Autotransformers

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bill talbot

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A customer wants us to use a 45kVA 480/575V 3 phase auto transformer for a test/research project. On the 575V side we are required to install two fusible safety switches, one a 30A 3P for control, the other switch will be a 60A 3P for load protection, which I would classify as two taps. The 480V side will be protected by a single 70A 3P CB.

The building distribution system is 480/277V 3 PH 4W.

Under testing functions the current will initially flow from the 480V side through the transformer to the 575V load side. After a short time the current will the automatically reverse, flowing from the 575V side to the 480V side supplying a minimal amount of reverse power which will be absorbed by the building power needs but not enough to export into the utility.

We advised our client that Art 450.4(A) prohibits an overcurrent device in series with the shunt winding which, in our case, is the 480V side where we have the 70A CB. Our client who is an EE, is urging us to complying with his requirements, confident that the AHJ will have no problem with this arrangement. I addition, we are not comfortable with power flowing backwards through the fusible switches either but these could be changed to CB's.

What are your thoughts?
 
I believe that on a three phase 480-575 V auto-transformer the shunt windings are between the 480 V tap and the neutral. (Refer to figure 450.4 in the NEC, B is the neutral, A is the 480V tap and the unmarked terminal is the 575V tap.)


Backfeeding the transformer should not be much of a problem except the fuses in the switch could be energized with the switch in the off position. Your idea of a CB sounds good.
 
senior member

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Yes, but the 480V is derived from tapping into the three windings of the wye configuration, the neutral is not involved in the conductor connections. So you think it is ok to have overcurrent protection on both sides of the transformer?
 
bill talbot said:
Yes, but the 480V is derived from tapping into the three windings of the wye configuration, the neutral is not involved in the conductor connections. So you think it is ok to have overcurrent protection on both sides of the transformer?
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Yes, it is allowed to have OCPD on both input and output sides of the transformer. You are not allowed to have it "in the middle" of the transformer. The intent is not to prevent you from turning off the entire transformer, but rather to prevent you from turning off only a single winding.

Look at the diagram included with 450.4(A) and notice where your OCPD location is in relation to the identified points A and B. Notice how these are not the input or output terminals of the transformer.
 
Senior member

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To: Bob Wilson and Jim Dungar,
Thank you for your remarks, I feel more comfortable with the autotransformer now that I see your reasoning, I do feel it will be best to use CB's on both sides rather than fuses to open all three phases under fault conditions and for the reverse power condition that I mentioned in my initial question.
Bill.
 
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