Natural Selection

[BillyMac]

Can anybody explain the following senario to me?
Sunday night we had a 66kv insulator track and burn off the X-arm, on it's way down it clips the 11kv circuit running on the same structure. All is good repairs made and from previous experiences expect there to be some 11kv T/X's around with blown Lightning Aresstors.
Myself and college restored 11kv supply from Recloser (which for some reason didn't see the fault, protection settings???) after interrogating the recloser we noticed a lot less amps on C phase, obviously the phase the 66Kv contacted.

So I guess what I want to know is with all the T/X's on this 11Kv feeder, why did it choose only a certain few 3 Phase T/X's to blow the C phase arrestors on, and come to think of it all the Single phase T/X's on C Phase just popped the EDO fuses and went straight back in, no blown arrestor.

Could it be something to do with the integrity of the earthing at the T/X's with blown arrestos? or simply the EDO fuses were older/fatigued?

Any Ideas?

[Edge]

wer they ungrounded wye/delta or "floating" wye/delta? need more info on the banks... but I'm guessing that they were...

Edge

[lewy]

There are a lot of unknowns about the condition of your system, but arrestors do not blow on the first over voltage they see, otherwise we would be changing far more than we do now.

[Edge]

There are a lot of unknowns about the condition of your system, but arrestors do not blow on the first over voltage they see, otherwise we would be changing far more than we do now.

yeah I agree wit ya lew but he said just the arrester on the banks blew only fuses on the single phase pots... leads me to believe that they 3 phase "c"phase pots were tied up in an ungrounded wye/delta config... 66 dropped onto 11 would over voltage the bank causing ferosonance... the floating neutral would pop the arrestor and the bank would counter balance... hence no fuse being blown only the arrestor and it cleared itself because of the over voltage...

it's theory but it's all I got after 8 brews..

for what it's worth...

Edge

[MI-Lineman]

Are your arrestors located before or aft of the trnsfrmr fuses? Arrestors are not supposed to blow on first strikes but do get weakened over time, maybe some were older than others and took more hits in the past? Definitely sounds like that **** ferosonance had something to do with?

[T-Man]

If the arrestors were at the end or near the open end of the line the magnitude of the voltage increases at that point and those LA's will take the brunt of it all. MI is right, the age has much to do with it. . . .they can take more than one hit, but after a while they give up.

[BigClive]

My guess is that the first arrestors to shunt the current clamped the line and protected everything else until the 66kV was clear of the 11kV.