Lightning protection to structures and power systems

LIGHTNING PROTECTIONTO STRUCTURES AND POWER SYSTEMS

Prof. Suryanarayana Danturti

What is Lightning?

Lightning is a discharge of electricity between two clouds or a cloud and earth.

The magnitude of current is in the range of about 2kA to 200kA.

The current rises rapidly typically in about 5-10 microseconds and falls to a low value in about 100microseconds.

EFFECTS OF LIGHTNING

When the lightning discharge passes through a conducting path like a lightning conductor heat is produced but the amount of heat produced is very small as the phenomena is short lived and lasts for 100 micro seconds or less.

When the discharge current (i) passes through the lightning conductor a large voltage drop is produced as the magnitude of the current and also the rate of rise of current (di/dt) are very high .

The voltage drop is given by iR+ l di/dt Even though the resistance(R) and the

inductance of the lightning conductor(l) are small, the voltage drop, as given above, is high as i and di/dt are very high. This rises the potential of the lightning conductor momentarily to a very high value during discharge and may cause arcing at sharp bends in the lightning conductor.

When the lightning strikes a structure made up of non conducting materials like brick work, masonry or concrete, it is shattered.

Since most of the structures are made up of non conducting materials , structures which are prone to lightning are provided with a lightning conductor system to protect the structure against lightning.

PRINCIPLE OF LIGHT PROTECTION

A lightning conductor works by diverting to itself a stroke which might otherwise strike part of the building being protected

The zone of protection is the space with in which a lightning conductor provides protection by attracting the stroke to itself.

It has been found that a single vertical conductor attracts to itself strokes of average or above average intensity which in the absence of the conductor would have struck the ground within a circle having its centre at the conductor and a radius equal to twice the height of the conductor. The protective angle works out to 63.5 degrees as shown in the next slide.

For weaker than average discharges the protective area becomes smaller.

Practical design rule Statically satisfactory protection can be

provided to a zone consisting of a cone with its apex at the top of the vertical conductor and a base of radius equal to the height of the conductor. Thus the protective angle becomes 45 degrees.

A horizontal conductor can be regarded as a series of apexes coalesced into a line and the zone of protection thus becomes a tent like space shown in the next slide.

Protective zone of a single vertical conductor

Courtesy: IS 2309:1989

Protective zone of a single horizontal conductorCourtesy: The Design of Electrical Services for Buildings by F.Porges

Zone of protection for a building provided with a lightning conductor of square shape laid along the periphery of the roof is shown in the next slide

RISK INDEX

Whether or not a building needs protection is decided based on probability of lightning stroke and its consequences. The risk index is calculated based on the following factors.

Index numbers 1 to 10 are allotted for each factor. Higher index figure indicates greater risk.

Use of Structure :- Residential buildings or houses in a large colony of similar houses have lower risk index (2to4).

Public places like big office buildings, churches, assembly halls, auditoriums, post offices, stadiums, hospitals and schools etc have higher risk index (6 to 10).

Type of construction:- Buildings made up of concrete, masonry, brick work have lower risk index (2 to 4) while made up of timber or metal roofs have larger risk index (7to9).

Contents or Effects :-Domestic buildings, small workshops or factories have smaller risk index(2 to 4), while power stations, telephone exchanges, petroleum and gas installations, key industrial plants, monuments, historical buildings, schools, hospitals have greater risk index( 6 to10).

Degree of Isolation:-buildings surrounded by buildings of similar height have lesser risk index(2 to 4),while isolated buildings or buildings twice the height of surrounding buildings or trees have higher risk index (8to10).

Type of Country:- Flat country or hill country at lower altitude below 300 Mtrs has a lower index (2 to 6),while mountainous country at higher altitudes have higher index ( 8 to 10).

Height of structures:- Risk index increases for higher structures from 9 to 53 Mtrs (from 2 to 30).Higher than 53 Mtr structures require protection in all cases.

Lightning prevalence:-Number of thunder storm days in a year varies from 3 to 200 or more. Risk index is equal to the number of thunder storm days up to 21. Above 21 days risk index remains 21.

Total risk index based on above factors is calculated. If the total is more than 40 lightning protection is generally recommended.

DESIGN LIGHTNING PROTECTION SYSTEM Hence a structure can be protected by

providing a set of horizontal conductors to attract the lightning strokes and a set of down conductors which are earthed to discharge the current to the earth.

Number of horizontal conductors:- For buildings with flat roof no part of the building should be more than 9 m from the nearest horizontal conductor .In case of buildings with inclined roof an additional 0.3 m may be added for each 0.3 m by which the part to be protected is below the nearest conductor.

Hence the maximum spacing between conductors shall not be more than 18 m. for flat roofs

For inclined roofs, the spacing can be beyond 18 m as per rule stated above.

Normally lightning conductor is provided along the periphery of the roof. Extra horizontal conductors are fixed if required as shown

Horizontal conductors for a flat roof

Arrangement of horizontal conductor system for buildings with flat roof are shown in the following slides. These arrangements ensures no part of the building is more than 9 Mtrs from the nearest horizontal conductor.


Elevation showing protective angle

18m (max.)


Plan showing zone of protection at ground level

18m (max.)


Isometric View of building showing horizontal conductors

Courtesy: The Design of Electrical Services for Buildings by F.Porges

Plan of the building showing horizontal conductors


Part elevation of the building


Horizontal conductor system for low factory building with large horizontal roof

Horizontal conductors for a inclined roof

Following slides show two examples of common profiles for inclined roofs covering large areas.

