Transformer for deriving neutral

[304sparky]

Hey guys,

Need some help. To preface, we installed a 150kva xfmr to derive a neutral. The xfmr is delta to wye. We had 480V and needed a neutral for a distribution panel. We terminated both the line and load on the load side of the xfmr (wye) to achieve this. However when we turned on power to the xfmr, we blew the bus line that was feeding it. Although, there was no load on the xfmr, all breakers were off in the panel.

My question is, was the initial start up of the xfmr too much? Or was the xfmr terminated incorrectly?

Thanks!

 

[augie47]

sounds like it was possibly terminated incorrectly,
Your Line terminations should be on the 3 Line terminals
Your load terminations should be on the 4 Load terminals.

 

[infinity]

My question is, was the initial start up of the xfmr too much?

Sounds like the initial starting current spike was too high for your primary OCPD. What size OCPD is on the primary?

We terminated both the line and load on the load side of the xfmr (wye) to achieve this.

What does this mean?

 

[304sparky]

Sounds like the initial starting current spike was too high for your primary OCPD. What size OCPD is on the primary?

What does this mean?

The OCPD is 200 amps.

As far as the terminations. The xfmr is delta to wye. All we needed to do was create a neutral. So we terminated the conductors coming into the xfmr on the secondary side (wye) and the conductors leaving the xfmr on the secondary side (wye).

Also, we performed a megger test on all conductors and all results were good

 

[augie47]

As far as the terminations. The xfmr is delta to wye. All we needed to do was create a neutral. So we terminated the conductors coming into the xfmr on the secondary side (wye) and the conductors leaving the xfmr on the secondary side (wye).

I find that confusing but it could be just me.....(I'm used to 'primary', 'secondary')

If it's connected like the diagram in Post #2 then the next probability is infinity's post.
Depending on the trip curve, a 200 amp breaker is marginal for that size transformer (NEC would allow 400 amps dependent on secondary protection)

 

[304sparky]

As far as the terminations. The xfmr is delta to wye. All we needed to do was create a neutral. So we terminated the conductors coming into the xfmr on the secondary side (wye) and the conductors leaving the xfmr on the secondary side (wye).

I find that confusing but it could be just me.....(I'm used to 'primary', 'secondary')

If it's connected like the diagram in Post #2 then the next probability is infinity's post.
Depending on the trip curve, a 200 amp breaker is marginal for that size transformer (NEC would allow 400 amps dependent on secondary protection)

It is connected this way because we are not changing voltage at all. Just creating a neutral for our system to power a distribution panel.

 

[winnie]

As described, the transformer is being used as an autotransformer to derive a neutral, not as a normal isolation transformer.

1) If you use an ordinary delta:wye transformer in this fashion, you will get much higher than normal inrush current. You are energizing what is normally the secondary.

2) The derived neutral needs to be grounded, and will ground the entire system. If the supply comes from a grounded wye source, then you'll be introducing a second neutral ground.

3) If you don't intend this as a grounding transformer for the entire system, then it should be wired as a normal isolation transformer even though you don't need a voltage change.

Jonathan

 

[augie47]

Jonathan, can you provide a diagram ?

 

[infinity]

It is connected this way because we are not changing voltage at all. Just creating a neutral for our system to power a distribution panel.

So this is just a basic 480 Delta to 480Y/277 transformer?

 

[304sparky]

So this is just a basic 480 Delta to 480Y/277 transformer?

This is a 480V delta to 120/208V wye transformer. However is we never terminate on the delta side, and only terminate on the wye side, would this not create a neutral legally?

 

[winnie]

Jonathan, can you provide a diagram ?

This is from a utility document on 'effective grounding': https://www.pacificpower.net/conten...unding_for_Customer_Generation_Facilities.pdf

Utility effective grounding has different requirements than adding a grounding transformer to an ungrounded system, but the 'transformer deriving a neutral' is the same physics and the same transformer connections.

-Jonathan

 

[winnie]

This is a 480V delta to 120/208V wye transformer. However is we never terminate on the delta side, and only terminate on the wye side, would this not create a neutral legally?

Well there is your problem!

You are trying to energize a 208V transformer winding at 480V.

A properly rated delta:wye transformer can be used as a grounding transformer. But as I stated it will create a neutral for the _entire_ system, not just the feeder you're supplying. It is not clear yet from your posts if this is what you want to do, or if it is legal.

But on top of that you don't have a properly rated transformer. In the grounding transformer use, you need the 'secondary' voltage to match the voltage you are using.

-Jonathan

 

[304sparky]

Well there is your problem!

You are trying to energize a 208V transformer winding at 480V.

A properly rated delta:wye transformer can be used as a grounding transformer. But as I stated it will create a neutral for the _entire_ system, not just the feeder you're supplying. It is not clear yet from your posts if this is what you want to do, or if it is legal.

But on top of that you don't have a properly rated transformer. In the grounding transformer use, you need the 'secondary' voltage to match the voltage you are using.

-Jonathan

Can you please dumb this down for me? lol

 

[augie47]

Can you post a picture of the transformer nameplate

 

[jaggedben]

I also have to wonder if/how the 480 supply is grounded.

 

[304sparky]

I also have to wonder if/how the 480 supply is grounded.

There is a grounding bus bar in the bus line.

 

[jaggedben]

There is a grounding bus bar in the bus line.

That does not answer the question at all.
What is the voltage to ground of all three conductors on the 480V supply?

Potentially you are violating 215.11.

 

[304sparky]

That does not answer the question at all.
What is the voltage to ground of all three conductors on the 480V supply?

Potentially you are violating 215.11.

Going back and reading 215.11, to my understanding it states that you can not use a derived neutral to feed a panel, unless you electrically connect the neutral from the supply side. If I’m correct, I’m violating 215.11. Am I close to understanding or am I way off?

 

[Hv&Lv]

This is a 480V delta to 120/208V wye transformer.

Wait…

You put 480 on a 208 transformer??

 

[304sparky]

Wait…

You put 480 on a 208 transformer??

Yes, however both sides are terminated on the wye side to create a neutral.