protection against overvoltages

Protection Protection Against Against

OvervoltagesOvervoltages

What are Overvoltages?What are Overvoltages?

According to IEEE standard for Insulation According to IEEE standard for Insulation Coordination, Overvoltage is defined as:Coordination, Overvoltage is defined as:

“ “ Voltage between one phase and Voltage between one phase and ground or between two phases, having a ground or between two phases, having a crest value exceeding the corresponding crest value exceeding the corresponding crest of maximum system voltage.”crest of maximum system voltage.”Overvoltages may be classified by shape Overvoltages may be classified by shape and duration as either temporary or and duration as either temporary or transient.transient.

What are Overvoltages?What are Overvoltages?Temporary Overvoltage:Temporary Overvoltage:An Oscillatory phase-to-ground or phase-to-An Oscillatory phase-to-ground or phase-to-phase overvoltage that is at a given location of phase overvoltage that is at a given location of relatively long duration(seconds, even minute) relatively long duration(seconds, even minute) and that is undamped or only weakly damped. and that is undamped or only weakly damped. Temporary overvoltage usually originate from Temporary overvoltage usually originate from switching operation or faults (e.g load rejection, switching operation or faults (e.g load rejection, single-phase fault, fault on a high-resistance single-phase fault, fault on a high-resistance ground or ungrounded system) or from ground or ungrounded system) or from nonlinearities (ferroresonance, harmonics), or nonlinearities (ferroresonance, harmonics), or both. They are characterized by the amplitude, both. They are characterized by the amplitude, the oscillation frequencies, the total duration or the oscillation frequencies, the total duration or the decrement.the decrement.

What are Overvoltages?What are Overvoltages?Transient Overvoltage:Transient Overvoltage:A short-duration highly damped, A short-duration highly damped, oscillatory, or nonoscillatory overvoltage, oscillatory, or nonoscillatory overvoltage, having duration of few milliseconds or having duration of few milliseconds or less. Transient overvoltage is classified as less. Transient overvoltage is classified as one of the following types: one of the following types:

Lightning OvervoltageLightning Overvoltage Switching OvervoltageSwitching Overvoltage Very fast front, short duration Very fast front, short duration

overvoltageovervoltage


Lightning Overvoltage:Lightning Overvoltage:

A type of Transient voltage in which a fast A type of Transient voltage in which a fast front voltage is produced by lightning or front voltage is produced by lightning or fault. Such overvoltage is usually fault. Such overvoltage is usually unidirectional and of very short duration. A unidirectional and of very short duration. A typical waveform is shown in figure.typical waveform is shown in figure.


Switching Overvoltage:Switching Overvoltage:

A transient overvoltage in which a slow A transient overvoltage in which a slow front, short-duration, unidirectional or front, short-duration, unidirectional or oscillatory, highly damped voltage is oscillatory, highly damped voltage is generated (usually by switching or fault). A generated (usually by switching or fault). A typical waveform is shown in figure typical waveform is shown in figure


Very fast front, short-duration overvoltage:Very fast front, short-duration overvoltage:

A transient overvoltage in which a short A transient overvoltage in which a short duration, usually unidirectional, voltage is duration, usually unidirectional, voltage is generated (often by GIS disconnect switch generated (often by GIS disconnect switch operation or when switching motor). High-operation or when switching motor). High-frequency oscillation or often frequency oscillation or often superimposed on the unidirectional wave. superimposed on the unidirectional wave. A typical waveform is shown in the figure. A typical waveform is shown in the figure.

Causes of OvervoltageCauses of Overvoltage

The overvoltages on a power system may The overvoltages on a power system may be broadly divided into two main be broadly divided into two main catagories:catagories:

1)1) Internal CausesInternal Causes

(i) Switching surges (ii) Insulation (i) Switching surges (ii) Insulation FailureFailure

