Heat transfer at electrical lug connections.

[dbecker]

Hello.


I am faced with a real problem. I need to perform an analysis on electrical lug connections used in very large circuit breakers.

The lug connections use about 1" thick solid copper bus and terminate to 4 large DLO 313kcmil cables. Each side of bus has two lugs, a total of 4 per end of bus.

Now comes the problem. I need to assume some heat generation. If I am given the current and voltage that this unit (per bus connection) will be seeing, what and how do I assume contact resistance and Q heat generation?

Also, where should I apply this Q? I am assuming I shall apply this Q right at the interface of the two surfaces (Bus/Lug) in the form of heat flux at those surface areas.

If my assumptions are correct, please confirm and if not, please advise.

Thank you and I love this forum.


- Dan

 

[hacksaw]

It is a very real problem, your initial contact resistance is in milli-ohms range and is a function of the bolt torques used in making the connections. The conductivity of Cu is such that the temperature in the lug itself is quite uniform so the big issue is the resistance at the bolted interface not so much the temperature gradients.

The resistance increases with time due to oxidation and varous environmental corrosion effects, and the connections need to be checked regularly for critical applications.

 

[electricpete]

I think your question about assumptions belongs in electrical, not heat transfer. Once you figure out your assumptions these guys can help.

I think the answer is: both. The volume resistivity is well known and quantifiable. The contact surface resistance exists but will be relatively small under usual conditions of a good connection, and difficult to quantify.

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[dbecker]

Thanks for the responses.

Hacksaw, I am actually interested in the steady state heat generation at the interface. Do you know of some correlation? Should I post this same question in electrical?

pete, the model I am running is trying to simulate what happened to one of our components that failed. We know the lugs get hot, but we want to try and predict lug temps.

 

[IRstuff]

Don't forget than one of the biggest, hidden, resistances is the spreading resistance. Everywhere that the conductor either changes cross-sectional area, materials, or even direction, incurs an additional resistance factor above the basic resistances of the materials.

So, therefore, you'll need to determine the actual flow of the current through the copper, spreading or turning as it gets to its contact with lug, the spreading or turning within the lug and toward the connection, and then the connection itsef, and everything past that. It's a fairly non-trivial problem, and you also need to determine how much accuracy you need versus how much modelling you're going to have to apply to achieve that accuracy, and whether it's worth the effort.

TTFN

FAQ731-376

 

[StoneCold]

I might point out that thermal imaging of several existing units at different loads would allow you to create a correlation that could help you predict installed lug temperatures.

Regards
StoneCold

 

[dbecker]

IRstuff, the current flow prediction is going to be beyond my work scope. And I think we arent going to get that detailed. We just want a rough idea of lug temperatures or even deltas from case to case.

StoneCold that actually is a good idea, but I dont know if we have access to thermal imaging right now.

 

[electricpete]

Just to clarify, the failure you think occurred based on heat generated between lug and bus... or at the crimped barrel of the lug?

Either way, I don't think you will get far with an analysis that starts with an assumption about the unknown your are trying to find. In other words, you have to assume some degradation of the interface but how much? That's what you're trying to figure out. And again for normal connection that contact resistance at the plane of the contact is small compared to other effects... smaller than the current distribution effect which has been mentioned I would bet.

You could perhaps work backwards and say "I know the temperature go this high (2000F to melt copper for example). What would the contact resistance have to be to cause that? Even then it's not particularly valuable because the contact degrades... obviously it was bad just before failure but what made it start is the question.

Maybe I'm not understanding exactly the situation...

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[IRstuff]

Without some detailed knowledge of how the current flows, or could flow, you'll just get a gross approximation, which may tell you absolutely nothing useful.

TTFN

FAQ731-376

 

[hacksaw]

bdecker,

we had a dc drive 950 amps, the connections buss bar and very heavy yet they were running about 100-150C within a year or so. When the connections were re-made, the temperature was 40-50C but life-time was 6-12 mos. At issue was sulfide corrosion that persisted even with a/c. Plating did not help, just regular checks, and taking advantage of down time.

The heat generation was from the mating junction. The bars had enough surface area to dissipate most of the heat and only heated up as the connection resistance builtup. Seems that we were getting thermal trips rather than meltdown failures.

Clean connections and properly torqued bolts were required. The contact resistance was less than a milliohm so we could not measure it directly; We could measure the junction voltage, but you had to be careful crawling in the switch gear. IR temp scanner was a lot safer.

good luck

 

[Compositepro]

There are greases available to protect the lugs from corrosion.

 

[desertfox]

Hi dbecker

Have a look at this site it might help:-

http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec7.htm#Joint

Resistance


desertfox

 

[electricpete]

My mistake - Based on that copper-to-copper chart linked there can be quite significant heat generated at the interface.

The vertical axis of the chart is labeled microOhm/mm^2.

A little thought tells that these cannot be correct units since contact resistance is inversely proportional to contact area. I assume it should be microOhm*mm^2.

Now convert it to an equivalent length of copper (in direction of current flow using R = rho*d/A
d = R*A/rho
Choose R*A = 1000 microOhm*mm^2 from the curve

d = 1000 microOhm*mm^2 / (1.7E-8 * ohm*meter)
d = 0.06 m (effective length of copper which creates same resistance as the contact).

