Lightning Arrestar

Engr.Muhammad Younis (MSEES)

Lightening Arrester


OBJECTIVE

What is Lightning

Mechanism of Lightning Discharge

Types of Lightning Strokes

Lightning Arresters

Types of Lightning Arresters


4

Lightning &

Transient Protection


What is Lightning

“An electric discharge between cloud and earth or

between the charge centers of the some cloud is known as

Lightning”.

Lightning is a huge spark and take place when cloudare charged to such a high potential (+ve or –ve)

With respect to earth.



Play a major role in lightning formation

Mechanism of Lightning Discharge

During the up rush of warm moist air from earth ,the friction between the air and

tiny particles of water

causes the building up charges.

When drop of water are

formed, the large drop become

positively charged and smaller drop

become negatively charge. When the

Drop of water accumulate, they form

Clouds and hence cloud may be posses

Fig.01 Engr.Muhammad Younis (MSEES)

cont…either a positive or negative charge, depending

Upon the charge of drop of water they contain.

the charge on a cloud may become so great that It may discharge to another cloud or earth. we call this discharge as lightning . The thunder which accompanies lightning is due to the fact that lightning suddenly heats up the air , thereby causing it to explain. the surrounding air pushes the explained air back forth causing wave motion of air which we recognize as thunder .


Lightning Discharge Diagram

Fig.02

Returnstreamer

Leader

Steppedleader

Cloud Cloud


+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Negative stepped leader

Positive answering leaders

Return stroke

_ _ _ _ _ _ _ _ _ _ _ _ _ _

Cont… As the charge acquired by the cloud increases, the

potential between cloud and earth increases and therefore ,gradient in the air increases.

when the

Potential gradient is sufficient

(5kv/cm to 10kv/cm) to

Break Down the surrounding

air, The lightning stroke

starts.

Fig : 03Engr.Muhammad Younis (MSEES)

Types of Lightning Strokes

There are two main ways in which a lightning

may strike the power system (e.g. overhead line or tower, sub-station etc.)

1. Direct stroke

2. Indirect stroke

Direct stroke: In the direct stroke, the lightning

discharge is directly from cloud to the subject equipment, like overhead line or other surface.

The overvoltage set up due to the stroke may be large enough to flashover this path directly to the ground.


Indirect stroke

Indirect stroke: Indirect

strokes result from the electro statically

induced charges on the conductors due to

the Presence of charge

clouds. A positive charge cloud is above the

line and induces a negative

charge on the line by electrostatic induction

This negative charge , however , will be

only On that portion of the line right under

the cloud and the portion of the line away

from, it will be positive charge.

When the cloud discharge to earth

or an other cloud ,the negative on the wire

is isolate as it cannot floe quickly to earth

over the isolator. Fig:04Engr.Muhammad Younis (MSEES)

Lightning Arresters

“A lightning arrester is a protective device which

Conducts the high voltage surges on the power system to the ground”.

The earthing screen and ground wires can well protect the electric system against direct lightning strokes but the fail to provide protection against traveling waves which may be

reach the terminal apparatus.

the lightning arresters or surge divertersprovide protection against such surges.


Working of lightning arrester

Power circuit

RNon-linearresistor

Sparkgap

Lightningarrester

Fig:04

Fig. o4 show the basic of asurge diverter. It consists of a spark gap

Is series with a non-linear resistor.one end of the diverter is connect to

The terminal of equipment to be Protected and the other end is effectivelyGrounded. The length of gap is so setthat normal line voltage is not enough

To cause an are across the gap but a Dangerously high voltage will break downThe air insulation and from an arc.

“The property of the non-linear Resistant is that its resistancedecreases as the voltage or current

Increases”.


Types of Lightning Arresters

There are several type of lightning arrester in general use.

They differ only in constructional details but operate the same principle viz. providing low resistant path for surge to ground.

We shell discuss the following types.

1. Rod gap lightning arrester

2. Horn gap lightning arrester

3. Multigap lightning arrester

4. Expulsion type lightning arrester

5. Valve type lightning arrester


1. Rod gap lightning arresterRod gap lightning arrester. It is a very

simple type of diverter and consist of two rods. One is connected to the line and other on to earth. the distant between gap and insulator must not be less then one-third of the gap length so that the arc may reach the insulator and damage it.

