Protection against Over voltages due to Lightning Interview Questions and Answers:
1. What is meant by voltage surge ?
Ans. By voltage surge is meant any sudden excessive rise in voltage that may be dangerous to the electrical equipment of an installation.
2. What are the internal causes of overvoltages ?
Ans. Internal overvoltages arise during sudden changes in the operating conditions of an installation such as a switch-in, switch-out or sudden change in load, the opening of circuits with large inductance or capacitance, the interruption of a short-circuit current, the appearance of an arc fault between one phase and ground, as well as other circuit disturbances.
3. What is meant by lightning ?
Ans. Lightning is a huge spark, which is due to the electrical discharge taking place between the clouds, within the same cloud and between the clouds and ground.
4. What is difference between A stroke and B stroke ?
Ans. A stroke is characterized by the comparatively long time to produce discharge, while the B stroke is characterized by its rapidity. A stroke strikes the highest and most sharply pointed object in the neighborhood while B stroke ignores the tall objects and reaches the ground in a random manner. B stroke is most dangerous.
5. What is earthing screen ? What type of protection is provided by it ? What is it application ?
Ans. Earthing screen consists of a network of copper conductors, earthed at least on two points, over all the electrical equipment in the substation. An electrostatic shield against external field is presented by such a screen and so protects the system. Earthing screen protects the system from direct lightning strokes, reduces the voltages induced electrostatically or electromagnetically but does not provide any protection against voltage waves which may still reach the terminal equipment.
Substations, interconnectors and powerhouses are protected from direct strokes by earthing screen.
6. What is an earth wire ?
Ans. An earth wire is a form of lightning protection using a conductor or conductors, well grounded at regular intervals, preferably at each support, and attached from support to support above the transmission line.
7. Why earth wire is provided in overhead transmission lines ?
Ans. Earth wire is provided above overhead transmission lines for protection from lightning strokes. The ground wire shields the phase or line conductors by attracting itself the lightning strokes which, in its absence would strike the phase conductors. Besides it, the ground wire reduces the voltage electrostatically or electromagnetically induced in the conductors by the discharge of a neighboring cloud. It also provides additional protective effect by causing attenuation of travelling waves set in lines by acting as a short-circuited secondary of the line conductors.
8. What is meant by protective angle ? Give its value for reliable protection.
Ans. Protective angle is the angle between the vertical line through the ground wire axis and the line passing from the ground wire axis to the outermost phase conductor. Its value for reliable protection is taken equal to 20-30 degrees.
9. What is meant by coupling factor ?
Ans. The ratio of the induced voltage in the line conductors to the potential of ground wire is called the coupling factor.
10. What is meant by a lightning arrester or surge diverter ?
Ans. A surge diverter is a device that is connected between line and earth, i.e., in parallel with the equipment under protection at the substation. It limits the duration and amplitude of the follow current.
11. What is meant by impulse ratio of any lightning arrester ?
Ans. Impulse ratio of any lightning arrester is the ratio of breakdown voltage of a wave of special duration to the breakdown voltage of a 50 Hz wave.
12. What is the application of an electrolytic arrester ?
Ans. Electrolytic arrester is very delicate, needs daily supervision, and the film is required to be reformed whenever destroyed. It is used in conjunction with an impulse gap.
13. What is the main advantage of oxide film arrester ?
Ans. The main advantage of an oxide film arrester is that it does not require daily replacement and it may thus be installed at points in transmission systems where daily attendance is difficult or expensive to provide.
14. How metal oxide lightning arrester is superior to thyrite arrester ?
Ans. Metal oxide lightning arrester is simple in construction and has superior energy dissipation capacity, better surge protection, more stable protective characteristics and substantial reduction in overvoltage across the equipment as compared to thyrite arrester.
15. What is meant by “follow current” ?
Ans. Follow current is the current that flows from connected supply source through lightning arrester following the passage of discharge current.
16. What is meant by surge absorber ?
Ans. The device, which reduces the steepness of the wave front of a particular surge and thus minimizes the danger due to overvoltage is called the surge absorber.
17. What is the difference between a lightning conductor and lightning arrester ?
Ans. Lightning conductor is installed at the top of tall structures and is connected to the general mass of earth through a system of connecting conductors and the earth electrodes so that the structure under protection is not subjected to the dangerously high potential of the cloud discharges. While lighting arrester is a device that is connected in parallel with the equipment, under protection from lightning strokes, at the substation.
18. What is the difference between a lightning arrester and surge absorber ?
Ans. Lightning arrester limits the duration and amplitude of the follow current while a surge absorber reduces the steepness of the wave front of a particular surge.