VFD Contribution to Short Circuit Study

[Dumbo2929]

Does anyone know the recommended industry standard for VFD motor contribution back to the system when performing a short circuit study?

 

[mpparent]

I don't know if there is a standard, but obviously, worst case would be at full speed. Typical induction motors are 6x fla.

Mike

 

[Dumbo2929]

I had someone question whether I should be using the full contribution of the motor, or something less because there is a VFD involved. Since the software I use doesn't allow for VFD modelling, the full contribution gets used.

 

[davidbeach]

If the VFD has bypass contactors, you need to account for full contribution. If the VFD does not have bypass contactors, it it highly likely that there will not be any contribution.

 

[Dumbo2929]

Are you aware of any standard or any place I can quote that would have me include or exclude VFD contribution from my study?

 

[davidbeach]

No, I know of no standard; but unless there are bypass capacitors or the VFD is a four quadrant VFD (capable of regenerative breaking) there is no path from the motor through the DC and back to the line AC.

 

[Dumbo2929]

davidbeach,
In one email you mentioned bypass contactors and in one you said bypass capacitors. Which do you mean?

 

[jraef]

I'm sure he meant contactors, there is no such term as bypass capacitors in VFDs. And I agree that a basic VFD has no path back to the line to allow contribution of motor regen during a fault. Look to see if your software desribes "power conversion equiment" or some such wording . Same issue. A power supply is not considered to contribute for the same reason. As far as coordination studies go, a VFD diode front end it just a power conversion unit. The motor on that VFD is considered to be fed from a "new" source on the load side of that power conversion unit.

Bypass contactors allow the possibility of the motor being connected to the line directly, therefore adding them back into the equation. Regenrative drives can also be thought of as contributors, but you get to add anyinductors (reactors or bus chokes) when considering it.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[davidbeach]

Contactors; typing too fast, spell check doesn't do too well with correctly spelled wrong words.

 

[RalphChristie]

typing too fast, spell check doesn't do too well with correctly spelled wrong words.

How true.... especially correctly spelled wrong words.... and you see them first after you have pressed the "submit post" button

 

[jraef]

0.432ms after you hit the Send button to be precise, at least for me.