Wye to Delta Conversion inside Transformer

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eeee

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I have been told by the chief electrician that a three phase transformer being fed a wye on the primary can be changed to a delta primary. I believe this is done by rewiring the transformer connections inside the transformer as I recollect him saying to me.

I want to do this because I am replacing a wye-wye 3 phase aerial distributed ground mounted 208/120 volt 3000KVA transformer with a 3 phase pad mounted 208/120 volt 3000KVA transformer.

I don't want a wye-wye configuration because there are too many harmonics and unbalanced loads. I want a delta wye configuration.

Comming from the substation to my 3 phase 300KVA pad mounted transformer is a 3 phase 12KV wye primary distribution.
 
Re: Wye to Delta Conversion inside Transformer

I believe he is referring to pole mounted transformers. You can re-arrange the secondary from 240 volt to 120 volt and connect in a wye configeration. The new transformer can be ordered in a delte wye config.

[ December 08, 2005, 05:14 PM: Message edited by: bob ]
 
Re: Wye to Delta Conversion inside Transformer

If the terminals are accessible, you might be able to do this, BUT, the voltage applied to each primary would be 1.73 times what it was before. This means your secondary phase voltages would be 1.73 times what they were before.

BUT, before that happens, the iron would saturate and the primary would fry.

Let's hear some comment on this.
 
Re: Wye to Delta Conversion inside Transformer

eeee,
You can only do this if you have a transformer bank made up of individual transformers. It cannot be done with a single three phase transformer.
Don
 
Re: Wye to Delta Conversion inside Transformer

You can't change the primary voltages, because the coils are fixed in most cases. Electricians can often change a secondary from 120/240 to simply 120 and reconfigure the secondary connection, but not the primary.

I encourage you to challenge the electrician and see what he intends to do.

Jim T
 
Re: Wye to Delta Conversion inside Transformer

Originally posted by don_resqcapt19:
eeee,
You can only do this if you have a transformer bank made up of individual transformers. It cannot be done with a single three phase transformer.
Don
Click to expand...
I agree with Don. Three phase transformers have their primary and secondary connections welded to a piece of busbar in the configuration listed on the nameplate. They can't simply be rewired.
 
Re: Wye to Delta Conversion inside Transformer

if I had a three phase 480 volt wye system, and i purchased a three phase 480v delta, to three phase 120/240 wye transformer , would that work?
 
Re: Wye to Delta Conversion inside Transformer

i asked that because as i read the posts by eeee i thought he was going to buy a new transformer anyway.
 
Re: Wye to Delta Conversion inside Transformer

you can't buy a 240/120 V wye you can buy a 208/120 V wye.
 
Re: Wye to Delta Conversion inside Transformer

if I had a three phase 480 volt wye system, and i purchased a three phase 480v delta, to three phase 120/240 wye transformer , would that work?
Click to expand...
You can certainly connect a delta wound transformer in a wye system if it has the same L-L rating as the wye system with no adverse effect, but as MJJBEE points out, there is no such thing as a 120/240 wye secondary.

I understood eeee to ask if an existing transformer bank could be rewired on the primary side, and the answer to that is no.

Jim T
 
Re: Wye to Delta Conversion inside Transformer

My mission is to go underground to eliminate the NEC code violation with an aerial ground mounted transformer having exposed live parts near workman.

I can not have underground feeds to and out of the transformer if I buy three individual 100KVA single phase transformers to make up my 3 phase and rewire to make my primary a delta when it is being fed a wye. That is my understanding.

Therefore I will be stuck with a wye-wye 3 phase 208/120 volt pad mounted transformer and have to live with the harmonics and unbalanced loads.
 
Re: Wye to Delta Conversion inside Transformer

eeee - You can buy a 12 kV delta to 208/120 V three-phase padmount transformer and connect it to the phase wires of the wye 12 kV system. Just because the system is wye does not mean that you have to have wye connected transformers. Only the phase to phase voltages of the system and the transformer have to match.

You might be able to reconnect the existing 3 phase transformer as a 6.9 kV delta to 120/208V wye, but that would not be any benefit.
 
Re: Wye to Delta Conversion inside Transformer

eeee your profile says ee ... does that mean electrical engineer? just curious.
 
Re: Wye to Delta Conversion inside Transformer

I wonder if you have ever heard of an "H" structure for pole mounted transformers.

Underground feeds leave an "H" structure rather easily.
 
Re: Wye to Delta Conversion inside Transformer

I must be missing something. I agree with jester..as long as his L-L voltage match the utility voltage, would the delta transformer not be fine connected L-L on the wye distribution.
 
Re: Wye to Delta Conversion inside Transformer

Originally posted by augie47:
I must be missing something. I agree with jester..as long as his L-L voltage matches the utility voltage, would the delta transformer not be fine connected L-L on the wye distribution.
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Sure Augie, but the original question was in so many words, "Is it permissible to rewire a wye primary into a delta config?".

If you have 13KV L-L, then you have 7.5KV L-N on the wye primaries. If you do manage to rewire these primaries into a delta, you you have 13KV on each primary. Not recommended!
 
Re: Wye to Delta Conversion inside Transformer

i think that we have determined that we cannot rewire the tx.

the question is can we use the wye 480 primary to serve a delta 480 primary tx .. that is new.

the answer i saw was yes.
 
Re: Wye to Delta Conversion inside Transformer

eeee
I can not have underground feeds to and out of the transformer if I buy three individual 100KVA single phase transformers to make up my 3 phase and rewire to make my primary a delta when it is being fed a wye. That is my understanding.
Click to expand...
Buy a 3 phase pad mt transformer 12kv delta primary and 120/208 wye secondary. This is a standard transformer.
 
Re: Wye to Delta Conversion inside Transformer

JB, I am an electrical engineer. I have a masters in electrical engineering (and working towards my PE), worked in the power generation industry (hdro) for 2 years; now I am in distribution for the past 1 year. Before that, I was in Telecom (SONET/ATM/Frame Relay) for 15 years. Before that, defense. Also JB, I have a 12KV primary, not 480 volt and wanted to make that correction and hope I understand you comment.

Rattus, you say it is not recommended to replace my three phase wye-wye connected, 100KVA single phase aerial distributed ground mounted transformers with a delta-wye pad mounted transformer by just connecting to the phases of the incomming wye-wye primary emanating from the substation. I believe your concern is a loss in voltages based on past E-Mails from others I have seen.

Bob, you seem to indicate I could do the operation of installing a 3 phase pad mounted delta-wye transformer and connecting it to my wye primary with no suffering or embarassment contrary to what Rattus says.

I have a very experienced EE colleague that will be judging my actions and don't want to get it wrong.
 
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