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100kV isolation transformers, high precision voltage and current sensing

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Spirit532:

--- Quote from: David Hess on February 11, 2019, 01:27:09 am ---One thing I learned messing around with high voltages is that it is amazing what becomes conductive if the voltage is high enough.  Do not forget to isolate the windings from the transformer core or cores.

--- End quote ---
As mentioned, isolation is going to happen on both sides. Below and above the winding, so that the spacing to the ferrite core is the same as it is to everything else. The low voltage coil will be wound directly on the ferrite.


--- Quote from: duak on February 10, 2019, 11:37:37 pm ---I would look at Color TVs for prior art, especially those with a vacuum tube HV rectifier as they had to provide the heater voltage for the rectifier floated at the HV DC output voltage.

--- End quote ---
Sadly the way isolation done there is not going to work here - it was usually potted alongside the main transformer, and I'm keeping the HV section very low power(~7W total input accounting for all losses).

jbb:
Hmm
Well, a great big core and appropriate lots of potting should see you right.

It’s unusual, but have you considered PCB windings for the transformer secondary? You can keep very good control of the windings which might help with the potting. It may also be possible to cut some spokes and slots to help locate the windings in the core (but this might cause other issues...).

Berni:
What you need in my opinion is a massive ferrite C core.

You can then turn a custom massive thick bobbin on a lathe to hold the secondary coil with lots of clearance to all parts of the core. Ferrite is not conductive at low voltage but certainly is at high voltage since its essentially very fine iron filings held together with a non conductive binder. Then to get power into the transformer you just wind a primary onto the other leg of the C core, without any special clearance requirements since both the primary and core are sitting at ground potential. To be sure the core stays at ground potential it might also be a good idea to stick some copper foil to both core halves using conductive glue and ground it.

The ferrite also lets you operate the transformer at high frequency (20 to 500KHz) so you only need a handful of turns on the coils to get the few volts you need. Such a large core would have a good bit of energy storage capacity so switchmode topologies that make use of this are proabobly a good idea such as flyback. Tho these topologies tend to be fairly electrically noisy and winding the secondary so far away from the core causes extra leakage inductance that makes it worse. A good alternative might be driving it using a Royer oscillator or a ZVS oscillator, these provide a more sinusoid like drive current and don't care about leakage inductance

As for the HV side sensing circuitry you can simply have an extra winding to create a supply voltage for it and treat the -100kV as being ground, this lets you use normal opamps up there to sense all those parameters you want. Tho i would recommend putting the elctronics in a shielded box that's also connected to -100kV to protect the circuitry from any potentially very strong fields drawn to anything grounded.

Doctorandus_P:
Some time ago I saw a few pictures of very specialised high voltage transformers.
These are used in big antenna systems (100kW or more, etc) where some auxilary power supply has to be coupled into the antenna.
These do not only need high voltage separatrion, but also very low capacitive coupling.

I tried an image search, but that did not work for me this time.
It was a toroidal transformer with a diameter of around 40cm and a relatively small cross dection of the ferrite / iron core (Few square cm).
The primary winding was wound tightly around the core around the whole circumference.
The secundary winding was wound around a spool with the same diameter as the toroidal core.
The two toroids were both mounted individually in an external frame to keep the separation at a maximum.

So if the diameter of the 2 toroids is 40cm you can have an air gap of 18cm.

NiHaoMike:
Have you considered a BLDC motor driving another through a plastic rod?

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