Asymmetrical SC current

[hhhansen]

Hi there.
When calculating the aymmetrical short circuit current this is defined as Ib_asym= sqrt(Idc^2+Ib_sym^2).

My question is:

Shall c factor 1,1 be applied to calculation of Ib_asym ?

Best Regards, hhhansen.

 

[Kiribanda]

hhhansen,

No. The C fator per IEC is applied only for calculating the initial SYMMETRICAL short circuit current.

 

[hhhansen]

Hi Kiribanda
Are you able to quote in which part & chapter of IEC this is mentioned ?

Best Regards
Hans-Henrik

 

[Kiribanda]

hhhansen,

If you go the basics, whether you adopt IEC or ANSI,the Thevenin's per phase equavalent circuit used for calculation always give the Initial Symmetrical Short circuit current (IK") only.After calculating initial sym sc current,we are able to derive peak, breaking etc etc.Therefore,the subject C factors (Table 1 of IEC 60909)used for calculating minimum & maximum short circuit current always give sym only.

 

[veritas]

Hi

I only work with the IEC 60909 standard and thus cannot comment on the ANSI one. First of all I would suggest that there be clear definitions as to what we're about. In the attached document (I compilid a while ago)distinction is made, amongst others, between I"k, Ib (whether symmetrical or asymetrical), Ip and Ik. Note that all are rms values except for Ip which is an instantaneous one. I will only comment on I"k and Ib (symmetrical). Essentially they are snapshots of the same current waveform at different times. I"k is at t = 0 and Ib at the time when the circuit breaker interrupts the fault current, typically taken to be at 55ms.

Note that if there is no AC decay (far from rotating machine faults) then Ib sym and I"k are the same.

The above is thus not quite in line with what Kiribanda stated - if the C factor influences I"k then it must do so to Ib as well since both are dependent on the same voltage and if no AC decay they are one and the same current. See V term in equation (2) of summary.

Equation (1) relates Ib(asym) to Ib and IDC. Since both of these are dependant on V then so must be Ib(asym). It is also intuitive that the pre-fault voltage whether defined by C or otherwise will effect all the fault current parameters.

Hope this helps.

 

[karalahana]

now the answer to ue answer is well explained in this document, according to the formula as can be seen easily no c factor needs to be applied since the c factor is already got involved in the initial symmetrical s.c current so why take it again in consideration? what will determine the magnitude of the s.c current is the a.c decay (position of the motor or generator whichever) component at 55 ms instant (for example)since it is stated that if it is far from a generator it means Ik'' = Ib ,if i am wrong pls discard this message, i just wrote here what i know what i got from this subject...