Main-Neutral Interchange? 2

[FarmallFan]

Hello,
Could you guys please explain to me what a main-neutral interchange procedure is for generator maintenance? I am currently working with some winders and they mentioned this type of work.
Thank You

 

[rbulsara]

Not enough information to comment. In fact I am not sure what you are asking. Describe your system and question better or leave it to your electrical folks.

Only thing I can think of is if you have multiple generators and a single point of grounding for all. There is usually procedure in place to isolate the generator completely from other system for maintenance. This includes isolating the neutral of the particular geneator in such way that other generators remains grounded. But then again I am guessing, something I should not.

 

[waross]

It may involve reversing the connections of the generator coils so that the leads that were the mains leads become collectively the neutral, and the leads that were formerly the neutral point become the mains leads.
I'm guessing also.
respectfully

 

[rbulsara]

waross:

That is interesting. Can you shed more light as to why is it done? I think that is what the OP is asking..

 

[waross]

Hi Rafiq
As you know, if one coil is reversed, the phase angles change from A-B 120, B-C 120, C-A 120 to A-C 60, C-b 60, B-A 240.
This would be corrected by interchanging the leads of the reverse connected coil. I have been wondering if there is a reason why all coils would be reversed.
Still guessing.
I hope one of the winding shop gurus drops in.
respectfully

 

[rbulsara]

Bill:

Thanks. I was wondering why would you do it if there is nothing wrong with windings.

 

[FarmallFan]

hi guys,
i am currently working an outage shift, and a generator is being rewound. the winders on this project told me that other companies employ this procedure to minimize maintenance costs. the winders explained to me when the main and neutrals are swithed the life of the generator can be extended. the time it takes to do this procedure is approx a 1/4 of the time to totally refurbish a generator. this is the first time i have seen the guts a huge generator, its been an experience to say the least! i imagined that there would be a group of people in lab coats doing the work, but i was surprised to see large rubber mallets, and crowbars being used. any of you guys know the advantages/disadvantes of doing this kind of procedure?
thanks for the replies!
FarmallFan

 

[ScottyUK]

I guess this depends on just how big a generator we are talking about. Swapping the line and neutral over on a big set would involve either re-routing the winding ends, or else re-engineering the phase isolated bus to the neutral cubicle and installing a neutral bar at the line cubicles. Both would be a massive undertaking. Seems a damned strange idea.


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Sometimes I only open my mouth to swap feet...

 

[Skogsgurra]

One of the workshop myths we see now and then? I have never heard of this swapping thing. And I see no technical, economic, environmental, or whatever reason is popular today, to do it.

Workshop myths are long-lived and strange creatures.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Hoxton]

I think I have heard of this , but not recently. The stress on the insulation is at the line end, where the voltage to earth is maximum. So swapping line and neutral ends of the phases brings the relatively unstressed neutral end up to full voltage, and gives the stressed line end a rest. No change in the middle of the phase, of course.

I think that modern insulation systems (Class F - epoxy)are less susceptible to voltage stress than the older systems (Class B - bitumen based), so the practice died out. Also, moving the connections may introduce mechanical stress on an older winding.

 

[Skogsgurra]

That IS a valid reason. But never heard of it before. You learn something new (even if it seems to be old knowledge) every day!

PLS for you.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[TurbineGen]

If I may jump in here... isn't the insulation damaged mainly by Partial discharge/corona? If that is the case aren't the distributions of those events fairly uniformly distributed throughout the generator? Or could I be missing something?

Also, wouldn't this practice require swapping phases as well? When doing that would the mechanical stresses felt by the coils be reversed? This idea of a main neutral interchange is new to me as well.

 

[Skogsgurra]

The voltage between turns is very much dependent on how the winding is made. But is is usually so, that the "hottest" part of the winding has more potential difference to the steel/iron parts. And that is where P.D. usually happens in a generator or motor. So changing that *could* be effective in some designs. But, as I said earlier, it seems to be a shop myth with reasons that are not very valid today. But interesting thread.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[davidbeach]

Well, if we rotate tires, why not swap ends of generator windings to even out the wear??? ;-) ;-)

 

[FarmallFan]

thanks guys,
well i have found that this isnt really a myth. hoxton pretty much explained why this practice isnt used much. i also found out the generating company that has used method somewhat recently. all i need to do now is to find out who their outage supervisor was.
farmallfan

 

[ScottyUK]

FarmallFan,

See if you can find out how big the unit was and how the exchange was achieved, particularly:
[ol]
[li]Whether the line cubicle became the neutral and the neutral cubicle the line;[/li]
[li]Whether the endwinding connections were re-routed internally (and was the bracing re-calculated for fault withstand);[/li]
[/ol]


I asked this question of one of our outage supervisors and he looked at me very strangely. I tried to explain further - with this thread - and he indicated that he thought I was mad. Maybe it is a practice which has never been used in the UK?


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Sometimes I only open my mouth to swap feet...

 

[Hoxton]

ScottyUK has made my point clearer:

If it is a large generator (or motor), line and neutral may be on opposite sides of the machine, so you have to re-lay or replace the external cabling (or bus-ducts on very large machines). If you re-connect internally, then you are interfering with existing connections which have to be re-made, and re-braced, thereby putting mechanical stress on the insulation.

First Law of motorcycle maintenance: things that aren't broken don't need mending!

 

[TheBlacksmith]

During an IRIS Power Engineering Seminar I attended, this was discussed. The idea is that the last few turns attached to the mains are where the potential to earth is high enough to cause PD, etc. and swapping would expose the unstressed ends of the winding and pretty much stop further damage on the former output ends. Seems like the generator should be designed to accomondate this rather than an afterthought.