Measurenents of DC Resistivity ( High voltage Engineering)

1. Written By
Engr. Muhammad Yaseen
Reviewed By
Engr Muhammad Imran

2. Measurement of d.c. Resistivity
Specimens and Electrodes
 The specimen shape and the electrode arrangement should be
such that the resistivity can be easily calculated. For a solid
specimen, the preferable shape is a flat plate with plane and
parallel surfaces, usually circular.
 The specimens are normally in the form of discs of 5 to 10cm
diameter and 3 to12mm thickness.

4.  If the electrodes are arranged to be in contact with the surfaces
of the specimen, the measured resistance will be usually greater
due to the surface conductivity effects.
 The electrode which completely covers the surface of the
specimen is called the “unguarded” electrode and is connected to
the high voltage terminal.
 The third electrode which surrounds the other measuring
electrode is connected to a suitable terminal of the measuring
circuit.
 The width of this "guard“ electrode must be at least twice the
thickness of the specimen, and the unguarded electrode must
extend to the outer edge of the guard electrode.
 The gap between the guarded and guard electrodes should be as
small as possible.

5.  The effective diameter of the guarded electrode is greater than
the actual diameter and is given as follows.
 Let r1, r2, and r be the radii of the guarded electrode, guard
electrode including the gap, and the effective radius of the
guarded electrode. Let the gap width = g and the specimen
thickness = t.

6. Electrode Materials
 For accurate measurements, the electrodes should have very
good contact with the surface of the insulator specimen.
 Hence, it is necessary to use some type of thin metallic foil
(usually of lead or aluminum of about 10 to 50micro meters
thickness), usually pressed on to the surface by a roller and
made to stick by using a conducting adhesive like petroleum
jelly or silicone grease.
 The electrodes are made simultaneously by cutting out a narrow
strip by means of a compass provided with a narrow cutting
edge.
 Some times conducting silver paint is also used for electrode
deposition.

7. Measuring Cells
 The three terminal electrode system and the measuring cell used
are shown in Fig.
 The measuring cell is usually a shallow metal box provided with
insulating terminals.
 The box it self is connected to the guard electrode and is
grounded if the guard terminal is grounded.
 The connecting lead for the guarded electrode is taken through a
shielded wire.
 In case the unguarded electrode is grounded, the entire box is to
be placed on insulated supports and is to be placed in a grounded
shield to eliminate induced voltages, and the lead from the guard
electrode is doubly shielded.

9.  In the simple two terminal system, the measuring cell itself is
the grounded support for the specimen and a small solid wire is
connected to the high voltage terminal of The measuring circuit,
as can be seen from below Fig this is a simple and compact
arrangement for quick measurements and requires less skill.

10.  The arrangement used for the
study of liquids is shown in
Fig. This consists of an outer
cylindrical case and an inner
cylinder with a cylindrical
guard electrode.
 The opposing surfaces of the
measuring electrodes should be
carefully finished to give a
polished surface, and a uniform
spacing of about 0.2 5mm is
maintained.
 The insulation should be able
to maintain the alignment of
the electrode even at the
highest temperatures used and
should still allow easy
disassembling and cleaning.

11.  Another simple arrangement of
the three electrode system for
the study of liquids is shown in
Fig.
 This consists of a metallic
cylindrical container with
concentric hollow cylindrical
electrodes as guard and
guarded electrodes.
 The inner surface of the
container electrode(unguarded)
and the outer surfaces of the
guard and unguarded
electrodes should be carefully
finished and a clearance of
about 0.25 to 0.5mm should be
accurately maintained.
 The arrangement requires less
liquid (usually only about 1 to
2ml).

12. Partial Discharge Measurement
Introduction
 Earlier the testing of insulators and other equipment was based on
the insulation resistance measurements, dissipation factor
measurements and breakdown tests.
 It was observed that the dissipation factor (tan ) was voltage
dependent and hence became a criterion for the monitoring of the
high voltage insulation.
 In further investigations it was found that weak points in an
insulation like voids, cracks, and other imperfections lead to
internal or intermittent (stopping and starting at irregular intervals)
discharges in the insulation.

