high volatge sytem overvolatge

8/14/2019 High Volatge Sytem Overvolatge

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Over Voltage

Dr. Oladokun Sulaiman


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Slide navigation Overview and Source of overvolatge Nature of danger

Lightening overvolatge Switching overvolatge


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When the voltage in a circuit or part of it is raised above its upper designlimit, this is known as over voltage . The conditions may be hazardous. Depending on its duration, the over

voltage event can be permanent or transient, the latter case also beingknown as a voltage spike.

Electronic and electrical devices are designed to operate at a certain

maximum supply voltage, and Considerable damage can be caused byvoltage that is higher than that for which the devices are rated. For example an electric light bulb has a wire in it that at the given rated

voltage will carry a current just large enough for the wire to get very hot(giving off light and heat), but not hot enough for it to melt.

The amount of current in a circuit depends on the voltage supplied: if the

voltage is too high, then the wire may melt and the light bulb has "burnedout". Similarly other electrical devices may stop working, or even maybe burst

into flames if an over voltage is supplied to the circuit of which thesedevices are part.

Overview and source of overvolatge


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Overview and source of overvolatge

Natural - A typical natural source of transient over voltageevents is lightning.

Man-made- sources are spikes usually caused byelectromagnetic induction when switching on or off inductive

loads (such as electric motors or electromagnets) One of the purposes of electromagnetic compatibilitycompliance is to eliminate such sources.

Conduction path- The transient pulses can get into theequipment either by power or data lines, or over the air from a

strong electromagnetic field change - an electromagnetic pulse(EMP). Filters are used to prevent spikes entering or leaving the

equipment through wires, and the electromagnetically coupledones are attenuated by shielding.


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Overview and source of overvolatgePower systems are always subjected to over voltages that have their origin in :Atmospheric discharges in which case they are called external or

lightning over voltages, or - The latter type are called internalover voltages - This classes may be further subdivided into

(iii)Temporary over voltages, if they are oscillatory of power frequency or harmonicsTemporary over voltages occur almost without exception under no

load or very light load conditions.whereas that of internal or switching overvoltages increases with

increasing the operating voltage of the system.(ii) Switching over voltages, if they are heavily damped and of shortduration they are generated internally by connecting or disconnecting the system, or due to the systems fault initiation or extinction.


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Overview and source of overvolatge The magnitude of the external or lightning over voltagesremains essentially independent of the systems design,

Hence, with increasing the systems operating voltage a pointis reached when the switching over voltages become thedominant factor in designing the systems insulation

Because of their common origin the temporary and switchingover voltages occur together and their combined effect has to

be taken into account in the design of h.v. systems insulation. Up to approximately 300 kV , the systems insulation has to be

designed to withstand primarily lightning surges. Above that voltage, both lightning and switching surges have

to be considered. For ultra. systems , 765 kV and above switching over voltages

in combination with insulator contamination become the predominating factor in the insulation design.


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Nature of danger

The degree of hazard depends on circumstances . To minimize the chances of being struck by lightning

during thunderstorm, one should be sufficiently far away from tall objects likely to be struck , remaininside buildings or be well insulated.

A direct hit on a human or animal is rare; they are more at risk from indirect striking, usually: (a) when the subject is close to a parallel hit or other

tall object, (b) due to an intense electric field from a stroke can

induce sufficient current to cause death, and


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Nature of danger (c) when lightning terminating on earth sets up high

potential gradients over the ground surface in anoutwards direction from the point or object struck.

The potential difference between the persons feetwill be largest if his feet are separated along a radialline from the source of voltage and

will be negligible if he moves at a right angle to such

a radial line. In the latter case the person would be safe due toelement of chance.


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Nature of danger


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Traveling Wave on Transmission Line

Any disturbance on a transmission line or system such as asudden opening or closing of line, a short circuit or a fault resultsin the development of overvoltage or overcurrent at that point.

This disturbance propagates as a traveling wave to the endsof the line or to a termination, such as, a sub-station.

Usually these traveling waves are high frequencydisturbances and travel as waves. They may be reflected, transmitted, attenuated or distorted during propagation until theenergy is absorbed.

Long transmission lines are to be considered as electricalnetworks with distributed electrical elements.


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Attenuation and Distortion of Traveling Waves

As a traveling wave moves along a line, it suffers both attenuationand distortion . The decrease in the magnitude of the wave as it

propagates along the line is called attenuation The elongation or change of wave shape that occurs is called

distortion. Sometimes, the steepness of the wave is reduced bydistortion. Also, the current and voltage wave shape becomedissimilar even though they maybe the same initially.

Attenuation is caused due to the energy loss in the line anddistortion is caused due to the inductance and capacitance of theline.


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Reflection and Transmission of Waves at Transition Points Whenever there is an abrupt change in parameters of a

transmission line, such as an open circuit or atermination, the traveling wave undergoes a transition,

part of the wave is reflected or sent back and only a portion is transmitted forward.

At the transition point, the voltage or current wave mayattain a value which can vary from zero to two times itsinitial value.

The incoming wave is called the incident wave and theother wave are called the reflected and transmitted waves at the transition point.


