Protection Against Overvoltages

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Overvoltages i.e. voltages greater than the normal value.

These overvoltages on the power system may be caused due to many reasons such as

lightning,

the opening of a circuit breaker,

the grounding of a conductor etc.

Most of the overvoltages are not of large magnitude but may still beimportant because of their effect on the performance of circuitinterrupting equipment and protective devices.

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Voltage Surge

A sudden rise in voltage for a very short duration on the power systemis known as a voltage surge or transient voltage.

The steeper the wave front, the more rapid isthe build-up of voltage at any point in thenetwork, this build-up is rapid, being of the orderof 1–10 µs.

Voltage surges are generally specified in termsof *rise time t1 and the time t2 to decay to halfof the peak value.

For example,

a 1/50 µs surge is one which reaches its maximumvalue in 1µs and decays to half of its peak value is50 µs.

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Causes of Overvoltages

The overvoltages on a power system may be broadly divided into two main categories viz.

1. Internal causes

(i) Switching surges (ii) Insulation failure(iii) Arcing ground (iv) Resonance

2. External causes i.e. lightning

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(i) Switching surges

(1) Case of an open line.

The overvoltages produced on the power system due to switchingoperations are known as switching surges.

When the unloaded line isconnected to the voltagesource, a voltage wave is set upwhich travels along the line. Onreaching the terminal point A,it is reflected back to thesupply end without change ofsign.

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(2) Case of a loaded line.

Overvoltages will produce during the switching operations of a loaded line.

Suppose a loaded line is suddenly interrupted. This will set up a voltage of2*Zn*I across the break (i.e. switch) where I is the instantaneous value ofcurrent at the time of opening of line and Zn is the natural impedance of theline.

For example, suppose the line having Zn = 1000 Ω, carries a current of 100 A(r.m.s.) and the break occurs at the moment when current is maximum.

The voltage across the breaker (i.e. switch) = 2 √2 × 100 × 1000/1000 = 282·8kV. If Vm is the peak value of voltage in kV, the maximum voltage to which theline may be subjected is = (Vm + 282·8) kV.

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(3) Current chopping.

While interrupting highly inductive current,like no-load current of transformer, therapid deionization of contact space andblast effect may cause current interruptionbefore its natural zero.

Such an interruption of current before itsnatural zero is termed as “currentchopping”.

This phenomenon is more pronounced in caseof air-blast circuit breakers which exertsthe same deionizing force for all currentswithin its short-circuit capacity.

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(ii) Insulation failure

The failure of insulation between the lineand earth is very frequent. When thereis a breakdown of insulation to earth, thepotential at fault suddenly falls frommaximum to zero and, therefore, anegative voltage wave of very steepfront in the form of surge travels fromthe fault in both directions. PRKREDDY.GNITS

(iii) Arcing ground

Arcing ground is the surge, which isproduced if the neutral is not connectedto the earth.

The phenomenon of arcing ground occursin the ungrounded three-phase systemsbecause of the flow of the capacitancecurrent.

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Resonance in an electrical circuit implies that the impedance of the circuit ispurely resistive and the power factor is unity. Thus at resonance theinductive reactances and capacitive reactances cancel out.

In usual transmission lines the capacitance is usually so small that resonancecannot occur at the fundamental supply frequency, but if the generator emfwave is distorted, trouble may be experienced due to resonance at one atthe higher harmonics.

(iv) Resonance

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Mechanism of Lightning Discharge

An electric discharge between cloud and earth(CG), between clouds(CC) orbetween the charge centres of the same cloud(IC) is known as lightning.

According to most accepted theory is that during the uprush of warm moist airfrom earth, the friction between the air and the tiny particles of water causesthe building up of charges.

When drops of water are formed, the larger drops become positively chargedand the smaller drops become negatively charged.

When the drops of water accumulate, they form clouds, and hence cloud maypossess either a positive or a negative charge, depending upon the charge ofdrops of water they contain.

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A negatively charged cloud inducing a positive charge on the earth.

When the potential gradient is sufficient (5 kV*/cm to 10 kV/cm) to break downthe surrounding air, the lightning stroke starts.