13. Introduction
 These imperfections being small were not exposed in capacitance
measurements but were exposed as power loss components in
contributing for an increase in the dissipation factor (loss factor).
 In modern terminology these are designated as “partial discharges”
which in course of time reduce the strength of insulation leading to
a total or partial failure or break down of the insulation.
 Electrical insulation with imperfections or voids leading to partial
discharges can be represented by an electrical equivalent circuit
shown in Fig.

14. Introduction

15. Partial Discharge Phenomenon
 The following terminology is often used in partial discharge
detection and as such their definitions are of importance:
Electrical Discharge
 The movement of electrical charges through an insulating
(dielectric) medium, initiated by electron avalanches.
Partial Discharge
 An electrical discharge that only partially bridges the dielectric or
insulating medium between two conductors.
 Examples are: internal discharges, Surface discharges and corona
discharges.
 Internal discharges are discharges in cavities or voids which lie
inside the volume of the dielectric or at the edges of conducting
inclusions in a solid or liquid insulating media

16. Partial Discharge Phenomenon
 Surface discharges are discharges from the conductor into a gas
or a liquid medium and form on the surface of the solid
insulation not covered by the conductor.
 Corona is a discharge in a gas or a liquid insulation around the
conductors that are away or remote from the solid insulation.

17. Gaseous, Liquid & Solid Breakdowns
Gas/Vacuum as Insulator
 Air at atmospheric pressure is the most common gaseous
insulation.
 The breakdown of air is of considerable practical importance to
the design engineers of power transmission lines and power
apparatus.
 Breakdown occurs in gases due to the process of collisional
ionization.
 Electrons get multiplied in an exponential manner, and if the
applied voltage is sufficiently large, breakdown occurs.
 In some gases, free electrons are removed by attachment to
neutral gas molecules; the breakdown strength of such gases is
substantially large.

18. Gaseous, Liquid & Solid Breakdowns
 An example of such a gas with larger dielectric strength is
sulphur hexafluoride (SF6) .
 The breakdown strength of gases increases steadily with the gap
distance between the electrodes; but the breakdown voltage
gradient reduces from 3MV/m for uniform fields and small
distances to about 0.6MV/m for large gaps of several meters.
 For very large gaps as in lightning, the average gradient reduces
to 0.1 to 0.3 MV/m.
 High pressure gas provides a flexible and reliable medium for
high voltage insulation.
 Nitrogen (N2) was the gas first used at high pressures because of
its inertness and chemical stability, but its dielectric strength is
the same as that of air.

19. Gaseous, Liquid & Solid Breakdowns
 Other important practical insulating gases are carbon
dioxide(CO2), dichlorodifluoro-methane (CCl2F2) (popularly
known as Freon), and sulphur hexafluoride (SF6).
 SF6 has been found to maintain its insulation superiority, about
2.5 times over N2 and CO2 at atmospheric pressure, the ratio
increasing at higher pressures.
 SF6 gas was also observed to have superior arc quenching
properties over any other gas.
 The breakdown voltage at higher pressures in gases shows an
increasing dependence on the nature and smoothness of the
electrode material.
 Under high vacuum conditions, where the pressures are below
10-4 torr, the breakdown cannot occur due to collisional processes
like in gases, and hence the breakdown strength is quite high.

20. Gaseous, Liquid & Solid Breakdowns
 Vacuum insulation is used in particle accelerators, x-ray and field
emission tubes, electron microscopes, capacitors, and circuit
breakers.
Ionization Process
 Ionization is the process of producing ions in dielectric by virtue
of external arrangement.
OR
 Ionization is the process of converting the neutral atom in to the
charged atom is called ions.
 Ions are the charged atoms, they are produced when the neutral
atom gains or losses the electron.
 If the neutral atom losses an electron then it has deficiency of
electron and hence positive ions are produced.

