Fault current calculation

[Sparkeusa]

A plan reviewer is asking me to consider two DC motors contribution (50 and 75 HP, 500 V) in my Fault Current Calculation of a 480/277 V, 3 phases system where they are connected through two rectifiers. Does somebody know about to do that?
Thanks.

 

[dpc]

It's unlikely that there will be any contribution on the ac side from the dc motors. If there is some type of dynamic braking/energy recovery, it may be possible. As a worst-case, if you assume a fault contribution equal to the full load kVA rating of the motor, you should be conservative. Since this is to satisfy a plan checker, I would just make a conservative assumption and not spend too much time trying to convince him that it is not an issue.

 

[7anoter4]

I agree with dpc, of course. See:
IEC 60909-0 3.9 Static converters
"Reversible static converter-fed drives (for example, rolling mill drives) are considered for threephase short circuits only, if the rotational masses of the motors and the static equipment provide reverse transfer of energy for deceleration (a transient inverter operation) at the time of short circuit. Then they contribute only to the initial symmetrical short-circuit current I"k and to the peak short-circuit current ip. They do not contribute to the symmetrical short-circuit breaking current Ib, and the steady-state short-circuit current I k .
As a result, reversible static converter-fed drives are treated for the calculation of short-circuit currents in a similar way as asynchronous motors….
All other static converters are disregarded for the short-circuit current calculation according to this standard."