Protection against over voltage
1. Why protection of transmission line important?continuity of service and electric supply.
2. What are the causes of over voltages in electric system?
External Cause – Lightning Internal Cause – Switching
3. What are the causes of power frequency over voltages?
Sudden load rejection Disconnection of inductive loads or capacitive loads
Ferranti effect
Unsymmetrical fault
Saturation in the transformers
Tap changing operations
4. Name the source of switching surges.
Opening and Closing or switchgears High natural frequency of the system
Damped normal frequency voltage components
Restriking between the contacts
5. What is lightning phenomenon?
Lightning phenomenon is a peak discharge in which charge accumulated in the clouds discharges intoa neighbouring cloud or to the ground.
6. How does switching over voltage originate?
Switching over voltages originate in the system itself by connection and disconnection of circuitbreaker contact or due to initiation or interruption of faults.
7. What is Isokeraunic level?
Isokeraunic level is defined as the number of days in a year when thunder is heard or recorded inparticular location. It is given by, 0.1 0.2TD/strokes/km - year 2 Ng to
8. State the different methods of protection against over voltages.
Overvoltage due to lightning strokes can be avoided or minimized in practice by,i. Shielding the overhead lines by using ground wires above the phase wires,
ii. Using ground rods and counter-poise wires, and
iii. Including protective devices like expulsion gaps, protector tubes on the lines, and surge diverters at the line terminations and substations.
9. What is a ground wire or shielded wire?
Ground wire is a conductor run parallel to the main conductor of the transmission line supported on the same tower and earthed at every equally and regularly spaced towers. It is run above the main conductor of the line.10. Define tower footing resistance.
It is the effective resistance offered against drainage of the charge and lightning current through the tower structure and tower foot.11. Define Shielding Angle.
Shielding angle or Protective angle of the ground wire is defined as the angle between the vertical line passing through the ground wire and the line passing through the outermost power conductor.Protective zone is the zone which is a cone with apex at the location of the ground wire and surface generated by line passing through the outermost conductor.
12. What are the measures to control over voltages due to switching and power frequency?
a. Energization of transmission lines in one or more steps by inserting resistances and withdrawing them afterwardsb. Phase controlled closing of circuit breakers,
c. Drainage of trapped charges before reclosing,
d. Use of shunt reactors, and
e. Limiting switching surges by suitable surge diverters.
13. What are the factors that influence the lightning induced voltage on transmission lines?
i. The ground conductivity ii. The leader stroke current iii. Corona14. How is transmission lines classified?
i. Lines with no loss or ideal lines ii. Line without distortion or distortion less lines iii. Line with small losses iv. Lines with infinite and finite length defined by all the four parameters15. Define attenuation and distortion.
The decrease in the magnitude of the wave as it propagates along the line is called attenuation. The elongation or change of wave shape that occurs is called distortion.16. How is attenuation and distortion caused?
Attenuation is caused due to the energy loss in the line and distortion is caused due to the inductance and capacitance of the line.17. What are the causes for the change of induction on transmission line?
The changes in the inductance are due to the skin effect, the proximity effect and non-uniform distribution effect of the currents and the nearness to steel structures.18. What are the causes for the change of capacitance on transmission line?
The variation in capacitance is due to capacitance change in the insulation nearest to the ground structures, etc.19. What is the effect of corona on transmission lines?
The effect of corona is to reduce the crest of the voltage wave under propagation, limiting the peak value to the critical corona voltage. Hence, the excess voltage above the critical voltage will cause power loss by ionizing the surrounding air.20. What are the principles observed in the lattice diagram?
i. All waves travel downhill, i.e, into the positive time.ii. The position of the wave at any instant is given by means of the time scale at the left of the lattice diagram.
iii. The total potential at any instant of time is the super position of all the waves which arrive at that point until that instant of time, displaced in position from each other by time intervals equal to the time difference of their arrival.
iv. Attenuation is included so that the amount by which a wave is reduced is taken care.
v. The previous history of the wave, if desired can be easily traced. If the computation is to be carried out at a point where the operations cannot be directly placed on the lattice diagram, the arms can be numbered and the quantity can be tabulated and computed.
21. What are the components of switching surges?
Switching surges may include high natural frequencies of the system, damped normal frequency voltage component or the restriking and recovery voltage of the system with successive reflected waves from terminations.22. How does switching surges cause damage to circuit breaker?
In circuit breaking operation, switching surges with a high rate of rise of voltage may cause repeated restriking of the arc between the contact of a circuit breaker, thereby causing destruction of the circuit breaker contacts.23. Give the factor for over voltages generation in EHV system.
Over voltages are generated in EHV system when there is sudden release of internal energy stored either in the electrostatic form in the electromagnetic form.24. What are the different method by which switching over voltages of short duration and long magnitude be calculated?
i. Mathematical modeling of a system using digital computerii. Scale modeling using transient network analyzers
iii. By conducting field tests to determine the expected maximum amplitude of the over voltages and their duration at different points on the line. 25. Where is surge arrester placed in substation? Surge arresters are devices used at substations and at line terminations to discharge the lightning over voltages and short duration switching surges. These are usually mounted at the line end at the nearest point to the substation. They have a flash over voltage lower than that of any other insulation or apparatus at the substation.
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