MV Cable Short Circuit Rating vs Clearing Time and Screen Rating 1

[JezNZ]

Hi,

I've had a search through the forum but can't find something specifically applicable to my query.

I normally work in the design of LV works, but in this instance am involved in the design and build of a handful of 11/0.415kV distribution substations connected to an existing industrial plant.

My work is being overseen and signed off by a registered engineer but I'm trying to undertake my own learnings at the same time.

With regards to the cable design and short-circuit withstand, on LV works, this is purely determined in relation to the clearing time of the upstream protective device and the earth conductor the same.

However with regards to MV, I understand in some circumstances, such as tolerable step & touch potentials, the back-up protection is considered rather than the immediate upstream protective device. Is this also the case with short-circuit withstand of a cable, should I consider up-stream or back-up protection clearing time?

Further, with the cable screen, since, with respect to short-circuit withstand, is only in relation to an earth fault, is it considered that the withstand is only considered with respect to part of the earth fault short-circuit current, given that a part of the current returns via the earth?

I had read some literature that indicated the screen needed to withstand the full E.F. short-circuit current for the case of insulation failure, where the arc pressure will likely rupture the cable and result in a two or three phase to earth fault.

Thanks

 

[7anoter4]

See:

https://electrical-engineering-port...des/power-system-protection/backup-protection

 

[7anoter4]

See: IEEE Std 242-2001 IEEE Recommended Practice for Protection and Coordination of Industrial and Power Systems Commercial
9.4.3.4 Backup protection
9.4.2.2 Temperature rise of shield and sheath

 

[JezNZ]

Thanks 7anoter4 - I have actually read those in advance of asking, but I felt the guidance quite vague. I will have a re-read and a ponder.

As a follow-up question, in evaluating the equivalent short circuit current from the 1s duration to the clearing time, I had read one cable manufacturer which stated the calculation (division by root t) was only valid for a time period of between 0.2s to 5s. Yet other cable manufacturers showing examples with a t = 0.04s (for example). I couldn't find any literature guiding the validity of the calculation and the limits of its application.

 

[7anoter4]

First of all, I have complete confidence in the author of the IEEE 80 standard and I agree with the corresponding equation. Anyway, I will now try to prove this relationship.
If we neglect the heat evacuation to the ambient-as in adiabatically way-all the losses generated in the conductor will be transformed in heat inside of conductor.
I^2*R*dt=Cth*γ*vol*dθ
Where:
I=short circuit current[rms and constant for now]
R=conductor resistance at θ temperature
R=Ro*[1+α*(θ-θo)]
Ro=ro/A per unit length
α=temperature coefficient of the resistance
ro=specific conductor resistance at θo temperature.
Cth=thermal capacity of the material per unit weight
γ=specific gravity of the conductor
vol=conductor volume per unit length
A=conductor cross section area
I^2*ro/A*[1+α*(θ-θo)]*dt=Cth*γ*A*dθ
Integrating we get:
I^2/A^2=ln[(1+α*(θ-θo)]/ k/t
where k=ro*α/(Cth*γ).

 

[7anoter4]

Please excuse. I failed to transcribe the end of the excel demonstration.
ʃdθ/[1+α*(θ-θo)]|θ=θi to θ=θf|= ln{[(1+α*(θf-θo)]/[(1+α*(θi-θo)]}
ro/(Cth*γ)=k
I^2/A^2=ln{[(1+α*(θf-θo)]/[(1+α*(θi-θo)]}/α/k/t
(1+α*(θf-θo)/(1+α*(θi-θo)=[1/α+(θf-θo)]/[1/α+(θf-θo)] dividing up and down by α and changing the θo place we get:
(1/α-θo+θf)/(1/α-θo+θi)
If we put instead 1/α-θo=Ko ; θf=Tm; θi=Ta; Cth=TCAP/10^4; ro=ρ
we have:
I^2/A^2=TCAP/10^4*ln[(Ko+Tm)/(Ko+Ta)/α/ρ/t
I=A*sqrt{TCAP/10^4/α/ρ/t*ln[(Ko+Tm)/(Ko+Ta)]} [IEEE 80/2013 formula 37]
Anyway, it's very boring and I promise not to do this another time ...