Regenerative VFD Fault Contribution Duration

[dpc]

We're trying to fine tune some short circuit calcs involving a large number of crane VFDs. These are all regenerative drives with dynamic braking so I know (or think I know) that there will be some contribution from these into a fault on the line side of the drive.

We've been assuming that the duration of the contribution would be similar to a motor running across the line, but now I'm wondering if the dc bus capacitors in the drive would keep the motor excited longer during a fault condition.

Any data on fault contribution magnitude and duration for a four-quadrant VFD?

Thanks,

Dave

 

[Marke]

Hello dpc

I presume that we are talking about active front end type regenerative drives rather than drives with a braking resistor.
The regenerative drive is able to continue to provide regeneration as the vars required for the stator magnetizing can be supplied by the capacitors and load. Some drives will adjust the rate of deceleration to ensure that sufficient energy is pulled out of the load to keep the bus voltage up when the input voltage is removed.
I would expect that the peak energy from the drives will be much less than direct connection, but the contribution would be for a longer period.

I do not have any figures or calculations, but I would suggest that you could asume that the maximum energy would be limited to around 150% of the motor rating (by the inverter operation) but the period of enegy flow could be dependent on the load inertia.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[dpc]

Thanks, Mark. For some reason, I'd never thought about the duration issue, only the magnitude.

This isn't really covered in the standard short circuit calculation procedures in ANSI and IEEE, at least that I'm aware of.

Thanks again.

Dave

 

[thinker]

Generally, I would say, the active front end VFD contribution during fault conditions is defined (and limited) by the input regen converter rated current and timed overload capability.

 

[gepman]

I asked Square D this question on standard (not regenerative) drives because I had a system that had over 75% of the power utilization through VFD's and I wanted to verify my assumption that they could NOT supply fault current back into the system bus. Square D informed me that they could NOT guarantee no fault current and that I should design for the standard four times the FLA of the motor loads. Although I am not experienced in regenerative VFD's I think that to be on the safe side I would model it as a generator if I could find the subtransient and transient reactance. You may also want to consult your VFD supplier. If it is one of the major suppliers like A-B, Square D, or C-H then they probably have good engineers who have already studied this problem.

 

[dpc]

Thanks for the responses. My main concern is the duration of the fault contribution. I'm pretty sure a regenerative drive will have some fault contribution - just not sure how long. In conventional fault analysis, induction motor contribution is assumed to be gone within 5 cycles at the most. But it appears that contribution from a regen drive could be longer than that, although at a lower magnitude than a motor running across the line.