3phase to 1ph transformer

[kjm]

I'm looking for a three phase to single phase transformer with the following specs:

Primary: delta connected, 3-ph 13.8kV
Secondary: 480V, 1-ph
dry type, VPI etc

Can any one have experience with this kind of 3-ph to 1-ph transformer. Someone suggested me the below

THere seems to be severe imbalance on the primary using this configuration.So, I m not totally comfortable. Can someone comment on this or suggest me any working solution

Thanks

 

[beanland]

Bizarre

Bizarre

You need 833A @ 480V single phase? That is very unusual. The transformer may work but it is an inefficient use of iron and copper.

 

[kjm]

You are right, I need 400kVA transformer (833A) at 480V, 1ph. Have you done any transformer for 3-ph to 1-ph?

 

[winnie]

I presume that your concern is the loading of the primary feeder, not optimal use of your transformer.

Unfortunately, unless you add some energy storage into the equation, you will not be able to evenly load the three phase supply with a single phase load. The power delivered to a single phase load drops to 0 twice per cycle; the power delivered by a three phase balanced system remains constant. Therefore you _must_ have an unbalanced load on your three phase supply if you only want a single phase load.

Rather than a transformer, you could use a system such as a motor-generator set; this could consume balanced three phase power yet deliver single phase power to your load, with the inertia of the system acting as the needed energy storage.

I believe that the standard approaches to your situation would either simply accept the primary unbalance, or redesign the load to be a balanced three phase load. For the unbalanced approach, you would use only two primary phases and a single phase transformer.

-Jon

 

[kjm]

Thanks Jon. Unfortunately I do not have the privilige to adjust the load to three phase. I'm only asked to supply 1-ph power for a welding system.

Does the primary unbalance makes it difficult to coordinate the breaker? or could it have any impact on the rest of the system? In my case this trafo is one small fed from MV breaker and there are 12 other transformers on the same line-up which are larger than this. My concern is will this effect the entire 13.8kV power flow?

 

[beanland]

It might work, but at what cost

It might work, but at what cost

According to my calculations, you would need two 13800V-277V 230kVA transformers to get 400kVA. You would need to buy 250kVA (standard sizes). Looks like you get the resulting currents you indicate. Or, you could get one 13800V-480V 500kVA transformer. What you propose would work.

Maybe we should step back further: what is the 480V 833A powering? Why does it need to be single-phase? Is it converted to DC? Is it just heaters? It is not a motor.

 

[kjm]

According to my calculations, you would need two 13800V-277V 230kVA transformers to get 400kVA. You would need to buy 250kVA (standard sizes). Looks like you get the resulting currents you indicate. Or, you could get one 13800V-480V 500kVA transformer. What you propose would work.

Maybe we should step back further: what is the 480V 833A powering? Why does it need to be single-phase? Is it converted to DC? Is it just heaters? It is not a motor.

It is a for welding process in a hot dip galvanizing steel line. It may or may not be converted to DC later since all of it will be part of the welding equipment supplier scope and thats the reason I do not have chance to redesign the power requirements

You are right, a single 500kVA might be a better solution. I may go for it. Thanks

Cost is not the major concern!!! I am expecting 500kVA transformer would be around $35K

 

[beanland]

Systems Engineering

Systems Engineering

This is a case where some systems engineering and coordination may make everyone happier. If this is going to become the feed to a DC welder, the harmonics and performance will be worse if fed from single-phase than 3-phase. If the welder supplier offers a 3-phase version, maybe that would be worth looking into.

Another interesting thing: if the welder has unity power factor, I think the power factor on the different legs of the 3-phase open-delta transformer will be +/- 86%.

 

[jim dungar]

If all you need is single phase out, then all you need is single phase in, the only problem you will have is an unbalanced load on your MV system. Do you own the MV grid, or is it coming from the utility? With a load of about 400 kVA you are only pulling about 30A so the effect of the unbalance depends on the size of the 13.8kV service.

 

[kjm]

If all you need is single phase out, then all you need is single phase in, the only problem you will have is an unbalanced load on your MV system. Do you own the MV grid, or is it coming from the utility? With a load of about 400 kVA you are only pulling about 30A so the effect of the unbalance depends on the size of the 13.8kV service.

The 13.8kV service is not in my control. Its a 1200Amp horizontal bus, 3ph 13.8kV line-up. I am given one feeder breaker in the long lineup of 15 fdr breakers. This means, I have to have 3ph input power - am I right?

 

[jghrist]

The 13.8kV service is not in my control. Its a 1200Amp horizontal bus, 3ph 13.8kV line-up. I am given one feeder breaker in the long lineup of 15 fdr breakers. This means, I have to have 3ph input power - am I right?

It means you will have a 3? breaker. It doesn't mean you have to use all three phases. The simplest solution would be to run two phases (and a ground if the phase cable shields are not adequate for the fault current) to a 13800-480 volt single phase transformer. If the 13.8 kV system is solidly grounded, you could run a single phase with full concentric neutral to an 8000-480 volt single phase transformer.

Are the other 14 feeders perfectly balanced? If not, connect the single phase load to the least loaded phases.

 

[jim dungar]

The 13.8kV service is not in my control. Its a 1200Amp horizontal bus, 3ph 13.8kV line-up. I am given one feeder breaker in the long lineup of 15 fdr breakers. This means, I have to have 3ph input power - am I right?

You only need two phase conductors in order to feed a single phase transformer. Your 3-pole breaker does not care that one pole is not being used. I would stick with using the 13.8kV to feed the transformer as this will cause the least amount of unbalance to your system.