Fault current on system
Fault current on system
(OP)
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..
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..






RE: Fault current on system
Regards
Marmite
RE: Fault current on system
And also, is it not required to consider motor fault current contribution if VFDs are used?
Pl. suggest me.
Thanks a lot...
RE: Fault current on system
RE: Fault current on system
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.
RE: Fault current on system
RE: Fault current on system
Thanks..
RE: Fault current on system
RE: Fault current on system
RE: Fault current on system
RE: Fault current on system
System is 9-generators connected in parallel without grid. We asked contractor to carry transient stability analysis.
Selected breaker:Icu=Ics=Icw=75kA
RE: Fault current on system
Regards
Marmite
RE: Fault current on system