04a Breakdown of Solid Insulation.pdf

The Hong Kong Polytechnic University
Department of Electrical Engineering
High Voltage Engineering
EE509/EE509A/EE509B/EE509D
Prof. Derek S. W. Or
2020 / 2021

EE509/EE509A/EE509B/EE509D High Voltage Engineering vi
Chapter 4 Breakdown of Solid Insulation..........................................................................................4-1
4.1 Introduction.......................................................................................................................................4-1
4.2 Breakdown Due to Treeing...............................................................................................................4-2
4.2.1 Occurrence and Causes.......................................................................................................4-2
4.2.2 Types of Electrical Trees....................................................................................................4-7
4.2.3 Electrical Trees and Water Trees .......................................................................................4-8
4.3 Breakdown Due to Surface Flashover ..............................................................................................4-9
4.3.1 Occurrence and Causes.......................................................................................................4-9
4.4 Breakdown Due to Surface Tracking .............................................................................................4-11
4.4.1 Occurrence and Causes.....................................................................................................4-11
4.4.2 Methods of Minimizing Tracking ....................................................................................4-12
4.5 Breakdown in Composite Insulation ..............................................................................................4-13
4.5.1 Properties of Composite Insulating Systems....................................................................4-13
4.5.2 Matching Dielectric Permittivities ...................................................................................4-15
4.6 References.......................................................................................................................................4-16

EE509/EE509A/EE509B/EE509D High Voltage Engineering 4-1
Chapter 4
Breakdown of Solid Insulation
4.1 Introduction
Solid insulating materials are widely used in electrical engineering systems to insulate one current-carrying
conductor from another when they are operated at different voltages. If a solid insulating material is truly
homogeneous and free from imperfections, its dielectric (intrinsic) breakdown strength can be as high as 10,000
kV/cm. This value is about 10 times higher than liquids and 100 times larger than air.
In practice, the breakdown of a solid insulator usually occurs at an electric field stress much lower than the
dielectric breakdown strength of the solid insulator. In fact, the breakdown process often starts at imperfections
either at the interfaces between solid insulators and conductors or inside solid insulators. The mechanism by which
breakdown occurs is called treeing. The said imperfections include: (1) micro-hills on the surfaces of conductors
(depending on the surface quality of the conductors); and (2) gas bubbles (voids), water droplets, and metallic or
carbon particles inside solid insulators.
In general, the breakdown of a solid insulator occurs more frequently on the surface of the solid insulator
compared to inside the solid insulator so that failure on surface insulators is the most common cause of trouble in
practice. When a breakdown occurs on the surface of a solid insulator, it can be a flashover on the surface or a
formation of conducting paths (or tracks) on the surface called tracking, both resulting in the degradation of the
solid insulator.

4.2 Breakdown Due to Treeing
4.2.1 Occurrence and Causes
Treeing is the term used for the mechanism by which breakdown occurs in solid insulating materials. Treeing
can be described by the erosion of a solid insulator at the tips of a spark (i.e., spark erosion of a solid insulator),
leading to an irregular tree-like fashion, either as a 2D or 3D electrical tree, with each tree branch consisting of
tubular channels/tunnels (or hollow tubes), as shown in Fig. 4.1. These partially conducting micro-channels/tunnels
ultimately lead to an early breakdown and hence an operational failure of the solid insulator. Treeing is often
observed in capacitors, cables, and transformers.
(a) (b)
Fig. 4.1 (a) 2D carbonized electrical trees as observed on the surface of a polycarbonate (PC) plate and
(b) 3D electrical trees occurred in a 1.5” cube of polymethyl methacrylate (PMMA).

