deadend insulation with synthetic insulators 1

[gozneli]

Hi,

It is a typical practice to add additional bells on deadend or heavy angle structures. I thought this was just a mechanical requirement, i.e. it is much more difficult to replace a broken bell on a deadend or heavy angle insulator string due to conductor tension, so you can afford a broken bell or two without compromising the insulation.

I want to use synthetic insulators instead of porcelain bells. In this case, is anybody aware of a requirement for increased electrical insulation? We have a debate going on, and I believe if you are usign long rod syn. insulators on deadends or heavy angles, you do not need additional electrical insulation.

Can anybody comment?

 

[jghrist]

Lightning surges will increase at large angles and can double at deadends. This is the reason for increasing insulation. I think this used to be mentioned in earlier revisions of the NESC (US National Electrical Safety Code).

 

[Marmite]

These days with the correct equipment it's as easy to change an insulator on a tension set as it is a suspension set. The additional insulator on heavy angles is to increase the distance of the conductor to the crossarm to compensate for the decrease caused by the heavy angle. Synthetic insulators are manufactured as like for like replacements for the equivalent string of disc insulators, depending on the system voltage. Providing you specify the correct polymeric insulator in terms of system voltage, Wet 1 minute power frequency withstand voltage, dry impulse withstand voltage and creepage distance, then you shouldn't need extra insulation. An exception would be for heavily polluted atmospheric conditions or salty atmospheres where you would increase the minimum creepage distance. Creepage distance is the shortest distance along the insulating surface of the insulator between points that have the operating voltage between them.
Regards
Marmite

 

[gozneli]

jghrist,

Any explanation on why/how the lightning surge increases/doubles on deadends? Is it due to the shape of the conductor, i.e. deadend conductor is cut and continue through a compression or bolted clamp? I find it interesting that this is not called in any of the standards I am aware of.

Marmite,

Generally the proximity due to angle of the insulator is resolved by adding cold-extension links between the insulator and the structure - adding bells seems to be a very expensive solution to a very simple problem.

Regards,

 

[jghrist]

The propogation of lighting along a line is a wave which has reflections at discontinuities. At a deadend, the full wave reflects and doubles. At other discontinuities, such as a bend in the conductor, there is only a partial reflected wave. See

http://www.usace.army.mil/publications/armytm/tm5-689/c-7.pdf

 

[gozneli]

Just in case anybody interested, I did some more research and found the following:

1-Number 1 reason is the difficulty of changing broken bells on a deadend structure-so practice is to put more bells than needed just to be able to afford (utility prcatice) one or two broken bells without having to replace the damaged bells.

2-Horizontally mounted insulators are not washed as good as they would be when mounted vertically. Therefore additional bells compensate for the reduced effective creepage distance.

3-Some people think that for a given surge, higher insulation forces the failure to take place on tangent structures, rather than on deadends - for the same reason as 1 above.

4-REA substation manual recommends 1 additional bell upto 45 degree installation, and 2 additional bell between 45 degree and horizontal (90 degree). This seems to make sense for reason 2.

5-I still think that, despite accepted practices, the additional number of bells for reason 2 should be a function of voltage - how can we specify 2 more bells for 69kV line 230kV line? This makes me believe that it does not have much to do with the doubled surges on the deadend terminations, otherwise they would have asked for twice as many bells.




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