High Voltage Connections for Dummies 2

[Summitak]

Hi Everyone,

First I would like to give some background on myself. I have an Electronics Technician degree from the early eighties. I’ve been working in the oil industry on gas turbine controls since graduation. My experience has been mechanical drives, compressors, pumps etc. In the last two years I’ve been working at electrical utilities as an electrician / controls technician.

I’ve had the occasion to help in the bus work (13.8 KVA) of the generator and the placing of load break elbows on 12470 VAC.

These are the questions that I’ve asked with only vague answers as the response.

Q. What the purpose of the semiconductor layer on the cable and how does work?
A. To relieve electrical stress.
Q. How?
A. I don’t know.

Q. What the purpose of the Stress Cone and how does work?
A. To relieve electrical stress.
Q. How?
A. I don’t know.

Q. Why are we using mesh tape on the bus splice and how does work?
A. To relieve electrical stress.
Q. How?
A. I don’t know.

It seems to me that the people doing the work know how to do it, but not why. I’ve searched the internet and found only brief explanations as to the “why” of proper high voltage techniques.

I was hoping some of you might like to enlighten someone without an engineering degree or at least point me to a text that would be like High Voltage Connections for Dummies.

Thanks,

Andy

 

[rbulsara]

For starters review this link and see page 11. Its on website of 3M.

http://multimedia.mmm.com/mws/mediawebserver.dyn?TTTTTTB_LdgTmwUTfwUTTTxUzDPssssr-

You may check website of other termination manufacrurers such as Raychem and look for their product application sheets or white papers. Even you can contact their engineering dept toprovide you more information.

 

[rbulsara]

sounds like the link does not work.

google for High voltage terminations. Find 3M site. Find intallation manual for terminations for 2-79 kV.

 

[oldfieldguy]

I used to teach a class on high voltage cables and terminations.

The semicon stuff: Imagine the high voltage as the air in a balloon. The insulation is the balloon itself. The semicon spreads the stress to the insulation, just as if you squeezed the balloon between your hands. Without the semicon, the stress is concentrated in areas at ground potential. Imagine concentrating the stress on your balloon with the point of a pin.

The semiconducting layer and shield tape simply insure that the electrical stresses in the insulation are spread evenly and safely.

old field guy

 

[Summitak]

Thanks for the replies.

rbulsara,

I couldn't find the information in which you were referring.

oldfieldguy,

I think I'm beginning to see the picture. What are some examples of areas of ground potential that would cause the pinpoint stress? Are the stresses normally present by proximity to earth such as in conduit?

Thanks,

Andy

 

[oldfieldguy]

Summitak--

I have personally observed incorrect applications of unshielded cable in high voltage applications.

Upon initial energization, there was no fault and no indication of any sort of problem.

After teh equipment had been in service for several years, the swtichgear was opened up for maintenance and that's when the problems were noted. Where the unshielded cable's insulation had touched the metal cabinet, without fail there was indication of erosion due to corona developement and the effects of ozone on both the metal of the cabinet and the rubber insulation. There were pits in the rubber that were both visible and and measurable.

I have no doubt that continued service would have reuslted in continued erosion of the insulation to the point that an electrical fault would have occurred due to insulation failure.

A shielded cable with the shield properly grounded would dhave no probem in exactly the same service.

Conversely, shielded cabe applied without grounding the shield will result in some very bad problems very fast.

old field guy

 

[rbulsara]

Summitak:

I know 3M site is very difficult to navigate. The article is in 3M Tech Brief "High Voltage Cable, Power Cable, Splicing and Terminating" published in 1991. The number in the back reads 80-6104-3863-4(713.0)BP.2.

You may want to contact 3M and have them send you a copy.

 

[Summitak]

Hi Oldfieldguy,

So when terminating a cable for example, the insulation is close to ground so the semiconductor and ground shield protect the insulation from degrading by creating a high resistance bleed to ground. Does that sound like I'm getting it?

The stress cone is there to facilitae the grounding of the shield and to create a bump in the cable to prevent tracking. Do I understand that and is the shape of the cone critical?

Thanks for your help.

rbulsara,

Thank you for your help as well. I sent a request to 3M for a copy of the publication.

Andy

 

[rbulsara]

A definition from S&C:

Stress Cone (underground): a cone-shaped stress relief device used on high-voltage cable. Cable insulation is increased at the end of a shield termination to reduce the stresses that concentrate at the end of shield termination; these stresses could otherwise cause a breakdown of the cable insulation at this point.


A google search for stress cone will get a few hits.

Here is a rough explanation.
Stress cone essentially helps smooth the abrupt change in electrical field that causes electrical "stress" where a shield is cut at the termination. The shape of the cone helps taper the electrical field lines by forcing them to follow the contour of the stress cone. Stress if left alone will eventually breakdown the cable insulation.

 

[Summitak]

Hi rbulsara,

Thanks for the reply. I've been really busy and haven't had a chance to catch up. I did a search on google and found very little about stress cones. From your explanation I understand the basic function of a stress cone. Why is there stress at the termination? I'm guessing that the removal of the insulation is the cause of the stress. Is it because the air is ionized? This brings up the question of how does the insulation function for high voltages. Is it providing insulation solely by distance or is there another molecular way to create insulation? Does the insulation attenuate the emf and then the emf becomes greater where the insulation is removed? Is the stress crated due to and at the step change in emf?

I'm just talking outloud and wondering.

There must be a text somewhere on how to design and build a stress cone.

I did contact 3M, hopefully I can get a copy of that publication.

Thanks for listening to my rant.

Andy

 

[oldfieldguy]

Andy--

Insulation at its simplest implementation is a function of distance AND the type of material. Stress cones can be built of the same material as the cable insulation. The idea is to evenly and gradually spread the electrical stress over a larger area to eliminate high point stress on the termination. Think of a labyrinth seal for electrons.

This was and still can be done by adding layers of the same type of insulation to produce a bell or cone shape, hence the term "stress cone".

New termination kits incorporate "Hi-Z" material which has different electrical characteristics instead of wrapping layer upon layer of EPR tape. It's a big savings in manhours and requires a much lower level of skill and information to achieve a similar result.

old field guy

 

[dewone]

Hope this helps you figure the how.
When you apply your stress cone or the build up of insulation.
You first gap the cable by peeling the layers back down to the conductor. So from the end of the conductor where you crimp on the connection back, you peel so many inches of insulation to the semi con then again so many inches of insulation to the shield. That gives you clearance ie so many inches in open air from one un-insulated conductor to another or to ground. The next step is to clean the areas to where the insulation is applied. Then the wrap goes on with bulk of the insulation build up over the first gap conductor to semi con. Failure is when voltage jumps the gaps. Letting out the magic smoke.