conductioni in media

Waktu : 2x50 menit (2 sks)Isi: - Media gas sebagai isolator. - Proses ionisasi. - Kriteria Townsend untuk breakdown. -Teori streamer dari breakdown dalam gas. - Hukum Paschen. - Pertimbangan praktis dalam menggunakan gas untuk tujuan isolasi.Interaksi : Tanya-jawab, tugas

Minggu-11: Conduction and breakdown in gases (1)

Dr. Ir. Hermagasantos ZeinDr. Ir. Hermagasantos Zein

Week-11 : Conduction and breakdown in gases (1)

Topic: Gases as insulating mediaGases as dielectrics is the simplest and the most commonlyMost of electrical apparatus use air as insulating mediumGases as insulating media such as: -Nitrogen (N2)-Carbon dioxide (CO2)-Freon (CCl2F2)-Sulfhur hexafluoride (Sf6)

Various phenomena occur in gaseous dielectrics: When the applied voltage is1. Low: small current flow between electrodes and the insulation retains its

electrical properties2. Large: The current flow in the insulation increases very sharply and an

electrical breakdown occurs.3. A strongly conducting spark formed during breakdown practically produces

a short circuit between electrodes.4. The maximum voltage applied to the insulation at moment of breakdown is

called the breakdown voltage.5. Phenomena breakdown in gases as electric properties that produce high

current is essential.


The electrical discharges in gases are of two type:-Non-sustaining -Self-sustaining

Definition: -The breakdown in gases is called spark breakdown is as the transition of a non-sustaining discharges into a self-sustaining discharges.

-The build-up of high currents in a breakdown is due to the process known as ionization in which electrons and ions are created from neutral atoms or molecules, and their migration to the anode and cathode respectively leads to high currents.

Occur two theories: -Townsend theory and -Steamer theory (mechanism for breakdown)

The various physical conditions of gases: Presure, temperature, electric configuration, nature of electrodes surfaces and the availability of initial conducting particles are known to govern the ionization process.


Topic: Gases as insulating media


A gas in its normal state in almost a perfect insulator. The high voltage applied, the gas becomes a conductor and an electrical breakdown occurs

The processes of breakdown are ionization by collision, photo ionization and the secondary ionization process.In insulating gases (called electron attaching gases) the process of attachment also plays an important role.

Ionization by collision.

Photo ionization: called radiation.The process of radiation can be absorbed by atom or molecules:

-excitation of the atom to a higher energy state-continuous absorption by direct excitation of the atom or dislocation of

diatomic molecule or direct ionization, etc


Topic: Ionization process

eAeAe

iE

hcAAhv when occurs ionization ,*


Where: h is the Plank’s constant c is the volocity of light is the wavelength of incident radiation Ei is the ionization energy of atom

Substitution value of h and c, We get



cmEi

61027,1

Ei is in eletron volts (eV): The higher the ionization energy, the shorter will be the wavelength of the radiation capable of causing ionization.

If a wavelength of radiation 1250 A(amtrong) is capable of causing photo-ionization of almost all gases.


Secondary ionization process.By which secondary electrons are produced are the one which sustain a

discharge after it is established due to ionization by collision and photo-ionization. They are briefly described below:

a. Electron emission due to positive ion impact.Positive ions are formed due to ionization by collision or photo ionization and being positively charged, they travel towards the cathode.

b. Electron emission due to photosTo cause an electron to escape from a metal, it should be given enough energy to overcome the surface potential barrier. Electron emission from a metal surface at critical condition:



0A 2755frequecy thresholdand eV 4,5 with surface nickelclean a :example

frequecy. threshold theasknown is

iprelationsh by thegiven isit andfrequency theis

electrode metallic theoffuction work theis ,

hf

f

hf


Electron attachment processThe type of collisions in which electrons may become attached to atoms or moleculesTo from negative ions are called attacthment collisions. Electron attachment process depands on the energy of the electron and the nature of the gas. It is a very important process from the engineering point of view.

