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#178366 05/29/08 12:04 AM
Joined: Mar 2001
Posts: 2,056
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Redsy Offline OP
Member
I need to install a step-up transformer from a 240 panel to get 480 volts for 3-phase equipment. No 277 required. However, in order to avoid the debate about grounded v. ungrounded 480 v. 3 phase 3 wire delta, I'm thinking about specifying a 277/480Y secondary and grounding the neutral, but not using it.

Comments, please?

Thanks,
John

Joined: Jun 2004
Posts: 1,273
T
Member
Probably not economic.

Stay with a pure delta-delta transform.

Monitor the power for phase loss.

Is this a VFD driven load?


Tesla
Joined: Mar 2005
Posts: 1,213
S
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If you're worried, why not just use a delta-wye transformer, ground the secondary neutral, and just not run a 4th wire? If you don't have to worry about phase shift, it's really the way to go.

Though the coils in a delta secondary aren't grounded, the core is, and magnetic core conductance and reactance will hold a delta secondary near equal potential, especially if the phases are balanced. Think of it like a wye-wye with a high impedance ground on the neutral.

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G
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Corner ground it?


Greg Fretwell
Joined: Mar 2001
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Redsy Offline OP
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Thanks guys!

I hesitate to corner ground the delta because 480 to ground concerns me. However, I'm not sure I want to float the delta either, hence my question.

However, a delta wye step-up xfmr seems to be special order and costs a premium. I will let the customer decide.

The other question...

The xfmr rep. says that although their step-down xfmr can be connected backwards, the inrush will be 10X primary current. A true step-up xfmr inrush would be 4-5X primary current, but a true step-up xfmr would cost more.

I am concerned about tripping primary OCPD if I connect backwards.

Joined: Jun 2004
Posts: 1,273
T
Member
The power calculations on three phase transformers are identical to three phase motors. ( Truly identical )

So just treat the transformer as if it were a motor load and protect it accordingly from the high side.

You'll still need protection on the secondary ( higher ) side because transformer faults still have to be guarded against. Use the Separately Derived System rules.


Tesla
Joined: Apr 2002
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Redsy:
FWIW, I never had an issue with a back-feed on 75 to 150 kva 480-120/208 delta/wye xfrs. All were for 480 machine loads; balance was not an issue.


John
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Redsy Offline OP
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Thanks everyone.

Thanks John,

Do you prefer to corner ground, or float the 480 volt side?

Last edited by Redsy; 06/02/08 07:12 PM.
Joined: Aug 2001
Posts: 7,520
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If you don't want to ground a delta secondary solidly but are concerned about just letting stray capacitances balance out the voltage to ground on the phases, how about using a bank of three resistors to provide a ground reference?

Joined: Apr 2002
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Redsy:
I float the 480 side. 3 phase, 3 wire.


Last edited by HotLine1; 06/03/08 03:46 PM.

John

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