Ground Fault Current on AFD Output

[dpc]

So I have a medium-voltage AFD fed from an ungrounded delta supply. Is there a zero sequence current path from drive to motor that would result in significant GF current for a GF in the motor, even with an ungrounded supply?

 

[davidbeach]

If the output of the AFD is a separately derived source then the delta supply would be an irrelevant red herring. If the AFD can produce zero sequence current it will produce zero sequence current and feed the fault.

 

[dpc]

Yep, I had that much figured out - I'm just not sure how the typical inverter would respond to ground fault.

 

[davidbeach]

That's a different question. ;-)

Probably depends on the specific inverter design. In that regard the inverter is probably a very weak source.

 

[jraef]

Most have residual current protection built-in, especially in the MV world.

 

[Marke]

You need to consider the high frequency paths, not just the low frequency paths. Although there may be a very high impedance to ground at 50/60Hz, there may be a very low impedance to earth at 20KHz due to capacitances in cables, transformers filters etc. This can result in GF currents (High Frequency only)
Best regards,

Mark Empson

http://www.lmphotonics.com