VACUUM INSULATION

Introduction
The idea of using vacuum for insulation purposes is very old. According to the
Townsend theory, the growth of current in a gap depends on the drift of the charged
particles. In the absence of any such particles, as in the case of perfect vacuum, there
should be no conduction and the vacuum should be a perfect insulating medium.
However, in practice, the presence of metallic electrodes and insulating surfaces
within the vacuum complicate the issue and, therefore, even in vacuum, a sufficiently
high voltage will cause a breakdown.
In recent years a considerable amount of work has been done to determine the
electrical properties of high vacuum. This is mainly aimed at adopting such a medium
for a wide range of applications in devices such as vacuum contractors and interrupters,
high frequency capacitors and relays, electrostatic generators, microwave tubes,
etc. The contractors and circuit breakers using vacuum as insulation are finding
increasing applications in power systems.

What Is Vacuum?
A vacuum system which is used to create vacuum is a system in which the pressure is
maintained at a value much below the atmospheric pressure. In vacuum systems the
pressure is always measured in terms of millimetres of mercury, where one standard
atmosphere is equal to 760 millimetres of mercury at a temperature OfO0C. The term
*'millimetres of mercury" has been standardised as "Torr" by the International
Vacuum Society, where one millimetre of mercury is taken as equal to one Torn
Vacuum may be classified as
High vacuum : 1 x 1(T3 to 1 x IGT6 Ton-
Very high vacuum : 1 x IQT6 to 1 x 1(T8 Ton-
Ultra high vacuum : 1 x 10~9 torr and below.
For electrical insulation purposes, the range of vacuum generally used is the* 'high
vacuum", in the pressure range of 10~3 Torr to 1(T6 Torr.

Vacuum Breakdown
In the Townsend type of discharge in a gas described earlier, electrons get multiplied due to various ionisalion processes and an electron avalanche is formed. In a high vacuum, even if the electrodes are separated by, say, a few centimetres, an electron crosses the gap without encountering any collisions. Therefore, the current growth prior to breakdown cannot be due to the formation of electron avalanches. However, if a gas is liberated in the vacuum gap, then, breakdown can occur in the manner described by the Townsend process. Thus, the various breakdown mechanisms in high vacuum aim at establishing the way in which the liberation of gas can be brought about in a vacuum gap.
During the last 70 years or so, many different mechanisms for breakdown in
vacuum have been proposed. These can be broadly divided into three categories
(a) Particle exchange mechanism
(ft) Field emission mechanism
(c) Clump theory
(a) Particle Exchange Mechanism
In this mechanism it is assumed that a charged particle would be emitted from one
electrode under the action of the high electric field, and when it impinges on the other electrode, it liberates oppositely charged particles. These particles are accelerated by the applied voltage back to the first electrode where they release more of the original type of particles. When this process becomes cumulative, a chain reaction occurs which leads to the breakdown of the gap.
The particle-exchange mechanism involves electrons, positive ions, photons and
the absorbed gases at the electrode surfaces. Qualitatively, an electron present in the vacuum gap is accelerated towards the anode, and on impact releases A positive ions and C photons. These positive ions are accelerated towards the cathode, and on impact
each positive ion liberates B electrons and each photon liberates D electrons. This is
shown schematically in Fig. 2.24. The breakdown will occur if the coefficients of
production of secondary electrons exceeds unity. Mathematically, the condition for
breakdown can be written as
(AB + CD) > I (2.32)
Later, Trump and Van de Graaff measured these coefficients and showed that
they were too small for this process to take place. Accordingly, this theory was
modifie4 to allow for the presence of negative ions and the criterion for breakdown
then becomes
(AB + EF) >l (2.33)
Where A and B are the same as before and E and F represent the coefficients for
negative and positive ion liberation by positive and negative ions. It was experimentally
found that the values of the product EF were close enough to unity for copper,
aluminium and stainless steel electrodes to make this mechanism applicable at
voltages above 250 kV.
(b) Field Emission Theory
(O Anode Heating Mechanism
This theory postulates that electrons produced at small micro-projections on the
cathode due to field emission bombard the anode causing a local rise in temperature
and release gases and vapours into the vacuum gap. These electrons ionise the atoms
of the gas and produce positive ions. These positive ions arrive at the cathode, increase the primary electron emission due to space charge formation and produce secondary electrons by bombarding the surface. The process continues until a sufficient number of electrons are produced to give rise to breakdown, as in the case of a low pressure Townsend type gas discharge.

Cathode Heating Mechanism
This mechanism postulates that near the breakdown voltages of the gap, sharp
points on the cathode surface are responsible for the existence of the pre-breakdown
current, which is generated according to the field emission process described below.
This current causes resistive heating at the tip of a point and when a critical current density is reached, the tip melts and explodes, thus initiating vacuum discharge. This mechanism is called field emission as shown schematically in Fig. 2.26. Thus, the initiation of breakdown depends on the conditions and the properties of the cathode surface. Experimental evidence shows that breakdown takes place by this process when the effective catliode electric field is of the order of 106 to 107V/cm.

Clump Mechanism
Basically this theory has been developed on the following assumptions (Fig. 2.27):
(O A loosely bound particle (clump) exists on one of the electrode surfaces.
(11) On the application of a high voltage, this particle gets charged, subsequently
gets detached from the mother electrode, and is accelerated across the gap.
(///) The breakdown occurs due to a discharge in the vapour or gas released by the
impact of the particle at the target electrode.
Cranberg was the first to propose this theory. He initially assumed that breakdown
will occur when the energy per unit area, W9 delivered to the target electrode by a
clump exceeds a value C, a constant, characteristic of a given pair of electrodes. The quantity W is the product of gap voltage (V) and the charge density on the clump. The latter is proportional to the electric field E at the electrode of origin. The criterion for breakdown, therefore, is
VE = C

In case of parallel plane electrodes the field E = VId9 where d is the distance
between the electrodes. So the generalised criterion for breakdown becomes
V=(Cd)^ (2.35)
where C is another constant involving C and the electrode surface conditions.
Cranberg presented a summary of the experimental results which satisfied this
breakdown criterion with reasonable accuracy. He stated that the origin of the clump
was the cathode and obtained a value for the constant C as 60 x 101^ V2/cm (for iron
particles). However the equation was later modified as V = C d01, where a varies
between 0.2 and 1.2 depending on the gap length and the electrode material, with a
maximum at 0.6. The dependence of V on the electrode material, comes from the
observations of markings on the electrode surfaces. Craters were observed on the
anode and melted regions on the cathode or vice-versa after a single breakdown.

Summary
Although there has been a large amount of work done on vacuum breakdown
phenomena, so far, no single theory has been able to explain all the available
experimental measurements and observations. Since experimental evidence exists for
all the postulated mechanisms, it appears that each mechanism would depend, to a
great extent, on the conditions under which the experiments were performed. The
most significant experimental factors which influence the breakdown mechanism are:
gap length, geometry and material of the electrodes, surface uniformity and treatment
of the surface, presence of extraneous particles and residual gas pressure in the
vacuum gap. It was observed that the correct choice of electrode material, and the use of thin insulating coatings in long gaps can increase the breakdown voltage of a
vacuum gap. On the other hand, an increase of electrode area or the presence of
particles in the vacuum gap will reduce the breakdown voltage.