Types of Lightning Arrester

Types of Lightning Arrester: Working, Advantages & Limitations

Introduction – Types of Lightning Arrester

The lightning arrester provides protection against surges. In this article lets see the various Types of Lightning Arrester, Its working, Advantages & Disadvantages.

What is Lightning?

Lightning is a phenomenon in which negative electric charges generated in a thunderstorm discharge to the ground as a result of dielectric breakdown in the air. A lightning surge, even an indirect one, causes a surge voltage on the cable lines, and transmits a momentary high voltage impulse to the sensors/transmitters in the field, or to the inputs of computers and also instruments in the control room.

Lightning Arresters

It is a protective device which conducts the high voltage surge on the power system to ground. The earthing screen and also ground wires fail to provide protection against traveling waves. The lightning arrester provides protection against surges.

Need for Lightning Arresters

The earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against travelling waves which may reach the terminal apparatus. The lightning arresters or surge diverters provide protection against such surges.

Working Principle of Lightning Arresters

Under normal operation, the lightning arrester is off the line i.e. it conducts no current to earth or the gap is non-conducting.

On the occurrence of overvoltage, the air insulation across the gap breaks down and an arc is formed, providing a low resistance path for the surge to the ground. In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.

Types of Lightning Arrester

There are several types of lightning arresters in general use. They differ only in constructional details but operate on the same principle viz. providing low resistance path for the surges to the ground. We shall discuss the following types of lightning arresters:

1. Rod gap arrester

2. Horn gap arrester

3. Multigap arrester

4. Expulsion type lightning arrester

5. Valve type lightning arrester

Types of Lightning Arrester: Working, Advantages & Limitations

Rod Gap Arrester

• It is a very simple type of diverter and consists of two 1·5 cm rods which are bent at right angles with a gap inbetween. One rod is connected to the line circuit and the other rod is connected to earth. The distance between gap and also insulator (i.e. distance P) must not be less than one-third of the gap length so that the arc may not reach the insulator and damage it. Its is one of the widely used Lightning arrester among the other types of Lightning Arrester.

Working:

• 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 also the surge current is conducted to earth. In this way, excess charge on the line due to the surge is harmlessly conducted to earth.

Limitations

• After the surge is over, the arc in the gap is maintained by the † normal supply voltage, leading to a short-circuit on the system.
• The rods may melt or get damaged due to excessive heat produced by the arc.
• The climatic conditions (e.g. rain, humidity, temperature etc.) affect the performance of rod gap arrester.
• Lastly the polarity of the surge also affects the performance of this arrester. Due to the above limitations, the rod gap arrester is only used as a ‘back-up’ protection in case of main arresters.

Horn Gap Arrester

• Next in the list of types of Lightning Arrester is Horn gap arrester. Figure shows the horn gap arrester. It consists of two horn shaped metal rods A and also B separated by a small air gap. The horns are so constructed that distance between them gradually increases towards the top as shown. The horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance R and also choke coil L while the other end is effectively grounded.
• The resistance R helps in limiting the follow current to a small value. The choke coil is so designed that it offers small reactance at normal power frequency but a very high reactance at transient frequency. Thus the choke does not allow the transients to enter the apparatus to be protected. The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc across the gap.

Working:

• Under normal conditions, the gap is non-conducting i.e. normal supply voltage is insufficient to initiate the arc between the gap. On the occurrence of an overvoltage, spark-over takes place across the *small gap G. The heated air around the arc and the magnetic effect of the arc cause the arc to travel up the gap. The arc moves progressively into positions 1, 2 and 3.
• At some position of the arc (perhaps position 3), the distance may be too great for the voltage to maintain the arc. Consequently, the arc is extinguished. The excess charge on the line is thus conducted through the arrester to the ground.

Advantages

• Firstly the arc is self-clearing. Therefore, this type of arrester does not cause short-circuiting of the system after the surge is over as in the case of rod gap.
• Lastly Series resistance helps in limiting the follow current to a small value.

