Problems with connections

[Bronzeado]

Hi folks,

I made this same questions for the forum of "Corrosion Engineering". However I would like to rise the same question in this forum as people may not visit the corrosion forum.

We are facing some problems with electrical connections in our old system (>50 years old) due to short circuits.

Our cables are all made of aluminiun (from 266 to 945 MCM, ACSR) with the majority of the connection being Al-Al. Also, we have Al-Cu connections.

I would appreciate if somebody could give me some tips to understand better what is happens (connetions failure during short circuits) and also how to cope with this old problem. The frequency of this failure are increasing.
Is it material fatigue???

We use only bolted connectors. Someone has suggested to use connectors type "ampact". Tips are welcoming!

Best regards,

Herivelto Bronzeado

 

[desertfox]

Hi Bronzeado

You need to give more information on the failures just telling us it happens under short circuit is not enough.
We need to know which part as failed and what it looks like,
can you post a picture?
Your failure could be a lot of things one example is the bolt you make the connection with, may have lost sufficient preload over time so that when it see's a short circuit it cannot hold the joint together. Another question is there any signs of corrosion when you inspect the joints on a rountine basis.

Regards

desertfox

 

[dpc]

Yes, we need a more specific description of the failures you are seeing.

Aluminum has cold flow tendencies that can cause loose connections, overheating and fires. This is not an issue if properly terminated, but could be suspected on very old installations.

But a description of "short circuits" is more indicative of insulation failure, not so much terminations.

 

[Bronzeado]

Hi desertfox and dpc,

Sorry for not being clear and thank you for your interest in help me. I will try to give you a better "picture" of my question.

Our connections (cable-cable and cable-flat terminal) were working fine. Last month, during some short-circuits due to sugar cane fire, three connections failured:

1. the cable (Al, 266 MCM) broken exactly at the end of the connector;
2. The cable (Al, 266 MCM) broken at 40 cm from the terminal;
3. The cable (Al, 266 MCM) did not brokem but seems to have twisted.

Unfortunately, I did not have a picture of these failures.

I find it strange as we had no big problem with connections. Also, I could not identify exactly what happens.

As an emergency solution, I recommended to change the cable and connectors to one of bigger ampacity. However, I am not sure if this will really the final solution.

I have just start to study this question.

I think that the connection changed its characteristics due to the high current caused by the short-circuit. But it should withstand that current as it was design for.

Also, electromechanical forces may be involved, with the cable forcing the conectors. This will make much difficult to visualize the problem.

I know that it is very difficult to find the cause of these failures as there are a lot of lack of information.

So, the question may be re-write as "has anybody experience on that problem?".

Anyway, thank you for "thinkink" on this problem.

Regards,

H. Bronzeado

 

[desertfox]

Hi Bronzeado

Firstly am I right in thinking your connectors are similar to these at this site:-

http://www.termofit.ru/index.php?option=com_content&task=view&id=297&Itemid=323.

From what you now describe it appears to me that its the cable failing not the connector.
I think you need to start looking before the problem ocurred

1/ what rountine inspection/testing on the
cables/connectors prior to the fire and fault starting.

2/ What was done immediately after the problem with the
cables and connectors.

3/ The failures you are seeing now are they under normal
running or again under fault conditions.

To answer you latest question directly no I have never experienced your situation.
Finally I think given the age of the installation dpc's point about insulation breakdown particularly after the recent short circuit you have had maybe very important.

Regards

desertfox

 

[Bronzeado]

Hi desertfox,

The problem may be the cable. But the connection is, also, a weak point.

Answering your questions:

1. We do infrared routine inspections on cables and connections. For those failures, we did a infrared inspection a day before. Everything seemed to be normal.

2. Due to short-circuit, the breaker made a reclose whithout success. After some time, a manual breaker closing (the third) was done and the cable/connection failured.
This has shown that the connection (Al, bolted) did not withstand the short-circuit current. WHY?
The connection was fine by the infrared inspection (before short-circuit).
Now, we are thinking to make infrared inspections after every short-circuit as we do not know what is happens. In this way, we may avoid some failures but is very hard and expensive.

3. Both. The failures where the cable broke was during the short-circuit. The "twisted"cable was seeing after the fault by infrared inspection.

Again, thanks for your help!

Reagards,

Herivelto Bronzeado

 

[desertfox]

Hi Bronzeado

What information does your infrared inspection give you, does it give temperatures at various locations? if so that means you need to build up a time history of temperatures at the same locations so you can see that any increase in temperature will show a possible problem in that area, is this what was done prior to your problem?
Again here with the bolted connection if the pre-load in the bolt was lower than it should be, then under short circuit conditions that could quite easily fail, however if you have a time history of that joint then any increase in temperature should have shown up on inspection, I suppose it depends how sensitive your infrared inspection is.
We could really do with some pictures or sketches of the failures to be of any further help, is there no chance you can take photo's of the next failure/failures.

regards

desertfox

 

[Bronzeado]

Thaks desertfox,

As I am now in charge with this problem, I hope get much inormation in near future.

Regards,

Herivelto Bronzeado

 

[desertfox]

hi Bronzeado

Keep us posted then

Regards

desertfox