Treeing always starts at an imperfection which can be: (1) a pointed micro-hill on the surface of a conductor or
(2) an inclusion inside a solid insulator such as gas bubbles (voids), water droplets, or metallic/carbon particles, as
shown in Fig. 4.2. These indicate that the surface quality of conductors and the quality of insulators are very
important for (1) and (2), respectively.
Because of the concentration of localized high electric fields at and around the imperfection, electrons being
emitted under such high electric fields (i.e., high electron emission currents) have sufficiently high kinetic energy
to erode the solid insulator, thereby creating a void and developing it into a tree branch-like partially conducting
tubular channel/tunnel (or hollow tube) in the insulator.
(a) (b)
Fig. 4.2 Schematics showing (a) an imperfection in a solid insulator in the form of a pointed micro-hill on the surface
of a conductor and (b) an electrical tree due to the imprefection in (a).

In more details, there appear to be three mechanisms by which a tree grows upon spark erosion of the solid
insulator by high-energy electrons.
(a) Breaking of Chemical Bonds by Electron Impact (Chemical Effect):
The high-energy electrons may have energies greater than those of the chemical (molecular) bonds
of the solid insulator; and if so, they break the bonds. In the case of polymers having long molecular
chains, this bond-breaking action results in shorter molecular chains, and the solid insulator will
probably become a liquid or even a gas (Fig. 4.3).
Fig. 4.3 Chemical (molecular) bonding in polymers.

(b) Melting or Decomposition due to Electron Impact-Induced Overheating (Thermal Effect):
The high-energy electrons may heat up the area of impact and eventually cause melting or
decomposition (as appropriate) of the solid insulator.
(c) Mechanical Cracking by Charge Repulsion (Mechanical Effect):
The high concentration of electrons (or positive ions) accumulated on the inside surface at the tip of
the branch produces strong forces of electrostatic repulsion to repel each other, causing cracking or
splitting of the solid insulator. These cracks will tend to open up where there is weakness or fault in
the insulator and account for the randomness in the direction of branch growth.
As in Fig. 4.4, trees form more easily in an insulator which is under tension but less easily under
compression. An example is a high voltage cable which is bent in an arc: the insulation on the
outside of the curve has more trees than that on the inside.
Fig. 4.4 Trees form more easily in a cable insulator under tension.

Physically, when a solid insulator lies between two conductors 1 and 2 as shown in Fig. 4.5, there is a
possibility for two different dielectric media: air in gap and solid insulator itself, to come in series. If the voltages
across the two media are: V1 across the air gap with gap distance d1 and V2 across the solid insulator with thickness
d2, V1 can be expressed as
𝑉1 =
𝑉𝑑1
𝑑1 + (
𝜀1
𝜀2
) 𝑑2
(4.1)
where V is the applied voltage, 𝜀1 is the relative dielectric permittivity of air (= 1.00054, 1 for vacuum), and 𝜀2 is
the relative dielectric permittivity of solid insulator (= 2.1 for Teflon, 2.25 for polyethylene, 7 for rubber, 11.68 for
silicon, 88–80.1–55.3–34.5 at 0–20–100–200 °C for water).
Since 𝜀1 < 𝜀2, a large portion of the applied voltage V will appear as 𝑉1 and across the limited d1, giving 𝐸1 → 𝐸.
Sparking will occur in the air gap, and charge accumulation will take place in the gap, leading to spark erosion of
the insulator at the tips of the spark. As time passes, breakdown channels (branches) spread through the insulator
irregularly – “Treeing”.
Treeing requires high voltages and can occur in a few minutes or several hours under ac voltage conditions.
Hence, care must be taken to avoid air gaps or other weaker insulation gaps. Treeing can also be prevented by
having clean, dry, undamaged surfaces and a clean environment.
Fig. 4.5 Arrangement for the study of treeing phenomenon.

