High Voltage Disconnect Switch derating by high X/R>17

[cuky2000]

Does any one know a methodology to derate a 230 kV disconnect switch rated for 3000 A continuous, 88 kA symmetrical SC current, 230 kV peak, and 142 kA asymmetrical.The switch should not be derated as a circuit breaker since will operate in close position during SC event with the following condition:

a) Magnetic Force: Driven by the peak current in approx. 1/4 of a cycle. The DC decay imposed by the X/R will not be significant with the short time.
b) Heating effect (I²R): Do not act instantaneous and driving by the RMS current. Since the switch is tested for 3 sec (180 cycles) and the protective device much shorter (let's say 18 cycles), by the time the switch component reach critical temperature, the circuit will be de-energize.

Please let us know your though regarding the impact of the X/R on disconnect switch rating.

 

[7anoter4]

I never did such a specific calculation for the equipment which I designed the physical part of a power station except if the equipment comply with IEC 60865-1/1993 chapter 3.2.4.1.
However, from the same standard the well known formula (63) may be employed in order to calculate the thermal equivalent short-time current:
Ith=I"k*sqrt(m+n)
where m=1/[2*f*Tk*ln(ϰ-1)]*[e^4*f*Tk*ln(ϰ-1)-1] Annex A pos. A8
ϰ=1.02+0.98*e^-3*R/X formula (55) from IEC 60909-0/2001.
For I"k/Ik=1 n=1 [usually]
However, from the same Annex A [IEC 60865-1] you may calculate n for I"k/Ik>1