in this topics we have study the different types of material used in Solid, Liquid and Gases state then their properties describing and study change in material and Breakdown voltage and strength can be measured.

1. Unit 2
A) Dielectric Breakdown
1

2. • Electrical breakdown or dielectric breakdown is when
current flows through an electrical insulator when the
voltage applied across it exceeds the breakdown voltage.
This results in the insulator becoming electrically
conductive.
• The ratio of breakdown voltage and thickness or density of
dielectric material is known as Electrical breakdown.
• 𝐵𝑟𝑒𝑎𝑘𝑑𝑜𝑤𝑛 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ =
𝐵𝑟𝑒𝑘𝑑𝑜𝑤𝑛 𝑉𝑜𝑙𝑡𝑎𝑔𝑒
𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑟 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝐷𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐
2

3. • Breakdown Voltage:- the dielectric materials remains stable
in its property but results in distraction of insulating
properties is called “Breakdown Voltage”.
• Breakdown voltage is normally in KV.
• The concept of breakdown voltage and breakdown strength
is very important in design of electrical apparatus like
machine, transformer etc.
• Factor affecting breakdown voltage:-
1. Thickness of material
2. Temperature
3. Moisture contents
4. Time for application of voltage
5. Composition
• Factor affecting breakdown strength of solid insulating
materials:-
1. Defects & non homogeneity of material
2. Thickness of specimen
3. Area & volume of solid dielectric
4. Shape of electrodes
5. Time of application of voltage
6. Moisture and other contamination. 3

4.  Dielectric breakdown of the Gases:-
• Any gas is good insulator. Air is an ideal gas, contains only neutral molecules which
do not conduct electric current.
• A various external influences like ultra violet radiation of sun a small quantity of
ions & electrons, gas definite conductivity are always present.
• The process of electron from a gas molecule with stimulation production of positive
ion is called ionization.
• Primary Ionization:-
a) Ionization by collision:-
b) Photo ionization:-
• Factor affecting breakdown strength of gaseous dielectric materials:-
1. Electrode configuration
2. Gap spacing
3. Pressure
4. Moisture and Humidity
5. Effect of atmospheric condition
• Factor affecting breakdown strength of liquid dielectric materials:-
1. Dissolved gases
2. Distance between electrodes
3. Dust & dirt particles
4. Moisture or water particles
5. Impurities a) Temperature b) shape of electrode c) viscosity of liquid d) electrode
material 4

5. • Application:-
1. Its used in Transformer, Cables, Capacitors, Circuit Breakers.
 Mineral Insulating Oil:- (Petroleum Oil or Transformer Oil)
• Properties:-
1. Most widely used liquid insulation in industry.
2. Dissipation factor (tan δ) at 90⁰C is 0.001 to 0.005
3. Breakdown voltage is 30 -50 KV/2.5mm
4. Permittivity is 2.1 to 2.5
5. Its resistivity at 90⁰C is 22000Ωm.
• Application :-
1. Mostly used as insulation as coolant in transformer.
2. Low viscosity oil is used in high tension oil filled cables, transformer.
3. Medium viscosity oil is used in switchgear & cables.
4. High viscosity oil used in gas filled cables & solid cables.
 Synthetic Liquids:-
 Properties:-
1. These are more expensive than mineral oil due to their high manufacturing cost.
2. It is non inflammable & non explosive. 5

6. • It has high breakdown strength 40 to 60 KV/2.5 mm.
• Dissipation factor (tanδ) is less than 0.0005
• Permittivity is 2.1 to 2.2 e.g. Arodors, Askarels, Pyranols etc.
 Application:-
1. Used in high voltage transformer as coolant and insulation.
2. Used in circuit breaker, high pressure gas filled power cables & in
d.c.capacitors.
 Askarel :-
 Properties:-
1. It is costlier than transformer oil.
2. Its fire resistant, because has good insulating properties.
3. Its breakdown voltage is 20 to 45 KV/2.5mm.
4. Its resistivity is 10¹²Ωm.
5. Its permittivity is 4.8 to 5.3
 Application:-
1. Used as a coolant for transformer and for capacitors operating at higher
voltage .
2. Used in circuit breakers.
6

7.  Varnish :-
 Properties:-
1. Varnishes protect the
materials against moisture,
dirt & dust.
2. Improve insulation
properties. Increase
mechanical strength.
3. Reduce degradation caused
by their oxidation.
4. Protect from atmospheric
corrosion & moisture, fire
proof finish.
 Enamel:-
 Properties:-
1. It is a fusible insulated
coating of organic base
material.
2. It is applied on conducting
surface.
3. The maximum thickness of
enamel coating is 0.05mm
4. The enamel coating is
provided on copper or
aluminimum wire which are
used for winding in case of
transformer motor etc.
5. They are also used to furnish
a heavy protective coating on
electronic equipment. 7

8. Gaseous Insulating Materials
 Types:-
1. Simple Gases:- e.g. Air, Nitrogen (N2), Hydrogen (H2),
Helium (He), Argon (Ar) etc.
2. Oxide Gases :- Carbon Dioxide (CO2), Sulphar Dioxide
(SO2)
3. Electronegative Gases:- Sulphar Hexafluoride (SF6)
4. Hydrocarbon Gases:- Methane (CH4), Ethane, Propane.
8

