Over Voltage in Power System

8/10/2019 Over Voltage in Power System

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By

Anish John paul. M

Head of SchoolSchool of Electrical & Electronics Engineering



Lightning Lightning is a sudden electrostatic discharge during

an electric storm between electrically charged regionsof a cloud (called intra-cloud lightning or IC), betweenthat cloud and another cloud (CC lightning), orbetween a cloud and the ground (CG lightning).

Lightning is generated in electrically charged storm

systems, but the method of cloud charging stillremains elusive.



Water Cycle Evaporation is the

process by which a liquidabsorbs heat andchanges to a vapor.

Condensation is theprocess by which a vaporor gas loses heat andturns into a liquid.



Electrical Storms

In an electrical storm, the storm clouds are charged likegiant capacitors in the sky. The upper portion of the cloudis positive and the lower portion is negative.

As the process of evaporation and condensation continues,these droplets collide other moisture that is in the processof condensing as it rises. Also, the rising moisture maycollide with ice or sleet that is in the process of falling to

the earth . The importance of these collisions is that electrons are

knocked off of the rising moisture, thus creating a chargeseparation.



The newly knocked-off electrons gather at the lowerportion of the cloud, giving it a negative charge.

The rising moisture that has just lost an electroncarries a positive charge to the top of the cloud.

Beyond the collisions, freezing plays an importantrole. The frozen portion becomes negatively charged

and the unfrozen droplets become positively charged.





When there is a charge separation in a cloud, there isalso an electric field that is associated with theseparation. Like the cloud, this field is negative in thelower region and positive in the upper region.

The strength or intensity of the electric field is directlyrelated to the amount of charge build up in the cloud.

As the collisions and freezing continue to occur andthe charges at the top and bottom of the cloudincrease, the electric field becomes more and moreintense.



As the separation of charge proceeds in the cloud, thepotential difference between the centers of chargesincreases and the vertical electric field along the cloud also

increases. The total potential difference between the two main charge

centers may vary from l00 to 1000 MV.

Only a part of the total charge-several hundred coulombs-

is released to earth by lightning; the rest is consumed inintercloud discharges.

All that is needed now is a conductive path for thenegative cloud bottom to contact the positive earth surface.



Air Ionization The strong electric field causes the air around the cloud to

"break down,“

the air breakdown creates a path that short-circuits the

cloud/earth as if there were a long metal rod connectingthe cloud to the earth.

The electric field causes the surrounding air to becomeseparated into positive ions and electrons -- the airis ionized.

the process of ionization, which creates plasma, also occursthrough the loss of electrons.

After the ionization process, the path between the cloudand the earth begins to form.

http://science.howstuffworks.com/environmental/earth/geophysics/earth.htm




Step Leaders

The channel to earth is first established by a stepped discharge called a leader stroke. There are usually many separate paths of ionized air stemming

from the cloud. These paths are typically referred to as stepleaders.

The step leaders propagate toward the earth in stages, which donot have to result in a straight line to the earth. The air may notionize equally in all directions.

If the lower surface of the cloud is not straight, the flux lines will

not be uniform. The leader that reaches the earth first reaps the rewards of the

journey by providing a conductive path between the cloud andthe earth. This leader is not the lightning strike; it only mapsout the course that the strike will follow.






Stepped Leader



Positive Streamers and Exploding

Air As the step leaders approach the earth, objects on the

surface begin responding to the strong electric field.The objects reach out to the cloud by"growing" positive streamers.

The human body can and does produce these positivestreamers when subjected to a strong electric fieldsuch as that of a storm cloud.

The streamers wait patiently, stretching upward as thestep leaders approach.



Streamers



After the step leader and the streamer meet, the ionized air (plasma)has completed its journey to the earth, leaving a conductive path fromthe cloud to the earth.

Since there is an enormous amount of current in a lightning strike,there is also an enormous amount of heat.

The air around the strike becomes extremely hot. So hot that itactually explodes because the heat causes the air to expand so rapidly.The explosion is soon followed by what we all know as thunder.

The corresponding current heats its path to temperatures up to20,000°C, causing the explosive air expansion that is heard thunder.

In air, sound travels roughly 1 mile every 4.5 seconds. Light travels at ablazing 186,000 miles (299,000 kilometers) per second.

