Fear of Changing from Wye-Wye to Delta-Wye

[eeee]

I have decided not to change my 3 phase, Wye-
Wye configured, 300KVA, 208/120 volt secondary, 12.4KV primary transformer configuration to Delta-Wye when I purchase my new trasformer for replacement of the original for one reason.

The facilities that are using this transformer and will be using the new transformer when the present transformer is replaced have MDPs which are delivering 208/120volts to the respective equipment. My MDPs will need to be configured for 240/120 volts and the equipment in the facility will see this voltage if I put in a Delta-Wye configuration. It may be that the facility was designed for a Wye-Wye in the first place because the equipment inside the facility requires 208/120 volts for 3 phase and would be harmed if it saw 240/120 volts from a delta-wye configuration.

Is my reasoning sound?

 

[sandsnow]

Re: Fear of Changing from Wye-Wye to Delta-Wye

A wye secondary cannot produce 120/240. A wye that is wound to produce 240 volts phase to phase would produce about 138 volts from any phase to XO.

The first term is always the primary windings. Do you mean to say wye - delta?? That could give you 120/240 on the secondary. In theory anyway, I don't know if anyone makes such an animal.

edit add: A wye secondary will produce 208Y/120.

[ January 20, 2006, 05:52 PM: Message edited by: sandsnow ]

 

[don_resqcapt19]

Re: Fear of Changing from Wye-Wye to Delta-Wye

The secondary voltage does not change when you change transformers as long as the primary winding rated voltage matches that of the primary supply.
Don

 

[eee]

Re: Fear of Changing from Wye-Wye to Delta-Wye

With a MSEE, I should have my thinking cap on straight by now. Yes, I thought about it this morning. If you change from a Wye-Wye to a Delta-Wye, the secondary voltage remains the same at 208Y/120 volts since it is still a Wye delivery system to the MDPs.

To bad I submitted the project with a Wye-Wye configuration. A delta-Wye configuration is much better for a commercial facility I hear. It is interesting though why there are so many Wye-Wye configurations installed throughout my facility from years past. There are more Wye-Wye configurations installed than Delta-Wye, although I have heard Delta-Wye is the much better for eliminating harmonics.

There may be a reason Wye-Wyes are installed, but I can't figure out what it is. Maybe it is better to install a Wye-Wye since that is what is predominanty installed now in most places so that I do not take any risk of embarassment.

Please Advise??

 

[jim dungar]

Re: Fear of Changing from Wye-Wye to Delta-Wye

The primary reason is probably: "that's the way it has always been done".

A long time ago many industrial plants were designed using three individual transformers just like the utility used, this allowed them to use the utility equipment in emergencies. Wye-wye was common among utilities because they could reconnect a delta primary into a highe voltage wye primary therby increasing the capacity of their system without haveing to run new lines.

Wye-Wye transformers constructed on a common core can cause many problems including ferroresonance. This is one reason single unit transformers are primarily delta-wye.

 

[eeee]

Re: Fear of Changing from Wye-Wye to Delta-Wye

I was trying to validate. Is it ture that three single phase, 100KVA, 240L-L/120L-N volt, 12.4KV primary transformers connected together to form a wye-wye configuration for a 3 phase distribution system from a 12.4KV primary feed can be done so in order that the 3 phase output of the transformer bank will be 3 phase, 208L-LY/120L-N, 300KVA with a 12.4KV primary.

This transformer bank is also presently feeding one single phase load from out of the single phase transformers' secondaries. This transformer also has an additional secondary which is one of three in as far as the 3 phase load the transformer bank is feeding.

An inspector told me that he thinks the 3 phase voltage on the output of the transformer bank is 240/120volts repeatedly. I know that this can not be with a wye connected load.

 

[eeee]

Re: Fear of Changing from Wye-Wye to Delta-Wye

Ok,

The inspector agreed that three 240/120 volts single phases do make up 208/120 volt, 3 phase. No more problem. Thanks,

 

[sandsnow]

Re: Fear of Changing from Wye-Wye to Delta-Wye

Originally posted by eeee:
Ok,

The inspector agreed that three 240/120 volts single phases do make up 208/120 volt, 3 phase. No more problem. Thanks,

Actually, he's partly correct in that three 120 volt sec. pole top XFMRS with the secondary connected as a wye will produce 208Y/120. I can't remember if the individual XFMRS could produce 120/240.