“Under normal operating conditions, the gap remains non-conducting . On the occurrence of a high voltage surge on the line , the gap sparks over and the surge current is conducted to earth .in this way ,excess change on the line due to the surge is harmlessly conducted to earth”.


Rod type lightning arresterLine

Rod connectedto line

Rod gap

Rod connectedto earth

Transformertank

TransformerBushing

Fig.05

P


Limitations of rod type The rod may malt or damaged due to excessive heat

produced by the are.

The climatic conditions

(e.g. rain, humidity, temperature etc.) affect the performance of rod gap arrester.

The polarity of surge also affected the performance of this arrester.

“Due to the above limitations the rod type arrester is only used as a ‘Back-up’ protection in case of main arrester”.


2.Horn gap lightning arresterIt consist two horn shaped metal rod

A and B separated by a small air gap. the Horns are so constructed that distance between them gradually increased.

one end is connected to the line through resistance R and choke coil L while the other is connected to effectively grounded .The resistance R helps in limiting in follow current to a small value. the choke coil is designed that it offers small reactance at normal power frequency but a very high reactance at transient frequency.


Horn gap lightning arrester Diagram

Fig:06Engr.Muhammad Younis (MSEES)

AdvantagesThe arc is self clearing. therefore this type of arresters

does not cause short circuiting of the system after the surge is over in the case of rod gap.

Series resistance helps in limiting the follow current to a small value.

Note : “This type of arrester is not reliable and can only be used a second line of defense like the rod gap arrester”.


3.Multigap lightning arrester It consist of series of metallic cylinders insulated from

one another and separated small intervals of air gaps.

First cylinder A in series is connected to the line and other to be ground through series resistance. the series resistance limits the power arc.

By the inclusion of series resistance the degree of protection against travelling wave is reduced. In order to overcome this difficulty some of gap(B to C) are shunted by resistance. such type arresters can be employed where system voltage does not exceed 33kv.


Multigap lightning arrester Diagram

Line

A

Series gaps Shunted gaps

Shunted resistance

Seriesresistance

Earth

B C

Fig:07


4.Expulsion type arrestersThis type of arrester is also called “protector tube

"and is commonly used on system operating up to 33kv.

It is essentially consist of rod gap AB in series with a second gap enclosed in a fiber tube. this gap in the fiber tube is formed by two electrodes.

The upper electrode is connected to rod gap and lower is earthed. One expulsion arrester is placed under each line conductor.


Expulsion type arresters Diagram

Fig:08

Line

A

B

Rod gap

Internal gap

Fibertube

Earth

(1) (2)

Horn gap

Protectortube


Advantages They are not very expensive.

They can easily installed.

They are improved form of rod gap arrestor as they block flow of power frequency follow currents.

Limitations this type of arrester can not be mounted in an enclosed

equipment due to discharge of gases during operation.

due to poor volt\ampere characteristics of the arrester it is not suitable for protection expensive equipment.


5.Valve type arrestorValve type arrester incorporate non linear resistor

and extensively used on system , operating at high voltages.

It consist of two assemblies (1)series spark gap (2)non linear resistor disc in series

The non linear elements are connected in series with the spark gaps and earth.


Valve type arrestor Diagram

Line

Spark gaps andGrading resistors

Set of non-linearResistors

(1)(2)

Fig:09

Currentterminal

Porcelainhousing

Spark gaps

Non-linearresistor


WorkingOn the occurrence of an over voltages, the break

down of series spark gap takes place and the surge current is conducted to earth via non linear resistor.

Since the magnitude of surge current is very large the non linear elements offer a very low resistance the passage of surge.

The result, surge will rapidly go to earth instead go back over the line.

When surge is over the non linear resistor to stop flow of current.


AdvantagesThey provide very effective protection (especially for

transformers and cables) against surges.

They operate very rapidly less than second.

ApplicationsStation type used for protection of important power

station operate on up to 220kv

Line type arrester station handling voltage up to 66kv.


Independenttype testing, IEC 60099-4 Surge Arrester Type Testing

Insulation withstand tests

Residual voltage tests

Long duration current withstand test

Operating duty test

Power frequency voltage versus time curve

Short circuit (pressure relief) test

Partial discharge testLatest edition of IEC 60099-4released 2006


THANK YOU


Date post:	15-Jul-2015
Category:	Engineering
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Lightning Arrestar

Engineering