Transient Over Voltage

8/8/2019 Transient Over Voltage

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Transient over voltages occur due to sudden

changes in the electrical conditions of a circuit andlead to the release of large amounts of energy

stored in the inductive and capacitive elements of

the system.


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There are mainly two sources of over voltage:` External over voltage :- Generated due to

atmospheric disturbances, lightning is the most

common and severe.

` Internal over voltage:- Generated by changes

in the operating conditions of the network.

Mainly switching surges.


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` Lightning falls directly on an overhead line:- The current

can be considered as an ideal current source. The

resulting over voltage therefore depends on theimpedance seen by the current source. The impedance

is the surge impedance of the line (200-400 ohm). Then

a very high voltage appears.

` Lightning falls near a overhead line:- The lightningcurrent generates over voltages through electromagnetic

induction in the distribution system. These over voltages

are propagated along the line.


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` Lightning falls directly on a building:-

The lightning current flows to earth via the more or

less conductive structures of the building with verydestructive effects.

` Lightning falls near a building:- The lightning

stroke generates the same types of overvoltage as

those described earlier. In addition, the lightning

current rises back from the earth to the electrical

installation, thus causing equipment breakdown.


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` A lightning strike can generate at an average 100

million volts of electricity.

` Current of up to 100,000 amperes.

` Can generate a temperature of54,000 deg F.` Lightning strikes kills hundreds of people every

year.


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` With the increase in transmission voltagesneeded to fulfill the required increase intransmitted powers, switching surges havebecome the governing factor in the design ofinsulation for EHV and UHV systems.

` High-frequency over voltages or burstdisturbance caused by a change in the steadystate in an electrical network.

` In the meantime, lightning over voltages come asa secondary factor in these networks.


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` There are a great variety of events that would

initiate a switching surge in a power network.

1. Energization of transmission lines and

cables.2. Re- energization of a line.

3. Load rejection.

4. Switching on and off of equipment.

5. Fault initiation and clearing.


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Switching surges due to energization of an unloaded transmission line


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` Protection against external overvoltages:-

The system can be protectedagainst external over voltagesusing shielding methods which donot allow an arc path to formbetween the line conductor andground. Therefore giving inherentprotection to the line design.

Use of ground wire is a shieldingmethod.


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` Protection against

internal over voltages:-

Normally non shieldingmethods are used which

allow an arc path between

the ground structure and

line conductor. Means areprovided to quench the arc.

Lightning arrester is an

example of a non shielding

method.


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Over voltages are very severe and cause damage

to life and property. Their occurrence cannot be

prevented but protective devices are used which

form 4-5% of the total cost in a power system.They ensure uninterrupted supply to the

customers and most importantly protection of life

and the very expensive equipments.


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Transient Over Voltage

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