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Alluminiun connecttion failure during a "long" short-circuit

Alluminiun connecttion failure during a "long" short-circuit

Alluminiun connecttion failure during a "long" short-circuit

(OP)
Hi folks,

I faced some failures in aluminiun connections ("T" joints)applied on 636 MCM ACSR cables, perpendicular to each other. According to the (last) termagraphy report the connections were fine. However, they failed during a "long" short-circuit of about 2s, with the cables being melted.

I wonder if anybody knows literature related to this failure, or could give an good explanation of which kind of phenomena happened inside the connectors during the short-circuit to melt the cable.

Best regards,

H. Bronzeado   

RE: Alluminiun connecttion failure during a "long" short-circuit

High resistance in the connection that produced enough heat during the fault to melt the conductor.  Frankly I'd be more worried about the 2 second clearing time than the connection failure if you were seeing normal temperatures under normal load.

RE: Alluminiun connecttion failure during a "long" short-circuit

(OP)
Thank you davidbeach,

If the connection was fine under normal load, so its  resistance should be low.
I agree that the duration of the short-circuit was long, but the connection should withstand the current during this time, anyway.

Do youthink that there some electric arc inside the connectors, which could trigger other reactions? Or is only "Ri2"?

Regards,

Herivelto

RE: Alluminiun connecttion failure during a "long" short-circuit

You didn't say the fault level. But if high enough, something is going to melt eventually. My guess connection before the cable would be typical.

=====================================
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RE: Alluminiun connecttion failure during a "long" short-circuit

You might want to check with the tap manufacturer to see if you are using the correct compression tool dies, conductor preperation procedures, etc.

RE: Alluminiun connecttion failure during a "long" short-circuit

A properly installed connection should not get any hotter than the conductor.  What type of connection is it?  If bolted, it could have experienced cold flow and become loose.  Careful attention to differential thermal expansion is needed for bolted connections if bolts are steel.   

RE: Alluminiun connecttion failure during a "long" short-circuit

(OP)
Thank all,

The short-circuit current was arround 10-14kA, 69kV. The connections are bolted (steel).

To anderstand what happens, I take a piece of cable (266 MCM) that was installed in the substation for more than 30 years, including its connections ("T" joints), and applied 13kA during 2s. The cable melted in the same way as in the field failure.

My coleague in the lab told me that this happens because an electric arc inside the connector, which was triggered by a voltage increasing in the small space between contacts. This voltage is caused by the current through the contact resistance.

I know that the temperature due to Ri2 play a crucial role in this phenomenon. However, as jghrist said, "a properly installed connection should not get any hotter than the conductor". So, during a short-circuit, if the connection is properly done, the cable coild be the weak point, as heating dissipation is greater in the connections.

What I am trying to do is to understand, in details, how this phenomenon really occurs. I would appreciate very much your comments. Thank you in advance.

Best regards,

H. bronzeado
      

RE: Alluminiun connecttion failure during a "long" short-circuit

When I brush aluminum cables prior to applying compression connections, I use a special brush and brush in the direction of the conductor strands. The brush applies presure to opposite sides of the cable and cleans as much of the arc of the cable as is visible. On seven strand cable that may be as much as a 300 degree arc. Many crews brush across the cable and don't clean nearly as great an arc of the strands. When the connection is made up there are arcs of aluminum oxide on each strand that are included in the joint. I am not surprised to hear of melting failures.
Can you quietly make some enquiries as to the method of preparing cable for splices? Also, if you are able to have a cable completely cleaned as I have described prior to making the connection and then running a comparison test with a normal connection, we will all be pleased to hear the results.
I'm short of time just now. If I am able to find a picture of the brush I will post it later.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Alluminiun connecttion failure during a "long" short-circuit

  
hi BronzeadoWe need more information, ie short circuit rating of cable etc.If you can post a picture that would help a lot more.One possible failure mode might be the clamping force on the lugs being either to high or to low, either way when the short circuit current heats the joint up, the electrical resistance increases not only due to the temperature rise, but also due to differential thermal expansion of the aluminium and the steel bolt (steel bolt assumed),which creates stresses beyond the yield stress of the materials and consequently loss of joint integrity.That said I have trouble imaging that all that could happen in 2 seconds.Can you tell us how many bolts are used per joint, bolt material, size and what they were torqued to on assembly.What was the ambient temperature of the joint during normal service, finally if you can give us the physical sizes of the lugs that would also help us to give you a better answer.


desertfox  
 

RE: Alluminiun connecttion failure during a "long" short-circuit

Hi Bronzeado

Please don't double post its against site policy, I answered this post in the heat transfer forum,but seeing as most answers are here, I have transfered my post here and red flagged the other one.

desertfox

RE: Alluminiun connecttion failure during a "long" short-circuit

(OP)
Hi desertfox,

I put some photos in the other post. Could you, please, have a look in thrm?

Sorry for the double post. I hope people forgive me for that!

Regards,

H. Bronzeado  

RE: Alluminiun connecttion failure during a "long" short-circuit

Hi Bronzeado

Can you post those photo's in this forum, the post in the heat transfer forum as been removed.

Those connectors you stated were 30 to 40 years old, judging by the corrosion product on the aluminium it doesn't surprise me that the failed.
The corrosion would certainly increase the electrical resistance and cause a temperature rise at the joint.
When was the joint checked for bolts being at the right tension and when?
I think your failure is down to age and loss of bolt tension.

desertfox

RE: Alluminiun connecttion failure during a "long" short-circuit

With steel bolted connections on aluminum conductor, the aluminum expands more than the steel during normal load heating.  The aluminum conductor will become permanently deformed.  When the connector cools, the connection will become loose.  Aluminum oxide can form where the loose connection is exposed to air.  This will increase resistance and may cause a failure when a large amount of current flows, even for a short time.

The solution is to:
1.  Avoid bolted connections on aluminum (preferable), or
2.  Use spring washers (like Belleville) to maintain pressure on the connection during heating and cooling cycles, or
3.  Use bolts with the same coefficient of expansion as the conductor (aluminum alloy).
 

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