4.2.2 Types of Electrical Trees
The electrical trees can further be classified depending on the different tree patterns. The two most commonly
found tree types are “bow-tie” trees and “vented” trees (Fig. 4.6).
(a) Bow-Tie Trees:
They start in a solid insulator from an inclusion imperfection inside the insulator (e.g., gas bubbles,
water droplets, or metallic/carbon particles) and grow symmetrically towards the two conductors. As
the trees start inside the insulator, they are difficult to have access to free air. Thus, these trees
usually have discontinuous and short growth such that they do not grow long enough to fully bridge
the conductors on the two sides [Fig. 4.7(a)].
(b) Vented Trees:
They start at an insulator-conductor interface from a pointed micro-hill on the conductor surface and
grow oppositely to conductor towards the insulator. These trees can have access to free air and thus
are able to grow continuously and long enough to bridge the conductors.
Fig. 4.6 Schematic showing “bow-tie” and “vented” trees.

4.2.3 Electrical Trees and Water Trees
Electrical trees grow in the absence of water [Fig. 4.7(a)]. If water is present, the polar nature of water molecule
allows it to penetrate into the electrical tree branches and form water trees. These water trees will grow several
times faster than the electrical trees but seldom result in complete breakdown. Water trees may be found in the
“bow-tie” formation inside the insulator or in the “vented” formation at the insulator-conductor interface [Fig.
4.7(b)]. They look diffuse and cloudy. An electrical tree may grow from a water tree [Fig. 4.7(c)]. Alternatively, an
electrical tree and a water tree may grow independently and then join together.
(a) (b) (c)
Fig. 4.7 (a) A “bow-tie” electrical tree in a PE cable, (b) a “vented” water tree on the inner conducting layer of a
20 kV corss-linked polyethylene (XLPE) cable after several years operation with water inside the cable, and
(c) a “bow-tie” electrical tree developed from the tip of a “vented” water tree in a XLPE cable.
Conducting
Particle

4.3 Breakdown Due to Surface Flashover
Flashover is the term used to describe the breakdown of solid (or liquid) insulation on the surface. The
phenomenon is fast, instantaneous to the human eye, like a spark. It is in fact a spark running across the surface of
a solid (or liquid) insulator.
Flashover is an instantaneous occurrence, being a breakdown via an avalanche travelling through the air close
to the surface of the insulator. The breakdown is at a lower voltage than would be expected from the apparent
values of the electric fields and the gas breakdown theories (i.e., Townsend and streamer theories) because of the
increased effects in secondary electrons and in electric field concentration.
Consider the three cases of plane conductor / air gap / plane conductor shown in Fig. 4.8, with Cases (b) and (c)
having an insulating spacer situated between the plane conductors. If the electric field is uniform and the same in
Cases (a) and (b) and for most of Case (c), flashover in Case (b) occurs at the lowest voltage level, while that in
Case (a) appears at the highest voltage level, provided that the spacer is very clean. Why?
Fig. 4.8 A case study of surface flashover.
Plane Conductor
Conductor
Plane Conductor
Conductor
Air Gap
Insulating
Spacer

Surface flashover usually occurs at a voltage lower than the gaseous breakdown because of:
(a) Enhanced Production of Secondary Electrons from Dirty (Unclean) Insulator Surface (with Impurities):
If the insulating spacer is not clean, there will be impurities on its surface. These impurities provide a
source of secondary electrons for emission upon an avalanche running up the surface of the
insulating spacer. Hence, the surface is more efficient to produce secondary electrons than the air and
thus has an enhanced ionization coefficient 𝛼
̃. This causes flashover on the insulating spacer surface
at a voltage well below the breakdown voltage of the air gap in the absence of the insulating spacer.
Dirty surfaces appear to produce much more secondary electrons than clean surfaces so that the
breakdown strength of an air gap with an insulating spacer will vary greatly with the cleanliness of
the insulating spacer.
(b) Triggering of Avalanches by Discharges in Irregularities at Three-Material Junctions:
In Case (b) of Fig. 4.8, since machining of the insulating spacer will not give a perfect right angle at
the ends, there will be an irregularly small air gap between the insulator and the conductor (Fig. 4.9).
The electric field will be greatly enhanced in this air gap, causing localized breakdown (partial
discharge) and injecting large numbers of electrons and ions into the air gap. This injection will be
multiplied through the normal ionization process, leading to breakdown at a voltage lower than the
expected value.
Fig. 4.9 A three-material junction.
Air
Conductor