The leader streamer being unable toreach the earth due to current in theleader streamer is low (<100 A) and itsvelocity of propagation is about 0·15m/µsthat of velocity of light. Moreover, theluminosity of leader is also very low.

The first discharge moves to earth in steps of about 50 meters in length each and is accomplished in about 1µs, therefore, termed the stepped leader.

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When the pilot steamer reaches near theearth, the electric field intensityincreases and due to this, the charges ofan opposite polarity in the form of ashort steam rises from the earth tomeet the tip of downward leader, this iscalled short upward leader

When a contact is made between thepilot leader and the short upwardsteamer, a return streamer travel fromthe earth to cloud along the ionisedchannel formed by the pilot leader.

The return steamer moves very fast and produces the well known,intensely luminous lightning flash, the current varies from 1kA to200kA and speed is 10% of light.

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There may be other cell ofcharges near the neutralizedcharged cell will try toneutralize through this ionizedpath, this streamer is calleddart leader, the velocity ofdart leader is 3% of the light.The effect of dart leader ismuch severe than returnstroke.

The discharge current in the return steamer is relatively large, but lasts for afew microseconds the energy contained in it is small. This streamer is calledcold lightning stroke, while the dart leader stroke is called Hot lightning stoke.

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A. A lightning discharge which usually appears to the eye as a single flash is in reality made up of a number of separate strokes that travel down the same path. The interval between them varies from 0·0005 to 0·5 second. Each separate stroke starts as a downward leader from the cloud.

B. It has been found that 87% of all lightning strokes result from negatively charged clouds and only 13% originate from positively charged clouds.

C. It has been estimated that throughout the world, there occur about 100 lightning strokes per second.

D. Lightning discharge may have currents in the range of 10 kA to 90 kA.

Facts about Lightning

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Over-Voltages due to Lightning

The ∝ and β are theconstants whichdetermine the shapes.The waves are defined bytimes t1 and t2 inmilliseconds.

If an overhead line is struck by lightning (direct stroke), thevoltage rise at the point is Vd = Id z0

where Z0 is the surge impedance, and ld is the discharge current.

But since travelling wave travel in both directions, the current ishalved and overvoltage is given by Vd = [1/2 (IdZ0)]

For example if Id is taken 30 k A and Z0 = 500 Ω, then – Vd =(1/2) x 30 x 1,000 x 500 = 7.5 x 106 V.

Due to direct stroke on a line, the nearby lines are also subjectedto over-voltages, but of less magnitude, through electromagneticcoupling. Experiences have shown that voltage rises induced byside strokes may attain a value of 2 million volts but that about90% of such voltages are below 500 kV. These voltages may alsospark over on the insulators.

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A direct stroke on the tower or earth conductor causes the rise of the voltage to a value given by Vd = Id RE + [L(di/dt)]

where Id is the discharge current, RE is the tower footing resistance, L is the inductances of the tower and di/dt is the slope of current wave.

For EHT towers of normal height, L is taken equal to 10 µH. Taking RE = 5 Ω, ld = 30 kA and di/dt = 10 kA/µs, we have –

Vd = [30 x 103 x 5 + 10 x 10-6 x (10 x 1000)/(1 x 10-6)] = 250 kV

If Vd exceeds the spark over voltage of the insulators a backwardflashover from tower to conductors will give rise to travelling waveson the conductors in both directions which on reaching the terminalequipment may subject the same to dangerous voltage rises. For allpurposes only travelling waves through the lines are considered forprotection. PRKREDDY.GNITS

Types of Lightning Stroke

The lightning stroke affects the lines in two ways

1. Direct stroke

2. Electrostatic induction.

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1. Direct Stroke

In the direct lightning strokes, the cloudattains a large amount of charge andinduces an opposite charge on tallerobjects such as temple, churches ormosques.

When the intensity of electrostatic fieldbecomes sufficiently great to ionize theneighboring air, the air break down anddischarge takes place between the cloudand the object.

