Unit 02

Dielectric breakdown

Q1) What is primary ionization? Define primary ionization co-efficient (Town-sends 1st ionization co-efficient)?

A1) Electron produced at the cathode by some external means, during its travel towards the anode due to the field applied, make collisions with neutral atoms/molecules and liberate electrons & positive ions. The liberated ions make future collisions and the process continue. The electrons and the ions constitute current. This process is called primary ionization.

The average number of ionizing collisions made by an electron per centimeter travel of the electron in the direction of the field is called Town-sends Ist ionization co-efficient. It depends on the gas pressure and E/P

Q2) What is secondary ionization also define secondary ionization coefficient?

A2) The librated positive ions, during the primary ionization process migrate towards cathode bombard and emit secondary electrons from the cathode.

The excited atoms/molecules, got excited during the collision of initial electrons, emit photons which bombard the cathode & emit secondary electrons

Metastable (excited particles) bombard the cathode metal surface & produce secondary electrons. The secondary electrons released as above make ionization collisions & produce additional electrons. The electrons again produce ionization collisions & the process repeats. This is called & secondary ionization. The discharge is self sustained because once the secondary electrons are formed. They take care of the situation. Where there are initial electrons at cathode or not ionization proceeds. That is called self sustained discharge. Secondary ionization coefficient (Town send second ionization coefficient – It is the net number of secondary ions produced per incident positive ion or photon or metastable particle.

Q3) Write down demerits of Town-sends theory.

A3) 1. Beyond a p.d > 1000 torr cm, this theory does net explain correctly.

2. Town sends theory says that current growth depends on ionization. But actually it depends on gas pressure and geometry of gap.

3. Town sends mechanism predicts time lag of 10-5 sec. But actually the time lag is 10-8 sec.

4. The discharge form is not as the one predicted by Town-sends theory. It is filamentary & irregular and not “diffused form” as predicted by town-sends.

Q4)( i)What are the characteristics of corona discharge?

(ii) What is corona inception field?

A4) (i) Characteristics-

It has bluish luminescence.

It produces hissing noise.

Air surrounding the corona becomes converted to ozone.

Creates loss of Power.

Create radio interference.

It causes deterioration of the insulation surface.

(ii)  Corona Inception field: The voltage gradient required to produce visual ac corona in air at a conductor surface is called corona inception field.

Q5) (i) Name a few liquid dielectrics.

(ii) List qualities of good dielectrics (liquid).

A5) (i) Following are liquid dielectrics-

Transformer oil

Synthetic hydro carbons – (Polyolefin’s)

Chlorinated hydro carbons: P.C.B. (Toxic)

Silicone oils. Alternative to PCB

Esters  1) Natural Esters : Castor oil    2) Organic Ester & Phosphate esters (synthetic Esters)

Hydrocarbons tetrachloro ethylene & per fluro poly ether

(ii) Qualities of good liquid  dielectrics-

High heat transfer capacity

Good dielectric strength

Good chemical satiety

Q6)  Distinguish between the BD in pure liquid & commercial liquid.

A6) Pure liquid, Theoretically only possible (hypothetical). BD is due to Electronic breakdown involving commission of electrons at fields greater than 100 KV/cm. Town-sends type of primary ionization & secretary ionization can be applicable. Commercial liquid contains suspended particles, bubbles of air or liquid etc. BD Mechanisms are influenced by these impurities. BD depends on several factors-

Nature & condition of electrodes.

The impurities present in the liquid. 

Physical properties of liquid.

Condition of electrodes.

Q7) List down Factors affecting –

A7) (i) breakdown strength of liquid dielectrics.

(ii) breakdown voltage of pure liquids.

(i) Following are factors affecting breakdown strength of liquid dielectrics-

Suspended particles.

Dissolved impurities. 

Physical properties like pressure, temperature.

Nature & conditions of electrodes.

(ii)  Factors affecting breakdown voltage of pure liquids are field applied, Gap separation, Density   ,Viscosity ,Cathode work function , Temperature ,Temperature of liquid, Molecular structure.

Q8) (i)What are the characteristics of a good solid dielectric?

(ii) How can solid dielectrics be classified?

A8) (i) Following are the characteristics of good solid dielectrics-

Low dielectric lo

High mechanical strength

Free from gaseous inclusions

Free from moisture

Resistance to thermal & chemical degradation

High BD strength.

(ii) classification-

Organic dielectric e.g. Paper, wood, rubber

Inorganic dielectric e.g. Mica, glass, porcelain, p v c, epoxy resins, perspex.

Q9) What is meant by Intrinsic strength of a solid dielectric ?

A9) All extraneous influences have to be isolated and the BD value which depends on the

Structure of the materials and the temperature is called intrinsic BD strength of solid dielectric.

Eg. Poly vinyl Alcohol

At – 1960c : 15 MV/cm (Intrinsic)

& at Normal. Temp : 5 MV/ cm to 10 MV/cm

Q10) What is ‘TRACKING’ and ‘TREEING’ is solid dielectric B.D ?

A10) TRACKING: Formation of a continuous conduction path across the surface of the insulation mainly due to surface erosion under voltage application is called ‘Tracking’. Water -Conduction path-heat -Drying - Conduction film -Carbonization -B.D TREEING: The spreading of spark channels during tracking in the form of the branches of tree is called Treeing.