Stress cones

[peter d]

Why are they required?

 

[stickboy1375]

what are they?

 

[mdshunk]

For stress relief. Limits the potential difference over a gradient. Keeps the cable insulation in tact at the terminal end.

 

[peter d]

For stress relief.

That's what I don't get. Where does this "stress" come from? What causes it?

 

[peter d]

what are they?

They are those pointy things you see on the end of underground high/medium voltage cable where it gets terminated to overhead lines.

 

[mdshunk]

That's what I don't get. Where does this "stress" come from? What causes it?

Just think of it like a plumbing "reducer" fitting. You have electrical energy coming down the pipe (wire) and if you just cut off the insulation, it will tend to blast right out the end. You need a stress cone to bring it to a subtle stop. Consider that the first layer of the insulation on the high voltage cable is semiconducting, so you need fitting on the end (the stress cone) that mimmics the conductor's insulation. Layer of semi-con, and a layer of dielectric. If the high voltage cable was just insulated with dielectric, that would have to be some pretty darned good insulation. They layer it first with semicon to limit the amount of actual energy that the outer insulation is exposed to. Same goes for the terminal end, thus you use a stress cone. It keeps the shield sufficiently far away from the conductor. The high voltage guys might have some other stuff to add, but that's about it in a simple nutshell.

 

[Nick]

It is pretty much semantics but you rarely see real "Stress Cones" any more. Stress cones are a form of geometric stress control. They get their name from their appearance. It is tape built up in the shape of a cone. The kits you see all over these days are forms of capacitive stress control. There is a material built in the kit that when installed is placed at the end of the shield cut back. It has a very high dielectric constant that capacitively changes the voltage distribution around the shield termination. By changing the electrical field surrounding the termination, the stress concentration is reduced from several hundred volts per mil to less than 50 volts per mil.

mdshunk,
The semi con layer that is in contact with the conductor is known as a strand shield. Its purpose is to eliminate voltage stress in voids between the conductor and the insulation. Since the potential on the conductor strand will differ from the potential on the insulation at the points between the strands there will be a voltage stress across the air gap. This can cause ionization of the air between the conductor strands resulting in insulation failure. The strand shield has the same potential as the conductor so there is no voltage stress across the air pockets.

 

[masterinbama]

Thanks Nick

Thanks Nick

not many here have hand wrapped or even seen a true stress cone for that matter. You tell a kid today to take down those potheads they would probubly tackle the long haired kid on the masons crew.

 

[72.5kv]

pictures of a pot head

 

[peter d]

Thanks for the info. Once again, I'm amazed at the amount of knowledge available here.

 

[mkgrady]

I can't say it in technical terms, but the energy within the conductor is trying to get to the shield that surrounds the insulation.

This shield is metalic and is grounded. The energy goes through the insulation evenly throught the length of the cable. The shield is constantly draining a small amount of energy to ground. If a stress cone is not used (or something that does the same thing) the energy comes out of the insulation at the point where the shield and the shield semiconductor end.

This abrupt ending of the shield causes the energy to concentrate the leakage at that point. Over time the insulation will deterioate at this location. The result will be a cable failure.

The stress cone, because it is cone shaped, causes the energy to leak through a wider, more spread out fashion. Because the leakage is not concentrated it doesn't harm the insulation.

 

[mattsilkwood]

i think marc hit it on the head as far as my understanding goes.
the electrons are moving so fast at the higher voltages that you have to funnel them where you want them otherwise they will blow out the end of the cable.

 

[72.5kv]