Short Circuit Contributors

[nightfox1925]

I am interested to ask whether we should include Frequency converters, harmonic filters and capacitors as contributors and model them in calculating short circuit currents.

I was having an impression that motors connected to VFDs do not contribute that much and are negligible, same as capacitors and harmonic filters.

 

[dpc]

Regenerative drives can contribute to external faults, the others can be ignored. Capacitors actually can contribute some current, but are universally ignored for a variety of reasons.

 

[nightfox1925]

Hi dpc, if it is not too much to ask, may I ask the variety of reasons why capacitors may be ignored (to educate myself further)?

 

[Rags004]

Hi Nightfox. Mabey this will help. Consider the natural response decay of a discharging capacitor. Now plug some realistic values in for R and C. You will have an extremely fast decay which makes the initial condition insignificant. Hope this helps.

BOb

 

[dpc]

Normally the power factor of fault current is very low - the current is highly lagging. Whatever capacitor fault current that occurs will be about 180 degrees out of phase with the inductive current. Also, the capacitor fault decays very quickly - normally within a cycle.

There is a lengthy discussion of capacitor fault currents in Conrad St. Pierre's book "A Practical Guide to Short Circuit Calculations."

But I don't think ANSI or IEC standards for short circuit calcs have any guidance regarding capacitors.

 

[davidbeach]

Motors on non-regenerative drives can be ignored only if there is not a bypass contactor. Regenerative drives will contribute some but not as high a magnitude as a motor directly on the line.

 

[nightfox1925]

Thank you very much for all your inputs. I will order the St. Pierrie Book.

Thanks Rags004 for the R-L-C circuit recap.

Thank you for the reminding me also about the bypass contactors...I am learning a lot from this forum.

 

[HVmariner]

Hi nightfox1925,

As a supplier of complete installations comprising converters and harmonic filters, I've been working on Isc calculation on marine HV networks. My conclusions are :

1. For converters:
Only converters with thyristor bridges on network side (ie static frequency converters) can contribute to a short-circuit. They will contribute only if the short-circuit occurs when the drive is regenerating. You can find quite good explanations in IEC61363-1. Calculations are quite difficult as the contribution depends on the operating point of the machine at the time the fault occurs;

2. For harmonic filters and capacitor banks:
Obviously they are never taken into account. Our calculations on a few typical projects have shown that due to the time constants of these R-L-C circuits, they do not contribute to the peak value of the short-circuit current, wich is usually the most critical value to verify.

Hope we are in phase guys?