4.4 Breakdown Due to Surface Tracking
While surface flashover presents an instantaneous phenomenon, surface tracking tends to take hours, days,
months, or even years, and usually occurs at very low voltages of the order of about 100 V.
By definition, surface tracking is a slow process of deterioration of the solid insulator surface by which dry
carbon tracks developed permanently across the insulator surface essentially act as conducting paths, leading to
gradual breakdown along the insulator surface. In most organic materials and plastics, the long carbon molecular
chains are repeatedly broken during tracking until they are reduced to carbon (Fig. 4.5). It follows that only organic
insulation, which can be degraded to carbon by heat, can ‘track’.
Tracking occurs when there is a conducting film of moisture across the surface of the solid insulator from the
high voltage conductor to the earth conductor. This tends to occur when the humidity is high and the temperature is
changing, causing condensation and later evaporation from the atmosphere. This moisture film will usually contain
contaminants, such as salt in coastal areas, carbonaceous dust from burning fuel or brush gear in industrial areas, or
cellulose fibers in a textile mill. Among these contaminants, salt is particularly important because it increases the
electrical conductivity of the moisture film to a certain extent.
Surface drying out is the usual mechanism whereby the leakage current flowing through the moisture film is
interrupted. Surface drying out occurs because of: (1) the heating effect of the leakage current itself, and
(2) evaporation through the rise in ambient temperature. These will eventually cause a break in the moisture film
and virtually stop the flow of the leakage current. Hence, all the voltage applied across the insulator is now across
the dried-out gap, called dry band.

If this dry band voltage exceeds the breakdown voltage of air, a spark occurs and bridges the dry band. The
bombardment of high-energy electrons carbonizes the insulator surface. The carbonized area becomes conductive,
and the conduction current flows. This conduction current will assist another “wet” area to dry out so as to form
another dry band, to break the area, and to cause a spark again. The cycle repeats until the carbon tracks link the
conductors together to result in a complete breakdown. In practice, it is likely that once a substantial proportion of
the insulator has been bridged by carbon tracks, the voltage gradient across the unbridged proportions will be high
enough to initiate flashover.
4.4.2 Methods of Minimizing Tracking
There are four major methods to minimize surface tracking:
(a) Ensuring that all surfaces are clean, dry, and undamaged. Cleaning is not often practicable but may be
used in some cases.
(b) Using non-tracking or track-resistant materials, such as porcelain and silicone rubbers (but they may not
be suitable for all applications). Porcelain does not contain carbon and so does not track. Silicone
rubbers exude low-molecular-weight silicones which are oily and break up the moisture into droplets,
thus preventing the formation of a continuous moisture film – so no conduction.
(c) Good design by (a) limiting the access of dirt, (b) avoiding dirt accumulation in areas between
conductors, (c) increasing creepage paths (the shortest path between two conductors of an electrical
circuit measured along the insulator surface) as much as is practical, and (d) for indoor installations,
ensuring either good ventilation or using air-conditioning when the humidity is high.
(d) Using moisture-repellant greases at the expense of requiring frequent cleaning and regreasing.