21. Gaseous, Liquid & Solid Breakdowns
 If the neutral atom gains an electron then it has excess of electron
and hence negative ion is produced.
According to High Voltage
 A gas in its normal state is almost a perfect insulator.
 However, when a high voltage is applied between the two
electrodes immersed in a gaseous medium, the gas becomes a
conductor and an electrical breakdown occurs.
 The processes that are primarily responsible for the breakdown
of a gas are ionization by collision, photo-ionization, and the
secondary ionization processes.
 In insulating gases (also called electron-attaching gases) the
process of attachment also plays an important role.

22. Gaseous, Liquid & Solid Breakdowns
Ionization By Collision
 The process of releasing an electron from a gas molecule with
the simultaneous production of a positive ion is called ionization.
 In the process of ionization by collision, a free electron collides
with a neutral gas molecule and gives rise to a new electron and a
positive ion.
 If we consider a low pressure gas column in which an electric
field E is applied across two plane parallel electrodes, as shown
in Fig. then, any electron starting at the cathode will be
accelerated more and more between collisions with other gas
molecules during its travel towards the anode.
 If the energy (Ԑ) gained during this travel between collisions
exceeds the ionization potential, Vi which is the energy required
to dislodge (shift) an electron from it’s atomic shell, then
ionization takes place. This process can be represented as

23. Gaseous, Liquid & Solid Breakdowns
Where, A is the atom, A+ is the positive ion and e
_
is the electron.

24. Gaseous, Liquid & Solid Breakdowns
 A few of the electrons produced at the cathode by some external
means, say by ultra-violet light falling on the cathode, ionize
neutral gas particles producing positive ions and additional
electrons.
 The additional electrons, then, them selves make ‘ionizing
collisions’ and thus the process repeats it self.
 This represents an increase in the electron current, since the
number of electrons reaching the anode per unit time is greater
than those released at the cathode.
 In addition, the positive ions also reach the cathode and on
bombardment on the cathode give rise to secondary electrons.

25. Gaseous, Liquid & Solid Breakdowns
Photo Ionization
 The phenomena associated with ionization by radiation, or
photo-ionization, involves the interaction of radiation with
matter.
 Photo-ionization occurs when the amount of radiation energy
absorbed by an atom or molecule exceeds its ionization potential.
 There are several processes by which radiation can be absorbed
by atoms or molecules. They are:
 excitation of the atom to a higher energy state
 Continuous absorption by direct excitation of the atom or
dissociation(removing from association) of diatomic molecule or
direct ionization etc.

26. Gaseous, Liquid & Solid Breakdowns
 Just as an excited atom emits radiation when the electron returns
to the lower state or to the ground state, the reverse process takes
place when an atom absorbs radiation.
This reversible process can be expressed as
 where, h is the Planck's constant, c is the velocity of light, ƛ is
the wave length of the incident radiation and Vi is the ionization
energy of the atom. Substituting for h and c, we get

27. Gaseous, Liquid & Solid Breakdowns
 Where Vi is in electron volts (eV). The higher the ionization
energy, the shorter will be the wave length of the radiation
capable of causing ionization.
 It was observed experimentally that a radiation having a
wavelength of 1250 A° is capable of causing photo-ionization of
almost all gases.

28. Gaseous, Liquid & Solid Breakdowns
Liquid Breakdown
 Liquids are used in high voltage equipment to serve the dual
purpose of insulation and heat conduction.
 They have the advantage that a puncture path is self-healing
(Automatic recovered).
 Temporary failures due to overvoltages are reinsulated quickly
by liquid flow to the attacked area.
 However, the products of the discharges may deposit on solid
insulation supports and may lead to surface breakdown over
these solid supports.