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Lightening Physical manifestations of lightning have been noted in

ancient times, but the understanding of lightning isrelatively recent.

The real incentive to study lightning came when

electric transmission lines had to be protected againstlightning.

The methods include measurements of:

(i) lightning currents,(ii) magnetic and electromagnetic radiated fields,(iii) voltages,(iv) use of high-speed photography and radar.


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lightening Fundamentally, lightning is a manifestation of a very large

electric discharge and spark. In an active thunder cloud the larger particles usually possess

negative charge and the smaller carriers are positive. Thus the base of a thunder cloud generally carries a negative

charge and the upper part is positive, with the whole beingelectrically neutral. There may be several charge centers within a single cloud.

Typically the negative charge centre may be located anywhere between 500m and 10 000m above ground.

Lightning discharge to earth is usually initiated at the fringeof a negative charge centre together with the current toground .


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lightening The stroke is initiated in the region of the negative charge

centre where the local field intensity approaches ionizationfield intensity ( D30 kV/cm in atmospheric air, or 10kV/cm in the presence of water droplets).

To the eye a lightning discharge appears as a single luminous

discharge - although at times branches of variable intensitymay be observed which terminate in mid-air - while theluminous main channel continues in a zig-zag path to earth.

High-speed photographic technique studies reveal that mostlightning strokes are followed by repeat or multiple strokes

which travel along the path established by the first stroke. The latter ones are not usually branched and their path is

brightly illuminated.


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Lightening process

Diagrammatic representation of lightning mechanism and ground current


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Lightening process The current in the return stroke is in the order of a few kA to

250 kA and the temperatures within the channel are 15 000C to20 000 C and are responsible for the destructive effectsof lightning giving high luminosity and causing explosive air expansion.

The return stroke causes the destructive effects generallyassociated with lightning

The return stroke is followed by several strokes at 10- to 300-m/sec intervals. The leader of the second and subsequentstrokes is known as the dart leader because of its dart-likeappearance.

The dart leader follows the path of the first stepped leader witha velocity about 10 times faster than the stepped leader. The

path is usually not branched and is brightly illuminated.


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Representation of various stages of lightning strokebetween cloud and ground

A diagrammatic representation of the various stages of the lightning stroke development fromcloud to ground in Figs 8.2(a) to (f) gives a clearer appreciation of the process involved.

In a cloud several charge centres of high concentration may exist. In the present case only twonegative charge centres are shown.

In (a) the stepped leader has been initiated and the pilot streamer and stepped leaderpropagate to ground, lowering the negative charges in the cloud. At this instance the strikingpoint still has not been decided;

in (b) the pilot streamer is about to make contact with the upwards positive streamer fromearth;

in (c) the stroke is completed, a heavy return stroke returns to cloud and the negative chargeof cloud begins to discharge;

in (d) the first centre is completely discharged and streamers begin developing in the secondcharge centre;

in (e) the second charge centre is discharging to ground via the first charge centre and dartleader, distributing negative charge along the channel. Positive streamers are rising up fromground to meet the dart leader;

(f) contact is made with streamers from earth, heavy return stroke proceeds upwards andbegins to is charge negatively charged space beneath the cloud and the second charge centre inthe cloud.


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Representation of various stages of lightning stroke between cloud and ground


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Distribution of times to crest of lightning strokecurrents


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Energy in lightning

Cumulative distributions of lightning stroke current magnitudes: After AIEE Committee.


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Energy in lightning

To estimate the amount of energy in a typical lightning discharge let usassume a value of potential difference of 107 V for a breakdown between acloud and ground and a total charge of 20 coulombs.

Then the energy released about 55 kWh in one or more strokes that makethe discharge.

The energy of the discharge dissipated in the air channel is expended inseveral processes.

Small amounts of this energy are used in ionization of molecules,excitations, radiation, etc. Most of the energy is consumed in the suddenexpansion of the air channel.

Some fraction of the total causes heating the struck earthed objects. Ingeneral, lightning processes return to the global system the energy that wasused originally to create the charged cloud.


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Over voltage Due to Switching Surges, System Faults andOther Abnormal Condition

Unlike the lightning voltages, the switching and other type of overvoltages depend on the normal voltage of thesystem and hence increase with increased systemvoltage.

In insulation coordination, where the protective level of any particular kind of surge diverter is proportional tothe maximum voltage, the insulation level and the cost of the equipment depends on the magnitudes of these

overvoltages. In the EHV range, it is the switching surge and other types of overvoltages that determine the insulation levelof the lines and other equipment and consequently, theyalso determine their and costs.


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Origin of Switching Surge

The making and breaking of electric circuits with switchgear may results in abnormal overvoltage in power systems having large inductance and capacitances.

The overvoltages may go as high as 6 times the normal power frequency voltage.

In circuit breaking operation, switching surges with ahigh rate of rise of voltage may cause repeated restrictingof the arc between the contacts of a circuit breaker,thereby causing destruction of the circuit breaker

contacts.


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Control of Over voltages Due to Switching

Insertion of Resistors Phase Controlled Switching

Drainage of Trapped Charge Shunt Reactor

high volatge sytem overvolatge

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