2 Phase 3 Wire to 3 Phase 4 Wire 208/120V- Scott Connected Xformers 2

[rhfii]

We have an existing 150kVA Dual Grid 2.4kV, 2 Phase 3 Wire primary service with Scott connected transformers for a 208/120V 3 Phase 4 Wire secondary in a building that is about to be retrofit into an automated parking garage. We must have 480V, 3 Phase, 4 Wire power.

The original manufacturer of the sub station is still in business and, because of the physical space and access restrictions we are faced with, we have asked them if they can convert it to a 480/277 3 Phase 4 wire arrangement.

Is there any such thing as a 2.4kV 2 Phase 3 Wire primary with a Scott connected secondary of 480V 3 Phase 4 wire service and if there is, is it true 3 Phase? We must have clean power for our variable speed drives and control system.

Assuming we get through this what other pitfalls are we in for? We've also asked the original manufacturer if they can convert it to a true 3 phase system. Has anyone lived through this experience.

Thanks - Industrial Guru

 

[Marmite]

I would think you might have problems. From my memory the three phase vectors for a Scott connected transformer look like an upside down letter T, rather than the 120 degree star. This might be a problem for VFD's expecting to see three phases nicely displaced by 120 degrees, but that's not my field so I'll leave it for someone else to explain.
Regards
Marmite

 

[tommom]

You might have to get something specially made for this. These probably aren't readily available, but they should be "makeable." The three-phase you get out of a Scott transformer is really three-phase. The mfr has to tap one of the coils at 87.7 % (I think that's the number) and connect that to the mid-tap of the other coil, and it comes out three phase.

 

[rhfii]

Thanks for the input.
Fortunately, the original manufacurer has the drawings for this sub-station. They are looking into a retrofit package to get us to 480V and the other improvements needed to bring this up to currect specs. The tap is correct @ 87.7% and they say it is true 3 phase, but there may be a little imbalance in the amperage department, little being the key word. The utility is still supporting 2 Phase and has no problem with the upgrade. We are looking at another project two blocks away and it to has the 2 Phase service, so, whatever we do for this one we'll duplicate for the next one. Please keep the good comments coming and I'll update when I hear back from the manufacturer.

Thanks - Industrial Guru