An electron attachmen process cn be represented as:



)(E ion atomic negative ke Atom a KK is the kinetic energyEa is the electron affinity


Townsend’s equation


Topic: Townsend’s criteria for breakdown

dd

eIe

I 0)1(1

1

1)1( deIf the current become un-finite,

breakdown

Self sustaining discharge

Non-sustaining discharge

Volt

T1To Ts

Vs

Io

Current

deII 0



Topic: Streamer theory of breakdown in gasesCausing of Townsend’s theory failFirst : Current growth occurs as a result of ionization process only. But in practice, breakdown voltage depend on pressure and geometry of gap.Second: The mechanism predicts time lags of the order 10-5 s. In actual practice, breakdown was observed to occur at very short time of the order 10-8s.

So, The Townsend mechanism failed to explain all these observed phenomena. Around 1940, Raether and, Meek and Loeb: independently proposed the streamer theory.

It (Pita teori) predict the development of a spark discharge directly from a single avalanche (longsoran) in which the space charge developed by the avalanche itself is said to transform the avalanche into a plasma streamer.



-+E

E3>E

E1>E

+ Anode

-Cathode

Topic: Streamer theory of breakdown in gases

1. A single electron start at cathode by ionization builds up an avalanche that cross the gap

2. The in avalanche move very fast compared with the positive ions.3. By the time the electron reach the anode the positive ions are virtually in original

position and form a positive space charge at anode.4. The secondary avalanches are formed from the few electrons produced due to

photo ionization in the space charge region.5. This occurs first near the anode where the space charge is maximum, in a

further increase in space charge.6. This process is very fast and the positive space extends to the cathode very

rapidly resulting in the formation of a streamer.

+ Anode

-Cathode

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Meek proposed a criteria to estimate the electric field that transforms an avalanche into a streamer:

Topic: Streamer theory of breakdown in gases

Volt/cm /

1027,5 7

px

exE

x

r

Where: r is radius alpha is Townsend’s first ionization coefficient. p is the gas pressure in torr x is the distance to which the streamer has extended in the gap.

According Meek, the minimum breakdown voltage is obtained when Er=E and x=d, the above equation become simple:

p

d

p

E

pad ln5,0ln5,14ln



Topic: Paschen’s law

Breakdown criteria in gases is given as

d

VE also , and

plet ,1)1( 21

p

Ef

p

Efe d

Then: 11 1

2

pd

Vpdf

epd

Vf

Where: f1 and f2 are natural function

Let V= f(pd), it is known as Paschen’s law and important for high voltage engineering.



Topic: Paschen’s law

Gas Vs(min) pd at Vs min

(Volt) (torr-cm)

Air 327 0,567

Argon 137 0,900

H2 273 1,150

Helium 156 4,000

CO2 420 0,510

N2 251 0,670

N2O 418 0,500

O2 450 0,700

SO2 457 0,330

H2S 414 0,600

Minimum sparking potential for various gases



Topic: Example

1. In an experiment in a certain gas it was found that the steady state current is 5,5x10-8 A at 8 kV at a distance of 0,4 cm between the plan electrodes. Keeping the

field constant and reducing the distance to 0,1 cm results in a current of 5,5x10-9 A. Calculate Townsend’s primary ionization coefficient alpha?

Solution: The current at the anode I is given by I=I0 exp(ad) For two conditions: we get I1/I2=exp a(d1-d2) Substitute, 10=exp a(0,3) or 0,3 a=ln(10) So a=7,676/cm.torr

2. What will the breakdown strength of air be for small gaps (1mm) and large gaps (20 mm) under uniform field conditions and standard atmospheric conditions?

Solution: approximately standard atmospheric condition: E=24,22+6,08/d0,5 kV/cm. For 1 mm gap : E=43,45 kV/cm For 20 mm gap : E=25,58 kV/cm


Minggu-11 : Conduction and breakdown in gases (1)

Topic: Practical considerations in using gases for insulation purposes

The properties of a gaseous dielectric for high voltage applications:1. High dielectric strength2. Thermal stability and chemical inactivity towards materials of construction 3. Non-flammability (bakar) and physiological intertness (bersifat menghilangkan)4. Low temperature of condensation

Home work:1. Explain properties of the gas when in the gas media is applied low and high

voltage.2. Why does Townsend’s theory fail to explain about gas phenomena.3. What are advantages of the streamer theory in the gas insulation.4. What is that mean with collision.5. Why do the electrons move fast to compare the ions.6. How will the gap distance of air be for breakdown strength (700 kV) under

uniform field conditions and standard atmospheric conditions? Note: Approximately standard atmospheric condition: E=24,22+6,08/d0,5 kV/cm


Thank’s a milion


Waktu : 2x50 menit (2 sks)Isi: - Cairan sebagai isolator. - Oli trafo. - Sipat-sipat listrik. -Test tembus. - Teori tentang dielectric cairan. - Interaksi : Tanya-jawab, tugas

Minggu-11: Conduction and breakdown in liquid (2)


Liquid dielectric strangth is more than gas about 10 time. In practice it commonly use as insulating materials than either salids or gases.

