VFD Application Question

[ThePunisher]

I have a VFD driven motor on a dredge and the only available supply in the area is 13.8 kV. One design was to provide a 13.8-4.16 kV step down power transformer ON THE DREDGE and then from there it would supply the 4.16 kV VFD and Station Transformer on the Dredge E-House.

I was thinking a different way to save some transformer cost and footprint. This is to supply 13.8 KV and will be connected to a 13.8kV Switchgear in the dredge and the VFD will have a 13.8-4.16 kV isolation transformer internally and the station transformer will be 13.8kV-600V. This will save us the cost and footprint of having a separate 13.8-4.16kV transformer on board the dredge. Is this possible? Did anyone have this arrangement with dredge supplier before?

 

[jraef]

I don't see much difference in the two scenarios, other than maybe that the isolation transformer for the VFD is in the VFD package and the station service xfmr is 13.8kV primary rather than 4.16kV? If that's the only difference, you have now added 13.8kV switchgear on the dredge and probably removed 4160V gear that splits the load, so what have you gained here? Maybe there is a legal way to feed the output of the 13.8kV - 4.16kV transformer to the two loads without added switchgear, i.e. two sets of fuses in the transformer secondary cabinet? Not sure if that's possible and then the primary fuses would still be common.

But be that as it may, I'd rather see the isolation transformer for the VFD be dedicated to the VFD alone and the station service be an entirely different dedicated transformer, rather than a shared 4.16kV transformer feeding both loads. My reasoning is, if the VFD does something horrible to the transformer and blows the primary fuses, you lose the station service as well. Murphy's law dictates that this will happen on a dark and stormy night, leaving the dredge crew in total darkness.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ThePunisher]

Thanks Jraef. Actually this was what I had in mind.

 

[LionelHutz]

That scheme would work fine as long as the VFD manufacturer will supply a 18.8kV input transformer for the VFD. There should be cost and equipment size savings possible.

 

[jraef]

Right. As opposed to this as proposed, correct?

So what is the type of 4.16kV VFD you are going to use? If it is a "transformerless" design like a Rockwell PF7000 CSI drive, then I see the viability of the way it was proposed originally, in that you are not duplicating the 13.8kV switchgear and you reduce the size and weight of the transformers. But if using a multi-pulse VSI drive design that will need a separate transformer feed anyway, then adding another step-down transformer ahead of it would be redundant, so your proposed alternate would make more sense as long as you can incorporate both the step-down and multi-pulse designs into one unit.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ThePunisher]

To update, I got a VFD supplier confirming such arrangement and application...I will get back to you guys once I get the details. Thanks for all your inputs.