Y-Y Transformer

[pete m.]

After reading 240.21(c) I know that the code will require secondary conductor protection for a 45kva 208y120 to 208y120 transformer. I am wondering why. Since the transformer has a 1:1 winding ratio wouldn't the current on A phase on the secondary be the same as the current on A phase on the primary regardless whether it is an overload or overcurrent condition?

Pete

 

[RB1]

Can't I load one arm of the secondary up to three times it's rated load current before full load current will flow in the primary?

 

[drbond24]

A three phase transformer is just 3 single phase transformers in the same box. If you pull 3 times the rated load current on only one phase, you are overloading only the corresponding phase on the primary and only the corresponding section of the interior of the transformer.

 

[coulter]

Pete -
Nothing to do with your question, just my own curiousity:

208/120Y to 208/120Y Supplied from a grounded feeder, and likely has a grounded secondary.

What is the purpose of this?

carl

 

[iaov]

I'm curious about the 208/120 P and 208/120 S?? Is this transformer an isolation transformer for a solid state drive. YY is unusual as it is because of harmonics generation. Whats the story with this??

 

[pete m.]

This transformer will feed two separate load centers. I do not know the purpose for the use of this configuration.

So is it a true statement that an overload or overcurrent condition on the secondary will have relatively the same ampacity on the primary? And, if this is true other than it being a code requirement what is the necessity for secondary conductor overcurrent protection?

Pete

 

[RB1]

Is that true even if the windings share the same core?

 

[drbond24]

Yes, that is a true statement Pete. I can't help on the reason for the requirement. Seems that an overcurrent on the secondary would trip the primary protection without any trouble.

Yes, RB1, it's still true. The core is just the magnet that facilitates the transfer of energy from one side to the other. The three windings still operate independently.

 

[coulter]

drb24 -
Help me out here - I've never put in a Y-Y xmf - likely have never even seen one, or didn't know it if I did see one.

So questions are:

1. The Y primary - is the feeder neutral connected up to the Y point? It would have to be for what you are saying to be true.

2. Consider the case of "A" secondary phase is loaded up , the other two are open circuit. What do we see for primary current? If I am understanding you , there will be Primary phase A current, and equal primary neutral current, and nothing but magnetizing current on phase B abd C.

Since I don't understand the physics, I'm looking at the code - specifically 240.21.C.1. For a Y-Y, it does not recognize secondary conductors as being protected by the primary OCP.
3. Is the NEC wrong on this issue?

I don't have a problem with the NEC being wrong - it would be just one of many. As I said earlier, I don't understand the physics.

carl

 

[drbond24]

I have never installed a Y-Y either. Never even seen one in fact. As iaov pointed out, harmonics are a big issue on this setup (especially the third harmonic) and other less troublesome transformers will do the same thing, so Y-Y is uncommon.

Now to your questions (as best I can):

1. If the transformer is Y-Y it needs fed with Y and will therefore have 3 phase conductors plus a neutral.

2. You just need to change the picture in your head. A three phase transformer is nothing but 3 single phase transformers. Delta and wye are just a matter of how the three are connected together. You have three independantly operating transformers. If you give two of them no power, those two will be off.

3. Delta-Delta is the only type recognized as being protected by the primary OCP. Delta-Y, Y-Delta, and Y-Y all must be protected on the secondary side. I can't explain why this is, so I have now pulled out my old college textbook. After choking on the dust, I see that delta-delta is the most stable type while Y-Y is the most unstable. Y-delta and delta-Y fall somewhere in between. I hypothesize that delta-delta is permitted because of the stability of the transformer while the others are not because they are less stable. Perhaps someone older and wiser can comment on this.

I'll not call the NEC wrong on this issue or any other. The end goal is to keep people safe and I say better safe than sorry. Err on the side of caution.