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Question about multiple VFDs on a single feed

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Noway2

Electrical
Apr 15, 2005
789
I have a question about an unusual installation. The current situation is as follows: 12.4KV utility service to a step down transformer that reduces the voltage to 4160V. The 4160 feeds a chiller VFD through a motor disconnect. We're looking at adding another variable speed chiller. One of the bids was for two (a dual stage) 480V chiller. This would require a second transformer, 480V switchgear and then two VFDs for 480V. See the attached pdf sketch.

The question that I've been asked is if the 480V drives could cause interference with the 4160V system on the same feed? All drives would have harmonic filters on the inputs to reduce the distortion. I am wondering if anyone has any real world experience with a situation such as this as I am not finding much information and the consulting design / engineering firm says that they don't think it would, but doesn't have a concrete answer.
 
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Since there would two transformers between the drives, it's unlikely that there would be a major problem with the drives. However, I'd recommend taking harmonic measurements of the existing system, preferably on the 12.47 kV side and then you can estimate the harmonic distortion when the second drive is added. The utility will also have harmonic distortion limitations at your "Point of Common Coupling" so you want to make a prediction for that issue anyway. If there are issues, adding a line reactor or if necessary, a harmonic filter on the new drive should address any issues found (if necessary). I'm guessing that with the buffering impedance of the two transformers, you should be OK.

 
By the time you get the 4160-480V transformer, the primary and secondary switchgear (you left out the primary because now you have two feeds off of that 12.47kV-4160V transformer), the two LV VFDs and some sort of harmonic mitigation for them, you could have probably bought 4160V VFDs that are IEEE519 compliant and be done with it.

"Will work for (the memory of) salami"
 
The front-end of VFD's are typically a rectifier that feeds a DC bus and then all the power and controls come off that DC bus. Using this DC bus means that the drive is fairly immune to any harmonics on the line power.
 
First, I would like to thank everyone for the replies.

The utility will also have harmonic distortion limitations at your "Point of Common Coupling" so you want to make a prediction for that issue anyway.
This is a subject we've been down before with this installation, to which we're expanding the capacity. On each of the installations, we've done the IEEE519 modeling and it has said put the filters in but in reality, we've questioned their necessity as has the design engineering firm, in part because of the extra stiff nature of our electric distribution grid. The problem is that the spec is based upon the PCC which is on the high voltage side and we have no good means to measure this. We're a University campus with an Electric Distribution department that we've talked to about this and we've been told that they can only measure on the low voltage side. The filters have been problematic for us, causing drive failures, leading power factors, and other assorted issues.

Using this DC bus means that the drive is fairly immune to any harmonics on the line power.
Good Point.

you could have probably bought 4160V VFDs that are IEEE519 compliant and be done with it.
This is what we want to do. Being a University, we fall under govt purchasing and we have a hostile vendor that submitted a quote that doesn't meet the spec in several ways, including the 4160 voltage requirement. Because of the size of the contract, it is going through the lawyers who are demanding we jump through a bunch more hoops to disqualify this vendor. One of the things we're looking into is potential issues with mixing the 480V and 4160V drives, but it doesn't look like we'll be able to add this to the reason list.
 
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