UNIT 1

OVERVOLTAGES IN ELECTRICAL POWER SYSTEMS AND INSULATION COORDINATION

Causes of over voltages and its effects on power system – Lightning, switching surges and temporary overvoltages, – Bewleys lattice diagram - Protection against over voltages, protection gaps, surge arresters - Insulation coordination.

TWO MARKS

1. Mention the different kinds of overvoltages  (OR)

What are the causes of overvoltages in power system

• Internal over voltages- i) Switching overvoltage  

                                    ii) power frequency overvoltage

• External overvoltages - Lightning

2. Define Lightning phenomenon.

Lightning phenomenon is a peak discharge in which charge accumulated in the clouds discharges into a neighbouring cloud or to the ground.

3. Define Thunderstorm (or) Isokeraunic level

Thunder storm days (TD) (is known as the Iso Keraunic level) is defined as the number of days in a year when thunder is heard or recorded in a particular location.

T.D is = 30 t0 50 in India

4. Classify the lightning stroke

• Direct lightning stroke
• Indirect stroke

5. What is stepped leader stroke?

When the electric field intensity at some point in the charge concentrated cloud exceeds the breakdown value of the most ionized air (=10 KV/cm) an electric streamer with plasma start towards the ground with a velocity of light. But after  short interval of about 100 micro sec the streamer again starts out repeating its performance.

6. Mention some sources causes switching surges/overvoltages in power systems.

• Disconnection of unloaded transformers, reactors, etc

• Sudden switching off of loads
• Short circuit and fault clearances

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

Counter-poise wires are 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.

8. What are the measures must taken to control or reduce the switching over voltages?

• One step or multi-step energization of lines by preinsertion of resistors
• Phase controlled closing or circuit breakers with proper sensors
• Drainage of trapped charges on long lines
• Limiting the overvoltages by using surge diverters

9. Define Insulation coordination

The selection of suitable values for the insulation levels of the various substation components in any electrical system and their arrangement in a rational manner is called inulation coordination.

10. State the importance of insulation coordination in power systems

• To ensure reliability and continuity to the utility concerns.
• To minimize number of failures of lines and substations due to overvoltages.

11. What is BIL (Basic Impulse Level)?

The basic insulation levels are reference levels fixed by standards for each voltage levels. It is expressed in terms of impulse crest voltage(Vp) with a standard lightning impuse voltage(1.2/50 micro seconds wave) for any apparatus the insulation level as demonstated by suitable tests should be greater than or equal to the BIL