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These overvoltages on the facility system are also caused because of several reasons like lightning, the gap of a circuit breaker, the grounding of a conductor, etc. Most of the overvoltages aren’t of huge magnitude however should still be necessarily attributable to their impact on the performance of circuit interrupting instrumentation and protecting devices. Associate in Nursing considerable range of those overvoltages square measure of comfortable magnitude to cause insulation breakdown of the instrumentation within the facility.

Therefore, facility engineers continually devise ways that and means that to limit the magnitude of the overvoltages created and to manage their effects on the in operation instrumentation. during this chapter, we have a tendency to shall confine our attention to the varied causes of overvoltages on the facility system with special stress on the protection devices used for the aim.

Voltage Surge:

Transients or surges square measure of temporary nature and exist for a really short period (a few hundred μs) however they cause overvoltages on the facility system.

Overvoltages Protection

They originate from change and from different causes however far and away from the foremost necessary transients square measure those caused by lightning placing a cable. once lightning strikes a line, the surge rushes on the road, even as a flood of water rushes on a slim natural depression once the wall of a reservoir at its head suddenly provides means. In most of the cases, such surges could cause the road insulators (near the purpose wherever lightning has struck) to flash over and will additionally injury the close transformers, generators or different instrumentation connected to the road if the instrumentation isn’t fittingly protected.

The voltage build-up is taken on the coordinate axis and also the time on the coordinate axis. it’s going to be seen that lightning introduces a steel fronted wave. The vessel the wavefront, the additional fast is that the build-up of voltage at any purpose within the network. In most of the cases, this build-up is relatively fast, being of the order of 1–5 μs. Voltage surges square measure usually per terms of *rise time t1 and also the time t 2 to decay to half the height worth.

Causes of Overvoltages:

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

  • Internal causes: 
  1. Switching surges
  2. Insulation failure
  3. Arcing ground
  4. Resonance
  • External causes:

Internal Causes of Overvoltages:

Internal causes don’t turn out surges of enormous magnitude. expertise shows that surges thanks to internal causes hardly increase the system voltage to double the traditional price. Generally, surges thanks to internal causes area unit taken care of by providing correct insulation to the instrumentation within the grid. However, surges thanks to lightning area unit terribly severe and will increase the system voltage to many times the traditional price. If the instrumentation within the grid isn’t protected against lightning surges, these surges could cause sizable injury.

  • Switching Surges.
  • Insulation failure.
  • Arcing ground.
  • Resonance.

Switching Surges:

Internal causes of overvoltages on the facility system area unit primarily thanks to oscillations started by the unexpected changes within the circuit conditions. This circuit amendment is also a standard switch operation like the gap of a breaker, or it should be the fault condition like the grounding of a line conductor. In apply, the traditional system insulation is appropriately designed to resist such surges.

The overvoltages created on the facility system thanks to switching operations area unit called switch surges. a couple of cases are going to be mentioned by the manner of illustration :

Overvoltages Protection

  1. Case of Associate in Nursing open line. throughout switch operations of Associate in Nursing unloaded line, traveling waves area unit started that turn out overvoltages on the road. As Associate in Nursing illustration, think about Associate in Nursing unloaded line being connected to a voltage supply If
    Er.m.s. is that the provide voltage, then instant voltage that the road can stand up to are going to be two E . This overvoltage is of temporary nature. it’s as a result of the road losses attenuate the wave and {in a|during a|in an exceedingly|in a terribly} very short time, the road settles right down to its traditional provide voltage E. Similarly, if Associate in Nursing unloaded line is shifted, the road can attain a voltage {of two|of two} 2 E for an instant before subsiding right down to the traditional price.
  2. Case of a loaded line. Overvoltages will be created throughout the switch operations of a loaded line. Suppose a loaded line is suddenly interrupted. this may start a voltage of two Zn I across the break (i.e. switch) wherever i is that the instant price of current at the time of gap of line and Zn is that the natural resistance of the road.
  3. Current chopping. Current chopping leads to the assembly of high voltage transients across the contacts of the air blast breaker as elaborated in chapter nineteen. it’s shortly mentioned here. not like oil circuit breakers, that area unit freelance for the effectiveness on the magnitude of the present being interrupted, air-blast circuit breakers retain constant termination power regardless of the magnitude of this current. once breaking low currents (e.g. electrical device magnetizing current) with air-blast breaker, the powerful de-ionizing impact of air-blast causes the present to fall dead to zero well before the natural current zero is reached.

