of ambient gas also allows for a high-voltage withstand across a small contact gap and therefore permits the creation of relatively compact vacuum interrupter designs. Even with these advantages, the development of practical vacuum interrupter contact materials continues to be limited by the traditional compromise between the desired electrical and mechanical properties and the contact material’s own limitations [2]. Table 3.1 presents a matrix showing the complex interaction between the contact material properties and a typical vacuum interrupter’s performance requirements.

Copper and Copper-Based Contact Materials That Have Been Developed Following the Initial Experiments on High Current Vacuum Arcs Using Copper Contacts

Much of the initial experimental research on high current, vacuum arcs has been performed on contacts made from pure Cu [3]. While valuable information on the nature of the vacuum arc was obtained from these studies, the tendency of Cu to form strong welds when closing on high currents in vacuum prevented its use in practical vacuum interrupter designs. This welding property, in fact, has prevented the use of all pure metals except perhaps the limited application of W contacts.

The first successful high current contact material developed for vacuum interrupters used in vacuum circuit breaker is a Cu-based material with other metals added to increase its weld resistance by reducing its mechanical strength. This material combined Cu with a small percentage of Bi. The addition of a small percentage of Bi to molten Cu results in the Bi migrating to the grain boundaries of the Cu during solidification. This makes the resulting contact more brittle than pure Cu. The inherent defects of this material are its high erosion rate, mechanical weakness, and its high level of chop current. Further development of this concept led to a whole class of Cu-Bi type of binary alloys [4, 5] (e.g. Cu-Sn, Cu-Pb, Cu-Sb, Cu-Zn, etc.). A very high percentage of Bi is required (> 5%) to obtain a very low chopping current. This amount of Bi made the resulting contact material mechanically very weak, its current interruption ability and its high voltage-withstand ability undesirably low, and its erosion undesirably high.

An effective contact material evolved when a very small amount, about 0.5% Bi, was added to Cu. This material had satisfactory current interruption ability, reasonably high voltage withstand capability and did not readily weld [4]. This class of materials was later expanded to include all materials where the major constituent had a boiling point of less than 3500K and where the minor constituent had a freezing temperature lower than that of the major constituent. The minor constituent of these materials also had to have a substantial solubility in the liquid state of the major constituent and little solubility in the solid state of the major constituent [6]. In fact, when reading the old patents for these materials, it seems as if the authors were trying to patent the whole periodic table. Research continued to improve this class of contact material using ternary systems [7, 8]. In the end, this research effort proved to be a dead end.

With the advent of the Cu-Cr contact material all research on the Cu-Bi type of material eventually ceased and the Cu-Cr contact material superseded its use in practical, high current vacuum interrupters. Even so, research continued on high Cu content, contact alloys such as Cu-Co-Ta [9], which showed some promise, but did not seem to compete with the Cu-Cr contact materials. Table

3.2 gives the material properties for this class of vacuum interrupter contact material.

Refractory Metals Plus a Good Conductor

One of the earliest contact materials applied in low current vacuum switches depended upon the use of a refractory material. The most common type of material in this class is W-Cu or Mo-Cu [10] and variations of them, e.g.. W-Cu-Ti-Bi, W-Cu-Ti-Sn [11], W-Cu-Ti. W-In-Cu [12], W-Cu-Zr [13, 14], W-Zr [15], and of course W-Cu-Bi [16]. In these materials the refractory W is usually a sintered matrix (> 50% by volume), which is infiltrated with the good conductor. The infiltrate is usually Cu or Cu alloy. The elements like Ti, Zr, and In are used to aid the vacuum infiltration of the W and the elements Bi and Sn are there to aid the chopping and also the anti-welding capability.

TABLE 3.1

Contact Performance Needs and Contact Properties

Contact

Performance

Material Properties

Gas

Content

Melting

Point

Vapor

Pressure

Work

Function & Electron Emission

Ionization

Potential

Electrical & Thermal Conductivities

Residual Gas Gettering

Structural

Quality

Smooth Surface, no Cracks and Pits

Circuit Interruption & Dielectric Recovery

/

/

/

/

/

/

/

/

/

Endurance & Resistance to Erosion

/

/

/

/

/

Dielectric Strength

/

/

/

/

/

/

Curtent Carrying Capability

/

/

/

/

Chopping Current

/

/

/

/

/

/

/

Resistance to Welding

/

/

/

/

Ability to interrupt high frequency currents

/

/

/

/

/

/

/

/

/

TABLE 3.2

Material Properties of Copper-Based Contact Materials

Contact

Material

Material Properties of Cu Based Contacts

Weight %

Electrical Conductivity MS.irr1

Hardness x102 Nmm'!

