UNIT 2

BREAKDOWN MECHANISM IN GASES, SOLIDS AND LIQUIDS

PART A

1. State and explain paschen’s law (or) Write paschen’s law

Paschens law explains the relationship between the breakdown voltage and the product of pressure (p) and gap (d), in the case of Breakdown in gas.

It states that,  V = f (p.d)

The Breakdown voltage is a function of p.d.

2. What is meant by corona discharges?

If the electric field is uniform, a gradual increase in voltage across a gap produces a breakdown of the gap in the form of a spark without any preliminary discharges. On the other hand, if field is non-uniform an increase in voltage will first cause a discharge in the gas to appears at points with highest electric field. This form of discharges is called a CORONA DISCHARGES.

3. What are electronegative gases?

Electronegative gases are gases that have affinity towards electrons. When electron comes into contact with these gas molecules, gas molecule attracts the electrons and becomes negative ion.

Examples: Carbon dioxide, SF₆

4. Define uniform and non-uniform fields and give example for each.

Uniform fields – If the field is uniform, voltage across the gap increases and ionization process take place and breakdown of gap in the form of a spark occur.

Example: Sphere plane gap at different gap spacing

Non-uniform fields – If the field is non-uniform, voltage across the gap increases and causes a discharge in the gap at sharp points where the electrode is curved is known as corona discharges.

Example: SF₆ and N₂, rod-rod gap in air

5. What is Town-sends condition for Breakdown?

                        Town-sends criterion for Breakdown  g (exp (ad) -1) = 1

Since e ad >>>1

The criterion becomes g e ad = 1

Where g = Town-sends secondary ionization coefficient a = Town-sends primary ionization coefficient d = gap

6. Write the criterion for breakdown in non-uniform fields.

The criterion for breakdown in non-uniform fields

7. List out the various mechanism of breakdown in solid dielectrics?

The breakdown mechanism in solid dielectrics are classified as

• Intrinsic or ionic breakdown
• Electromechanical breakdown
• Thermal breakdown
• Electrochemical breakdown
• Failure due to treeing and tracking
• Breakdown due to internal discharges

8. What are the properties of solid dielectrics?

• Low dielectric loss
• Free from gaseous particles and moisture
• High mechanical strength
• Resistant to thermal and chemical deterioration
• High breakdown strength than liquid and solid breakdown strength

9. Define intrinsic strength (or) What do you mean by ‘intrinsic strength’ of a solid dielectrics?

When voltages are applied only for short durations of the order of 10^-8 s, the dielectric strength of a solid dielectric increases very rapidly to an upper limit called the intrinsic electric strength.

10. Define treeing and tracking.

Treeing – The spreading of spark channels during tracking, in the form of the branches of a tree is called treeing.

Tracking – Tracking is the formation of a continuous conducting paths across the surface of the insulation mainly due to surface erosion under voltage application.

11. What do you mean thermal breakdown in solid dielectrics?

When a field is applied to a solid dielectric at room temperature, a small conduction current flows and its value increases and heats up the dielectric and the lattice temperature increases. The heat generated by the current is conducted to the surrounding and is absorbed to increase lattice temperature

Condition for instability or thermal breakdown

Heat generation > Heat conduction

12. What are the two main reasons for long term breakdown in composite dielectrics?

The main main two resons for long term breakdown in composite dielectrics are

• Partial discharges normally occur within the volume of the composite insulation system.
• The charge accumulation and conduction on the surface of the insulation also contributes significantly towards the ageing and failure of insulation.

13. What are the different theories related with liquid dielectric breakdown?

Breakdown mechanism in commercial liquids may classified as

5. Suspended particle mechanism
5. Cavitation and bubble mechanism
5. Stressed oil volume mechanism

14. What are the electrical properties of liquid dielectrics?

• Capacitance per unit or its relative permittivity
• Resistivity
• Loss tangent (tan delta)
• Its ability to withstand high electric stresses

15. What are the different mechanism in vacuum breakdown?

5. Particle exchange mechanism
5. Field emission theory

• Anode heating mechanism
• Cathode heating mechanism

5. Clump theory

16. What is short-term breakdown?

When the applied electric field is high, failure may occur in second or even faster without damaging the insulating surface prior to breakdown is called as short-term breakdown.

17. What is composite dielectrics?

Different dielectric materials can be in parallel with each other (air or SF₆ gas in parallel with solid insulation) or in series with one another. Such insulating systems are called as composite dielectrics.

18. What is photo-ionization?

It occurs when the amount of radiation energy absorbed by an atom or molecule exceeds its ionization potential.

19. What is secondary ionization process?

Secondary ionization processes 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.

20. Define primary ionization co-efficient.(Town-sends first  ionization co-efficient)

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

21.  What is Town-sends secondary ionization co-efficient:

It is the net number of secondary ions produced per incident positive ion (g1) or

photon (g 2) or metastable particle (g 3)

g = (g 1 + g 2+ g 3)                    g = f (E/P)

PART-B

1. State the criteria for sparking potential and hence obtain the relation between sparking and (pd) values (paschen’s law). Discuss on the nature of variation of sparking potential with (pd) values. (16)
2. Explain in detail the breakdown mechanism in non-uniform fields and phenomenon of corona.(or) Explain the phenomenon of corona discharges and breakdown in non-uniform fields (16)
3. Explain briefly various theories of breakdown in liquid dielectrics. (16)
4. Explain the breakdown mechanism involving in solid dielectrics breakdown. (16)

Hint: Cutshort points each for mechanism

Describe thermal breakdown mechanism of solid dielectrics.(8)

5. Explain composite dielectrics and how the breakdown occurs in it? (16)

Describe the ageing and breakdown in composite dielectrics due to partial discharges. (8)

6. Explain clearly various process which explain electric breakdown in vacuum.(8)
7. Deduce the expression for Townsend’s current growth equation.(16)
8. Explain the streamer theory of breakdown in gases. (8)
9. List out the problems caused by corona discharges.(8)
10. Explain the townsends criterion for a spark.
11. Discuss about the properties of composite dielectrics.(6)
12. Discuss the classification and characteristics of liquid dielectrics.(8)
13. Explain the Time lags for breakdown in electronegative gases.(8)