Turning 240v to 480v

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detroitelectrician

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I have a job at a small factory where they need 480v 3 phase 150 amps, the poco supplies them with 3 phase 240 with a high leg. The owner went out and purchased a used (480v to 240v) 75 kva transformer and asked me to reverse it to step up the voltage. I hooked up three phase 240v to x1,x2 and x3 and grounding x4 which is the way it was hooked up as a step down transformer. Only this time the input voltage was on the 240 volt side. Before energizing the transformer I did a continuity check between ground and x1 and got 0 ohms. I told the owner the transformer was defective and I refused to energize it. He did not want to believe that so he energized it as I ran outside. Once outside I saw sparks shooting from the bus service and I heard the sound we all know (nannggggg!) as a high amperage fault on the service. He quickly opened the disc switch , the 300 amp fuses did not blow. I still believe the transformer was bad but wonder about the connections. What do you guys think about the connections? Can the connections remain the same when you reverse a transformer?
 
In general, you must only connect your three supply phases to the primary of the transformer. The neutral or center tap connection on the _primary_ side is not necessary to supply the load.

If you don't have everything phased correctly, than the neutral or center tap on the primary side will be at a different potential than the primary system ground. In this case grounding that terminal will cause a high current fault.

If everything is phased correctly, then you can still see problematic current flows from harmonic or imbalance situations.

The DC resistance between any of the transformer terminals will be very low, it could be measured with a milli-ohm meter, but not with an ordinary multimeter. The DC resistance is not relevant. What concerns you is the AC impedance which sets magnetizing current flow.

How will you be grounding your secondary side, or are you installing an ungrounded system?

-Jon
 
The transformer you have is delta primary by wye secondary.
When you reverse it, then is wye primary and delta secondary. NEC 250.30 and 250.20 require this system to have a grounded secondary, with out it the 480 will be 480 to ground which is very dangerous. Preferred is 480/277 which is then 277 to ground.
I would suggest the owner purchase a step up transformer, its delta x wye.
Square D makes them....
this has come up before with no clear answer. I maintain you can reverse a standard transformer unless it shows the alternate connections
 
transf.

transf.

disconnect the neutral. the readings you have are not that abnormal.
 
Tom,
NEC 250.30 and 250.20 require this system to have a grounded secondary, with out it the 480 will be 480 to ground which is very dangerous. Preferred is 480/277 which is then 277 to ground.
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There is no code rule that requires this 480 volt system to be a grounded system.
Don
 
Looks like it's a piece of cake to me.
To install the 480v delta-240v delta transformer as a step up, simply terminate your 240 3ph source at X1, X2, and X3 bring the EGC along to bond to the enclosure. . Ignore X4 if you have one and confirm that it is not bonded to the enclosure. X4 shound simply be insulated and isolated.
Then the H1, H2, H3 provides 480v delta. It is my personal preference to ground the 'B' phase. It provides an opportunty to trip the OCPD should either two of the ungrounded conductors go to ground. If left ungrounded there's not reason for the OCPD to trip should any of the 3 ungrounded conductors goes to ground. But thinks can get a little exciting should one ungrounded conductor go to ground and another goes to ground in another locaton.
Corner grounding also provides the opportunity for an EGC to be taken from the grounded phase to ground the equipment.
 
templdl said:
But thinks can get a little exciting should one ungrounded conductor go to ground and another goes to ground in another locaton.
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That's why the grounded conductor should be colored white like any other.
 
Templdl,
Corner grounding also provides the opportunity for an EGC to be taken from the grounded phase to ground the equipment.
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Just to be clear an EGC is required for both grounded and ungrounded systems. In an ungrounded system nothing happends when the first ground fault path is made. The EGC is part of the fault clearing path when a second ground fault on a different phase occurs.
Don
 
Lou,
The OP was using a Delta high leg service to hook up to the Wye secondary of a transformer. Wouldn't that create some issues since one leg would be 208 to neutral (X0)?
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No, because you don't connect the grounded conductor to the transformer when using it in this manner.
Don
 
When you're reverse connecting a transformer, you'll need to size the primary OCPD with more care. It will take a lot to excite the transformer and get things going when you reverse connect it. If you size the primary OCPD device on a reverse connected transformer too tightly, it will trip on energizing every time. Make liberal use of the rules...
 
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