VFD Isolation Transformer Connection

[mull982]

We have a 480V 10hp VFD with an 480V-480V isolation transformer on the line side of the VFD. The transformer has both a delta and wye winding.

The engineering drawings that we have show to connect our incoming 480V power to the wye side windings and connect the outgoing side to the VFD on the delta windings. The wye side windings has a center tap but the drawings show not to use it. The ground basically is not used on a transformer connection and passed right through to the VFD. (The transformer casing is grounded)

The thing that is confusing us in the field is that the wye side is labeled with X terminals and acts as the transformer priamary and the secondary is labeled with H but this is the way the drawings show it wired.

I'm not familiar with isolation transformer connections as was hoping someone could explain this connection and how it works. Is this the right connection?

 

[mull982]

I was able to track down the system designer and he admitted that his drawing was wrong and that the transformer should be connected delta-wye with the wye center tap being grounded. I am going to change these connections in the field.

Just for fun, though I'm curious to hear what is wrong with the connection in my origonal post, and what kind of problems can exist. We had some nusience trips of the drive last week which I'm thinking may have been related.

 

[mull982]

The manufacturer has revised the drawings however on the secondary wye side of this transformer he is still not grounding the XO bushing. The ground which comes from our 480V wye system basically just bypasses any connection in the transformer and goes right to the VFD itself. (The ground may connect to the transformer casing)

The argument that I am trying to make with the manufacturer is that the ground is coming from a different source from which the VFD is being supplied. For a ground fault what will happen if this ground is not tied to the XO bushing in the isolation transformer? Will it trip a breaker upstream instead of the VFD?

I always thought that for a ground fault the fault current had to make its way back to its source which in this case is the wye side of the isolation transformer. What happens if the current goes back to a different source?

 

[jim dungar]

Current returns to its source, it does not go to a different one. Your first "faulted" conductor is not really a ground fault; it is an unintended ground bond.

An ungrounded wye connection is treated no differently than an ungrounded delta connection. Starting in 2005 the NEC finally required detectors on ungrounded systems.

Your designer is missing some of the advantages of an isolation transformer by not bonding the X0, but it is strictly a design issue.

 

[mull982]

Current returns to its source, it does not go to a different one. Your first "faulted" conductor is not really a ground fault; it is an unintended ground bond.

So in this case would a single ground faulted conductor trip the breaker upstream on our plant 480V system?

I know with an ungrounded system the first ground fault does nothing, and its not until the second ground fault then creates a L-L fault that the OCPD will trip. Like you mentioned I now understand that the same holds true for an ungrounded wye system. What will happen though for a single ground fault with this unintended ground bond?

 

[Besoeker]

We have a 480V 10hp VFD with an 480V-480V isolation transformer on the line side of the VFD. The transformer has both a delta and wye winding.

This may seem to be a silly question but why would you want an isolation transformer? What purpose does it serve?

 

[mull982]

This may seem to be a silly question but why would you want an isolation transformer? What purpose does it serve?

The main purpose for using an isolation transformer on a VFD is to eliminate harmonics that are caused by VFDs from getting onto the rest of the distribution system. These harmonic currents (mostly 3rd harmonic) get trapped in the isolation transformer and are dissipated as heat rather then getting transfered onto the rest of the distribution system and causing problems.

There are other applications besides harmonics that an isolation transformer can be used for, but harmonics is the biggest one when dealing with VFD's.

 

[Besoeker]

The main purpose for using an isolation transformer on a VFD is to eliminate harmonics that are caused by VFDs from getting onto the rest of the distribution system. These harmonic currents (mostly 3rd harmonic) get trapped in the isolation transformer and are dissipated as heat rather then getting transfered onto the rest of the distribution system and causing problems.

There are other applications besides harmonics that an isolation transformer can be used for, but harmonics is the biggest one when dealing with VFD's.

Your transformer is 3-phase so I'm assuming that the VFD is also 3-phase. If so, the lowest order harmonic of any significance is the 5th. They are in a 6n+/-1 sequence i.e. 5, 7, 11, 13 etc.
There are no significan triple-n harmonics. These are usually produced by single-phase non-linear loads.

 

[Strahan]

This may seem to be a silly question but why would you want an isolation transformer? What purpose does it serve?

I would think more of a line reactor. What size drive (sorry maybe I missed it) with drives 10hp and larger also require a choke. oops just seen it is a 10hp so you may want to consider a choke also.