the BackFlashover

7/25/2019 the BackFlashover

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An insulator flashoverAn insulator flashover

that leads to a systemthat leads to a system

ground fault that inground fault that inturn causesturn causes

The BackflashThe Backflash

Possible conductor damagePossible conductor damage

momentary ormomentary orlong term outageslong term outages

possibly insulator damagepossibly insulator damage

breaker agingbreaker aging


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It can only occur on aIt can only occur on a

shielded line orshielded line or

where the top phasewhere the top phase

acts like a shieldacts like a shield

It can be eliminatedIt can be eliminated

It is usually causedIt is usually caused

by lightningby lightning



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This is how it happensThis is how it happens

on a typical shieldedon a typical shielded

69kV or 115kV69kV or 115kVtransmission line.transmission line.

Normal Voltage


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In most cases, the CriticalIn most cases, the Critical

Flashover Voltage of theFlashover Voltage of the

insulators is not reached,insulators is not reached,so everything returns toso everything returns to

normal after the stroke.normal after the stroke.

Note, the

duration ofthe impulse is

many times

shorter than

the power

frequency


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If conditions are notIf conditions are not

optimum, a backflash andoptimum, a backflash and

ground fault will occurground fault will occur

Note, the

powerfrequency

voltage is

now reduced

to near zero


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The follow currentThe follow currentcontinues until a breaker orcontinues until a breaker or

other device interrupts theother device interrupts the

event.event.

Breaker clear the

circuit


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System

voltagereturns to

normal

After the event isAfter the event is

interrupted, the systeminterrupted, the system

returns to normal.returns to normal.

But only after a momentaryBut only after a momentary

outage for an untoldoutage for an untold

number of customers.number of customers.


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The Backflash fromThe Backflash from

beginning to end withbeginning to end with

all its harmful effectsall its harmful effects


Possible conductor damagePossible conductor damage

momentary ormomentary orlong term outageslong term outages

possibly insulator damagepossibly insulator damage

breaker agingbreaker aging

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the BackFlashover