Short Circuit Current Carryin capacity in multi run Cables

[abs710524]

I have to size the 11 kV cables for a very high fault level system. My requirement is 110 kA(rms) for 1 Second. I intent to put multi runs singel phase cables. I have the short time current ratings of 1 core cables. can we directly multiply with no of runs to get the fault level of mult run cables. Rated current carrying will not be the issue in deciding the cable size.

Need your opinion on this.

Thanks



Akhilesh Solanki
VA Tech Hydro (I) Ltd

 

[Kiribanda]

Hi abs710524,

My understanding is since the short circuit rating of a cable depends on the type of insulation it is possible to multiply by the no. of runs. (Reasonable assumption: Short circuit current is equally divided among the number of runs)

Kiribanda

 

[cuky2000]

It is through that the SC cable capability depends of the insulation material to withstand max. rated temperature. However, there are other important factors to be considered in the ability of the cable to survive during SC as follow:

a- Conductor area (Size).
b- Conductor material (Cu, Al,)
c- Impedance (Z = R+jX)
d- Protective device operating time (fuse, circuit breaker, etc).

For single conductor cable, the following formula applies:

I = A.[(k/t) .log(T₂-234)/(T₁-234)]<sup>1/2

Were:
A = Conductor size in circular mills
k= 0.0297 for copper
T₁- = Operating temperature
T₂- = Insulation SC rated temperature
t = protective device time to open the circuit.).</sup>

For multiple conductors in parallel, the impedance Z is:
Resistance: The total resistance is the resistance of one conductor divided by the number of parallel conductors. (R_n=R_{1 conductor}/n
Reactance: Is a function of the cable size, number of cable, distances among cable (physical configuration

 

[abs710524]

Thanks.

 

[jghrist]

The impedance of multiple cables in parallel, and the impedance of the sheaths varies according to the relative position of the cables. The short time rating of the cables depends not only on the current in the conductors but the current in the sheaths. The current in both the conductors and the sheaths may be different among parallel cables. Calculating these currents involves the solution of 2·n equations where n is the number of cables involved (per phase for Ø-grd faults and total for 3Ø faults).

 

[alehman]

At that level of fault current, I would expect the current division to be highly dependent on the physical arrangment of the cables - lengths, spacing, transposition, shielding, etc. Equalizing current division may require very careful design.

You may consider consulting a cable manufacturer who has experience in this type of application.

 

[RRaghunath]

110kA fault level for 11kV system suggests to me that the cables are in the generator system (say generator to generator transformer).
In such an event, I would have preferred bus duct (may be isolated type - presuming the generator is fairly large going by the fault level).

Are there any constraints!!!

If Cables are a must, then it would help if pilot wire differential protection or circulating current differential protection (high impedance, fast acting type) is also included.