(iii) arcing Ground(iii) arcing Ground (iv) Resonance (iv) Resonance

2)2) External CausesExternal Causes i.e. lightning i.e. lightning

Causes of OvervoltageCauses of OvervoltageInternal causes do not produce surgesInternal causes do not produce surges** of large of large magnitude. Experience shows that surges due to internal magnitude. Experience shows that surges due to internal causes hardly increase the system voltage to twice the causes hardly increase the system voltage to twice the normal value. Generally, surges due to internal causes normal value. Generally, surges due to internal causes are taken care of by providing proper insulation to the are taken care of by providing proper insulation to the equipment in the power system. However, surges due to equipment in the power system. However, surges due to lightning are very severe may increase the system lightning are very severe may increase the system voltage to several times the normal value. If the voltage to several times the normal value. If the equipment in the power system is not protected against equipment in the power system is not protected against lightning surges, these surges may cause considerable lightning surges, these surges may cause considerable damage. In fact, in a power system, the protective damage. In fact, in a power system, the protective devices provided against overvoltages mainly take care devices provided against overvoltages mainly take care of lightning surges. of lightning surges.

** A sudden rise in voltage for a very short duration on the A sudden rise in voltage for a very short duration on the power system is known as a Voltage surge.power system is known as a Voltage surge.

Lightning FactsLightning Facts

A strike can average 100 million volt A strike can average 100 million volt of of electricityelectricity

Current up to 100,000 amperes.Current up to 100,000 amperes. Can generate 54,000 Can generate 54,000 ooF.F. Lightning strike somewhere on the Lightning strike somewhere on the

earth every second.earth every second.

Mechanism of Lightning DischargeMechanism of Lightning Discharge

In an electrical storm, the In an electrical storm, the storm clouds storm clouds are chargedare charged like giant capacitors in the like giant capacitors in the sky. The upper portion of the cloud is sky. The upper portion of the cloud is positive and the lower portion is negative.positive and the lower portion is negative.

When there is a charge separation in a When there is a charge separation in a cloud, there is also an cloud, there is also an electric fieldelectric field that is that is associated with the separation.. associated with the separation..


The strength or intensity of the electric field The strength or intensity of the electric field is directly related to the amount of charge is directly related to the amount of charge buildup in the cloud. As the electric field buildup in the cloud. As the electric field becomes more and more intense -- so becomes more and more intense -- so intense, in fact, that the electrons at the intense, in fact, that the electrons at the earth's surface are repelled deeper into the earth's surface are repelled deeper into the earth by the strong negative charge at the earth by the strong negative charge at the lower portion of the cloud. This repulsion of lower portion of the cloud. This repulsion of electrons causes the earth's surface to electrons causes the earth's surface to acquire a strong positive charge.acquire a strong positive charge.


All that is needed now is a All that is needed now is a conductive conductive pathpath for the negative cloud bottom to for the negative cloud bottom to contact the positive earth surface. The contact the positive earth surface. The strong electric field, being somewhat self-strong electric field, being somewhat self-sufficient, creates this path. sufficient, creates this path.


When the electric field becomes very When the electric field becomes very strong (on the order of tens of thousands strong (on the order of tens of thousands of volts per inch), conditions are ripe for of volts per inch), conditions are ripe for the air to begin breaking down. The the air to begin breaking down. The electric field causes the surrounding air to electric field causes the surrounding air to become separated into positive ions and become separated into positive ions and electrons -- the air is electrons -- the air is ionizedionized. .


The importance of this separation is that The importance of this separation is that the electrons are now free to move much the electrons are now free to move much more easily than they could before the more easily than they could before the separation. So this ionized air (also known separation. So this ionized air (also known as as plasmaplasma) is much more conductive than ) is much more conductive than the previous non-ionized air. the previous non-ionized air. The ionization of air or gas creates plasma The ionization of air or gas creates plasma with conductive properties similar to that of with conductive properties similar to that of metals. After the ionization process, the metals. After the ionization process, the path between the cloud and the earth path between the cloud and the earth begins to form. begins to form.


Once the ionization process begins and Once the ionization process begins and plasma forms, a path is not created plasma forms, a path is not created instantaneously. In fact, there are usually instantaneously. In fact, there are usually many separate paths of ionized air many separate paths of ionized air stemming from the cloud.stemming from the cloud.These paths are typically referred to as These paths are typically referred to as step leadersstep leaders. The step leaders propagate . The step leaders propagate toward the earth in stages, which do not toward the earth in stages, which do not have to result in a straight line to the earth. have to result in a straight line to the earth.


