Grounding for Medium Voltage Drive

[strap89]

Curious what you guys think about this.

An 'MV' drive is equipped with only a ground bus and not a neutral bus, which makes sense because it's only powering a motor, thus no line to neutral loads. Consider if you have a delta:wye transformer feeding this load. Naturally, you will have an 'X0' bushing where you could attach a neutral in the future. If you bond the MV drive ground bus (this bus also is grounds the enclosures of the drive and the motor) with the grounding plate at the transformer via a bonding jumper, and also bond your 'X0' bushing to to grounding plate, would you have any reason for concern for circulating currents if line to neutral loads were connected to the transformer secondary at a later date? I'm concerned about equipment grounds carrying current and energizing enclosures unexpectedly. I really appreciate any input you may have. I've attached a sketch for reference.

View attachment MV Drive Grounding.pdf

 

[petersonra]

I am not entirely sure what you are asking. Do the two ground symbols shown on the sketch indicate some connection to earth?

Personally, I think what you have labeled as SSBJ is just an EGC.

Any future line to neutral loads would go between X123 and X0. I don't see a current path thru what you are labeling as SSBJ.

 

[Jraef]

An isolation transformer ahead of a MV drive is for the specific propose in isolating Common Mode noise from the rest of your system (other than the voltage change). That transformer is typically sized specifically for that drive, which is also going to be sensitive to any voltage imbalance. Connecting other loads to that transformer secondary comes with a host of other problems that would make it a very bad idea. Circulating currents in line to neutral loads, if any, would be way down the list of reasons to never do this in the first place. Bottom line, you are concerning yourself with something that should never happen.

 

[strap89]

An isolation transformer ahead of a MV drive is for the specific propose in isolating Common Mode noise from the rest of your system (other than the voltage change). That transformer is typically sized specifically for that drive, which is also going to be sensitive to any voltage imbalance. Connecting other loads to that transformer secondary comes with a host of other problems that would make it a very bad idea. Circulating currents in line to neutral loads, if any, would be way down the list of reasons to never do this in the first place. Bottom line, you are concerning yourself with something that should never happen.

It's actually a service transformer for a customer with a primary service. This is for an industrial plant. The transformer is actually sized larger than the motor load. I'm thinking about requesting a placard that discourages adding additional services from this transformer. The transformer is 2500 kVA and the motor is 1500 HP. In the future, though unlikely, the client could potentially think they could add a single phase motor, or unbalanced 4 wire system.

 

[strap89]

I am not entirely sure what you are asking. Do the two ground symbols shown on the sketch indicate some connection to earth?

Personally, I think what you have labeled as SSBJ is just an EGC.

Any future line to neutral loads would go between X123 and X0. I don't see a current path thru what you are labeling as SSBJ.

I think you could be right, but it is at least is sized as a SSBJ with the absence of a protective device on the secondary of the transformer.