Transformer bank for 120v phase to phase

[DRSHANN]

Is there a way I can bank transformers for wye-wye 3phase service with 120V phase to phase? This is for a special application.

 

[peebee]

Sure you can -- just specify a 120v wye secondary -- BUT -- realize that this BETTER be a VERY special application, as you would not be able to legally run most equipment thats readily available in the US from your 120v wye system in compliance with NFPA. Most US 120v utilization equipment would require a grounded neutral.

Can you give a better idea of why you want to do this? Can you give a better idea of the types of equipment you want to run? Are you planning to bond the center point of the secondary, or one of the phases?

My gut feeling is that you really really really don't want to do this.

 

[davidbeach]

peebee - I don't know what DRSHANN's application might be, but I've seen diagrams for the electrical system in older submarines where they used a 120V delta system, so a 120V wye system would at least add a ground to the system.

 

[mc5w]

You could make you transformer a wye primary delta secondary with the secondary corner grounded. Use of 3 pole circuit breakers on corner grounded systems is highly recommended and is required for motor circuits. The reason why is that the grounded circuit conductor needs to be protected against overload and opening the grounded circuit conductor can have a tingle voltage on it because it carries full load current.

This method could also use standard single phase transformers. Your circuit breakers would still need to be rated for use on corner grounded or ungrounded 240 volt systems.

In the 400 Hertz world there is such a thing as 120 volts delta and 120Y208 volts.

Mike Cole, mc5w@earthlink.net

 

[cuky2000]

Is there a way I can bank transformers for wye-wye 3phase service with 120V phase to phase? This is for a special application.

Yes, it is possible. Two ways to obtain secondary voltage close to 120V phase-to-phase is as follow:
1-Order special three-phase transformer with 120 V in the secondary.

2- Use standard transformer with taps rated for twice the primary voltage. Operate the transformer connected to a primary voltage half the transformer rating. Automatically the secondary voltage will be half.

For example, if a secondary delta connection is allowed, choose a transformer with rated for 240 V phase to phase. The resultant voltage will be around 120V phase-to-phase.

If a YG is require the secondary voltage will be 104 V. In this case, the transformer tap should be increased as high as possible to obtaing a sec voltage closes to 120 V

 

[stevenal]

Sometimes VTs can be found with 69V taps. They can be used to power small loads.

 

[busbar]

Although an inefficient use of transformer capacity, a short-run solution may be three 480:120V transformers configured grounded-wye—grounded-wye on a 480Y/277V source—yielding 120Y/69V on the secondaries. Acceptable load on transformer bank would be ~58% of the nameplate sum of transformer-kVA ratings.

 

[tinfoil]

If you can lay your hands on three 1:1 ratio isolating transformers, you could connect them Wye-Delta, transforming a standard four wire 120/208V supply into a 120V three-wire delta.

 

[tinfoil]

In addition, as a longshot...

The local utility used to use delta connections exclusively (way back when), but the system was subsequently converted to 4-wire wye system. They still have several old transformers in the storeyard (don't ask me why) that used to be connected phase-to-phase on the primary side and delivered 120V at the secondary. If these units were connected line-to-ground, they would be underexcited and deliver 69V nominal at the secondary, which could then be banked to create 69/120V 4-wire.

You could ask your local utility if they have such a thing available in thier 'boneyard'.