Example: used mainly as high voltage cable, capacitor, filling up transformers and circuit breaker, atc.

Composition of liquid dielectric: Mixtures of: Hydrocarbons and weakly polarisedFree from moisture, products of oxidation and other contaminantsAffect the electrical strangth of oil is the presence of water, even 0,01% water in oil tranformer reduce its electrical strangth to 20%.

Week-12 : Conduction and breakdown in liquid (2)

Topic: Liquids as insulators


Composition of liquid dielectric: Mixtures of: Hydrocarbons which include paraffins, iso paraffins, naphthalenes and aromatics.

Become darker due to the formation of acids and resins or sludgeSome of the acids are corrosive to solid insulating material and metal parts.Deposit of slugde on the transformer core, the coil and inside the oil ducts reduce circulation of oil


Topic: Transformer oil


Essential in determining the electric performance of liquidits capacitance per unit volume or its relative permittivetyIts resistivityIts loss tangen or its power factor which is an indication of the power

loss under ac voltage applicationIts ability to withstand high electric stresses.


Topic: Electrical properties

relative permittivity (50 Hz) :Transformer oil : 2,2 -- 2,3Cable oil : 2,3 -- 2,6Capacitor oil : 2,1Askerels : 4,8Silicone oil : 2 -- 73

In case of non-polar liquids, permittivity is independent of frequency.But for polar liquids, such as water, it changes with frequency. Example permittivity of water is 78 at 50 Hz and reduces to about 5 at 1 MHz.

Resistivities: for application in high voltage should be more than 1016 ohm- metre. Dr. Ir. Hermagasantos ZeinDr. Ir. Hermagasantos Zein


Topic: Electrical properties

Power factor : Under ac voltage will determine its performance under load conditions. It is a measure of the power loss and an important parameter in cable and capacitor systems. In transformer, the dielectric loss in the oil is negligible when compared to copper and iron losses.

Dielectric strength: is the most important parameter in the choice of a given liquid dielectric for a given application. It depends on the atomic and molecular properties. However, under practical conditions of dielectric strength depends on the material of electrodes, temperature, type of applied voltage, gas content in liquid, atc. Which change the dielectric strength by changing the molecular properties of the liquid.



Topic: Breakdown tests

Breakdown test use electrodes spheres of 0,5 to 1,0 cm in diameter with gap spacing of about 100-200 micro-metre. The test voltages required for these tests are usually low, of the order of 50-100 kV. DC conductivities obtained in pure liquids are very high of 10-18 – 10-20 mho/cm.

LiquidMaximum breakdown

strength (MV/cm)

Hexane 1,1 - 3

Benzene 1,1

Transformer oil 1,0

Silicone 1,0 - 1,2

Liquid oxygen 2,4

Liquid nitrogen 1,6 - 1,9

Liquid hydrogen 1,0

Liquid helium 0,7

Liquid argon 1,10 - 1,42



Topic: Theory about dielectric liquidIt is to explain the breakdown in the liquid, proposed any theory:

Suspended partical theory: In commercial liquids, the presence of solid impurities can not be avoided, like fibres or dispersed solid particles. Permittivity of these particles will be different from the permittivity of the liquid.

Let the particles is spherical with radius r, in the field is E, a force F.

2

21

123

grad22

1E

rF

Note: Index 1: liquid and 2: particle

Solid particles: permittivity great than liquidGas bubbles : permittivity less than liquid



Topic: Theory about dielectric liquid

Calvitation and the bubble theory: the breakdown field is given as

Note: Index 1: liquid and 2: bubble tho is the surface tension of liquid Vb is the voltage drop in bubble

Where: R = initial radius of the bubbles and betha is ratio of the longer to the shorter diameter of spherical, is about 1,85.