Such types of discharge take a long timeto produce, and it strikes the highestand the most sharply pointed building inthe neighborhood. PRKREDDY.GNITS

Consider the three clouds, clouds 1 and 3are positively charged, and cloud 2 isnegatively charged as shown in the figure

The potential of cloud 3 is reduced dueto the presence of the charged cloud 2.On the flash over from Cloud 1 to Cloud2, both these clouds are dischargedrapidly, and cloud 3 assumes a muchpotential and flashes to earth veryrapidly.

It is the most dangerous strokes becauseit ignores taller building and reachesdirectly to the ground. This stroke iscalled the ‘B’ strokes.

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2. Electrostatic Induction StrokeA cloud electrostatically charged, and lyingabove a transmission line, will induce in theadjacent section of the line a correspondingcharge of opposite sign known as a boundcharge.

This charge has the maximum value below thecloud and then gradually tails away. Thischarge will not flow since it is a boundcharge. The positive charge on the far endsof the line will however leak to earth slowlythrough insulators, metallic parts etc., thusleaving only the negative charge on the line.

The charge will flow from a higher to a lower potential and the result is travelling waves in both directions. The two waves will be equal and thus each wave will have half the potential of the charge at the time of discharge of the cloud; they will also have the space-voltage distribution of the original charge, as illustrated in Fig. 9.8.PRKREDDY.GNITS

The above travelling waves will be of quite large amplitude (10 to 15 kV) andshall have very steep wave fronts which can damage the unprotectedequipment connected to the line and hence these must be passed to theearth.

The charge induced on the conductor is given as Q = CV

where C is the capacitance between conductor and ground –

and V = E h

where E is the field gradient and h is the height of the line from ground.

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Basic design requirements for protection against direct lightning strokes:

1. The ground wire used should be mechanically strong and should be solocated that they provide sufficient shielding.

2. There should be sufficient clearance between the power conductors andthe tower structure.

3. There should be an adequate clearance between the line conductors andthe ground wires, particularly at the mid-span, so as to avoid flashover tothe power conductor upto the protective voltage level used for the linedesign.

4. The tower footing resistance should be as low as permissible.

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1. Mechanically strong wires

2. Proper clearance between conductor and structure

3.Adequate clearance between conductor and ground wire

4. Low tower footing resistance

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Protection of Power System against Lightning

All the electrical equipment must be protected from severe damage due to the lightning strokes.

1. Protection of power stations and substations from direct lightning strokes

2. Protection of overhead transmission lines from direct lightning strokes

3. Protection of electrical equipment from travelling waves.

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Protection of Power Stations and Substations from Direct Lightning Strokes:

Power stations are usually indoor while substations may be indoor or outdoor. For protection of a structure from direct strokes there are three requirements which are to be fulfilled.

These requirements are interception, conduction and dissipation.

These requirements involve:

(i) An object in good electrical connection with the earth so that the leader stroke may get attracted,

(ii) A low impedance path joining this object to earth so that the discharge follows it in preference to any other path,

(iii) A low resistance connection with the earth body.PRKREDDY.GNITS

For 1, the upper portion of a metal structure may be employed. Alternatively aseparate metallic system, often called the shield, either mounted on thestructure or near to and above it may be provided.

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For 2, the requirements are:

(i) Low resistance (i.e., adequate conductivity and cross- section, properly bounded joints, free from possible corrosion),

(ii) Low reactance (i.e., absence of sharp bends, or loops and short conductors),

(iii) And sufficient clearance from any other conducting objects that might provide separate uncontrolled path to ground.

Outdoor substations have much of equipment carried on metal gantries and theinterconnection of the upper portion of these will screen the apparatus.Usually, there is suitable grounding provided.

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Shielding of the station and the incoming lines(about 0.8 km out from the station) to restrict theseverity of the waves that can enter the stationthrough the lines is a desirable supplement,particularly in the case of hv lines (66 kV andabove) to the lightning arrester located in thestation [Fig. 9.10(b)].Where overhead ground wires cannot be providedon the incoming lines due to existingstructure/construction, additional protection ofthe station equipment against direct lightningstrokes can be provided by equipping each line withprotector tubes at the entrance to the structureof the station and at each tower for a distance ofabout 0.8 km out from the station, as illustrated inFig. 9.10(c). However, shielding of the powerstation/substation is the only way of eliminatingdirect strokes to the station itself.

