Power Cable Calculation MV/HV 1

[ohms]

Hi every one,

In our substation, the route of the HV power cable is through trench and pipe ducts. Pipe ducts consititute 25% of the route.

For Cable calculations, how should we consider the laying method of the cable. Trench, Pipe duct or the worst of both.

Or we do the calculation separately for Trench part and separately for pipe duct route and take into consideration the worst case.

What IEC recommends in such scenario.

Thanks for you time,

Ohms

 

[hbarton]

Hi OHMS,

Unless you consider using different cable sizes for the trench and duct sections and joining them(In my opinion not a good solution), you should consider the worst case which is pipe duct.

Hope this helps

HBarton

 

[ohms]

well, is there any recommendation by IEC on such particular cases?

 

[7anoter4]

Hi ohms,
The IEC STD 287 may be useful:
IEC 287-1-1: 1994, Electric cables - Calculation of the current rating - Part 1: Current rating equations (100 % load factor) and calculation of losses - Section 1: General
IEC 287-2-1: 1994, Electric cables - Calculation of the current rating - Part 2: Thermal resistance - Section 1: Calculation of thermal resistance
IEC 853, Calculation of the cyclic and emergency current rating of cables
regards

 

[flexagon]

Question to hbarton and to cable calc experts:

what if installation in pipe ducts constitutes 5% (or 1%) of the route? should we still consider this as the worst case for IEC287 cable derating calculations? is there a related clause in IEC or other standard that binds route length to what is considered "worst case".

example given: 100m in a trench and 5m in a pipe duct to enter a building. Could those 5m be safely "ignored"? How can you prove this?

thanks...

 

[Marmite]

In my experience ducted sections upto 15-20m are ignored if the remaining cable is laid direct. You can do detailed calculations if you want to, but this is only really done in practice at transmission and higher voltage distribution levels. Most Utility Companies use standard rating tables assuming certain load profiles, ambient temp,soil resistivities etc. Even if you do detailed calculations there will be a number of assumptions made to get the result.
Regards
Marmite

 

[rcw retired EE]

I beleive that "short" lengths of duct can be ignored because the cable conducts heat along its length also. If the cable in trench is at 75C it seems unlikely that the cable 1 meter away in a duct can be at 90C.

Definition of "short" is left to engineering judgement.

 

[flexagon]

IMHO rcwilson is on a good path and we all agree that 5m can be safely 'ignored' from engineering point of view...but I am looking for a proven way/method to demonstrate this to my educated client.

My feeling is that IEC287 derating formulas are obtained for lim d->inf, where d is the length of cable. Thus, they are valid to apply for long runs but lead to overdimensioning when applied for 'short' runs.