IEEE 605-1998 and Bus Bar Short-Circuit current withstand capability 1

[Thienma]

Hi All,

I'm a bit puzzling by the Gamma Factor of 1, .866, and .808 listed on Table 2 of the IEEE standard as adjustment factors for the bus force depend on the type of fault and conductor location. I'm seeking advice on:
1) If you have any reference or document showing how these factors were derived.

2) Can these factors be used to calculate the actual bus bar fault current withstand capability (i.e. Multiply the Gamma factor to the asymmetrical current)???

I'm very much confused by this and any advice is greatly appreciated. Thank you for your time.

 

[desertfox]

Hi Thiena

This site might help:-

http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec6.htm#Electromagnetic Stresses

desertfox

 

[Thienma]

Thanks desertfox,

I have seen that document in the past, but not quite sure how did they come up with the .866 or .808 factors. Also, how can these factors be applied to calculate, electrically, the maximum fault current that the bus should be able to withstand. If I were to obtain the asymmetrical current of the bus using the X/R ratio at 1/2 cycle peak following fault, can I multiply this asymmetrical current with the decrement factor of .866 to get the maximum fault current that the bus must withstand?? I was talking with someone who proposed such, but I don't know how valid it is and been looking for some sort of dis-proof or confirmation.

 

[jghrist]

The 1, 0.866, and 0.808 factors are derived from calculating the peak forces using differentiation of the net sinusoidal forces on the busbars for the different faults. I don't have any references for the derivation, but I have done the calculations myself to confirm that the factors are correct.

 

[desertfox]

hi Thienma

Yes it doesn't say were the factors came from I assumed it was linked to the asymmetry of the fault current.
Not sure why you want the busbar fault current withstand, I assume that the withstand current would be determined by the temperature rise or how quick the heat can be dissapated from the busbar and also the stresses set up between adjacent busbars during a fault.
From my experience the fault ratings for busbars is either one or three seconds if during that time the temperature doesn't get to high and the mechanical stresses are exceptable ie:- they are not permanently deflected then the system should be okay.
If you go to the index on the link I provided in the previous post there are sections on the current ratings of busbars.

http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec3.htm

desertfox

 

[Thienma]

Desertfox,

The reason why i was looking for the short-circuit withstand capability is that we have one of the buses in our system which comes pretty close to the designed electrical current rating, and I was not certain if the same rating is adequate given the present configuration. Through talking with a fellow engineer, he suggested that the .866 factor can be used to derate the total fault current impacting the bus, and i'm do a little soul searching to see if indeed his suggestion is valid and if the same approach was being used elsewhere.

 

[desertfox]

Hi Thienma

In that case I would use the formula for:- Short Circuit Heating of Bars in the second link I posted earlier.
The time t calculated from this must be greater than the short circuit time rating for the system.

desertfox

 

[jghrist]

The 0.866 is applicable only for the short circuit force calculation, not for the thermal rating. It has to do with the fact that the currents in adjacent buses during a 3Ø fault are not 180° out of phase like they are during a Ø-Ø fault.

 

[patrick7961]

The reason why i was looking for the short-circuit withstand capability is that we have one of the buses in our system which comes pretty close to the designed electrical current rating, and I was not certain if the same rating is adequate given the present configuration.

Iin order to be assured that the bus in question is in complience with 605 you need to look at the continous current rating, the short ckt rating, check that the bus voltage gradient and thermal expansion are ok.
You will need to calculate the total vectorial force on the bus, then you can check the max span based on bus strength, bus deflection, and insulator strength. You will also have to know what loading district you are in light, medium, heavy. You also need to know what type of vibration dampers are installed.
you can check the fault current by
I=C*10^6*A*{log[(Tf-20+K/G)/(Ti-20+K/G)]/t}^.5