Transformer Burn Out Question

[jcall]

Greetings,

I have a swimming pool UV Lamp sanitizer wall mounted 3 phase 6 KVA buck/boost transformer 480v Delta Primary to 208v wye secondary. It is wired with the pri. and sec. reversed 208 v wye pri. up to 480 v sec..
The amp draw (measured at 480v while in use) is Phase A 6.1A, Phase B 5.7A and Phase C 5.8A. The secondary (208v) has no connections on the X0 tap.
This trans. has been in use for about two years, the outside of the case measured about 232 degrees F, this thing has burned out with no sign of lightning or other componant failure. I am trying to figure out why it burned out so soon.
Any help is appreciated!!!!
jcall

 

[MacGyverS2000]

Wow, I'm not a transformer guy, but I can't imagine too many components lasting at 200+ degrees for a long time... personally, I think I would be amazed it lasted that long. But maybe those things were designed for such high heat over the long haul...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[jcall]

This transformer has a temperature rating rise of 115 degrees celcius, housed in a NEMA 3R case, in a mechanical room that gets about 95 degrees F in summer.

jcall

 

[itsmoked]

No connection on the X0 tap? You mean it is not being fed three phase?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

X0 is the neutral, not needed for three phase. That said it would have been far better to have gotten the proper transformer for the application, a 208 delta-480/277 wye.

It is probably a good thing the neutral wasn't connected. We had a customer connect one just like this to feed 480V x-ray equipment from a 208V service. We had a phase to ground fault on our primary, cleared almost instantaneously by a fuse. Customer's wye-delta transformer (XO connected) continued to supply the ground fault until such time as X0-ground connection burnt open. Excessive currents in the delta damaged the transformer.

Like I say, best to use the proper transformer.

 

[VEBill]

Is it a heated pool...?

 

[jcall]

OK engineers, help me out here, if I am experiencing 6.1 amps/phasae A, 5.7 amps/phase B, and 5.8 amps/Phase C, at 480 volts, is a 6 KVA transformer cutting it too close for proper sizing? Also I am told that a 480 V Delta primary and 208 v wye secondary transformer wired in reverse is suitable for this application. Is thhis correct? Again my primary voltage is 208 wye.

jcall

 

[davidbeach]

Loading is in the 5kVA range, not inherently a problem. Wrong transformer for the application - works in theory but not well in practice, see above. Are you sure that it is a Y-[Δ] and not a T-T?

 

[jcall]

The manufacturer "Dongan" rep. says that this particular transformer is 480 V delta primary, 208 V wye secondary and it can be wired in reverse with no adverse effects.
He also says the 5 kva is big enough to handle the load.
So the unit will be wired 3 phase 208 V wye supply to the transformer "secondary" 208 V wye side and will get 480 v Delta output to the load.
I am thinking about replacing it with a 9KVA unit so as to laod the transformer to around 65%, less load/less heat generated. Thoughts.....

jcall

 

[ScottyUK]

Thought 1... as already posted by David

Buy a transformer with a 208V delta primary and 480V/277V star secondary. Earth the star point. Treat it as a separately derived source.

232F is about 111 celcius, not outrageously hot for a class F or class H winding. If you have a high ambient temperature make sure the transformer is adequately rated at the true ambient temperature with any covers on, panels closed etc. Elevated ambient temperatures require derating of the transformer.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Ditto.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sreid]

I'd like to add that the temperature of a transformer in a closed NEMA box can take hours to stabilize.

 

[jcall]

Thanks folks for all the technical information !!!!
A 208V delta primary and 480V/277V wye secondary will be the replacement unit enabling star point secondary grounding.

Great to have all this knowledge available to tap into.

Much Thanks,
Jcall

 

[Warpspeed]

The delta connected secondary makes me highly suspicious about circulating harmonic currents flowing around the three secondaries. These currents obviously will not show up in the external load connections. That is the reason most transformers run delta/star, to reduce harmonic problems.

This could be producing quite a fair bit of mysterious hidden internal heating of the secondary.

Not sure if this is really the cause, but there are two solutions. Run the secondaries in open delta, with a 480v load directly across each winding.

Run the secondaries in star, with 480v phase to neutral, and ground the secondary centre point.

 

[georgeshall]

I second Warpspeeds opinion. I've seen a power center transformer fail due to harmonics issues with UV banks at a water treatment plant, and this happens a lot if the correct type of transformer is not supplied. We actually change the way we design now and specify that the UV bank suppliers also supply the power center transformers and warrant the installation, but they always seem to take exception to this.

 

[jcall]

Looking back on this UV transformer failure, it gets me to thinking I might need to pass on this thread to them to consider a change in the way they are doing their UV instalations.

 

[LionelHutz]

If the current on the primary was measured you may have seen an overload in the transformer caused by some odd interaction between the power source and the transformer connections.

 

[dpc]

Most UV systems I have seen use high frequency electronic ballasts that generates a lot of harmonics. I agree with the others that harmonic currents could be at least part of the problem.