2

1

0

21

210 1

24

)2(21

rE

V

rE b

RTpv law gas ,60021

1

1

HG

RE c

The critical field is

11

cosh

1

12/12

1

2

G

22 1

123

2

H




Thermal mechanism theory: to explain breakdown under pulse condions is thermal breakdown. This mechanism is based on extremely large current just before breakdown.

Stressed oil volume theory: the Impurity of liquid will determine breakdown strength. The breakdown voltage is highly influenced by the gas content in the oil, the viscosity of the oil, and the presence of other impurities. The stressed oil volume is taken as the volume which is contained between the maximum stress (Emax) contour and 0,9 Emax contour. By this theory the breakdown strength is inversely proportional to the stressed oil volume. Increase in the stressed oil volume consequently results in a reduction in the breakdown voltage.

The high current belived projections on cathode surface with densities of order of 1 A/cm cubit. These high density current pulse give rise to localised heating of the oil which may lead to the formation of vapour bubble.




Questions:1. Explain the phenomena of electrical conduction in liquid. How does it differ

from that gas?2. What are commercial liquid dielectrics? How are they different from pure

liquid dielectrics?3. What are the factors that imfluence conduction in pure liquid dielectrics and

in the commercial liquid dielectrics?4. Explain the various theories that explain breakdown in commercial liquid

dielectrics.5. What is the stressed oil volume theory and how does it explain brakdown in

large volume of commercial liquid dielectrics?



Thank’s a milion


Waktu : 2x50 menit (2 sks)Isi: - Lingkup - Intrinsic or ionic nreakdown - Electromehanical breakdown - Failure due to treeing and tracking - Thermal breakdown - Electrochemical brealdown - Breakdown due to internal discharges - Interaksi : Tanya-jawab, tugas

Minggu-13: breakdown in solid dielectrics (3)


Introduction1. Solid dielectric materials are used in all kinds of electric circuits ad divices to insulate

one current carrying part fram another when they operate at different voltage.2. Solid dielectrics have higher breakdown srength compared to liquids and gases.3. Breakdown in solid dielectrics are of extreme importance in isullation studies. 4. The breakdown mechanism of solid is a complex phenomena, depand on the time of

voltage application. Shown in the ficture



Breakdown strength

Log Time

Intrinsic, electro-mechanical

streamer

thermal

Erosion and electrochemical

The various breakdown mechanism can be classified as1. Intrisic or ionic breakdown2. Electromechanical breakdown3. Failure due to treeing and tracking4. Thermal breakdown’electrochemical breakdown5. Breakdown due to internal discharges



Intrisic or ionic breakdown• Difinition: The intrisic electric strength is when voltage applied only short

durations of the order of 10-8s, the dielectric strength of a solid dieletric increases rapidly to an upper limit.

• The hghest dieletric strength depands only on the structure of the material and the temperature.

• The maximum electric strength is 15MV/cm for polyvinyl-alcohol at -1960C. And it is rangs from 5 MV/cm to 10 MV/cm.

• Intrisic breakdown depands upon the presence of free electrons which are capable of migration through the lattice of the dielectric. Usually, a small number of conduction electrons are present in solid dielectrics, structural imperfections and small amounts of impurities.

• Trapped electrons are released, and these electrons participate in the conduction procss. Base on this principle, two types of intrinsic breakdown mechanisms: electronic and avalanche/sreamer breakdown.



Electronic breakdown• Free electrons are also assumed to be large, and electrons collisions occur• When an electric field is applied, electrons gain energy from electric field

and cross the forbidden energy gap fram the valency to the conduction band.

• When this process is repeated, more and more electrons become available in the conduction band, eventually leading to breakdown.

Avalanche / steamer breakdown• This is similar to breakdown in gases due to cumulative ionization.• Conduction electrons gain sufficient energy above a certain critical electric

field and cause liberation of electrons from the lattice atoms by collisions. • An electron starts from the cathode will drift towards the anode and during

this motion gains energy from field and loses it during collisions.• In practice, breakdown does not occur by the formation of a single

avalanche itself, but occurs as a result of many avalanches formed within the electric and extending step by step through the entire of the material.



Electromechanical brakdown



conductioni in media

Documents

standard atmospheric

uniform field

stressed oil

electron emission

commercial

pure liquid

hermagasantos

secondary