Questions regarding 600\480 Autotransformer

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Azazael

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Automation Tech
First time posting but have been lurking in the background for years, so i would like to begin by saying this is a great site for information and member wise.
At my facility we have a 3 phase 600volt service feeding a panel which is protected by a 1000 amp breaker, ground rod supplied at the panel. A 600 amp breaker then supplies 600 volts and EGC to a 600v\480v Y-Y 500kva auto transformer, this is a HPS auto transformer part# Y500PKCH3LOU with the XO kit.
The 480v and ground then feeds another breaker panel for equipment, no connection at the auto transformer XO. When I take DMM measurements with everything off and isolated after the transformer i get odd readings. On the 600v tap i get H1=156, H2=480, and H3=481. On the 480 tap i get X1=86, X2=415, X3=416. Those readings are all line to ground, phase to phase readings are all correct, and phase to floating XO are all correct as well.
The equipment being feed doesn't require a nuetral so the persons installing this set-up did not land anything to the XO lug. I'm fairly certain the XO and EGC ground should be bonded together at the transformer but when i tried this it tripped the 1000amp breaker GFP. The 1000 amp main is a MCB a PowerPact PG1000 with a Micrologic 6.0. I was told to lift the EGC going out to the 480 panel and land it on the XO lug Could some of you more experienced gentlemen please advise on the XO connection?
Thanks for any help on this.
 
electrofelon said:
It sounds like the 600 volt system is ungrounded, is that correct? You should NOT ground the XO on an auto.
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Yes that is correct it is ungrounded, and i'm assuming they thought the ground rod would make it a grounded system? I believe they should have bought a delta-wye 600/480 transformer, i think it may be called a corner ground? And then i would be able to bond the XO and ground, is that correct?
 
electrofelon said:
In an ungrounded system your line to ground voltages will be all over and are generally meaningless.

Yes, if you do want to make a grounded system, a delta-wye transformer would do it.
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Thank you for your help. I am going to argue for replacing with a D-Y asap. We do not run this equipment as of yet, but everything else in the plant is on a grounded system and this one could cause other issues in the future.
 
I could be over my head here, but perhaps you just need to run a neutral from the service to the transformer XO. That should make your line-ground voltages roughly the same as your line-neutral voltages, which you previously described as 'correct'. There may be other reasons you should have a delta-wye instead of a y-y but if it's just the voltages that are bothering you that should fix it.
 
jaggedben said:
I could be over my head here, but perhaps you just need to run a neutral from the service to the transformer XO. That should make your line-ground voltages roughly the same as your line-neutral voltages, which you previously described as 'correct'. There may be other reasons you should have a delta-wye instead of a y-y but if it's just the voltages that are bothering you that should fix it.
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My understanding is one should not use a regular autotransformer as a grounding transformer. I believe the voltage regulation is poor.
 
A 600 to 480 Autotransformer seems strange to me, the advantage of an autotransformer is small steps in voltage minimizing copper and steel as a cost savings. 600 to 480 seems like a major voltage step and the advantage of an auto transformer would be minimized?
 
brian john said:
A 600 to 480 Autotransformer seems strange to me, the advantage of an autotransformer is small steps in voltage minimizing copper and steel as a cost savings. 600 to 480 seems like a major voltage step and the advantage of an auto transformer would be minimized?
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We did a PV system once where the designer used 480 to 208 autos to interconnect 480 inverters to the 208 service. I am not really sure why, as I imagine when you are at over a 2x change with an auto, the advantages over iso's are pretty much gone. (also an iso gives you taps which can be handy for PV).
 
An autotransformer for 600 to 480 would need to have a KVA rating of (600-480)/600 = 0.208 or ~21% of the load KVA. So if you don't need isolation that's still a considerable advantage.
For 480 to 208 the autotransformer would need to be (480-208)/480 = 0.556 or ~57 percent of the load KVA, so it's probably not worth considering unless the KVA is very high.
 
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