4.5 Breakdown in Composite Insulation
4.5.1 Properties of Composite Insulating Systems
Almost no complete electrical insulating system consists of one insulating material. Usually more than one
insulating materials will be involved, either in series, in parallel, or both.
A composite insulating system generally consists of a large number of insulating (material) layers arranged one
over the other. This is called the layered construction and is widely used in cables, capacitors, and transformers.
Three properties of composite insulting systems which are important to their performance are given below.
(a) Effect of Multiple Layers:
The simplest form of composite insulating system contains two layers of the same insulting material.
In this case, advantage is taken of the fact that two thin material layers have a higher dielectric
strength than a single material layer of the same total thickness. In other cases, composite insulating
systems occur either due to design considerations (e.g., paper with an impregnating liquid) or due to
practical difficulties of fabrication (e.g., air in parallel with solid insulation).
(b) Effect of Layer Thickness:
Increase in layer thickness normally gives increased breakdown voltage. In a layered construction,
breakdown channels mainly occur at the interfaces and not directly through another layer. Also, a
discharge having penetrated one layer cannot enter the next layer until a part of the interface also
attains the potential which can produce an electric field stress comparable to that of the discharge
channel. For the design with insulating paper, special care must be paid to the uniformity of paper
thickness as it affects significantly the homogeneity of dielectric strength across its surface. In fact,
the difference in the thickness imparts a rough surface to the paper which can produce an electric
field stress comparable to that of the discharge channel, causing breakdown at the rough points at
considerably lower voltages.

(c) Effect of Interfaces:
The interfaces between insulating material layers play an important role in determining the pre-
breakdown and breakdown strengths of the composite insulating system. Discharges usually occur at
the interfaces and the magnitude of the discharge depends on the associated surface resistance and
capacitance. When the surface conductivity increases, the discharge magnitude also increases,
resulting in damage to the insulation. Hence, insulating materials with low dielectric losses are
crucial to the operation at high electric field stresses. However, even in an initially pure liquid
insulation, when used under industrial conditions for impregnating solid insulation, impurities arise,
resulting in increased dielectric losses (see Chapter 3 for details).
In certain cases, the behavior of a composite insulation system could be predicted from the behaviors of the
components. But in most cases, the system as a whole has to be considered, and the following four considerations
determine the performance of the system as a whole.
(a) The stress distribution at different parts of the insulating system is distorted due to the components’
dielectric permittivities and electrical conductivities.
(b) The breakdown characteristics at the surface are affected by the insulation boundaries of various
components.
(c) The internal or partial discharge products of one component invariably affect the other components in
the system.
(d) The chemical aging products of one component also affect the performance of other components in the
system.

4.5.2 Matching Dielectric Permittivities
When a composite insulating system has components with different relative dielectric permittivities, utilization
of the materials may be weaken. This is especially true in oil/transformer board dielectrics. This is because the oil
has a lower dielectric constant and lower dielectric strength compared to those of transformer board.
For that case, the dielectric components are in series, sharing the same Q as
𝑉1
𝑉2
=
𝐶2
𝐶1
=
𝐴 𝜀o 𝜀2
𝑑2
∙
𝑑1
𝐴 𝜀o 𝜀1
=
𝜀2𝑑1
𝜀1𝑑2
𝑉 = 𝑉1 + 𝑉2
𝑉1
𝑉
=
𝑉1
𝑉1 + 𝑉2
=
𝜀2𝑑1
𝜀1𝑑2 + 𝜀2𝑑1
𝐸1 =
𝑉1
𝑑1
=
𝜀2
𝜀1𝑑2 + 𝜀2𝑑1
∙ 𝑉
𝐸2 =
𝑉2
𝑑2
=
𝜀1
𝜀1𝑑2 + 𝜀2𝑑1
∙ 𝑉

4.6 References
1. Of Dwyer, J.J., Theory of Dielectric Breakdown in Solids, Clarendon Press, Oxford (1964).
2. Whitehead, S., Dielectric Breakdown of Solids, Oxford University Press, Oxford (1951).
3. Von Hippel, A., Dielectric Materials and Applications, John Wiley, New York (1964).
4. Mason, J.H., “Electrical insulation”, Electrical Energy, Vol. 1, 68-75 (1956).
5. Taylor, H.E., Modern Dielectric Materials (Ed. J.B. Birks), Chap. 9, Haywood, London (1960).
6. Clark, P.M., Insulating Materials for Design and Engineering Practice, John Wiley, New York (1962).
7. Bradley, A., Electrical Insulation, Peter Peregrinus, London (1984).

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