Most transformers of the HV type are either potted or under oil. This is to absolutely avoid all contact with air (corona) or the environment (moisture, dirt, etc.) If you can't do either of these, you may be limited to 5kv in an open frame transformer design.
All major HV transformers are oil tanked. Magnetically shunted or low duty cycle/low power units can be potted.
Most HV transformers are either single ended grounded units (potential types) or ground center tapped on the secondaries. (neon, oil burner, etc)
The best designs invovle several pie sections in the secondary. (isolated secondaries hooked in series.)
Central E core wound transformers are efficient and usually lower voltage units. Outer leg wound units are often of the HV type.
I have depotted and restored many neon sign transformers over the years in my Tesla coil venue. To rewind one is a study in human torture and self abuse.
I have dealt directly with a local firm who custom wind HV distribution transformers. They say that to rebuild a transformer they furnace the entire unit to ignition and the paper, potting and oil burn off and the copper/aluminum melts away or is isolated in a mass is lower temps are used. The core is then tank dip cleaned and the process starts with the clean core and fresh wire.
I had them do soemthing they rarely do. design and air operating 13.5kv 5kva transformer. They used 15kva iron due to the required extra insulation over an oil based unit. They wound in the conventional E fashion, but isolated secondary layers with plastic strips for air flow and standoff in a form of modified pie winding effort. They finally hot tanked the thing in a formvar coating glop that coated and sealed the thing against contact moisture and dust. The unit is NOT potted! Air can circulate up through the HV windings. It was a real nice job and I used it for many years in my huge Nemesis Tesla coil system at 13KW (short 4 minute runs.)
I can't imagine an amateur wound 20kv or higher transformer. All the potential transofrmers I have in the 34.5kv design are monsters!! A 1.5 kva 40kv potential transformer is 4 feet tall, weighs 300 lbs, and takes up the room of a phone booth. I will not use one of these even though I have one. I plan on using a single eneded 20.4kv 1kva unit I have that is managable at 115lbs. I will voltage double it for fusor use.
There is no way a significant neutron producing fusor can be run on a neon sign xfrmer system and provide workable HV in the 30KVDC class without a complete and expert rewind and oil environment with voltage doubling. Even this would be a stretch.
So many folks think a 15kv neon transformer rated @ 30ma will give 30ma @15kv. It can't! At 30ma most neon transformers of this type are warranted to supply 1-2kv! you can actually place a pair of high brighness LED's hooked back to back right across the HV knobs and plug the thing in and they light normally and will not blow out! I have done this before.
I would recdommend against your own rewinding. Most likely you will spend a lot of time on a doomed effort. Of course, you will be smarter and better educated in transformer hands-on. Given enough passes, you will ultimately succeed. Got time?
Richard Hull
Created on Thursday, March 15, 2001 11:14 AM EDT by Richard Hull