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Protection of Overhead Transmission Lines from Direct Lightning Strokes:

The two methods of protecting overhead transmission lines against lightning strokes are:

(i) Overhead ground wires and

(ii) Expulsion protector tubes.

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A ground wire is a form of lightning protectionemploying a conductor or conductors, well-grounded at regular intervals, preferably at eachsupport (pole or tower), and attached fromsupport to support above the transmission lineconductors.

Running of ground wire above the line isconsidered better as it provides more effectiveshield.

Protection of Overhead Transmission Lines from Direct Lightning Strokes by Ground Wires:

For reliable protection the protective angle α is taken equal to 20-30 degrees.This is the angle between the vertical line through the ground wire axis and theline passing from the ground wire axis to the outermost line or phase conductor.

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Protection of Overhead Transmission Lines from Direct Lightning Strokes by Protector Tube:

Even after reduction in the induced voltage by using a ground wire, there still exist over-voltages in the system which must be removed by using additional protective devices such aslightning arrester that bypasses the surges to the ground. Another device that is quitecommon in use is the protector tube.Expulsion protector tube consists of a backlisted fibre tube containing two built-inelectrodes between which an internal gap is provided. An outer electrode or arcing horn,made from steel wire 5 or 6 mm in dia is attached to the bolt at the upper end of theprotector tube (Fig 9 15) to provide an external spark gap between it and a line conductor.The internal gap can be adjusted by turning the tube about the bolt which secures the tubeto the bracket. The external gap is adjusted to templates made from wire 3 or 4 mm indiameter.

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When a surge of sufficient voltage travels along the phase conductor and reaches the pointwhere the expulsion tube is mounted, both of the series air gaps (internal air gap G1 andexternal air gap G2) breakdown and drain the surge current to ground through the tube and itsearthing conductor; thereby reducing the crest value of surge voltage.On breakdown of the tube gaps on two or three phases, or on one phase of solidly-groundedneutral circuits, the operating voltage simultaneously initiates a flow of short circuit or powercurrent. This current must flow through the tubes and set up arcs between their spark gaps.The high temperature of the arc across the gap in the tubes then produces a large amount ofgases due to decomposition of some of the tube material. These gases flash out of the tubeunder pressures reaching from 100 to 500 atm and intensely deionize the arc. The latter isthus extinguished and the circuit insulation returned to its normal value with respect to earth.Arc extinction duration will be only one or two half-periods. This interval is too short for theprotective relays of the line to come into action, the circuit breaker remains closed and theline remains in operation. Immediately after the gases have been expelled and the arcsuppressed, every tube is ready for a new operation.The purpose of external air gap G2 is to isolate the expulsion tube from the line conductor.Failure to provide the external gap would otherwise place the tube at the operating potentialof the conductor and cause flow of leakage currents over the tube surface and to eventualcarbonizing of the tube material and final destruction of the tube.

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The ground wire or earthling screen used for the protection of overhead linesand power stations and substations not only provides an adequate protectionagainst lightning but also reduces the over-voltages induced electrostaticallyor electromagnetically, but such shielding is inadequate in providing protectionagainst travelling waves which may reach the terminal equipment and causedamage to it.

The damages that may be caused by travelling waves are:i. The high peak or crest voltage of the surge may cause flashover in the

internal winding thereby spoil the winding insulation.ii. The steep wave front of the surge may cause internal flashover between

inter-turns of the transformer.iii. The high peak voltage of the surge may cause external flashover, between

the terminals of the electrical equipment which may result in damage toinsulators.

iv. The steep wave front resulting into resonance and high voltages may causeinternal or external flashover of an un-predicable nature causing building upof the oscillation in the electrical apparatus.

Protection of Electrical Equipment from Travelling Waves:

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Thus it is absolutely necessary to provide some protective device at thepower stations or substations to prevent transformers and other equipmentfrom being subjected to travelling surges reaching there.

The most common devices used for protection of equipment at thesubstations against travelling waves are lightning arresters or surgediverters.

A surge diverter is a device that is connected between line and earth, i.e., inparallel with the equipment to be protected at the substation.

