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Sherlock Ohms

Southern Fried Transformer

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William K.
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Platinum
Re: I really DON'T get it!
William K.   5/12/2014 7:49:08 AM
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OldJim, thanks for the clraification about why the failures appeared. For that very same reason it has always been a policy of ours to run a system or machine for 24 hours non-stop before delivering it to our customers. The time and power for the runoff cost less than even one service call, and fixing any problems prior to delivery means that the customers never see them. Shipping systems that don't have problems does a lot to enhance an organizations reputation, which is also quite valuable. And, in addition, we are able to learn from any problems that develop and avoid those design or assembly problems in the future. 

Removing the neutral connection on the primary side would seem to be a reasonable solution, and it appears that nobody initially asked why that connection should be there in the initial design, since it did not carry any load current.

oldjimh
User Rank
Silver
Re: I really DON'T get it!
oldjimh   5/11/2014 11:18:01 PM
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@ William K

 you asked  "What Changed"

Oops - sorry ,  you're right i didnt  make that clear.

 

What changed the first time was the duration of our test run.

Our usual run was for just an hour.

The transformers failed after seven hours at full  load. They'd never before seen a run of that duration at that power. Even when we'd used the generators for longer periods it was at quite low load  where third harmonic currrent wasn't  excessive.

 

At low load the third harmonic current was modest enough,  but it increased with excitation which increased with load.

Below half load they'd have lasted quite a long time.

 

The replacement transformers that failed more quickly were lower impedance. Engineering felt they'd be less lossy.  

We reasoned that's the reason they lasted less time.  Their delta secondary is a dead short for third harmonics because they're in phase as mentioned earlier.  A hundred pound transformer just can't absorb all the harmonic from a two thousand pound generator.   All the transformer has available  to oppose the generator's ~5% third harmonic voltage content is its own impedance;  and less impedance allowed more current.

Removing the neutral connection between the huge generator and the small transformer blocked third harmonic current,  at the expense of   mild neutral instability on the small transformer's primary side..  It's run fine since.

Thanks  guys,  for the questions -  helps me improve my technical writing.

old jim

 

oldjimh
User Rank
Silver
Re: I still don't think you get it...
oldjimh   5/11/2014 11:01:40 PM
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@ Jlawton

 You offered that presence of third harmonic current in the neutral  suggests imbalance as the culprit.

Well,  actually if you plot third harmonic for each of the phases versus time ,  they're in phase with one another.

That's because 1/3 cycle  at 60hz is a whole cycle at 180 hz. Count the milliseconds. Or plot the sinewaves.

So for third harmonic , what returns via the neutral is the SUM of the individual phase currents, not their difference or imbalance as with fundamental.

Third harmonic currents in the neutral  do NOT indicate imbalance. That's where they go naturally.  They'll be there in a perfectly balanced system.

 I stand by the analysis as presented. You might brush up on your three phase.

 old jim

oldjimh
User Rank
Silver
Re: Read both?
oldjimh   5/11/2014 10:50:15 PM
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@ Cabe Atwell

 

 

The generator and its excitation came as a package.  The installer just hooked up the wires as  shown. 

The oversight originated at the supplier who integrated the package,  which included the engine,  generator, excitation,  engine cooling and  short term  fuel delivery systems from several different manufacturers.

Our design reviews also failed to catch it,  as did A/E's.

Lest i sound harsh on them,

it took us quite a while to find from the generator's  original manufacturer just  what was its harmonic content. It certainly wasn't given on the nameplate or in the instruction manuals.

 

old jim

 

 

 

 

 

William K.
User Rank
Platinum
I really DON'T get it!
William K.   5/9/2014 6:38:19 PM
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I can understand that an excess amount of third hyarmonic power dissipated in the transformer would cause it to overheat, but I don't have a clue about why this problem suddenly appeared. And there is always that question about "what changed", when a working system stops working. So either that explanation was left out, or else I am missing something.

jlawton
User Rank
Gold
I still don't think you get it...
jlawton   5/6/2014 9:30:33 AM
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You leave the reader with the impression that it's the magnitude of the third harmonic content going into the smaller transformer that's the cause of the problem. But most power transformers can handle a somewhat higher flux level at 180 Hz than at 60 Hz. The real point here is it's the IMBALANCE at 180 Hz that's the real culprit (because this showed up in the neutral lead), and that could as well have been an imbalance in the load at that frequency as imbalanced generation across the phases of 180 Hz coming out of the generator. If you're going to publish a resolution of the problem please try at least to be accurate!

Cabe Atwell
User Rank
Blogger
Read both?
Cabe Atwell   5/5/2014 11:28:07 PM
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Would this have been prevented if the installer read the info off of the generator, or was that not an option?

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