29. Gaseous, Liquid & Solid Breakdowns
 Highly purified liquids have dielectric strengths as high as
1MV/cm.
 Under actual service conditions, the breakdown strength reduces
considerably due to the presence of impurities.
 The breakdown mechanism in the case of very pure liquids is the
same as the gas breakdown, but in commercial liquids, the
breakdown mechanisms are significantly altered by the presence
of the solid impurities and dissolved gases.
 Petroleum oils are the commonest insulating liquids. However,
askarels, fluorocarbons, silicones, and organic esters including
castor oil are used in significant quantities.
 A number of considerations enter into the selection of any
dielectric liquid.

30. Gaseous, Liquid & Solid Breakdowns
 The important electrical properties of the liquid include the dielectric
strength, conductivity, flashpoint, gas content, viscosity, dielectric
constant, dissipation factor, stability, etc. Because of their low
dissipation factor and other excellent characteristics, polybutanes are
being increasingly used in the electrical industry.
 Askarels and silicones are particularly useful in transformers and
capacitors and can be used at temperatures of 200oc and higher.
 Castor oil is a good dielectric for high voltage energy storage
capacitors because of its high corona resistance, high dielectric
constant, non toxicity, and high flash point.
 In practical applications liquids are normally used at voltage stresses of
about 50-60kV/cm when the equipment is continuously operated.
 On the other hand, in applications like high voltage bushings, where
the liquid only fills up the voids in the solid dielectric, it can be used at
stresses as high as 100-200kV/cm.

31. Gaseous, Liquid & Solid Breakdowns
Breakdown of commercial liquids
 When a difference of potential is applied to a pair of electrodes
immersed in an insulating liquid, a small conduction current is first
observed. If the voltage is raised continuously, at a critical voltage a
spark passes between the electrodes.
 The passage of a spark through a liquid involves the following.
a) Flow of a relatively large quantity of electricity, determined by the
characteristics of the circuit,
b) A bright luminous path from electrode to electrode,
c) The evolution of bubbles of gas and the formation of solid products of
decomposition (if the liquid is of requisite chemical nature).
d) Formation of small pits(cavity or hole) on the electrodes,
e) An impulsive pressure through the liquid with an accompanying explosive
sound.

32. Gaseous, Liquid & Solid Breakdowns
Breakdown of commercial liquids
 Tests on highly purified transformer oil show that
a) Breakdown strength has a small but definite dependence on electrode
material,
b) breakdown strength decreases with increase in electrode spacing,
c) breakdown strength is independent of hydrostatic pressure for degassed
oil, but increases with pressure if oil contains gases like nitrogen or
oxygen in solution.
 In the case of commercial insulating liquid, which may not be subjected to
very detail purifying treatment, the breakdown strength will depend more
upon the nature of impurities it contains than upon the nature of the liquid
itself.

33. Gaseous, Liquid & Solid Breakdowns
Breakdown due to liquid globules
 If an insulating liquid contains in suspension a globule of another liquid, then
breakdown can result from instability of the globule in the electric field.
 Consider a spherical globule of liquid of permittivity immersed in a liquid
dielectric of permittivity When it is subjected to an electric field between
parallel electrodes, the field inside the globule would be given by
 where E0 is the field in the liquid in the absence of the globule.
 The electrostatic forces cause the globule to elongate and take the shape of a
prolate (elliptic) spheroid (i.e. an elongated spheroid).
 As the field is increased, the globule elongates so that the ratio ᵧ of the longer
to the shorter diameter of the spheroid increases. For the same field E, the
ratio ᵧ is a function of

34. Gaseous, Liquid & Solid Breakdowns
Breakdown due to liquid globules
 When (generally when ), and the field exceeds a critical
value, no stable shape exists, and the globule keeps on elongating eventually
causing bridging of the electrodes, and breakdown of the gap.When the
critical field at which the globule becomes unstable no longer depends on the
ratio, and is given by Ecrit.

35. Insulation Material

36. Electrode Material

37. Raychem Kit