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When a travelling wave reaches the diverter, itsparks-over at a certain prefixed voltage asillustrated by point A in the figure, and provides aconducting path of relatively low impedance betweenthe line and ground.

The surge impedance of the line restricts theamplitude of current flowing to ground. This isnecessary in order to protect the insulation of theequipment. Fig. 9.17 shows the shape of voltage andof current at the diverter terminals.

It should, however, be noted that the surge divertershould provide a path of low impedance only when thetravelling surge reaches the surge diverter, neitherbefore it nor after it.

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It should not draw any current during normal operating conditions, i.e., itsspark over voltage must be above the normal or abnormal power frequencythat may occur in the system.

Any abnormal transient voltage above the breakdown value must cause it tobreakdown as quickly as possible so that it may provide a conducting path toground.

When the breakdown have taken place, it should be capable of carrying theresulting discharge current without getting damaged itself and without thevoltage across it exceeding the breakdown value.

The power frequency current following the breakdown must be interrupted assoon as the transient voltage has fallen below the breakdown value.

An ideal surge diverter should have the following characteristics:

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There are many types of surge diverters are used to protect the power system.

The choice of lightning arrester depends upon the following factors:

(i) Voltage of the line.

(ii) Frequency of the lightning.

(iii) Cost.

(iv) Weather conditions.

(v) Reliability.

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1.Road Gap Arrester

2.Sphere Gap Arrester

3.Horn Gap Arrester

4.Multiple-Gap Arrester

5.Impulse Protective Gap

6.Electrolytic Arrester

7.Expulsion Type Lightning Arrester

8.Valve Type Lightning Arresters

9.Thyrite Lightning Arrester

10.Auto valve Arrester

11.Oxide Film Arrester

12.Metal Oxide Lightning Arresters

The lightning arrestor is mainly classified into twelve types.

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1. Rod Gap ArresterIt is one of the simplest forms of the arrester.

The gap setting of the arrester should be such that it shouldbreak before the damage.

When the high voltage occurs on the line, the gap sparks andthe fault current passes to the earth.

The difficulty is that once the spark taken place it maycontinue for some time even at low voltages.

To avoid it a current limiting reactor in series with the rod isused.

The resistance limits the current to such an extent that it issufficient to maintain the arc.

Another difficulty with the road gap is that the rod gap isliable to be damaged due to the high temperature of the arcwhich may cause the rod to melt.

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2. Sphere Gap Arrester

The air gap is provided between two different spheres.

The spacing between the two spheres is very small.

A choking coil is inserted between the phase winding ofthe transformer and spheres is connected to the line.

The air gap between the arrester is set in such a wayso that the discharge must not take place at normaloperating condition.

The arc will travel up the sphere as the heated air nearthe arc tend to rise upward and lengthening till it isinterrupted automatically.

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3. Horn Gap Arrester

It consists of two horns shaded piece of metalseparated by a small air gap and connected inshunt between each conductor and earth.

The distance between the two electrodes is suchthat the normal voltage between the line andearth is insufficient to jump the gap.

But the abnormal high voltage will break the gapand so find a path to earth.

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4.Multiple-Gap Arrester

The multiple gap arrester consists aseries of small metal cylinderinsulated from one another andseparated by an air gap.

The first and the last of the series isconnected to ground. The number ofgaps required depends on the linevoltage.

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5.Impulse Protective Gap

The protective impulse gap is designed to have a low voltageimpulse ratio, even less than one and to extinguish the arc.

The auxiliary needle is placed between the mid of two sphereS1 and S2.

At normal frequency, the impedance of the capacitance C1 isquite large as compared to the impedance of resistor R.

If C1 and C2 are equal the potential of the auxiliary electrodewill be midway between those of the S1and S2 and theelectrode has no effect on the flash over between them.

When the transient occurs the impedance of capacitor C1 andC2 decrease and the impedance of the resistor now becomeeffective.

Due to this, the whole of the voltage is concentrated acrossthe gap between E and S1.