The current pulse rises to its crest in a few micro seconds and decaysover a period of tens or hundreds of microseconds.



LIGHTNING PHENOMENON



Facts about Lightning A strike can average 100 million volts of Electricity

Current of up to 100,000 amperes

Can generate 54,000 oF Lightning strikes somewhere on the Earth every

second

Kills hundreds of people every year



Types of Strikes and Types of

Lightning

Cloud to ground -Discussed previously Ground to cloud -The same as above with the exception that

usually a tall, earth-bound object initiates the strike to the cloud Cloud to cloud - Also the same mechanics as discussed above

except the strike travels from one cloud to anotherTypes of Lightning Normal lightning - Discussed previously Sheet lightning - Normal lightning that is reflected in the

clouds Heat lightning - Normal lightning near the horizon that is

reflected by high clouds Ball lightning - A phenomenon where lightning forms a slow,

moving ball that can burn objects in its path before exploding orburning out



http://images.google.com/imgres?imgurl=http://epod.usra.edu/archive/images/earth_storm.jpg&imgrefurl=http://epod.usra.edu/archive/epodviewer.php3?oid=161632&h=574&w=850&sz=127&hl=en&start=3&tbnid=2kFXAaNXCSckMM:&tbnh=98&tbnw=145&prev=/images?q=cloud+to+cloud+Lightning&svnum=10&hl=en&lr=



RECAP On Earth, lightning begins with the water cycle. As the sun heats the planet,

moisture heads for the skies in the form of vapor. Given enough moisture,

a cloud will form. Due to updrafts, downdrafts, freezing and particle collisions, storm clouds

become positively charged at the top and negatively charged at the bottom.

This separation of charges goes hand in hand with an electric field. As thecharge separation grows stronger, so does the associated electric field.

Eventually, an intense electric field can cause the air around the cloud to“break

down,” or become ionized, allowing current to flow through the ionized air (orplasma) and potentially neutralize the charge separation. The path of ionizedair is called a step leader.

Meanwhile, the positive charge is getting bigger on the Earth’s surface below,and objects (including people) respond locally to this strong electric field by

sending out positive streamers. When a streamer and a step leader meet, they can form a complete path for

lightning to travel from the cloud to the ground (other types of lightning followa slightly different process). After this fateful meeting, the lightning strikeoccurs.

Lastly, the air around the strike heats up and expands so much that it causes ashock wave in the form of a sound wave to radiate away from the strike path.

’

http://science.howstuffworks.com/dictionary/meteorological-terms/cloud.htm

http://science.howstuffworks.com/nature/climate-weather/storms/question182.htm








Lightning Rods Lightning rods were originally developed by Benjamin

Franklin.

A lightning rod is very simple -- it's a pointed metalrod attached to the roof of a building. The rod mightbe an inch (2 cm) in diameter.

It connects to a huge piece of copper or aluminum

wire that's also an inch or so in diameter. The wire is connected to a conductive grid buried in

the ground nearby.

http://history.howstuffworks.com/revolutionary-war/benjamin-franklin.htm







Lightning Safety

More than 1,000 people get struck by lightning every year in the United States, and more than 100 of themdie as a result of the strike.

Appropriate shelter would be a building or a car.

If you do not have anywhere to go, then you shouldavoid taking shelter under trees. Trees attract

lightning. Never lay down on the ground.

If you are indoors, stay off the phone

http://electronics.howstuffworks.com/telephone.htm

http://electronics.howstuffworks.com/telephone.htm



Lightning Voltage Surges The most severe lightning stroke is that which strikes a phase

conductor on the transmission line.

It produces the highest overvoltage for a given stroke current.

The lightning stroke injects its current into a terminationimpedance Z, which in this case is half the line surge impedanceZo. since the current will flow in both directions as shown inFigure. Therefore, the voltage surge magnitude at the strikingpoint is

V =( ½)Izo……………

(1) The lightning current magnitude is rarely less than 10 kA.

For typical overhead line surge impedance Zo of 300 Ω, thelightning surge voltage will probably have a magnitude in excessof 1500 kV.



EFFECT OF LIGHTNING

Over Voltage in Power System

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