Bushing - electrical field

[daniele039]

Good Morning

I´m drawing a bushing for 24 kV . The bushing consists of a rod ( made of metal),a tube(epoxy/glass fiber) and a flange (stainless steel).
I would to calculate the electrical field at the flange , to have an idea of the filed magnitude´and to define the right proportions for the rod and the tube .
I´m treating the problem like a cylindrical capacitor with two dielectrics : air and epoxy . The equipotential surfaces are then the rod (24kV) and the flange (0V).
The question is : how high can be the electrical field around the conductor ( between rod and tube )?In my opinion , the electrical strenght of the air should not be overcomed (3kV/mm ) .

Is this correct ?

To calculate the electrical field , i have do the following considerations :

1 - I have evaluated the capacitanche of each cylindrical capacitor
2 - The two capacitors are a series -> they have the same charge
3 - I have evaluated the voltage distribution between the two capacitors
4 - Field calculation

How high are the typical values of electrical field at the surface of the rod ( in air ) ?

Thank you a lot

Can you give me some suggestion ?

 

[ScottyUK]

Bushings usually employ some form of electrical stress control. Typically it uses foils within the bushing to form a capacitor grading structure which ensures the electrical stress is predictable and uniform. Discontinuities such as an air-resin interface within the stress field are consciously designed out. Most bushings designed for (e.g.) 24kV utility service aren't designed to withstand 24kV, they're designed to withstand (e.g.) 95kV, the basic insulation level or BIL.

I have to ask why you aren't just buying a bushing from one of the numerous manufacturers: a 24kV oil-air or air-air bushing is a commodity item and priced accordingly; exotic types are more expensive such as an oil - SF₆ bushing for CGIT or an H₂ - air bushing for a generator.

Have a look at some of the bushing manufacturers literature: for starters, ABB Micafil give some outline constructional details for their bushings, although the detail of the manufacturing process is necessarily commercially sensitive.

http://www.micafil.ch/0320.html

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