Insulation failure:

Overvoltages Protection

The most common case of insulation failure in a very grid is that the grounding of the conductor (i.e. insulation failure between line and earth) which can cause overvoltages within the system. Suppose a line at potential E is earthed at purpose X. The grounding of the road causes 2 equal voltages of −E to go XQ and XP containing currents −E/Zn and +E/Zn severally. each these currents meet up with X to earth so current to earth is two E/Zn.

Arcing ground:

Within the time period of transmission, the neutral of three-phase lines wasn’t earthed to achieve 2 blessings. Firstly, within the case of a line-to-ground fault, the road isn’t placed out of action. Secondly, the zero sequence currents area unit eliminated, leading to a decrease in interference with communication lines. Insulated neutrals offer no drawback with short lines and relatively low voltages. However, once the lines area unit long and operate at high voltages, a significant drawback known as the arcing ground is commonly witnessed.

The transients made because of the arcing ground area unit additive and should cause serious harm to the instrumentation within the grid by inflicting the breakdown of insulation. The arcing ground is prevented by grounding the neutral.


Resonance in AN electrical system happens once an inductive electrical phenomenon of the circuit becomes an adequate electrical phenomenon. below resonance, the resistivity of the circuit is adequate the resistance of the circuit and also the p.f. is unity. Resonance causes high voltages within the electrical system. within the usual transmission lines, the capacitance is extremely tiny so resonance seldom happens at the basic offer frequency. However, if generator e.m.f. wave is distorted, the difficulty of resonance could occur because of fifth or higher harmonics and just in case of underground cables too.


An electric discharge between cloud and earth, between clouds or between the charge centers of a similar cloud is understood as lightning. Lightning may be a Brobdingnagian spark and takes place once clouds square measure charged to such high potential (+ve or −ve) with relevancy the planet or a neighbor cloud that the nonconductor strength of neighbor medium (air) is destroyed. There square measure many theories that exist to clarify however the clouds acquire charge. the subsequent points could also be noted concerning lightning discharge :

  1. A lightning discharge that sometimes seems to the attention as one flash is in point of fact created of a variety of separate strokes that travel down a similar path. The interval between them varies from 0·0005 to 0·5 second. every separate stroke starts as a downward leader from the cloud.
  2. it’s been found that eighty-seven of all lightning strokes result from charged clouds and solely thirteen originate from charged clouds.
  3. it’s been calculable that throughout the globe, there occur concerning one hundred lightning strokes per second.
  4. The lightning discharge could have currents within the very often Ka to ninety Ka.

Types of Lightning Strokes:

There are two main ways in which a bolt of lightning may strike the power system (e.g. overhead lines, towers, sub-stations, etc.), namely;

  1. Direct stroke
  2. Indirect stroke

Direct stroke:

Overvoltages Protection

In the direct stroke, the lightning discharge (i.e. current path) is directly from the cloud to the topic instrumentality e.g. AN overhead line. From the road, this path could also be over the insulators down the pole to the bottom. The overvoltages found out because of the stroke could also be massive enough to discharge this path on to the bottom. The direct strokes will be of 2 sorts viz.

  • Stroke A and
  • Stroke B.

In stroke A:

The lightning discharge is from the cloud to the topic instrumentality i.e. AN overhead line during this case (i). The cloud can induce a charge of opposite sign up the tall object (e.g. AN overhead line during this case). once the potential between the cloud and line exceeds the breakdown worth of air, the lightning discharge happens between the cloud and also the line.

In stroke B:

The lightning discharge happens on the overhead line as a result of stroke A between the clouds (ii). There square measure 3 clouds P, Q, and R having positive, negative and positive charges severally. The charge on the cloud Q is sure by the cloud R. If the cloud P shifts too close to the cloud Q, then lightning discharge can occur between them and charges on each these clouds disappear quickly.

Indirect stroke:

Overvoltages Protection

Indirect strokes result from the electrostatically induced charges on the conductors because of the presence of charged clouds. this can be illustrated. A charged cloud is on top of the road and induces a charge on the road by electricity induction. This charge, however, is solely thereon a portion of the road right beneath the cloud and therefore the parts of the road far away from it’ll be charged. The induced electric charge leaks slowly to earth via the insulators.

