VFD Voltage 1

[BRIS]

We have a design with 12 motors each 3.3 kV (power ratings from 300 Kw to 1000 Kw) 9 motors have VSD drives the other 3 have soft start.
Incoming power supply is 11 kV. My question is why do we not transform the incoming supply to 3.3 kV instead of having one 3.3 kV transformer on each drive (12 No 11/3.3 kV transformers in total) ?

 

[itsmoked]

Only a guess but the transformers provide galvanic isolation and impedance for each drive so they don't interact with each other.
I recall that old drives used to all require transformers.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

At that combined power level the use of phase shifting transformers to limit harmonics is certainly a possibility.

 

[LionelHutz]

The transformer is likely phase shifting to reduce harmonics so it would be there no matter what. So, changing the primary voltage is rather simple to match the available power. So, 11kV might be because other equipment in the building already relies on 11kV power making it readily available. Possibly the decision was made because the 11kV wiring and equipment doesn't have to handle as much total current as the same equipment using 3.3kV would have to handle.

 

[waross]

The individual transformers also lower the available fault currents.
The lower fault available currents may have allowed the selection of less expensive 3.3 kV equipment and less stringent Arc Flash requirements.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

All of the above.

1) You need to step down the voltage.
2) You need to mitigate harmonics created by the VFDs (assuming these are VSI drives).
3) You need some galvanic isolation between drives for Common Mode Noise attenuation.
4) You need to limit the available fault current feeding each drive.

One large transformer ahead of all of them solves #1 only.

You don't need an individual transformer ahead of each soft starter, but it could be that they may want to some day upgrade those to VFDs as well.


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