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oldjimh
User Rank
Silver
Re: Fried Transformer
oldjimh   8/20/2015 10:49:19 AM
NO RATINGS
The transformer  was  more than adequate to power its intended load,  the excitation system . It just couldn't sink the harmonic current that generator produced..

Disconnecting the transformer's neutral from   the generator's neutral  blocked those pesky circulating third harmonic currents .

Only downside to that fix was it'd no longer operate with loss of a single primary side phase.  That was demonstrated when a fuseholder failed to make contact.

To your question: if the designer  intended for that transformer  to carry the generator's third harmonic current,  he needed a   bigger transformer.

I maintain if he'd used a delta-wye instead of wye-delta he'd have been okay ,  just without the ability to run with one phase missing..  which  we gave  up anyway.

old jim

 

 

Papa Smurf
User Rank
Iron
Fried Transformer
Papa Smurf   7/14/2014 3:45:24 PM
NO RATINGS
Did it occur to anyone that the transformer might be substantially under-sized?

William K.
User Rank
Platinum
Re: WYE-WYE?
William K.   6/2/2014 7:40:49 PM
Old, You are certainly correct. The only reason to use the Wye connection is when feeding lighting circuits, which need one side at common.

Stephen
User Rank
Gold
Re: I really DON'T get it!
Stephen   6/2/2014 5:10:50 PM
"what changed?"

They ran the Genset at full rated load for an extended period (as a test) for the first time. Prior, it had not run at full power for long durations, so the transformer didn't overheat enough to fail.


An "oops" for the system integrator!

Old TV tech
User Rank
Iron
WYE-WYE?
Old TV tech   6/2/2014 5:08:46 PM
NO RATINGS
Pardon the pun, but why were the transformers connected WYE-WYE?  Everything I have ever used was either Delta-WYE, or Delta-Delta.  There are too many problems WYE-WYE which is why it is almost never used.

oldjimh
User Rank
Silver
Re: Neutral Grounding Resistor
oldjimh   5/24/2014 12:57:06 AM
NO RATINGS
@ Rosek :  Now there's a thought - lessen the heat and dissipate most of remainder there instead of  the transormer windings.

Sure wish you'd been there !

brett_cgb
User Rank
Gold
Re: I really DON'T get it!
brett_cgb   5/20/2014 12:13:07 PM
NO RATINGS
Ah! Forgot about the MagAmps. Another harmonic load.

Now all you have to do is rewind the way-back machine, and replace those MagAmps with PFC power supplies.

Not gonna happen.    B)

 

rosek
User Rank
Silver
Neutral Grounding Resistor
rosek   5/20/2014 10:20:05 AM
NO RATINGS
Sometimes, those high power gen-sets need to have a neutral grounding resistor installed to prevent that very problem. Most of my experience is with solid-state inverter units but the principals are still the same.

oldjimh
User Rank
Silver
Re: I really DON'T get it!
oldjimh   5/20/2014 1:37:39 AM
NO RATINGS
Thanks,  Bret  for your interest . 

Indeed a rectifier followed by a filter capacitor  takes its current in gulps near the voltage peaks,  as you described.

Before pfc supplies this was a real problem for installations with a lot of computers.  Here's a Square D  paper on that.

http://static.schneider-electric.us/docs/Electrical%20Distribution/Low%20Voltage%20Transformers/Harmonic%20Mitigating/0104ED9501R896.pdf

Our neutral current was a very clean looking 180 hz sinewave,  not narrow peaks from capacitive input power supply filters.   VA going into the transformer was a lot more than came out.

But your point is well taken; load side harmonics count too,  and our magnetic amplifiers were a non-linear load.

old jim

 

brett_cgb
User Rank
Gold
Re: I really DON'T get it!
brett_cgb   5/19/2014 4:14:42 PM
NO RATINGS
Harmonic currents make me think of rectifier-capacitor style power supplies - these generate a large current pulse just before the voltage crest as the filter capacitor charges. I suspect this is where the load-side 3rd harmonic current was actually coming from.

Today, we would install power factor corrected power supplies that force current to be in proportion to the voltage, and make the load (field winding power) appear almost purely resistive. A smaller field transformer would have worked quite well in that situation as there would have been little or no neutral current, even at full load.

Before PFC power supplies were common, oversized transformers or harmonic filters were just about the only way to deal with harmonic currents.

 

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