9. • Air:-
• Properties:-
1. Its naturally & free gas.
2. It needs no processing & can
be used directly.
3. Its dielectric strength is 30
KV/cm at 50 Hz.
4. Its dielectric strength increase
linearly with increase in gas
pressure.
• Application :-
1. Its provides insulation
between overload
transmission lines.
2. Used for cooling rotating parts
of machine.
3. Used in capacitors as
dielectric.
4. Used in small transformer as
coolant.
• Hydrogen:-
• Properties:-
1. Very light gas.
2. Thermal conductivity is 6.69
times of air.
3. It has density 0.07 times that
of air, so windage losses in
machines can be minimized.
• Application:-
1. Used as coolant in electric
machine due to which
efficiency increase.
2. Used to reduce windage loss
in high speed machines.
9

10. • Nitrogen:-
• Properties:-
1. Its density is 0.97 times
that’s of Air.
2. Its thermal conductivity is
1.08 times that of air.
3. In many high voltage
applications air is reduced
by nitrogen to prevent
oxidation of the other
insulating materials.
• Sulphar Hexafluoride (SF6):-
• Physical Properties:-
1. Colorless
2. Odourless
3. Non-toxic
4. Non inflammable
5. Heat transfer ability is 2.5 times greater
than Air.
• Chemical Properties:-
1. Its stable up to 500⁰C.
2. Its chemically inert so life of metallic
parts, contacts is more.
3. Its electro negative gas.
4. It has excellent arc quenching
properties.
• Dielectric properties:-
1. Dielectric strength of SF6 gas at
atmosphere pressure is 2.35 times that
of air but less than oil by 30%.
2. When pressure of SF6 gas more than 3
kg/cm², its dielectric strength is higher
than oil.
10

11. Unit No 2
B) Testing of Materials
11

12. Aim- To measure dielectric strength of solid insulating materials
• Apparatus -
– High Voltage Testing Kit
– Verniercalifer
– Sheets of different dielectric Material e.g. Epoxy, Glass, Paper,
Pressboard, Hylum etc.
• Circuit Diagram –

14. • Procedure –
1. Place the insulating material between the electrodes
2. Ensure the voltmeter shows zero reading
3. Switch on the main supply
4. Switch on the H.T. supply
5. Raise the Voltage gradually
6. Note down the reading of voltmeter in observation table
when breakdown occurs
7. Repeat the above procedure for each specimen of
insulating material.

15. • Observation Table –
Sr.No. Name of Material Thickness (mm) Breakdown
Voltage (KV)
Breakdown
Strength (KV/mm)
1 Epoxy 0.4mm 23KV 57.5 KV/mm
2 Hylum 0.2mm 22KV 110 KV/mm
3 Paper 0.1mm 20KV 200 KV/mm
4 Acrylic 0.3mm 21KV 70KV/mm
5 Glass 0.4mm 21KV 52.5 KV/mm
6 Pressboard 0.4mm 24KV 60 KV/mm

16. • Calculation –
Name of Material Breakdown strength =
Breakdown Voltage / Thickness of
Material
Result
Epoxy 23/0.4 57.5 KV/mm
Hylum 22/0.2 110 KV/mm
Paper 20/0.1 200 KV/mm
Acrylic 21/0.3 70KV/mm
Glass 21/0.4 52.5 KV/mm
Pressboard 24/0.4 60 KV/mm

17. Aim – To measure dielectric strength of liquid insulating material
• Apparatus –
– High Voltage Oil
Testing kit
– Transformer oil

18. Circuit Diagram -

19. Glass Vessel
Breakdown Voltage in Testing Oil
Kit

20. • Procedure –
– Adjust the Gap between the electrode to 2.5mm using the filler
gauge with punch mark.
– Fill the test vessel / Glass vessel or Cup with the dielectric oil
sample to be tested and place it on H.T. electrode.
– Ensure that voltmeter shows zero reading
– Switch on the mains supply
– Switch on H.T. supply
– Raise the voltage automatically gradually by rotating auto
transformer knob in clockwise direction.
– Note down reading of voltmeter in observation table when
breakdown occurs.
– Repeat steps for number of reading

21. • Observation Table –
– Distance between Electrode - _2.5_mm
Sr.No. Liquid Insulating Material Breakdown Voltage (KV)
1
Transformer oil
27
2 29
3 30
4 32
5 34
Average Breakdown Voltage - 152 / 5 = 30.4 KV

22. • Calculation –
• Average Breakdown Voltage =
Sum of Total breakdown voltage
Number of reading
=
27+29+30+32+34
5
=
152
5
= 30.4 KV

23. Aim – To measure Dielectric strength of gaseous insulating material using
Sphere Gap Unit
• Apparatus –
– High voltage testing Transformer 230v / 100Kv along with
voltage regulator and Control Panel.
– Sphere Gap Unit
– Discharge Rod
• Theory –
– Breakdown in gases are initiated by ionization due to collision of
electrons, this cumulative process into breakdown of gas.

