Singel Phase Operating Conditon for a 3-Phase Transformer

[rcw retired EE]

A client's RFP specifies that the "480 Volt Secondary unit substation shall be rated to supply the total 480 volt auxiliary load plus 20 percent under all single Phase Operating Conditions."

How do we size/specify the transformer to continue carrying three-phase loads when the primary is single phased? The "Single Phase Condition" is not defined. We assume it refers to the primary side and not loss of one phase of 480V.

The motor loads will help maintain the lost phase, but I am having difficulty determining the benefit of oversizing a transformer to help a condition that is going to be detrimental to the plant loads.

Any ideas on why this is specified or how to comply?

These are standard delta-wye transformers feeding mostly 3-phase motor loads. Upstream is all circuit breakers and switchgear to the generator bus, no fused switches to create single phase conditons.

 

[rcw retired EE]

I should have spell checked the title.

 

[jraef]

Hmmm... does seem odd, I have never seen that in a spec before. The motors would fry long before the transformer would.

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[jghrist]

I agree with jraef. If you lose a phase on the primary, you will have severe unbalance on the secondary (two phases at 1/2 voltage and 120 deg out of normal phase). The motors will burn up if not protected for this. There may be increased current in the transformer until the motors fry to the point of failing. If protected, they will trip and reduce the current in the transformer.

 

[dpc]

You might ask the client to clarify the specification requirement. Something might have been changed in the wording somewhere along the way.

 

[burnt2x]

Are you sure you didn't leave out phrases or digits behind when you copied the specifications? Or is the figure "20 percent" something like "200 percent"?

Generally, an accidental break (on the secondary of the transformer) on a supply line to motors will raise the current on the remaining motor lines by as much 200% of the normal three-phase amps (1.732X but can reach 2X due to power factor changes). OTOH, a single-phasing on the primary of the supply transformer (wye-delta or delta-wye) can raise the motor current on the secondary phase by about 230%.

BTW, single-phasing is an abnormality and you are supposed to prevent those things from happening. Motors don't work well under single-phasing.

 

[wareagle]

This application would apply to a y-d or a d-d hook up rather than the one you have. Maybe your client thinks they know more that you.