temporary generator hookup to 240 midpoint ground system

[jim@airportelectric.com]

I want to connect a temporary generator (floating conductors) to supply a 240 volt midpoint ground (high leg) system. Can I ground the system by connecting 2 of generator legs to the secondary windings of a single phase 240 to 240/120 transformer and connecting the neutral tap to the neutral of the building panel which I assume is grounded. I will check this before proceeding. What about capacity? Do the windings need to be able to carry the max unbalanced current between the (2) 120v to ground legs? Would it be better to use the secondary of a 3 phase 240 volt high leg transformer connecting all 3 generator legs. I don't know if it is important to consider phase angles etc. Thanks for any help. Any other solutions?

Jim

 

[winnie]

What you are describing is using an 'autotransformer' to derive a neutral, similar to using a 'grounding transformer'. In _theory_ it should work, but may violate the code for proper use of autotransformers.

The autotransformer would need to have sufficient current handling capacity to supply any required neutral current. However the _primary_ coil is not needed at all. You would not need a three phase transformer; a single phase would be sufficient, and if you can get a single phase 120V:120V transformer you could connect the two coils in series to provide your required autotransformer. As I said, I don't know the code for this sort of application.

Are you sure you have a 240V generator, and not a '12 lead' generator connected for 240V? If you have a 12 lead generator, then you already have a midpoint tap available for grounding.

See: http://www.electrical-contractor.ne...cs/148753/12_Lead_Stator_Generators_Sche.html

-Jon

 

[ActionDave]

Winne gave some good info on generators and transformers but I can't help but think there is some over analysis at work.

What is this project you are working on?

 

[Birken Vogt]

If it is 12-lead then the answer is above, use the center tapped series delta connection.

If it is 10-lead or 4-lead (permanently wye-connected) then you may still be able to connect as 120/208Y if all your line-line loads are capable of running at 208 volts, and let the center point be the neutral as it was intended.

If you use an autotransformer, I would think that the autotransformer would have to be able to handle any faults from line to ground in order to allow fault clearing of the breakers/fuses.