High Voltage Engineering Important Questions and Answers

1. Define surge impedance of transmission line.

Answer:

Zn = √L/C

The ratio of inductance and capacitance of transmission lines is called as natural impedance or surge impedance of transmission line.


2. What is a composite dielectric?

Answer:

Different dielectric materials can be in parallel with each other or in series with one another. Such insulation systems are called as composite dielectrics.


3. Define Paschen’s law.

Asnwer:

V= f(P, d)

The breakdown voltage of a uniform field gap is a unique function of the product of gas pressure (p) and gap length (d) for particular gas and electrode material. This relation is known as Paschen’s law.


4. What is Tesla coil?

Answer:

Tesla coil is the commonly used high frequency resonant transformer, which is also the doubly tuned resonant circuit.


5. How is the circuit inductance controlled and minimized in impulse current generators?

Answer:

In order to minimize the effective inductance the capacitors are subdivided into smaller units. Also the arrangement of capacitors into a horse-shoe shaped layout minimizes the effective load inductance.


6. Define the front and tail times of an impulse wave.

Answer:

It is the time required for the response to raise from 10% to 90% or 0 to 100% of the final value is called front time. It is time required for the impulse wave to reach 50% of its peak value from peak point is called as tail time.


7. Why are capacitance voltage dividers preferred for high a.c. voltage measurements?

Answer:

  1. Capacitance ratio is independent of frequency.
  2. Capacitance voltage dividers are connected to the source through long leads which gives lead inductances and residual resistances.

8. Classify the types of tests to be conducted for the apparatus.

Answer:

  1. Power frequency test
  2. Impulse test

9. Define safety margin as applied to insulation co-ordination.

Answer:

The safety margin is defined as the difference between probability density function for failure (Pg(Vi)) and overvoltage probability density function (Po(Vi)).


10. What are the causes of over voltage in power system?

Answer:

The causes of over voltage in a power system are

  1. Lighting (External cause).
  2. Switching surges (Internal cause).

11. What is ionization by collision?

Answer:

When an electric field is applied between anode and cathode, any electron from cathode collide with a neutral molecule and produces a +ve ion and a new electron. This additional electrons again collide with a neutral molecule and this process repeats and ionizing collision takes place


12. What is “DELTATRON” circuit?

Asnwer:

Deltatron circuit is a combination of cockcroft-walton type voltage multiplier and a cascaded transformer with a dc rectifier and a cascaded transformer with a dc rectifier for producing very high DC voltage. This circuit produces HUDC with limited output current having high stability, small ripple factor and faster regulation.


13. What are the advantages of CVT?

Answer:

  1. CVT can be used as a voltage measuring device for meters.
  2. It can be used in power line carrier communication.
  3. It provide isolation between HV terminal and low voltage metering.
  4. Design in simple.
  5. Installation is easy.

14. What are merits of series resistance micro ammeter?

Answer:

  1. It provide crona free termination.
  2. It maintains very good temperature stability

15. Define 50% flash over voltage.

Answer:

The voltage which has a probability of 50% flashover, when applied to a test object. This is normally applied in impulse test in which the less of insulation strength is temporary.


16. What are the tests conducted on power transformers?

Answer:

  1. Induced over voltage test.
  2. Partial Discharge test.
  3. Impulse test.

17. What is known as Isokeraunic level?

Answer:

“Isokeraunic level” defined as the number of days in a year when thunder is heard or recorded in a particular location.


18. What do you mean by Intrinsic strength of a solid dielectric?

Answer:

When the voltages are applied only for short duration of the order of 10-8 S the dielectric strength of solid dielectric increases rapidly to an upper limit called the intrinsic electric strength.


19. What are the drawbacks of single stage circuit for the generation of very high impulse voltage?

Answer:

A single capacitor Ci may be used fir voltages up to 200kV. Beyond this voltage, a single capacitor and its charging unit may be too costly, and the size becomes very large. The cost and size of the impulse generator increases at a rate of the square or cube of the voltage rating. Hence, for producing very high voltages, a bank of capacitors are charged in parallel and then discharged in series. The arrangement fir charging the capacitors in parallel and then connecting them in series for discharging was originally proposed by Marx.


20. What re the significances of power factor tests?

Answer:

The dielectric power factor test is done using the high voltage Schcring bridge. The power factor or dissipation factor tan 5 is measured at 0.5, 1.0, 1.66 and 2.0 times the rated voltage (phase to ground) of the cable. The maximum value of the power factor and the difference in power factor between the rated voltage and 1.66 time the rated voltage, as well as between the rated voltage and two times the rated voltage are specified.


