Isolation transformer

[hhsting]

I have 75kva primary 480V three phase to secondary 480V/277V three phase isolation transformer dry type. 240.21(C) is confusing.

Will primary breaker protect the secondary conductors this sort of transformer?

 

[beanland]

Delta-wye

Delta-wye

The simple answer is no. A delta to wye transformer must have primary fault protection up to 225% and secondary overload protection up to 125%. A line ground fault on the wye side appears as a line-line fault on the primary. This changes the effective winding ratio and makes primary-only protection ineffective.

 

[hhsting]

The simple answer is no. A delta to wye transformer must have primary fault protection up to 225% and secondary overload protection up to 125%. A line ground fault on the wye side appears as a line-line fault on the primary. This changes the effective winding ratio and makes primary-only protection ineffective.

Where do you see delta to wye? Primary is 480V. Secondary is 480V/277V.

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[electrofelon]

Where do you see delta to wye? Primary is 480V. Secondary is 480V/277V.

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Although it is possibly to have a wye-wye and leave the primary wye point floating, a wye wye is quite a rare beast in the low voltage world. Plus the way you worded it implies a Delta primary.

 

[hhsting]

Although it is possibly to have a wye-wye and leave the primary wye point floating, a wye wye is quite a rare beast in the low voltage world. Plus the way you worded it implies a Delta primary.

To me it looks like 3 wire delta to 4 wire delta.

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[electrofelon]

To me it looks like 3 wire delta to 4 wire delta.

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You can't get 480/277 off a Delta. Closest thing you could get with a Delta is 480/240 if it had a center tap.

 

[hhsting]

You can't get 480/277 off a Delta. Closest thing you could get with a Delta is 480/240 if it had a center tap.

What gets 480/277V then?

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[winnie]

What gets 480/277V then?

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480/277V is a expected for wye.

-Jon

 

[LarryFine]

To me it looks like 3 wire delta to 4 wire delta.

Not the goal; He wants/needs delta/wye.

You can't get 480/277 off a Delta.

Wye not?

You can if the primary is delta and the secondary is wye.

 

[Besoeker3]

To me it looks like 3 wire delta to 4 wire delta.

The 277V would be line to neutral on a star (wye) secondary.

 

[m sleem]

I have 75kva primary 480V three phase to secondary 480V/277V three phase isolation transformer dry type. 240.21(C) is confusing.

Will primary breaker protect the secondary conductors this sort of transformer?

So many replies have nothing to do with the question,

Will primary breaker protect the secondary conductors this sort of transformer?

It depends, i think yes for un-grounded faults, for grounding faults i'm not sure!!

 

[electrofelon]

So many replies have nothing to do with the question,

It depends, i think yes for un-grounded faults, for grounding faults i'm not sure!!

Actually the winding configuration has everything to do with whether the primary OCPD can protect the secondary conductors.

 

[m sleem]

Actually the winding configuration has everything to do with whether the primary OCPD can protect the secondary conductors.

Yes that's right, the thing is how to control the prolongation from the original op. Thanks.

 

[GoldDigger]

So many replies have nothing to do with the question,

It depends, i think yes for un-grounded faults, for grounding faults i'm not sure!!

The basic principle is whether you can draw enough current through the primary side OCPD to overload any part of the secondary under odd overloads, not necessarily faults.

In the case of 240 to 120/240 you can overload one half of the secondary by a factor of two while still drawing only normal current through the primary.
In the case of delta/wye you can overload one hot to neutral let of the secondary (with the other three unloaded) while still drawing less than norwwmal current through the two hot leads for the corresponding side of the delta primary. Without any load on the other two legs of the delta, you can pull more current than normal through the single loaded side of the delta.

 

[Besoeker3]

So many replies have nothing to do with the question,

It depends, i think yes for un-grounded faults, for grounding faults i'm not sure!!

It's an isolation transformer. We don't know if the secondary star point is grounded.
Would be helpful to know that and why the need for an isolation transformer in the first place.

 

[m sleem]

It's an isolation transformer. We don't know if the secondary star point is grounded.
Would be helpful to know that and why the need for an isolation transformer in the first place.

Ok, let's go back, what is the meaning of secondary 480/277v, is it sort of auto transformer? I wish if it's not a typo by op.

 

[LarryFine]

Ok, let's go back, what is the meaning of secondary 480/277v, is it sort of auto transformer? I wish if it's not a typo by op.

Normal wye voltages.

 

[Besoeker3]

Ok, let's go back, what is the meaning of secondary 480/277v, is it sort of auto transformer? I wish if it's not a typo by op.

An autotransformer would not provide isolation. So you'd have to assume it isn't one of those.

 

[Besoeker3]

Normal wye voltages.

Yes.
But see post#5.

 

[kwired]

Normal wye voltages.

Yes.
But see post#5.

It has to be one or the other and can not be both. OP needs to clarify.

If secondary voltage is 480/277 then secondary has to be wye, it is not possible to get this voltage out of a delta secondary.

Primary configuration could be wye or delta, but most low voltage transformers like this would most commonly have delta wound primary.