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LONG FLASHOVER ARRESTERS

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CONTENTS

INTRODUCTION

WHAT IS A LIGHTNING STROKE?

TYPES OF LIGHTNING STROKES.

WHAT IS LFA-M?

BLOCK DIAGRAM

DESIGN OF LFA-M

THE LFA-M PRINCIPLE

HOW LIGHTNING AFFECTS OVERHEAD TRANSMISSION?

HOW CAN IT BE PROTECTED?

ADVANTAGES & DISADVANTAGES

FUTURE EXPANSION

CONCLUSION

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INTRODUCTION

• A new simple, effective and inexpensive method for lightning protection of medium voltage overhead distribution line is using long flashover arresters (LFA).

• The important feature of this modular long flashover arrester (LFA-M) is that it can be applied for lightning protection of overhead distribution line against both induced overvoltage and direct lightning strokes

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WHAT IS LIGHTNING STROKE ?

Lightning is an electrical discharge between cloud and the earth, between clouds or between the charge centers of the same cloud.

Lightning is a huge spark and that take place when clouds are charged to at a high potential with respect to earth object (e.g. overhead lines) or neighboring cloud that the dielectric strength of the neighboring medium(air) is destroyed

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TYPES OF LIGHTNING STROKES

DIRECT STROKE

INDIRECT STROKE

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DIRECT STROKE

Lightning discharge is directly from cloud to overhead line.

From the line, current path may be over the insulators down to the pole to the ground.

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

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DIRECT STROKE- TYPES

STROKE-A -The lightning discharge is from the cloud to the subject equipment(e.g. overhead lines).

The cloud will induce a charge of opposite sign on the tall object.

When the potential between the cloud and line exceed the breakdown value of air, the lightning discharge occurs between the cloud and the line.

STROKE-B – Lightning discharge occurs on overhead line due to stroke-A.

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INDIRECT STROKE

Results from electro statically induced charges on the conductors.

(+ve) ly charged cloud induces (-ve) charge right under the cloud on the line and (+ve) charge away from it.

When cloud discharges to earth, (-ve) charge rushes along both directions in form of travelling wave.

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WHAT IS LFA-M?

Long flashover arrestors are those devices which capture the high voltage sparks resulting out of lightening.

”M” indicates that LFA-M is for the protection of medium overhead transmission lines.

It can be applied for lightning protection of overhead distribution line against both induced over voltages and direct lightning strokes.

The induced over voltages can be counteracted by installing a single arrester on an overhead line support (pole).

For the protection of lines against direct lightning strokes, the arresters are connected between the poles and all of the phase conductors in parallel with the insulators.

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11 BLOCK DIAGRAM

BLOCK DIAGRAM

ARRESTER TESTING

DESIGN OF LFA-M12

An LFA-M arrester consists of two cables like pieces. Each cable piece has a semi conductive core of resistance R

The cable pieces are arranged so as to form three flashover modules 1,2,3 as shown in figure. Semi conductive core of upper piece, whose resistance is R

ELECTRIC SCHEMATIC

THE LFA PRINCIPLE

Probability of power arc flow (PAF) depends on many parameters: nominal voltage of the line Unom, length of the flashover path L,

The probability of PAF sharply decreases with a decrease in E

An analysis of available data on spark over discharge transition to PAF concluded that for E=7 to 10 kV/m probability of PAF is practically zero.

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THE LFA PRINCIPLETHE LFA PRINCIPLE14

1- Power line conductor; 2- Insulation tube;3- Metal tube clamp; 4- Insulator;5- Structure;6- Flashover channel; 7- Spark over horn;S- Spark over gap

HOW DOES LFA-M PROTECT THE

OVERHEAD LINES? First, a lightning impulse causes a spark over of the spark

gaps.

Next, the semiconducting core of the upper cable piece, whose resistance , carries the high potential to the surface of the lower piece at its middle. Similarly, the semiconducting core of the lower piece of the same resistance applies the low potential 0 to the surface of the upper piece, also at its center.

Thus the total voltage is applied to each flashover module at the same moment, and all the three flashover modules 1, 2 and 3 are assured conditions for simultaneous initiation of creeping discharges which, with respective modules flashed over, develop a single long flashover channel.

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LFALFA LOOP-TYPE16

LFALFA LOOP-TYPE17

1- Insulated steel loop;2- Clamp;3- Steel cross-arm;4- Metallic tube;5- Power line conductor;6- Flashover channel;7- Insulator;8- Structure.

ADVANTAGES

LFA-Ls help avoid both conductor burnouts and overhead power lines outages caused by lightning induced overvoltage's.

Extend the working life of high voltage circuit breakers.

Protect electric networks against arc overvoltage's.

Cannot be destroyed by lightning currents and power follow currents as metal oxide arresters, gapped silicon carbide arresters or expulsion tubes since currents are flowing outside of LFAs.

Are not stressed by operational voltage and do not require maintenance.

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DISADVANTAGE

We can improve only its efficiency by increasing the number of flashover modules then only it can withstand very high over voltages

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FUTURE EXPANSION

The LFA-M described here consists of three flashover modules. We can increase the flashover modules. If the number of flashover modules increases by increasing the cable pieces this LFA-M can be used for lightning protection of very high voltage lines.

When the modules increases the total arrester stressing is distributed these modules also. Then it can withstand very high over voltages

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CONCLUSIONS

The LFA-M is a simple, low weight and convenient apparatus.

To protect a line against induced over voltages; a single arrestor must be mounted on a pole

The voltage-time characteristics of this modular arrestor assure reliable protection of medium voltage overhead lines against both induced over voltages and direct lightning strokes.

A long flashover arrestor (LFA) comprising three flashover modules using the creeping discharge effect was presented in this report. Its resistors assure application of the total arrestor-stressing voltage simultaneously to all the modules.

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REFERENCES

PRINCIPLES OF POWER SYSTEM BY V.K. MEHTA ,ROHIT MEHTA

IEEE TRANSACTIONS ON POWER DELIVERY .VOL.18,NO 3,JULY 2003 PAGE NO. 781-787

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