As the step leaders approach the earth, As the step leaders approach the earth, objects on the surface begin responding to objects on the surface begin responding to the strong electric field. The objects reach the strong electric field. The objects reach out to the cloud by "growing" out to the cloud by "growing" positive positive streamersstreamers. These streamers also have a . These streamers also have a purplish color and appear to be more purplish color and appear to be more prominent on sharp edges. Next to occur prominent on sharp edges. Next to occur is the actual is the actual meeting of a step leader meeting of a step leader and a streamerand a streamer. .


After the step leader and the streamer After the step leader and the streamer meet, the ionized air (plasma) has meet, the ionized air (plasma) has completed its journey to the earth, leaving completed its journey to the earth, leaving a conductive path from the cloud to the a conductive path from the cloud to the earth. With this path complete, current earth. With this path complete, current flows between the earth and the cloud. flows between the earth and the cloud. This discharge of current is nature's way This discharge of current is nature's way of trying to neutralize the charge of trying to neutralize the charge separation. separation.

More on Lightning PhenomenonMore on Lightning Phenomenon

Lightning is a discharge of electrical Lightning is a discharge of electrical energy.energy.

It may occur:It may occur: Between cells in the same storm as Inter-Between cells in the same storm as Inter-

cloud Lightning or within a cloud as Intra-cloud Lightning or within a cloud as Intra-cloud Lightning (80%)cloud Lightning (80%)

Cloud to Air (1%)Cloud to Air (1%) Cloud to Ground (19%)Cloud to Ground (19%)


During a collision between heavy graupel During a collision between heavy graupel particles and lighter ice crystals:particles and lighter ice crystals: Negative charge is transferred to the graupel Negative charge is transferred to the graupel

(electrons gained)(electrons gained) Positive charge is transferred to the ice crystals Positive charge is transferred to the ice crystals

(electrons removed)(electrons removed) Graupel falls to the bottom of the cloud bringing Graupel falls to the bottom of the cloud bringing

the negative charge with itthe negative charge with it Ice crystals are transported to the upper levels Ice crystals are transported to the upper levels

of the thunderstormof the thunderstorm


++ +++

- - - - -The strong negative chargeat the base of the thunderstorminduces a positive charge atthe surface by repulsion of electrons

+ + + +

+ +

- - - - --- - -

-- -


If the electric field, or the difference between the negative and positive charge regions, is large enough, the insulator between the charge regions (the air) “breaks down” and the lightning discharge can occur between the regions of positive and negative charge.The break down voltage for air is about 10,000 Volts/meter


The lightning stroke begins when the electric fields exceed the break down voltage.

Initially streams of electrons surge from the cloud base toward the ground in steps of 50 to 100 m.

Start and stop steps as the stepped leader progresses toward ground.

This occurs over a few microseconds and is relatively invisible.


Stepped leader moving in ~50m steps.

More on Lightning PhenomenonMore on Lightning PhenomenonStreamer:Streamer:

When the stepped leader gets near the ground When the stepped leader gets near the ground within 100 m or so...within 100 m or so... Positive charge moves from the ground up toward the Positive charge moves from the ground up toward the

stepped leader -- these are called stepped leader -- these are called streamersstreamers.. The streamers may come from almost any pointed The streamers may come from almost any pointed

object on the ground:object on the ground: TreesTrees AntennasAntennas GrassGrass FlagpolesFlagpoles Telephone PolesTelephone Poles PeoplePeople Really Tall TowersReally Tall Towers

Electric fields are stronger around pointed objects.Electric fields are stronger around pointed objects.


A streamer rising from a part of a tree. The return stroke was created when the stepped leader met with a streamer from another part of the tree.


Stroke:Stroke: An electrical current of about 20,000 Amps An electrical current of about 20,000 Amps

flows, depositing the electrons on the flows, depositing the electrons on the ground.ground.

The current generated over the short time The current generated over the short time interval heats the surroundings to interval heats the surroundings to approximately 30,000 K (The sun’s surface approximately 30,000 K (The sun’s surface ~ 6000 K)~ 6000 K)

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protection against overvoltages

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