Horizontal conductors are fixed along the ridges bounded at either end by conductors following the roof profile.

A 10 Mtrs horizontal spacing is adopted between conductors because of site conditions as the same can be laid along the ridges only.

If S> 10+H additional longitudinal conductors are required.

Industrial building with inclined roof Courtesy: IS

2309:1989


Industrial building with a different roof profile

If the length of the roof exceeds 20 Mtrs additional transverse conductors are required as shown in the next slide.

Horizontal conductor system is quite effective in attracting the lightning strokes and use of pointed air terminations or vertical finials is, therefore not required as essential .

However a vertical conductor may be used to protect a special structure like a chimney or a narrow vertical structure like a pent house.(illustrated in the following slides)


Plan showing a building with penthouse


Elevation showing a building with penthouse

Number of Down Conductors

One conductor if the base area is not more than 100 Sqm.

If the base area is more than 100 Sqm adopt smaller of the following

One plus an additional one for each 300 Sqm or part there of

One for each 30 m of the perimeter of the structure.

A down conductor should follow the most direct path between the horizontal conductor network and the earth terminal network.

Where more than one down conductor is used the conductors should be arranged as evenly as practicable around the periphery of the structure.

Each down conductor is connected to a separate earth electrode.

Where there are more than one earth electrode, all the earth electrodes are interconnected by a ground ring . This is called earth terminal network.

The total earth resistance of the complete lightning protection system to earth should not exceed 10 ohms.

Protection to Buildings where Explosives are Manufactured, Processed or Stored

For Such buildings, smaller spacing between horizontal conductors, larger number of down conductors are adopted. Refer BS /BIS code of practice for details.

Protection to Power systems

Transmission lines are to be protected against

1. Direct strokes 2. Over voltages induced in the power system

due to indirect strokes

Protection against direct strokes is by shielding.

A shielding angle of 30 degrees is adopted

which means , all the live conductors below should be within an angle of 30 degrees as shown in the next slide

Field studies have indicated that a shielding

angle of 30 degree provides 0.1 % exposure. This means out of 1000 strokes 999 strokes will be shielded and only stroke may hit the protected object.

However the presence of shield wire does not provide protection against induced over voltages occurring in the power system due to indirect strokes.

Also over voltages occur in the power system due

to switching. As components of power system i.e.

transformers, transmission lines, cables etc possess both inductance and capacitance and transient over voltages are produced when ever such circuits are either switched on or off.

Transient Over Voltages

The frequency range of the switching transients are in the range of a few kilo cycles and last for a few hundred milliseconds.

The magnitude of the switching over voltages are in the range of 3 to 4 times the magnitude of the system voltage.

In the case of lightning over voltages the magnitude is independent of the system voltage and may reach values up to 1500 to 2000 KV and above.

The frequency range is in the range of a few hundred mega cycles and may last up to a few hundred micro seconds.

The most commonly used device for protection against high voltage surges is called a surge diverter. It is connected between each line and the earth

For a three phase system a set of three surge diverters will be connected between each line and earth as shown in the next slide.

The surge diverter diverts to earth any voltage surge in excess of a pre fixed voltage wave. When a voltage surge reaches the diverter, it breaks down and sparks over at a certain prefixed voltage as shown by point A in the next slide and provides a conducting path of low impedance to earth.

OPERATION OF SURGE DIVERTOR The voltage surge is, diverted to the earth and this

results in flow of current to earth through the diverter. Since the impedance of the path is low and the

voltage is very high, a very high current flows which is in the range of a few kilo amperes for the duration of the surge which may last for a few microseconds to few milliseconds.

The voltage drop across the surge diverter due to

flow of heavy current and due to the finite impedance of the diverter is called residual voltage which is shown in the previous slide.

In other words the insulation of the equipment ‘sees’ only the residual voltage as against being subjected to the peak voltage ‘P’ of the surge shown in dotted line in the previous slide. The figure also shows the resulting current wave.

It is very important to note that this low

impedance path should not exist before the over voltage appears and it should cease to as soon as the voltage returns to normal.

REQUIREMENTS OF IDEAL SURGE DIVERTOR. No current should flow through the diverter

during the normal system voltage .i.e its power frequency break down (or spark over) voltage must be above any normal or abnormal power frequency voltage that may occur.

Any voltage surge with a peak voltage exceeding the spark over voltage of the surge diverter must cause it to break down as quickly as possible and provide low impedance path to ground.

Having broken down, it must be capable of carrying the resulting discharge current which shall be in the range of a few kilo amperes for the duration of the surge ,without damage to itself and without the voltage across (residual voltage) it exceeding the breakdown voltage of the insulation of the equipment it is protecting.

The surge diverter must regain its high impedance as soon as the surge has passed to interrupt the flow of power frequency current due to normal system voltage. An ideal surge diverter shall offer infinite impedance under normal conditions and quickly breaks down and offers zero impedance as soon as a high voltage surge reaches it.

Surge diverters are also called lightning arrestors. As the device actually diverts the surge to ground and as it cannot arrest (stop) the surge ,the term surge diverter is technically correct.

METAL oxide type arrestors are used for

protecting the power systems against over voltages induced due to lightning strokes or switching.

For selecting the voltage rating and current rating of surge diverters, location and method of installation and for other details any book on Power System Protection may be referred.

REFERENCES

The Design of Electrical Services for Buildings by F.Porges

B S Code of Practice 326 of 1965 B I S Code of Practice 2309 of 1989 Power System analysis and Design by Glover

and Sharma

For suggestions and clarifications contact

[email protected]

Lightning protection to structures and power systems

Engineering

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