That's a pretty big equivalent lenght of copper... especially considering all the heat is generated right at the contact.

Obviously there are many factors - materials, roughness, contact pressure. The bottom line - I was going from memory and remembered wrong. Sorry.

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[dbecker]

Thank you all for the replies, please give me some time to study them.

desertfox, that website is fantastic. It really sheds light on some questions that I had.

hacksaw, if the contact resistance is so low. Than what is the driver of the temperature rise at lug connections? Is it the current density or 'current flow' that people have mentioned?

electricpete, I am not going to be assuming a degredation at the interface. I will assume the interface had just been made and there is no corrosion. I am performing this analysis with the least amount of variables at this point. Later, I will consider oxidation.

I just need to understand what drives temperature rise in lug connections.

And from the looks of it, the overlap/thickness ratio is a huge driver of resistance. As seen on the website provided by desertfox.

I may have more questions to come, as I study this article.

 

[desertfox]

Hi dbecker

Can you give more details on your failure, have you any pictures that you can post?
When the switchgear was first designed, did it undergo a temperature rise test? if so then you should have records of the temperature rise at the point of interest which should give you some idea of the heat generated in that area.
One possible failure scenario I thought about was, that the joint is bolted up using torque figures, however using torque figures is subject to an error of about +/-25% so you could end up with more or less preload in the bolt, which would reflect in joint pressure either being high or low.
Suppose the bolts had 25% extra preload when the gear is in service, is it possible that when the joint reaches its service temperature, that due to differential thermal expansion, the bolt washer has embedded itself slightly into the copper (ie copper has yielded), now lets say the gear is turned off and the joint cools down,the materials contract back to their nominal size but because the copper has yielded slightly (permanent deformation) then the joint has a reduced contact pressure or even worse no joint pressure at all and any subsequent use would result in joint failure.
Obviously without more detail of your joint and failure I might be way off the mark.

desertfox

 

[hacksaw]

bdecker,

say you've 1 milliohm resistance in the joint and 950 amps dc through the made up connection. Thats about 1 volt across the connection, and just shy of a kw of heat generation. The solid copper wasn't the source of heat, the conductor ohmage was good for only a 20-40C rise, we had a lot of cooling and open grills for an enclosure.

 

[dbecker]

Hello again to all,


I will try to answer the questions one at at time.

electricpete, correct me if I'm wrong but isn't contact resistance unnaffected by contact area given a constant surface pressure?

desertfox, I was given word that the failure did not occur at the lugs themselves (exceeding temperature limit) but in other places that wont pertain to this problem. Either way, I am still asked to perform a thermal analysis on the lugs.
Also, they are using belleville washers to keep the pressure somewhat constant during cooling/heating.

hacksaw, I was given information that the resistance at the joint was 12 micro-ohms. That is TINY!

According to my calculations, if 1300 amps flows through the joint. The voltage drop is 0.0156V. And the heat generated due to that drop is Q = VA = 0.0156x1300=20.28 Watts. That sounds like a very small amount of heat.

But Im not done, the contact surface area has been calculated to be 1736 mm^2. Which comes out to a heat flux (Q'') = 20.28 / (1736/1000^2) = 11,682 W/m^2

Please advise if this is a logical approach to obtaining heat generation due to contact resistance only.

Thanks again all.

 

[desertfox]

Hi dbecker

I am looking into your last post but I can't believe that the joint is dissapating 11.68 Kw/m^2.
Well I thought it was your lug joint that failed reading your other posts, but nevermind can you provide some more info on this failure, then you might get better assistance.

desertfox

 

[dbecker]

Hi Desertfox,

Yes I initially thought the lugs were exceeding rated temp, but that is not the case. Still, I am tasked to predict lug temps. And I'm looking for the best method to assume heat gen.

Yea I find it hard to believe too, that 11.68kw is being dissipated in the connection.

The actual failure did not occur at the lugs, that was the information given to me initially which turned out wrong.

In any case, I can say for certain that the lug connection is passing 1300 Amps and the terminal connection is 1736 mm^2 with all certainty. And that the terminal resistance was measured to be 12 micro-ohms. This data is to be taken as baseline input data.

Material 1 is silver plated copper, thickness is 20mm.

Material 2 is pure copper with a thickness of 12.8mm.

Overlap distance is 32 mm.

In the article you supplied to me desertfox, there is a correlation for resistance vs overlap/thickness.

Both of my conductors have different thickness, therefore how do I apply this correlation now?

Thanks for your time.

 

[desertfox]

hi dbecker

In that reference site a gave in my first post there is another chapter which deals with heat generated in a conductor see below.

http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec3.htm#Heat

Generated by a Conductor
In regard to your overlap the truth is I don't know, if you have different thicknesses however I would just use the thinner section for that calculation.
I think the heat generated at the joint is around the 28 Watts you calculated in your earlier post, although the overlap factor needs to be included.
I am not convinced that the heat generated is that helpful because there might be sufficient cooling aroud the joint by convection, radiation and conduction to dissiapate the 28 Watts, which is why I mentioned whether a temperature rise test had been done for this gear, if it as then you should be able to see what temperature the joint finally achieved under normal conditions.
Another question did this failure occur under normal conditions or fault conditions?

http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec3.htm#Heat

Generated by a Conductor