Gas Content ppm

Cu

100

60

4-6

5,0,

Cu-Bi

99.5-0.5

-55

7

-5.0,

Cu-Pb

99-1

-50

7

Cu-Co-Ta

90-5-5

7.1

10, 0,

80-15-5

13

8.9

20, N,

77-18-5

12

9.3

75-20-5

11

10.2

The W-Cu material will readily interrupt a low current, diffuse vacuum arc when switching an ac circuit. It does not, however, work well at currents where the plasma plumes from the cathode spots overlap close to the cathode or when the columnar vacuum arc forms. This class of material does have very good high voltage withstand ability, a resistance to welding and an acceptable level of chopping current, so it has found use in load-break switches and in capacitor switches with circuit currents less than about 2kA (see Chapter 5 in this volume).

Another commonly used contact material in this class is manufactured from WC and Ag. This material has found almost universal acceptance for application in vacuum interrupters for vacuum contactors over the range of circuit voltages 400V < U < 15kV; i.e., this material is used in motor switching applications. This material’s properties of its low-surge capability, which results from its inability to interrupt high frequency currents, its low chopping current, and its resistance to welding make it particularly attractive for this application. I will discuss this further in Chapters 4 and 5 in this volume. The WC-Ag material has the same limitation on interrupting higher currents, as does the W-Cu material: in ac circuits up to 7.2kV the limit is usually about 4.5kA. It has been found capable, however, of interrupting much higher currents (« 25kA rms.) if the contacts have a large enough diameter and if the high current vacuum arc is forced into the diffuse mode by the application of a high enough axial magnetic field (AMF) [17, 18]. It can also interrupt somewhat higher currents if the transverse magnetic field spirals are cut into its face. This material has been shown to be extremely resistant to welding, even when the circuit current is high enough for the blow-off force (see Section 2.1.4 in this volume) to open the contacts and for an arc to be established. This property is very desirable for contactor applications where the contact holding forces are generally low'.

The cost of the Ag in the WC-Ag contacts has resulted in a search for a lower cost alternative. Behrens et al. [19, 20] report work on WC-Ag, WC-Cu, W-Cu, and W-Cu-Sb for low' voltage contactor applications. They evaluate chop current values, susceptibility of re-ignition and contact resistance change during operating life. An example of their data is showrn in Figure 3.3. In general, all the contact materials have acceptable chop currents (i.e., I < 6A), the lowest being for the WC-Ag contacts. The Cu containing materials have lower re-ignition rates than the Ag containing materials. As I will discuss in Chapter 4 in this volume, I believe this will result in materials that do not have the same low-surge property of WC-Ag. Temborius et al. [21] show that the exact performance of these materials is strongly affected by the particle size and the size distribution of the W powders: an example is showrn in Figure 3.4. They also show' that contacts with a low chopping current also have a low current interruption capability. Pure W contacts have found a limited application in vacuum interrupters used for switching low' dc currents (a few amperes) in medium voltage circuits. The use of this material relies upon the property of W shown in Figure 1.84. At currents less than 20A the vacuum arc between W contacts is very unstable and w'ill chop to zero within a

Comparison of the 99.5% chopping current and reignition percentage for WC-Cu (30 wt.%), WC-Cu (40 wt.%), WC-Ag (40 wt.%) and WC-Cu (29 wt.%)-Sb (1 wt.%) [19, 20]

FIGURE 3.3 Comparison of the 99.5% chopping current and reignition percentage for WC-Cu (30 wt.%), WC-Cu (40 wt.%), WC-Ag (40 wt.%) and WC-Cu (29 wt.%)-Sb (1 wt.%) [19, 20].

Interruption frequency of WC-Ag (40wt.%) and WC-Cu (40wt.%) contacts manufactured with different WC grain sizes [21]

FIGURE 3.4 Interruption frequency of WC-Ag (40wt.%) and WC-Cu (40wt.%) contacts manufactured with different WC grain sizes [21].

few milliseconds even in a medium voltage circuit (5kV-15kV). Table 3.3 gives a general overview of the material properties for this class of vacuum interrupter contact material.

 
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