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#62612 - 02/21/06 01:53 AM Ferro Resonance
RODALCO Offline
Member

Registered: 12/08/05
Posts: 863
Loc: Titirangi, Akld, New Zealand
We have the odd problems relivening the older 11 kV polyloom cables in our network, after either transformer maintenance or upgrade.

These are 11 kV single core screened cables.

These circuits are usually fitted with ddo's and to avoid the inrush surge, we either liven 1Ø first, then the other 2Øs at the same time with 2 sets of fuse sticks.
Or the other way around, liven 2Øs first, then the third phase.

We isolate the secondary transformer fuses to avoid single phasing of motors, do a secondary voltage check before inserting the LV fuses.

Ok easiest would be to drop the feeder briefly, insert ddo's and liven all 3Øs at once, this usually avoids the ferro resonance problem. Of course our controlroom doesn't like that so it's out of the question.

I have heard of putting a resistive load on the TF secondary. like a 2 kW heater to "soften" the inrush to the transformer, Not had much luck with it yet.

Usually it's BANG !! and one ddo blows up, we have a cable fault on the polyloom and put in a generator(s) for the time being if we can't backfeed the 11 kV.

There are lots of theoretical explanations of above topic on Google but has any one practical experience with this fenomena and the best way to minimise the blow ups?

The best way is of course replace the cables, they are over 30 years old and are not armoured, only screened with a thin Cu sleeve and then black PVC sheeth.

Thanks, Ray
_________________________
The product of rotation, excitation and flux produces electricty.

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#62613 - 02/21/06 02:53 PM Re: Ferro Resonance
WFO Offline
Member

Registered: 09/03/05
Posts: 206
Loc: Cat Spring, TX
Not that I'm going to come up with anything practical here, but I'm a still a little confused on your hookup. What is the transformer primary and secondary voltage and configuration (ie-wye/wye, wye/delta, etc.)?



[This message has been edited by WFO (edited 02-21-2006).]

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#62614 - 02/21/06 03:26 PM Re: Ferro Resonance
RODALCO Offline
Member

Registered: 12/08/05
Posts: 863
Loc: Titirangi, Akld, New Zealand
Primary 11 kV Delta.
Secondary 230 / 415 Volts star.
M.E.N. system
Dy 11
_________________________
The product of rotation, excitation and flux produces electricty.

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#62615 - 02/22/06 05:19 PM Re: Ferro Resonance
WFO Offline
Member

Registered: 09/03/05
Posts: 206
Loc: Cat Spring, TX
Quote:
"I have heard of putting a resistive load on the TF secondary. like a 2 kW heater to "soften" the inrush to the transformer, Not had much luck with it yet."

The quick answer here is I don't know.
I do know that when we go to take our capacitor banks off line in the substation, they come off a whole lot easier if the buss is still loaded than after we've transferred all the load off.
I'm sure it's got something to do with resonance, but beyond that, I'm clueless.

You might try re-posting that question here http://www.eng-tips.com/threadminder.cfm?pid=238&page=1

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#62616 - 02/22/06 08:17 PM Re: Ferro Resonance
HighPotter Offline
Member

Registered: 08/29/04
Posts: 41
Loc: So. Cal
In my experience, a 3 gang operated switch is best used in situations like this.

But if ya dont have one, it's safer to close in WITH load, then without.

HP

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