Rated Short-circuit current vs Short-time withstand current 1

[kssschsekhar]

Dear Folks,

I am looking for clarification on selecting short-time withstand current for circuit breakers and Bus-bars.

In most of the calculations, I"k (Maximum symmetrical rms current) is taking as short-time withstand current. In fact, as per IEC 60909 / IEEE C37.013, short time withstand current is to be calculated as per attachment

In the attached formula, DC component is also be included.

Please suggest which one (I"k or Ith) needs to be considered for Generator circuit breakers and Isolated Phase bus ducts as short-time withstand current.

Advanced Thanks...

 

[7anoter4]

According to IEC 865-1 Short-circuit currents-Calculation of effects part1 [Quote:]
"3.2.2 Calculation of thermal equivalent short-time current
The thermal equivalent short-time current shall be calculated using the short-circuit current r.m.s. value and the factors m and n for the time-dependent heat effects of the d.c. and a.c. components of short-circuit current.
The thermal equivalent short-time current can be expressed by:
Ith=I"k*SQRT(m+n) I"k=the r.m.s. value of the initial symmetrical short-circuit current."
For m and n see fig 12a and 12b
For Electrical equipment:
"3.2.4.1 Electrical equipment
Electrical equipment has sufficient thermal short-circuit strength as long as the following relations hold for the thermal equivalent short-time current Ith:
Ith<=Ithr for Tk=<Tkr or Ith=<Ithr*sqrt(Tkr/Tk) for Tk>=Tkr.
Where Ithr is the rated short-time withstand current
Tkr is the rated short-time".

 

[7anoter4]

See m and n in the attached sketch

 

[slavag]

Hi kssschsekhar.
I would recommend to you only one thing.
Please ask GCB and bus duct mnf for this calculation and verification.
Them chek also asymmetrical SC.

Best Regards.
Slava

 

[7anoter4]

Nevertheless the above standard [60865-1] states

:
"For the calculation of the thermal equivalent short-time current in a three-phase system the three-phase balanced short-circuit is normally decisive", I agree with Slava it will be better if you could contact the manufacturer.
As the fig.12 a and b are not so clear here are some explanations:
In Fig.12a the axe y [from up to down] is the factor "m". On abscise [axe x] is the f [frequency]*Tk. The figures noted above the curve is k [kappa] =1.02+.98*exp (-3*R/X) from IEC 60909.
In Fig.12b the axe y [from up to down] is the factor "n". On abscise [axe x] is Tk. The figures noted above the curve is the ratio I"k/Ik.
When a number of short-circuits occur with a short time interval between them, the resulting thermal equivalent short-time current is obtained from:
Ith=sqrt{1/Tk*Sum[for i=1 to n](Ithi^2*Tki)} where Tk=Sum[for i=1 to n](Tki)
n=number of short-circuits.

 

[kssschsekhar]

Dear 7anoter4 /slavag,
Thanks for valuable information. I understood that in most of the cases, I"k is being taken as Ith, which is wrong.

Ith=I"k*SQRT(m+n) I"k=the r.m.s. value of the initial symmetrical short-circuit current."

 

[ElectricalPhil]

I'm told that, traditionally, breaker specifications usually consider the thermal effects of the fault current.
I haven't done a lot of work with breakers, but I've recently done some work with specifying temporary grounds. One thing I'd recommend, based on what I learned doing the work with grounds, is that it may also be prudent to consider the mechanical forces generated during a short circuit. The forces will be dependent on the peak current, and you can get very different peaks for different situations that have the same thermal effects.

In generating stations, where the X/R ratio is fairly high, you can get very high peak currents due to the slowly decaying DC component, and this may mean that you need to ensure very sturdy mounting hardware, etc. for your bus work.

You could inquire with manufacturers about whether they have any testing data (rms over some duration to cover the thermal effects, and peak current values to demonstrate an ability to handle the mechanical forces) to support the suitability of their breakers and bus work configuration in your application.

 

[7anoter4]

You are right ElectricalPhil, but if someone remains in IEC world then the test described above is required by IEC 694 (BS 6581) "Common Clauses for High-Voltage Switchgear and Controlgear Standards" as one test from a series of High-Voltage Apparatus tests. You referred to another test-not less important-the Peak Withstand Current Test.
One could find how to calculate this for a bus-bar in the above mentioned standard IEC 60865-1.

http://www.ewh.ieee.org/soc/pes/switchgear/currenx.htm