EE1402(OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS)

PART- A

UNIT-I

OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS

1. What are the 2 types of over voltages?

1. Lightning over voltages,

2. Switching over voltages.

2. Explain the various regions of the cloud.

The upper regions of the cloud are positively charged, whereas the lower region and the base are predominantly negative except the local region near the base and the head which is possible.

3. Mention the different theories of charge formation.

Simpson’s theory, Reynold’s theory and Mason’s theory

4. What does a thunder cloud consist?

A thunder cloud consists of super cooled water droplets moving upwards and large hailstones moving downwards.

5. Mention the requirements for the thunder clouds and charge formation of air currents.

Moisture and specific temperature range

6. What is back flashover?

When a direct lightning stroke occurs on a tower ,the tower has to carry huge impulse currents. If the tower footing resistance is considerable ,the potential of the tower rises to a large value, steeply with respect to the line and consequently a flashover may take place along the insulator strings .This is known as back flashover.

7. State the parameters and the characteristics of the lightning strokes.

Amplitude of the current, the rate of rise, the probability distribution of them and the waveshapes of the lightning voltages and currents.

8. Define Isokeraunic level or thunderstorm days.

It is defined as the number of days in a year when the thunder is heard or recorded in a particular location. Often it does not distinguish between the ground strokes and the cloud-to-cloud strokes.

9. State the factors influence the lightning induced voltages on transmission lines.

• The ground conductivity,
• The leader stroke current and
• The corona.

10. What are the sources (or) origin of switching surges?

* Opening and closing of switchgears.

* High natural frequencies of the system.

* Damped normal frequency voltage components.

* In circuit breaker operation, switching surges with a high rate of rise of voltage may cause repeated restriking of the arc between the contacts of the C.B.

11. What are the characteristics of switching surges?

* Deenergizing of lines, cables and shunt capacitor banks etc.

* Disconnection of unloaded transformers, reactors etc.

* Opening and closing of protective devices connected to lines and reactive loads.

* Switch off the loads suddenly.

* Clearing of the faults.

12. What are the causes of switching over voltages in EHV and HV system?

* Interruption of low inductive currents by high speed circuit breakers.

* Interruption of low capacitve currents by switching off the unloaded lines.

* Ferro-resonance condition.

* Energization of long EHV or UHV lines.

* Interruption of fault cuurent when the fault is cleared.

13. How to control the over voltages due to switching and power frequency?

* Inserting resistors

* Phase controlled closing of C.B. with proper sensors

* Drain the trapped charges before reclosing of the lines

* Using shunt reactors

* By using lightning arresters or surge diverters

14. When over voltages are generated in EHV system?

Over voltages are generated in EHV systems when there is a sudden release of internal energy stored either in the electrostatic form or in the electromagnetic form.

15. What are the causes for power frequency and its harmonic over voltages?

• Sudden loss of loads,
• Disconnection of inductive loads or connection of capacitive loads,
• Ferranti effect,
• Unsymmetrical faults and saturation in transformers etc.

16.What is ground 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.

17. What is the use of ground wire?

It shields the transmission line conductor from induced charges,from clouds as well as from a lightning discharge.

18. What is an expulsion gap?

Expulsion gap is a device which consists of a spark gap together with an arc quenching device which extinguishes the current arc when the gap breaks over due to overvoltage.

19. Mention the parts of an expulsion gap.

It consists of a rod gap in air in air in series with a second gap enclosed within a fibre tube.

20. What is a protector tube?

It is a device which consists of a rod or spark gap in air formed by the line conductor and its high voltage terminal.It is mounted underneath the line conductor on a tower.

21. How are the insulation level and the protective safety margin arrived?

Selecting the risk of failure,the statistical safety factor and by firing the withstand level of any equipment or apparatus corresponding to 90% or 95% of the withstand voltage.

22. Define Basic Impulse Level.

It is defined as the minimum insulation impulse withstand voltage of any power equipment or apparatus. The BIL of a power system is usually chosen as 25% to 30% more than the protective level offered by the protective devices.

23. Mention the various insulation levels in a substation.

The busbar insulation is the highest to ensure the continuity of supply in a substation. The circuit breakers, isolators, instrument and relay transformers are given the next lower limit level .The power transformers are the costliest and sensitive device and the insulation level for it is the lowest.

24. What is the use of ground rod?

Ground rods are used to reduce the tower footing resistance. These are buried into the ground surrounding the tower structure.These rods are a number of rods about 15mm diameter and 3m long driven into the ground

25. What are surge arresters?

Surge arresters are non-linear resistors in series with spark gaps which act as fast switches. Surge diverters or lightning arresters are devices used at sub-stations 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 sub-station.

26. State few high voltage applications.

• Electric Traction
• Particle accelerator
• Biomedical applications
• Lightning arrestors

UNIT 1:

PART-B

1. Explain the lightning phenomena (or) charge formation in clouds? – (8)
2. Explain the Mechanism of lightning strokes? –(8)
3. Derive the expression for the transmission lline (or) derive the telegraphic equation?- (10)
4. What is meant by switching surge, how it originates and explain their characteristics?-(16)
5. How will you control the over voltages due to switching?-(8)
6. Explain the protection of transmission line against over voltage?-(16)
7. Explain the principles of insulation coordination of HV and EHVpower system?
8. Explain about the surge arresters and how will you select the surge arrester?-(8)
9. Explain the reflection of transmission waves at transient point?-(8)
10. Explain about the successive reflection and lattice diagram?-(10)