Thermal Calculation on Cables

[sinhchoo]

My earlier thread 238-259943 was on Overheating of cables in trench. Now we end up having 21 x 1C, 630sqmm XLPE/PVC cables for the 2000kW 415V turbine generator system. The configuration of the cables laid flat spaced in trench is as follow, following IEE Wiring Regulation method 20:

R,Y,B,N,B,Y,R
R,Y,B,N,B,Y,R
R,Y,B,N,B,Y,R

Can anyone help me on the thermal calculation of these cables laid in a 700mmx600mm(D) trench and backfilled with dry sand.

There are many papers I found on thermal calculation but they are too complicated. Is there any simple way to represent or to calculate the amount of heat generated by
(a) each conductor
(b) mutual heating in each layer of conductors
(c) all three layers of conductors.

Another question is on IEE Wiring Regulation: Cable laying method 20, cables laid flat spaced in trench. In this laying method, do we assume the current rating given in this regulation is based on a trench filled with air or filled with backfills with a given thermal resistivity?

 

[desertfox]

Hi sinchoo

The current rating of the cables should be based on a trench filled with a backfill with a given thermal resistivity see links:-

http://tdworld.com/mag/power_underground_cables_need/

http://www.cyme.com/company/media/whitepapers/2005 03 UCA-FL.pdf

desertfox

 

[jghrist]

There are many papers I found on thermal calculation but they are too complicated. Is there any simple way to represent or to calculate the amount of heat generated

No

 

[dpc]

As jghrist says, there is no simple way. The Neher-McGrath equations are the accepted method. If you are buying new cable, your cable supplier may have access to computational software to provide you with an answer.



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[sinhchoo]

Thank you for all your replies.

Desertfox, for IEC standard a de-rating factor for different backfills is provided. But in IEE Wiring Regulation there is no de-rating factor provided. One can argue that the current rating given in IEE Wiring Regulation is based on an air filled trench, another can argue that it is based on a trench filled with backfills with a given thermal resistivity.

 

[desertfox]

Hi sinchoo

Well I think it will make a great difference to the heating calculation if the trench is backfilled as compared with an air filled trench.
I have no access to the IEE reference perhaps you could upload the page that you are referring to.

desertfox

 

[jghrist]

One can argue that the current rating given in IEE Wiring Regulation is based on an air filled trench, another can argue that it is based on a trench filled with backfills with a given thermal resistivity.

The wiring regulation should indicate what the basis for the ratings are.

 

[sinhchoo]

I am referring to the IEE Wiring Regulation. There is no web page I can find. Guess I can't scan and upload the Regulation here, can I?
Let me briefly describe the laying method in IEE Wiring Regulation.

The installation method is based on installation method 20 in Table 4A for cables laid in flat spaced formation in enclosed trench.

For cables' current carrying rating we look up Table 4E1A for cables installed in FREE AIR and apply a de-rating factor for method 20 in table 4B3.

There is no clear indication on whether the de-rating factor in table 4B3 is based on a trench filled with air or filled with backfills. Therefore, can we assume that the rating is based on trench filled with air as we are getting the rating which is based on Table 4E1A (current carrying rating of cables installed in FREE AIR)?

I have found a link on thermal calculation of underground cables to share here:

http://www.scribd.com/doc/22251909/Thermal-Analysis-of-Parallel-Underground-Energy-Cables

 

[ScottyUK]

Are you reading the 17th Edition Regs? Where is Table 4A? I assume you mean Table 4A2, in which case Method 20 is for cables fixed to or in close proximity to a wooden or masonry wall. Look at Table 4A2 methods 70 - 73 instead

Table 4B3 is quite explicitly "Rating factors for cables buried direct in the ground or in an underground conduit system to BS EN 50086-2-4 for soil thermal resistivities other than 2.5 K.m/W to be applied to the current-carrying capacities for Reference Method D". You also need to look at Tables 4B2 and 4C2 for de-ratings due to adjacent cables and correction factors for soil temperature.


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If we learn from our mistakes I'm getting a great education!

 

[sinhchoo]

Thanks ScottyUK for pointing out. I am having the 16th Edition. Method 20 is for cables in enclosed trench. I will get the new edition to read further.