electronic tap changer 1

[jramirez001]

I am designing an electronic load tap changer for distribution transformers, but the thyristors are incompatible with oil's transformer; its dielectric rigidity is diminished dramatically. Is there some protection for avoiding such problem?

 

[PowerfulStuff]

Hi
Transformer oil that has been kept dry is a really good insulator - fields of 50kV/cm shouldn't cause problems. The oil can absorb moisture from the air and degrade very quickly - is this a possibility?

 

[Alex68]

Hi
What do you mean with "the thyristors are incompatible with the oil transformer"?
What are the physical changes in the oil? High temperature?
You ask about a protection but.. Against what?
Have you recorded discharges inside the oil?
Are you completely sure that the SCR switchings happen exactly when the curent is zero?
I fear that a spurious switching could produce an overvoltage.

I don't imagine the origin of the moisture as suggested by PowerfulStuff, because the oil should be inside a sealed enclosure.

 

[jramirez001]

I appreciate your comments a lot. They were carried out tests of compatibility among the electronic devices (included the printed circuit board) and the oil. The oil changed color, but overalls, it lost its dielectric rigidity. Now no longer it is of 50 kV/cm but 1 kV/cm. The protection that I look for is some liquid or solid able of being applied on the electronic devices, so the oil doesn't lose its rigidity.

 

[mc5w]

Sounds like your printed circuit board material is dissolving in the oil. You could also have leftover solder flux contaminating the oil. You probably need different plastics for your boards and electronic components.

You might want to think about reengineering the transformer so that the windings run submerged in boiling sulfur hexafluoride or octofluoropropane. Hexafluoroethane has dielectric properties similar to sulfur hexafluoride but the vapor pressure is way to high for a boiling liquid design. If you use a stricly vapor phase design you will need internal blowers which is another part that can fail and you will have difficulty keeping SF6 liquid when it is cold outside meaning that all or your SF6 will condense on the bottom of the tank. Another design uses liquid pumps to pour liquid SF6 onto the windings but you could just as easily put the high current winding on the bottom and boil the liquid instead.

You also need small amount of inductance in series with each SCR or bank of SCRs. SCRs do not turn off instantly with the result that you will have 2 SCRs shorting out the small voltage between taps. This similar to how a 3-phase bridge rectifier needs a 3-phase inductance in the branch circuit to limit current flow during commutation.

You sould also consider using steel core printed circuit cards in your application. These are circuit cards that have a layer of sheet steel in them. Volkswagen found out the hard way that their fuse and relay panels need to use a steel core circuit boards instead of plastice core boards.

For an SF6 or C3F8 design you would also need refrigerant service valves top and bottom to facilitate draining the tank.

 

[Alex68]

I think that mc5w is right!
Your is a chemical problem.
I suppose that your printed boards were washed with CFC and this reacts with the oil.

Maybe my idea is stupid but... Why don't you put the printed board outside the transformer enclosure?
I know that you should add some bushings for the cables but you avoid any incompatibility!

 

[GamezBeCJ]

My idea is this. I don't know if you have seen how the computer circuit in a car is built. In that case the whole circuit board and components are embeded in a plastic.
I'm sure you can find a suitable/compatible (nitrile?, viton?, noprene?) material where you can embed the TC circuit in.


Carlos Gamez, P.E.
Industrial Consultant
Transformer Specialist

 

[mc5w]

I one time replaced the fuse and relay panel in a Volkswagen Scirocco. The old and new panels were of far different weights corresponding to the use of a steel core circuit board in the new board and plastic core in the old board.

If you have worked on 1A2 telephone key service units enough you know the difference between Western Electric (Lucent) line cards and ITT Kellogg line cards. The WE line cards are much heavier because they have a steel core card and the ITT Kellogg card uses a cheap plastic core card.