EE1402(unit II)

UNIT-II

1. What are the advantages of HVDC over HVAC?
HVDC transmission has the following advantages over HVAC
• Skin effect is absent in HVDC
• Corona is absent
• Power loss is minimum
• Number of conductors required for transmission is less in HVDC than HVAC
• Tie between two stations is very difficult in HVAC .But in HVDC it is very easy
2. State few high voltage applications.
• Electric Traction
• Particle accelerator
• Biomedical applications
• Lightning arrestors

3. Define dielectric strength. What are the types of Dielectrics? State few examples.
Dielectric strength is defined as the maximum applied voltage that a dielectric can withstand
Without causing breakdown. Types of Dielectrics and examples
• Solid dielectric - Mica ,ceramic,Plastic,PVC
• Liquid dielectric - transformer oil ,Askarels
• Gaseous dielectric - Air, oxygen,SF6,Freon

4. What are electronegative gases? Give some examples.
Electronegative gases are the gases that have affinity towards electrons.When electron comes into contact with these gas molecules, the gas molecule attracts the electrons and becomes negative ion
Examples:
• Carbondioxide
• Oxygen
• SF6

5. Define Paschen’s Law for breakdown?
The breakdown voltage of a uniform field gap is a unique function of the product of p, the gas
Pressure and d, the electrode gap, for a particular gas or for a given electrode material.
V= f(pd)

6. What are the discharges that occur after breakdown of gaseous dielectric?
The discharges that occur after breakdown of gaseous dielectric are
• Glow discharge
• Arc discharge

7. Why SF6 is most widely used as gaseous dielectric?
SF6 is most widely used as gaseous dielectric because it has
• High dielectric strength
• Low liquification temperature
• Good arc quenching property
• Good heat transfer
8. What is corona? What are the effects of corona?
Corona is a phenomena associated with non uniform field. When ac voltage is applied between the conductors, under normal voltage the air between the conductor remains as such and there will be no change in it. But when the voltage is increased beyond certain voltage called critical disruptive voltage, then the air gets ionized and blue violet luminent glow appears all along the conductor called corona.
Effects of corona:
• Hissing noise
• Air surrounding the conductor gets ionized resulting in the formation of ozone
• Radio interference voltage affects the communication lines
• Deteriation of insulating property of conductors

9. What are the practical considerations for selecting gases as dielectrics?
The practical considerations for selecting gases as dielectrics are
• High dielectric strength
• Low liquification temperature
• Good thermal stability
• Good heat transfer
• Available at moderate cost
• Chemical inactivity towards the material of construction
• Physiological inertness of gas

10. Define Townsend’s Primary ionization coefficient.
Townsends Primary ionization coefficient α is defined as the number of ionizing collision
made by the free electron per centimeter travel in the direction of applied electric field.


11. What are the different time lags that occur during breakdown?
Time difference between the application of sufficient voltage to cause breakdown and the real occurrence of breakdown itself is called time lags.. The different time lags that occur during breakdown are
• Statistical Timelag
• Formative Timelag
12. What are electronegative gases? Give some examples.
Electronegative gases are the gases that have affinity towards electrons.When electron comes into contact with these gas molecules, the gas molecule attracts the electrons and becomes negative ion
Examples:
• Carbondioxide
• Oxygen
• SF6

13. Define Paschen’s Law for breakdown?
The breakdown voltage of a uniform field gap is a unique function of the product of p, the gas
Pressure and d, the electrode gap, for a particular gas or for a given electrode material.
V= f(pd)

14. What are the discharges that occur after breakdown of gaseous dielectric?
The discharges that occur after breakdown of gaseous dielectric are
• Glow discharge
• Arc discharge

15. Why SF6 is most widely used as gaseous dielectric.
SF6 is most widely used as gaseous dielectric because it has
• High dielectric strength
• Low liquification temperature
• Good arc quenching property
• Good heat transfer

16. Define an attachment coefficient.
An attachment co-efficient is defined as the number of attaching collusions made by one electron drifting one centimeter in the direction of the field.

17. What is a statistical time lag of the gap?
The time which lapses between the application of the voltage sufficient to cause breakdown and the appearance of the initiating electron is called a statistical time lag of the gap.

18. Mention the different mechanisms for the breakdown in vacuum.
• Partial exchange mechanism,
• Field emission mechanism and
• Clump theory.

19. Write the two mechanisms used in field emission theory?
• Anode heating
• Cathode heating mechanisms.

20. What are the classifications of vacuum?
• High vacuum
• Very high vacuum and
• Ultra high vacuum

21. What are liquid dielectrics?
Liquid dielectrics are normally mixtures of hydrocarbons and are weakly polarized.

22. Mention some of the applications of liquid dielectrics.
They are used as impregnants in high voltage cables and capacitors, and for filling up of transformers, circuit breakers .They are also used as heat transfer agents in transformers and as arc quenching media in circuit breakers.

23. Name some examples of liquid dielectrics.
Petroleum oils, Synthetic hydrocarbons, halogenated hydrocarbons, silicone oils and fluorinated hydrocarbons.

24. What are pure liquid ? Give examples.
They are chemically pure and do not contain any other impurity even in traces of 1 in 109 and are structurally simple. Examples are n-hexane, n-heptanes and other paraffin hydrocarbons.

25. Mention the impurities added in liquid dielectrics.
Dust, moisture, dissolved gases and ionic impurities.

26. What are the different types of solid insulating materials?
Organic materials : paper, wood and rubber
Inorganic materials: Mica, glass and porcelain
Synthetic polymers : Perspex lex, pvc, epoxy resins

27. State the properties of good dielectrics
Low dielectric loss, high mechanical strength, should be free from gaseous inclusions and moisture and be resistant to thermal and chemical deterioration.

28. Name the two types of intrinsic breakdown mechanisms.
• Electronic breakdown .
• Avalanche or streamer breakdown