The gap at once breakdown, the rest of the length between E and S2immediately follow.PRKREDDY, GNITS

6. Electrolytic Arrester

In such type of arrester have high a largedischarge capacity. It operates on the fact thatthe thin film of aluminum hydroxide deposits onthe aluminum plates immersed in the electrolyte.The plate acts as a high resistance to a low valuebut a low resistance to a voltage above a criticalvalue.

Voltage more than 400 volts causes a punctureand a free flow of current to earth. When thevoltage remains its normal value of 440 volts, thearrester again offers a high resistance in thepath and leakage stops.

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7. Expulsion Type Lightning Arrester

Expulsion type arrester is an improvement over therod gap in that it seals the flow of power frequencyfollows the current. This arrester consists of a tubemade up of fibre which is very effective, isolatingspark gap and an interrupting spark gap inside thefibre tube.

During operation, the arc due to the impulse sparkover inside the fibrous tube causes some fibrousmaterial of the tube to volatile in the form of the gas,which is expelled through a vent from the bottom ofthe tube. Thus, extinguishing the arc just like incircuit breakers.

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8. Valve Type Lightning Arresters

Such type of resistor is called nonlinear diverter. Itessentially consists a divided spark gap in series with aresistance element having the nonlinear characteristic.

The divided spark gap consists of some identicalelements coupled in series. Each of them consists twoelectrodes with the pre-ionization device. Between eachelement, a grading resistor of high ohmic value isconnected in parallel.

During the slow voltage variations, there is no sparks-over across the gap. But when the rapid change involtage occurs, the potential is no longer evenly gradedacross the series gap. The influence of unbalancingcapacitance between the sparks gaps and the groundprevails over the grounded resistance. The impulsevoltage is mainly concentrated on the upper spark gapwhich in spark over cause the complete arrester to sparkover to. PRKREDDY, GNITS

9. Thyrite Lightning Arrester

Such type of arrester is most commonly used forthe protection against dangerous high voltage. Itconsists the thyrite which is an inorganiccompound of ceramic material. The resistance ofsuch material decreases rapidly from high valueto low value and for current from a low value tohigh value.

It consists a disc whose both the side is sprayedso as to give the electric contact between theconsecutive disc. The disc is assembled insidethe glazed porcelain container. It is used inconjunction with the container.

When the lightning takes place, the voltage israised, and breakdowns of the gaps occur, theresistance falls to a very low value, and the waveis discharged to earth. After the surge haspassed the thyrite again come back to its originalposition. PRKREDDY, GNITS

10. Auto valve Arrester

Such type of arrester consists some flat discs ofa porous material stacked one above the otherand separated by the thin mica rings. The discmaterial is not homogenous and conductingmaterial also have been added. Therefore theglow discharge occurs in the capillaries of thematerial and voltage drop to about 350 volts perunit. The discs are arranged in such a way thatnormal voltage may not cause a discharge tooccur.

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11. Oxide Film Arrester

It consists of pellets of lead peroxide with athin, porous coating of litharge arranged in acolumn and enclosed in a tube of diameter. Out ofthe two lead, the upper is connected to the line,while the lower is connected to the earth. Thetube contains a series spark gap.

When an overvoltage occurs an arc passesthrough the series spark gap and an additionalvoltage is applied to the pellet column and adischarge takes place. After the discharge, theresistance of the pellet gun increases till onlyvery small current flow through it. This smallcurrent is finally interrupted by the series sparkgaps.

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12. Metal Oxide Lightning Arresters

Such Types of diverter are also known as gapless surgediverters, or Zinc oxide diverter. The base materialused for manufacturing metal oxide resistor is zincoxide. It is a semiconducting N-type material. Thematerial is doped by adding some fine power ofinsulating oxides. The powder is treated with someprocesses and then it is compressed into a disc-shaped.The disc is then enclosed in a porcelain housing filledwith nitrogen gas or SF6.

This arrester consists a potential barrier at theboundaries of each disc of ZNO. This potential barriercontrols the flow of current. At normal operatingcondition, the potential barrier does not allow thecurrent to flow. When an overvoltage occurs, thebarrier collapse and sharp transition from insulating toconducting take place. The current start flowing and thesurge is diverted to ground.

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