Limitations

• Firstly the bridging of gap by some external agency (e.g. birds) can render the device useless.
• The setting of horn gap is likely to change due to corrosion or pitting. This adversely affects the performance of the arrester.
• Lastly the time of operation is comparatively long, say about 3 seconds. In view of the very short operating time of modern protective gear for feeders, this time is far long. Due to the above limitations, this type of arrester is not reliable and can only be used as a second line of defence like the rod gap arrester

Multigap arrester

• Next in the list of types of Lightning Arrester is Multigap type arrester. Figure shows the multigap arrester. It consists of a series of metallic (generally alloy of zinc) cylinders insulated from one another and separated by small intervals of air gaps. The first cylinder (i.e. A) in the series is connected to the line and also the other to the ground through a series resistance.
• The series resistance limits the power arc. By the inclusion of series resistance, the degree of protection against travelling waves is reduced. In order to overcome this difficulty, some of the gaps (B to C in Fig. 24.11) are shunted by a resistance

Working:

• Under normal conditions, the point B is at earth potential and the normal supply voltage is unable to break down the series gaps. On the occurrence of an overvoltage, the breakdown of series gaps A to B occurs. The heavy current after breakdown will choose the straight – through path to earth via the shunted gaps B and C, instead of the alternative path through the shunt resistance.
• When the surge is over, the arcs B to C go out and any power current following the surge is limited by the two resistances (shunt resistance and series resistance) which are now in series. Thus the current is too small to maintain the arcs in the gaps A to B and normal conditions are restored. Such arresters can be employed where system voltage does not exceed 33 kV.

Expulsion type arrester

• Next in the list of types of Lightning Arrester is Expulsion type arrester. This type of arrester is also called ‘protector tube’ and is commonly used on system operating at voltages upto 33 kV.
• Figure shows the essential parts of an expulsion type lightning arrester. It essentially consists of a rod gap A A′ in series with a second gap enclosed within the fibre tube. The gap in the fibre tube is formed by two electrodes. The upper electrode is connected to rod gap and also the lower electrode to the earth. One expulsion arrester is placed under each line conductor.
• Figure 2 shows the installation of expulsion arrester on an overhead line. On the occurrence of an overvoltage on the line, the series gap A A′ is spanned and also an arc is struck between the electrodes in the tube. The heat of the arc vaporises some of the fibre of tube walls, resulting in the production of a neutral gas.

Working:

• In an extremely short time, the gas builds up high pressure and is expelled through the lower electrode which is hollow. As the gas leaves the tube violently, it carries away ionised air around the arc. This de-ionising effect is generally so strong that arc goes out at a current zero and also will not be re-established.

Advantages

• Firstly they are not very expensive.
• They are improved form of rod gap arresters as they block the flow of power frequency follow currents.
• Lastly they can be easily installed.

Limitations

• Firstly an expulsion type arrester can perform only limited number of operations as during each operation some of the fibre material is used up.
• This type of arrester cannot be mounted in an enclosed equipment due to the discharge of gases during operation.
• Due to the poor volt/amp characteristic of the arrester, it is not suitable for the protection of expensive equipment.

Valve type arrester

Next in the list of types of Lightning Arrester is Valve type arrester. Valve type arresters incorporate non-linear resistors and are extensively used on systems operating at high voltages. Figure1 shows the various parts of a valve type arrester.

It consists of two assemblies

• Series spark gaps and
• Non-linear resistor discs (made of material such as thyrite or metrosil) in series. The non-linear elements are connected in series with the spark gaps. Both the assemblies are accommodated in tight porcelain container.

Working:

• Under normal conditions, the normal system voltage is insufficient to cause the breakdown of air gap assembly. On the occurrence of an overvoltage, the breakdown of the series spark gap takes place and also the surge current is conducted to earth via the non-linear resistors.
• Since the magnitude of surge current is very large, the non-linear elements will offer a very low resistance to the passage of surge. The result is that the surge will rapidly go to earth instead of being sent back over the line. When the surge is over, the non-linear resistors assume high resistance to stop the flow of current.

Advantages

• They provide very effective protection (especially for transformers and cables) against surges.
• They operate very rapidly taking less than a second.
• Lastly the impulse ratio is practically unity.

Limitations

• Firstly they may fail to check the surges of very steep wave front from reaching the terminal apparatus. This calls for additional steps to check steep-fronted waves.
• Lastly their performance is adversely affected by the entry of moisture into the enclosure. This necessitates effective sealing of the enclosure at all times.

Applications

According to their application, the valve type arresters are classified as

• Station type and
• Line type. The station type arresters are generally used for the protection of important equipment in power stations operating on voltages upto 220 kV or higher. The line type arresters are also used for stations handling voltages upto 66 kV.

Lightning Arrester FAQ

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