Thermal Stability calculation

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steven2020

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I am an electrical engineer with 2 years experience , I was designing a meduim voltage cable riser from a 10kv swgr to a double ended transformer
I was asked by the senior enginner on the project to acccount for thermal stability in the cables . . . which honestly i dont know what that is , could'nt find much infromation online. . . any information would be appreciated.

thnx
 
Thermal Runway

Thermal Runway

Not sure if this is what the engineer was considering but it is reasonable to examine the ampacity of the cable in the environment. There are a variety of methods used in NEC, IEEE, IEC, Neher-McGrath, FEA, etc.

If cable is located underground, as the losses heat the surrounding soil, the soil dries and its ability to conduct the heat away from the cable is worse. As a result, the cable gets hotter and dries out more soil, making the cable hotter... Also, the resistance of aluminum and copper increase with temperature so as the cable gets hotter, the amount of heat dissipated increases. Both of these are positive feedback leading to thermal runaway.

The engineer could also be considering the temperature rating of the insulation. Thermoplastic insulation (PVC) will soften and deform at elevated temperatures. Thermoset insulation (XHHW, XLPE, EPR) soften but do not deform.
 
thnx for the reply beanland.

the cable is not located underground, it is going up a 60 story highrise. I am under the impression that this could be part of the safety applied when we design the cable size .

I still need a reference or a source that has a definiton or at least an equation for thermal stability calculation. any other input is appreciated.

thnx
 
Steven2020 - I would strongly suggest that you ask the senior engineer exatcly what he/she wants. Admit that you don't know what it is, and that you need some help. A junior engineer should never have to appologize for asking questions.
good luck
db
 
barclayd he knows that I have no idea what it means , it was given to me to find out what it is as a challenge , online search didnt bring back much infromation, thats why am here.
 
steven2020 said:
barclayd he knows that I have no idea what it means , it was given to me to find out what it is as a challenge , online search didnt bring back much infromation, thats why am here.
Click to expand...
Are you sure that he isn't sending you out for a left handed crescent wrench?
 
steven2020 said:
barclayd he knows that I have no idea what it means , it was given to me to find out what it is as a challenge , online search didnt bring back much infromation, thats why am here.
Click to expand...

Well, I think the first place you should go is to the cable manufacturer's website for "contact-us-for-technical-information" and see what they say about thermal stability.
try www.okonite.com
db
 
Any Answer

Any Answer

If the senior is testing you, work up an analysis and let hm get back to you. For example, for a tall high rise, determine the amount of expansion and contraction that will occur in the cables; how often they are supported, how much bow (sideways movement) must be allowed for in the cable. Assume 0C-90C cable temperature extremes. Look at the change in length. Calculate the catenary sag for a change in length. This sag will be roughly the same as the sideways movement. Also, you can assume the cable will spiral in a duct, and can calculate the movement based on that. Work up an analysis and give it to the senior. Ask for his/her review and feedback. Maybe they don't know what is needed, they are just echoing client a request.
 
I have almost 40 years of engineering experience, MSEE courses on cable and insulation design, and several years experience testing cables and investigating cable problems in the ANSI and IEC worlds. But I cannot recall the term "thermal stability" being applied to cables.

Possibilities, as mentioned above:
1. Verify the steady state temperature of the conductor under maximum sustained load is within the insulation temperature rating.
2. With a riser cable going up 60 stories, the cable weight has to be held by clamps on the insulation. If the insulation or jacket is thermo-plastic, it will deform if it gets too hot and slip out of the clamps or flow away from the conductor and cause a fault. Thermo-set insulation materials maintain their shape at higher temperatures.

"Thermal stability" may refer to the mechanical or physical stability of the cable materials under high temperatures, in other words, their ability to hold their shape under heat and stress.
 
Call Okonite

Call Okonite

barclayd said:
Well, I think the first place you should go is to the cable manufacturer's website for "contact-us-for-technical-information" and see what they say about thermal stability.
try www.okonite.com
db
Click to expand...

Good advice, if someone at Okonite doesn't know then no one does. Okonite has knowledgeable reps. find the one around you and talk with him.
Okonite had a book called "Engineering Data For Copper and Aluminum Electrical Cables"
It's Bulletin EHB-90.
A very informative book and its ( or was ) free. Get one.
By stability I think your boss is refering to thermal runaway.
Every thing you need ( execpt the ambient temperature of the installation site ) is in the book.
 
wow , thnx for the feedback guys , i guess my next move would be contacting okonite for information and i will post back if i find an answer. I think it might be a physics issue than electrical . . .
 
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