Fault current on system

[power2020]

Dear Folks,
For Isolated generation system-LV (Without grid), our contractor submitted the following fault calculation.
0.5 cycle fault current:145kA Assymetrical Peak
1.5 to 4 cycle fault current-105kA Symmetrical rms
30 cycle fault current- 70 kA symmetrical rms (60 Hz system, )
Contractor selected circuit breaker with breaking current 75kA and short-circuit with-stand current :75kA, 1-sec (Scneider breaker), Making current-160kA. And contractor explanation is --The complete system is designed to short circuit with-stand current i.e. 75kA for 1-sec. Contractor is setting the protective relay to open the breaker with time delay 0.5 sec (fault current will decay to less than 75kA with this time delay)instead of designing 1.4 to 4 cycle fault current i.e.105kA
Is it allowable to keep the system without tripping for 0.5 sec, for the above fault currents however it is designed for 1-sec? What is effect of arc flash for 0.5sec?

And also contractor not considered motor contribution 500kW - motors as all motors are operated with Variable frequency drives (Altivar 61). It is a 6-pulse VFD. If VFD is there, is there no possibility of contributing fault current by motor?
Pl. share ur knowledge in this regard,
Advanced thanks a lot..

 

[Marmite]

It seems perverse to deliberately keep the fault on the system longer than is necessary, just so you can get away with cheaper switchgear. This would risk system stability and increase the risk of catastrophic damage.
Regards
Marmite

 

[power2020]

Thank you Marmite for coinciding with my views regarding the above issue. How can I express this risk to contractor with international standards? Contractor view is the system designed to withstand 75kA for 1 sec.
And also, is it not required to consider motor fault current contribution if VFDs are used?
Pl. suggest me.
Thanks a lot...

 

[dpc]

Motors supplied from standard AFDs will not contribute fault current to faults on the line side of the AFD. Regenerative drives are different and there can be some contribution.

 

[gepman]

Even if the short circuit withstand is 75kA for one second you need to look at the I2T for the whole time, not just the fault level when the circuit opens.

The incident energy in an arc flash is a function of the total amount of time of the flash so you are increasing the risk to service personnel.

I agree with dpc with his assessment on fault contributions of VFD's but I had an installation where I had about 1500kVa of VFD's installed so I asked Square D (Schneider) about the fault contribution since I was only about 800A short of 65kA. They told me that it shouldn't contribute but they wouldn't guarantee it (or state so in writing). I don't see how it could contribute but their response left me wondering why they wouldn't state it.

With that high of fault I think I would look into current limiting breakers or fuses.

 

[power2020]

Thanks a lot for sharing your experience. Regarding current limiting breakers, is it possible to use lower short-circuit breaking capacity breakers if prospective fault currents are higher than the rated.

 

[power2020]

I enquired some suppliers. They said current limiting feature is available with MCCBs only but not available with Air circuit breaker (ACB). Can anyone confirm this please?
Thanks..

 

[dpc]

Yes, that is generally true. Current-limiting circuit breakers that I'm aware of are MCCBs. The air breakers can generally be fitted with internal current-limiting fuses to handle high fault current, although this does limit the selectivity that can be achieved.

 

[QBplanner]

Not sure your mahcine can bear 0.5 second without loss of syn. Don't know your system " Isolated generation system-LV (Without grid)".

 

[alehman]

I think your contractor is forgetting that the circuit breaker and switchgear also have a short-time withstand rating that must not be exceeded. Typically that value does not exceed the interrupting rating of the breaker.

 

[power2020]

Thank you dpc for info.
System is 9-generators connected in parallel without grid. We asked contractor to carry transient stability analysis.
Selected breaker:Icu=Ics=Icw=75kA

 

[Marmite]

Power2020, you asked for international standards which would support your argument with the contractor. I haven't found international standards as yet, but the UK IEE wiring regulations, which are the bible in terms of UK installations and of equivalent stature to the North American NEC quite clearly state that you should consider the maximum prospective short circuit current at the location you propose to site the breaker, and that the breaker should be sized to cope with the maximum current. I've attached a link to the NICEIC guidance note. This is the trade body and approval organisation for electrical contractors in the UK.
Regards
Marmite

 

[power2020]

Thanks a lot Marmite for supporting doc.