Fault current calcs w/ VFD's & iso-Xformers

[STinSC]

Greetings from new member. The following 2 questions refer to fault current calculations concerning relatively large motors (300-700hp, some AC and some DC):

While calculating motor contribution for fault current (during arc flash analysis), what considerations must be given to variable frequency drives (Inverters/converters)? Do they affect clearing time, alter impedence, or are they to be considered circuit-invisible during the calculation?

Also, is any additional consideration given to isolation transformers upstream of these large drives other than their impedence losses?

Thank you, and I like the site.
ST

 

[beanland]

Vfd

Vfd

Unless the VFD is regenerative type, it cannot feed power backwards so cannot feed the energy from a spinning motor into a fault.

An isolation transformer is bi-directional so "can" feed energy into a fault. However, the specific type of transformation needs to be considered. A delta-wye transformer feeding a motor cannot feed a line-ground fault in the delta primary.

 

[STinSC]

Thanks beanland,
As is usual, it looks like a little more legwork is required to get further details. Thanks for the information.
ST

 

[rcwilson]

Isolation transformers will add impedance and reduce the fault current. The impedance of the VFD's electronics, filters, inductors, and other parts will reduce the fault current downstream at the load.

I have to believe that the VFD's protection system will also try protect itself and clear a downstream fault to by blowing a fuse, turning off the output or tripping a breaker. But I don't know how to account for that action in an arc flash study unless the VFD supplier can provide some time-current curves for the internal protection.

 

[jim dungar]

Isolation transformers will add impedance and reduce the fault current. The impedance of the VFD's electronics, filters, inductors, and other parts will reduce the fault current downstream at the load.

I have to believe that the VFD's protection system will also try protect itself and clear a downstream fault to by blowing a fuse, turning off the output or tripping a breaker. But I don't know how to account for that action in an arc flash study unless the VFD supplier can provide some time-current curves for the internal protection.

VFDs work by converting AC into DC and then inverting the DC into AC. This conversion process disconnects the VFD load from any available fault current on the line side of the VFD. A fault on the output side of the VFD would only see the current the VFD is capable of providing (which is rarely even as much as 1.5x rated current).

Of course, a VFD with regeneration capability is connected to the AC supply through SCRs so these systems should be analysed as if the VFD did not exist. Also if the VFD has a by-pass contactor it would be treated as a normal motor starter.

 

[weressl]

Greetings from new member. The following 2 questions refer to fault current calculations concerning relatively large motors (300-700hp, some AC and some DC):

While calculating motor contribution for fault current (during arc flash analysis), what considerations must be given to variable frequency drives (Inverters/converters)? Do they affect clearing time, alter impedence, or are they to be considered circuit-invisible during the calculation?

Also, is any additional consideration given to isolation transformers upstream of these large drives other than their impedence losses?

Thank you, and I like the site.
ST

ASD(Adjustable Speed Drive) - the appropriate IEEE designation - is a limited energy storing device and therefore have very limited short circuit contribution and even though I am not aware of the IEEE/ANSI position, but I would bet $ for doughnuts that it would not even be included as a contributor. The motor on the "other" side of the ASD is effectively isolated from your power system and does not need to be included as a contributor on the line side system calculation. To calculate the available arc-flash energy on the motor side of the ASD, you would need to get the data from the ASD manufacturer and calculate it the "utility contribution" of an independent power system.