24. • Circuit Diagram -

25. • Sphere Gap – • Transformer –

26. • Operating Panel –

27. • Procedure –
– Make the connections as shown in circuit diagram
– Switch on the main supply
– Insert the key in control panel lock and rotate its clockwise
– Switch on H.T. supply using H.T. ON push button.
– Raise the voltage gradually using variable on control panel.
– Note down reading of voltmeter in observation table when breakdown of air
occurs.
– Reduce the voltage gradually to zero using variable on control panel.
– Increase the gap between the sphere and repeat steps 2 to 6

28. • Observation table –
• Calculation –
– Dielectric Strength =
𝐵𝑟𝑒𝑎𝑘𝑑𝑜𝑤𝑛 𝑉𝑜𝑙𝑡𝑎𝑔𝑒
𝐺𝑎𝑝 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑠𝑝ℎ𝑒𝑟𝑒𝑠
= 40/0.2 = 200
KV/mm
Sr.No. Gaseous Insulating Material
Gap between
sphere (mm)
Breakdown
voltage (KV)
Dielectric strength
(KV /mm)
1
Air
0.2mm 40KV 200 KV/mm
2 0.3mm 42KV 140 KV/mm
3 0.5mm 45KV 90 KV/mm

29. – Dielectric Strength =
𝐵𝑟𝑒𝑎𝑘𝑑𝑜𝑤𝑛 𝑉𝑜𝑙𝑡𝑎𝑔𝑒
𝐺𝑎𝑝 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑠𝑝ℎ𝑒𝑟𝑒𝑠
=
42/0.3 = 140 KV/mm
– Dielectric Strength =
𝐵𝑟𝑒𝑎𝑘𝑑𝑜𝑤𝑛 𝑉𝑜𝑙𝑡𝑎𝑔𝑒
𝐺𝑎𝑝 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑠𝑝ℎ𝑒𝑟𝑒𝑠
=
45/0.5 = 90 KV/mm

30. Aim – Measurement of Tangent of Dielectric Loss Angle (Tan δ) by
Schering Bridge
• Theory –
– A Capacitor is nothing but a two conductors parts separated by a insulating
or dielectric material it is a static device.
– Insulator or dielectric are do not having free electrons but however no
insulators or dielectrics are perfect so when capacitor is connected across
e.m.f. source it do not conduct but they contains small amount of free
electrons which causes very small amount of current flow through them.
Therefore capacitor is having a small leakage conductance.
– A lossy capacitor can be electrically modelled as an ideal capacitor
connected in series with hypothetical resistance.
– One of the most commonly used method for measurement of capacitance
and dissipation factor is high voltage shearing bridge.
– In one arm, a standard capacitors C having negligible loss is placed. The
sample dielectric is represented by series model of Cx & Rx, while third
arm carries a variable resistor R3.

31. • There fore
𝑡𝑎𝑛𝛿 =
𝑉𝑟
𝑉𝑐
=
𝑅𝑥
1/𝑤𝐶𝑥
= 𝑤𝐶𝑥𝑅𝑥 −− −1
Phasor diagram for lossy CapacitorEquivalent Circuit for lossy capacitor

32. • The forth arm consist of variable capacitor C4, at
balanced condition.
𝑍1 𝑍3 = 𝑍2 𝑍4 −− −2
Where Z1, Z2, Z3 & Z4 are impedance of arms.
𝑍1 =
1
𝑗𝑤𝐶𝑠
−− −3
𝑍2 = 𝑅𝑥 +
1
𝑗𝑤𝐶𝑥
−− −4
𝑍3 = 𝑅3 −− −5
𝑍4 =
1
1
𝑅4 + 𝑗𝑤𝐶4
−− −6

33. Therefore,
1
𝑗𝑤𝐶𝑠
∗ 𝑅3 = 𝑅𝑥 +
1
𝑗𝑤𝐶𝑥
1
1
𝑅4
+ 𝑗𝑤𝐶4
−− −7
𝑍2 =
𝑍1 𝑍3
𝑍4
−− −8
Therefore,
𝑅𝑥 −
𝑗
𝑤𝐶𝑥
= −
𝑗𝑅3
𝑤𝐶𝑠
Therefore,
𝑅𝑥 −
𝑗
𝑤𝐶𝑥
=
𝐶4 𝑅3
𝐶𝑠
−
𝑗𝑅3
𝑤𝑅4𝐶𝑠
−− −9
Separating real and imaginary part,
𝑅𝑥 =
𝐶4
𝐶𝑠
𝑅3 −− −10
And
𝐶𝑥 =
𝑅4
𝑅3
𝐶𝑠 −− −11

34. Therefore,
𝑡𝑎𝑛𝛿 = 𝑤 𝐶𝑥 𝑅𝑥
• The effect of stray capacitance can be nullified by using earth.
• Dielectric loss (Tan δ) for such connection can be given
𝑡𝑎𝑛𝛿 =
1
𝑤𝐶𝑅
−− −12