Protection Against Lightning:

Transients or surges on the facility system could originate from the shift and from different causes however the foremost necessary and dangerous surges area unit those caused by lightning. The lightning surges could cause serious injury to the pricy instrumentality within the installation (e.g. generators, transformers, etc.) either by direct strokes on the instrumentality or by strokes on the transmission lines that reach the instrumentality as travel waves. it’s necessary to produce protection against each variety of surges. the foremost usually used devices for cover against lightning surges area unit :

  1. Grounding screen
  2. Overhead ground wires
  3. Lightning arresters or surge diverters

Earthing screen provides definition to power stations and sub-stations against direct strokes whereas overhead ground wires protect the transmission lines against direct lightning strokes. However, lightning arresters or surge diverters defend the station equipment against each direct strokes and therefore the strokes that get the equipment as travel waves.

Overhead Ground Wires:

The most effective methodology of providing protection to transmission lines against direct lightning strokes is by the employment of overhead ground wires. For simplicity, one ground wire and one line conductor are shown. the bottom wires are placed on top of the road conductors at such positions that much all lightning strokes are intercepted by them (i.e. ground wires). the bottom wires are grounded at every tower or pole through as low resistance as doable. Since Green Mountain State (= I1R1) is that the approximate voltage between tower and line conductor, this can be conjointly the voltage which will seem across the string of insulators. If the worth of Green Mountain State is a smaller amount than that needed to cause stuff discharge, no other results.

Overvoltages Protection

On the opposite hand, if Green Mountain State is excessive, the stuff discharge could occur. Since the worth of Green Mountain State depends upon tower-footing resistance R1, the worth of this resistance should be unbroken as low as doable to avoid stuff discharge.

When the direct lightning stroke happens on the conductor, it’ll be haunted by the bottom wires. The serious lightning current (10 Hindu deity to fifty kA) from the bottom wire flows to the bottom, so protective the road from the harmful effects of lightning. it’s going to be mentioned here that the degree of protection provided by the bottom wires depends upon the footing resistance of the tower.


  1.  It provides considerable protection against direct lightning strokes on transmission lines.
  2. A grounding wire provides a damping effect on any disturbance traveling along the line as it
    acts as a short-circuited secondary.
  3. It provides a certain amount of electrostatic shielding against external fields. Thus it reduces the voltages induced in the line conductors due to the discharge of a neighboring cloud.


  1. It requires an additional cost.
  2. There is a possibility of its breaking and falling across the line conductors, thereby causing a short-circuit fault.
  3. This objection has been greatly eliminated by using galvanized stranded steel conductors as ground wires.
  4. This provides sufficient strength to the ground wires.

Lightning Arresters:

The attachment screen and ground wires will well defend the electrical system against direct lightning strokes however they fail to supply protection against traveling waves which can reach the terminal equipment. The lightning arresters or surge diverters offer protection against such surges.

It consists of a spark gap nonparallel with a non-linear electrical device. One finish of the diverter is connected to the terminal of the instrumentality to be protected and also the different finish is effectively grounded. The length of the gap is therefore set that ordinary line voltage isn’t enough to cause associate degree arc across the gap however a perilously high voltage can break down the air insulation associate degreed type an arc. The property of the non-linear resistance is that its resistance decreases because the voltage (or current) will increase and vice-versa.

Overvoltages Protection

Two things should be taken care of within the style of a suppressor. Firstly, once the surge is over, the arc within the gap ought to stop. If the arc doesn’t leave, this would still flow through the electrical device and each electrical device and gap is also destroyed. Secondly, I R drop (where I is that the surge current) across the constraint once carrying surge current mustn’t exceed the breakdown strength of the insulation of the instrumentality to be protected.

Types of Lightning Arresters:

There are many styles of lightning arresters generally use. They take issue solely in constructional details however treat a similar principle viz. providing a coffee resistance path for the surges to the bottom. we tend to shall discuss the subsequent styles of lightning arresters :

  1. Rod gap arrester
  2. Horn gap arrester
  3. Multigap arrester
  4. Expulsion type lightning arrester
  5. Valve type of lightning arrester [ learn more ]