21. What is counter poise wire? Give its use.

Answer:

Counterpoise wires are wires buried in the ground at a depth of 0.5 to 1.0 m, running parallel to the transmission line conductors and connected to the tower legs. These wires may be 50 to 100 m long. These are found to be more effective than driven roads and the surge impedance of the tower may be reduced to as low as 25. The depth does not materially affect the resistance of the counter-poise, and it is only necessary to bury it to a depth enough to prevent theft. It is desirable to use a large number of parallel wires than a single wire. But it is difficult to lay counter-poise wires compared to ground or driven rods.


22. What are the two main reasons for long term break down in composite dielectrics?

Answer:

Long-term breakdown is also called the ageing of insulation. The principal effects responsible for the ageing of the insulation which eventually leads to breakdown arise from the thermal processes and partial discharges.

  • Ageing and breakdown due to partial discharges
  • Ageing and breakdown due to accumulation of charges on insulator surfaces

23. What are the advantages of cascaded transformer method?

Answer:

The insulation design of cascaded high voltage testing transformer is simple and less as compared to the same rated single transformer.


24. What are the limitations of series resistance micro ammeter method?

Answer:

The limitations in the series resistance micro ammeter design are:

  1. power dissipation and source loading,
  2. temperature effects and long time stabilit,
  3. voltage dependence of resistive elements, and
  4. sensitivity to mechanical stresses.

25. What are the tests conducted on surge arrester?

Answer:

  1. Various tests conducted on surge diverter are
  2. Power Frequency Sparkover Test,
  3. Hundred percent Standard Impulse Sparkover Test,
  4. Front of Wave Sparkover Test,
  5. Residual Voltage Test,
  6. Long Duration Impulse Current Test,
  7. Operating Duty Cycle Test,
  8. Mechanical tests like porosity test, temperature cycle tests and others,
  9. Pressure relief test,
  10. The Voltage withstand test on the insulator housing of the diverter,
  11. The Switching surge flashover test,
  12. The Pollution test.

26. What is insulation coordination?

Answer:

It is necessary to protection of transmission lines and power apparatus from the chief causes of over voltages in electric systems, namely lightning over voltages and switching over voltages. There are temporary over voltages of power frequency or its harmonic frequency either sustained or weakly damped and originate in switching and fault clearing processes in power systems. Although both switching and power frequency over voltages have no common origin, the may occur together, and their combined effect it important in insulation design. This is called insulation coordination.


27. List the sources of switching over voltage in power system.

Answer:

  1. Re-energizing of transmission lines
  2. Disconnection of unloader transformer
  3. Energization a reclosing of line
  4. Sudden switching of loads
  5. Shorts circuits and fault clearances
  6. Ferroseronance, arriving grounds, etc.

28. What are electronegative gases?

Answer:

The gain in which attachment plays an active role are called electronegative gases. These gases gives high breakdown strength in which free electrons are attached to neutral atoms to form negative ions.


29. State the disadvantages of CVT method of measurement.

Answer:

  1. The voltage ratio is susceptible to temperature variations.
  2. The problem of inducing few-resonance in power systems.

30. How the stray effect is reduced in resistive shunt type of measurement?

Answer:

  1. Bifilac flat strip design,
  2. Coaxial tube or parks shunt design,
  3. Coaxial squirrel cage design.

31. State the importance of insulation coordination in power system.

Answer:

  1. To ensure reliability and continuity to the utility concerns.
  2. To minimize the number of failures of lines and substations due to over voltages.
  3. To minimize the cost involved in the design.
  4. To maintain the flashover should be minimum for insulators.

32. Define lightning phenomenon.

Answer:

It is peak discharge in which charge accumulated in the laids discharge into a neighboring closed on to the ground.


33. List some  sources causing switching surges.

Answer:

  1. Opening and closing of switch gear,
  2. High natural frequency of the system,
  3. Damped normal frequency voltage components,
  4. Restriking and recovery voltage with successive reflected waves.

34. Define freeing and tracking.

Answer:

The spreading of spark channels during tracking in the form of branches of tree is called treeing. A formation of continuous conducting path across the surface of the insulation due to surface erosion is called tracking.


35. Give some merits of Van de graff generator.

Answer:

  1. Very high D.C. voltage can be generated easily.
  2. Ripple free output.
  3. Precision and flexibility of control.
  4. Stability of voltage can be achieved.

36. Explain the basic principle of Hall generator.

Answer:

The basic principle of Hall generator is Hall effect. It states that whenever an electric current flows through a metal plate placed in a magnetic field perpendicular to it. Lorentz force will deflect the electrons in the metal plats in a direction perpendicular to both the magnetic field and the flow of current.


37. List some advantages of Faraday generator.

Answer:

  1. No electric connection between the source and the device.
  2. No thermal problems even or large currents.
  3. No insulation problem arise for EHV system.

