High power bushing (HV power line insulator shapes)

[mendes]

Hello,

I would like to understand why "bushings" that are used as insulation for high power electric cables in transformers, and which are often made out of porcelain or glass, I would like to understand why they have this form of "skirts" ? Why aren't they just as a simple cylinder?

 

[mendes]

Have a look at this:

http://3.imimg.com/data3/TE/AF/MY-5753/porcelain-bushing-for-transformer-rectifier-250x250.jpg

 

[rumborak]

I could see two reasons:
1: Heat dissipation. The conductor inside might warm up, and this design creates a good amount of surface area.
2: Simply as a way to keep anything earthed away from it a certain distance, but without making the whole thing a big cylinder.

 

[mendes]

I think I found the answer:

Wikipedia:

http://en.wikipedia.org/wiki/Insulator_(electricity)

Dirt, pollution, salt, and particularly water on the surface of a high voltage insulator can create a conductive path across it, causing leakage currents and flashovers. The flashover voltage can be reduced by more than 50% when the insulator is wet. High voltage insulators for outdoor use are shaped to maximise the length of the leakage path along the surface from one end to the other, called the creepage length, to minimise these leakage currents.[3] To accomplish this the surface is moulded into a series of corrugations or concentric disc shapes. These usually include one or more sheds; downward facing cup-shaped surfaces that act as umbrellas to ensure that the part of the surface leakage path under the 'cup' stays dry in wet weather. Minimum creepage distances are 20–25 mm/kV, but must be increased in high pollution or airborne sea-salt areas.