شرح الإنهيار فى العوازل الصلبة و السائلة

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CONDUCTION AND BREAKDOWN IN LIQUID

DIELECTRICS

INTRODUCTION

• Liquid dielectrics are used mainly as impregnants in hv cables and

capacitor, and for filling up of transformers, circuit breakers etc.

• Act as heat transfer agents (in transformer), and as arc quenching media

(in circuit breakers).

• The most important factor that affects the electrical strength of an

insulating oil, is the presence of water in the form of fine droplets

suspended in the oil. Dielectric strength of oil reduces more sharply if it

contains fibrous impurities in addition to water.

• In practice, the choice of a liquid dielectric is made mainly on the basis of

its chemical stability.

• Clasification of liquid dielectrics: Mineral oil, synthetic hydrocarbons,

chlorinated hyfdrocarbons, silicone oils and esters

CONDUCTION AND BREAKDOWN IN PURE LIQUIDS

• At very low fields, the current is due to the dissociation of ions. With

intermediate fields, the current reaches a saturation value, and at high

fields the current generated because of the field-aided electron emission

from the cathode gets multiplied in the liquid medium by a Townsend

mechanisms.

• The current multiplication also occurs from the electrons generated at the

interfaces of liquid and impurities.

• The breakdown voltage depends on the field, gap separation, cathode

work-function and the temperature of the cathode. In addition, the liquid

viscosity, liquid temperature, the density and the molecular structure of

liquid also influence the breakdown strength of liquid.

• The type of breakdown process in pure liquids is called the electronic

breakdown, involves emission of electrons at fields greater than 100

kV/cm.

[In general the breakdown mechanisms are[/size
classified as follows

a ) Suspended Particle Mechanism

b) Cavitation and Bubble Mechanism

c) Thermal Mechanism

a) Suspended Particle Mechanism

• The impurities will be present as fibres or as dispersed solid particles.

• If the voltage is continously applied (d.c.) or the duration of the voltage is

long (a.c.), then this force drives the particles towards the areas of

maximum stress. If the number of particles present are large, they

becomes aligned due to these forces, and thus form a stable chain

bridging the electrode gap causing a breakdown between the electrodes.

• If there is only a single conducting particle between the electrodes, it will

give rise to local field enhancement depending on its shape. If this field

exceeds the breakdown strength of the liquid, local breakdown will occur

near the particle, and this will result in the formation of gas bubbles,

which may lead to the breakdown.

• The larger the size of the particles, the lower were the breakdown

strengths.

b) Cavitation and Bubble Theory

• The following processes have been responsible for the formation of the

vapour bubbles.

i) gas pockets at the surface of the electrodes.

ii) electrostatic repulsive forces between space charges which may be

sufficient to overcome the surface tension.

36

iii) gaseous products due to the dissociation of liquid molecules by

electron collisions.

iv) vapourization of the liquid by corona type discharge.

• The bubble will elongate in the direction of the electric field under the

influence of electrostatic forces.

• This theory does not take into account the production of the initial bubble

and hence the results given by this theory do not agree well with the

experimental result.

c) Thermal Mechanism

• Based on the experimental observations of extremely large currents just

before breakdown. The high current pulses originate from the tips of the

microscopic projections on the cathode surface with densities of the order

of 1 A/cm3. This high density current pulses give rise to localised heating

of the oil which may lead to the formation of vapour bubbles.

• When a bubble is formed, breakdown follows, either because of its

elongation to a critical size or when it completely bridges the gap

between the electrodes.

• The breakdown strength depends on the pressure and the molecular

structure of the liquid.

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BREAKDOWN IN SOLID DIELECTRIC

INTRODUCTION

• Good dielectric - low dielectric loss, high mechanical strength, free from

gaseous inclusions, and moisture, be resistant to thermal and chemical

deterioration.

• Solid dielectrics have higher breakdown strength compared to liquids and

gases.

• When breakdown occurs, solids get permanently damaged while gases

fully and liquids partly recover their dielectric strength.

• Breakdown mechanism varies depending on the time of application of

voltage and can be classified as follows;

a) Intrinsic or ionic breakdown

b) Electromechanical breakdown

c) Thermal breakdown

d) Electrochemical breakdown

e) Treeing and tracking

f) Internal discharges

1 INTRINSIC BREAKDOWN

• When voltage are applied only for short durations of the order 10-8 s the

dielectric strength of a solid dielectric increases very rapidly to an upper

limit called the intrinsic electric strength.

• Maximum strength usually obtainable ranges from 5 - 10 MV/cm.

• Intrinsic breakdown depends upon the presence of free electron which

capable of migration thru the lattice of the dielectric. Usually small

number of conduction electrons are present, with some structural

imperfections and small amounts of impurities. The impurity atoms or

molecules act as traps for the conduction electrons up to certain ranges of

electric fields and temperatures. When these ranges are exceeded,

additional electrons and trapped are released and participate in the

conduction process.

• Two types of intrinsic breakdown mechanisms;

a- Electronic breakdown

• Assumed to be electronic in nature (occurs in time 10-8 s)

• Initial density of conduction (free) electrons assumed to be large and

electron-electron collisions occurs.

• When electric field is applied, electrons gain energy and cross the

forbidden gap from the valency to the conduction band. This process

repeated, more and more electrons available in conduction band,

eventually leading to breakdown.

b- Avalanche or streamer breakdown

• Similar to breakdown in gases due to cummulative ionization.

• Conduction electrons gain sufficient energy above a certain critical

electric field and cause liberation of electrons from the lattice atom by

collisions.

• Motion of electron from cathode to anode will gain energy from the field

and losses it during collisions. When the energy gained by an electron

exceeds the lattice ionization potential, an additional electron will be

liberated due to collision of the first electron. This process repeats itself

resulting in the formation of an electron avalanche, and breakdown will

occur when the avalanche exceeds a certain critical size.

• In practice, breakdown does not occur by the formation of a single

avalanche, but occurs as a result of many avalanches formed and

extending step by step through the entire thickness of the material

2 ELECROMECHANICAL BREAKDOWN

• Failure occurs due to electrostatic compressive forces, which can exceed

the mechanical compressive strength.

3 THERMAL BREAKDOWN

• When an electric field is applied to a dielectric, conduction current flows

thru the material. Current heats up the specimen and the temperature is

rise.

• Heat generated is transfered to the surrounding medium by conduction

and radiation.

• Thermal breakdown sets-up an upper limit for increasing the breakdown

voltage when the thickness of insulation is increased.

• Heat generated is proportional to the frequency and hence thermal

breakdown is more serious at high frequency.

• Thermal breakdown stresses (MV/cm) are lower under a.c. condition then

under d.c.