38. Find and locate the fault during the impulse testing of transformer.

Answer:

  1. Faults can be located by one of the following methods.
  2. General observation.
  3. Voltage oscillogram method.
  4. Neutral current method.
  5. Transferred surge current method.

39. Define partial discharge.

Answer:

An electric discharge that partially bridges the dielectric or insulating medium between two conductors is called as partial discharge.


40. Mention the different kinds of over voltages.

Answer:

  1. Switching overvoltages.
  2. Lightning overvoltages.
  3. Power frequency and its harmonic overvoltages.

41. What is stepped loader stroke?

Answer:

The leader streamer moves towards earth, it is accompanied by points of luminescence which travel in jumps giving rise to stepped leader. Stepped leader will give rise to first visual phenomenon of discharge.


42. What is meant by corona discharges?

Answer:

If the electric field is non-uniform, an increase in voltage will first cause a discharge in the gas to appear at points with highest electric field intensity, namely at sharp points or where the electrodes are curved or on transmission lines. This form of discharge is called a corona discharge and can be observed as a bluish luminiscence. This phenomenon is always accompanied by a hissing noise.


43. Mention the specifications of the standard impulse voltage.

Answer:

  1. Peak : Tolerance ± 3%.
  2. Time to Front : Tf = 1.2 μsec ± 30%.
  3. Time to Half Value : Tt = 50 μsec ± 20%.

44. Give any two methods of switching surge generation in laboratory.

Answer:

  1. Impulse generator circuit modified to give longer duration waveshapes.
  2. Power transformers or testing transformers excited by d.c. voltages giving oscillatory waves and these include Tesla coils.

45. Define CVT.

Answer:

It is defined as Capacitance divider with a suitable matching or isolating potential transformer tuned for resonance condition is often used in power systems for voltage measurements.


46. Give the advantages of electrostatic voltmeter.

Answer:

  1. The electrostatic voltmeters have a small capacitance ( 5 to 50 pF).
  2. With compressed gas or vacuum as medium, the meter is compact and much smaller in size.
  3. The electrostatic voltmeters are used to measure high ac voltages.

47. what are type and routine tests?

Answer:

Type Test : They are intended to prove or check the design features and the quality. They are done on samples when new designs or design changes are introduced.

Routine Test : They are intended to check the quality of an individual test piece. They are done to ensure the reliability of an individual test object and quality and consistency of materials used in their manufacture.


 

48. State the sources which determine the wave shape of switching surges.

Answer:

  1. Power transformers or testing transformers excited by d.c. voltages giving oscillatory waves and these include Tesla coils.
  2. Impulse generator circuit modified to give longer duration waveshapes.

49. Write down the causes of power frequency and its harmonic over voltages.

Answer:

  1. Ferranti effect, unsymmetrical faults.
  2. Saturation in transformers.
  3. Sudden loss of loads.
  4. Disconnection of inductive loads or connection of capacitive loads.

50. What are the properties required for a gaseous dielectric for HV applications?

Answer:

  1. Low temperature of condensation.
  2. Non-flammability and physiological inertness.
  3. High dielectric strength.
  4. Thermal stability and chemical inactivity towards materials of construction.

51. What are the disadvantages of half wave rectifier circuit?

Answer:

  1. Ripple voltage is large.
  2. The other shortcoming of the single-phase half-wave rectifier concerns the possible saturation of the h.v. transformer, if the amplitude of the direct current is comparable with the nominal alternating current of the transformer.

52. What is the effect of dust particles on the measurement using sphere gaps?

Answer:

When a dust particle is floating between the gap this results into erratic breakdown in homogeneous or slightly inhomogeneous electrode configurations. Under d.c. voltages erratic breakdowns occur within a few minutes even for voltages as low as 80% of the nominal breakdown voltages. This a major problem, with high d.c. voltage measurements with sphere gaps.


53. List out the limitations of generating voltmeters.

Answer:

  1. Disturbance in position and mounting of the electrodes make the calibration invalid.
  2. They require calibration.
  3. Careful construction is needed and is a cumbersome instrument requiring as auxiliary drive.

54. Define disruptive discharge voltage.

Answer:

This is defined as the voltage which produces the loss of dielectric strength of an insulation. It is that voltage at which the electrical stress in the insulation causes a failure which includes the collapse of voltage and passage of current.


55. Mention the characteristics of the spray used in wet flashover test.

Answer:

  • Direction : 45° to the vertical
  • Water temperature : ambient ± 15°C
  • Precipitation rate : 3 ± 10% (mm/min)
  • Conductivity of water : 100 micro Siemens ± 10%.