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Re: [TCML] large DC coil power supply

David Sharpe wrote:

Since I am working with switchgear on a daily basis, I can give a little
guidance from the power side...


Clearly, this is bigger than typical residential loads, so that means
probably using a standalone generator, and in this class, that means 3 phase
too, which is nice.  So, does one start with 240 or 480?
120/208 wye is a standard secondary voltage for three phase medium
voltage dry type CPT's.  30kVA is smallest size, however it weighs as
much as 3 oil filled pole pegs ( ~ 700 lbs).  Primary (HV) has +/- 7.5%
or 2.5% taps.


So, a pair of pigs in open delta might be a smaller/lighter way to go to get from generator low voltage to HV, assuming the thermal ratings are managed properly. These won't be hanging on a pole in the hot sun, after all.

(of course, open delta won't get the inbetween phases if I wanted to do a 12 pulse rectifier)

Another point, stay away from 480V single or three phase.  Available fault
currents and arc flash/shock hazards are significantly higher with 480
then 240V or less.

Even on a generator? I would think that the fault current would be limited by the impedance of the generator. On conventional distribution, sure.. 480 will have a LOT less impedance, just because it's used for bigger loads, if nothing else.

 125kVA transformer or smaller @ 240V with available
bolted SC of 10kA or less has a AF HRC of '0' (street clothes, cotton;
voltage rated gloves).

And, as well, 240V switchgear/plugs/receptacles/wiring are cheaper, more available (e.g. one could conceivably buy stuff at Home Depot for on the spot repairs), now that I think about it.

  Would be excellent if the generator you select
has field control adjustability, then you simply adjust generator output
reactive power via field excitation.  Field excitiation DC power
requirements are approximately 0.5-2.0% of machine output
(construction, speed/poles dependent).

I'll have to look into that.. I'm thinking fairly standard rental trailer mount units in the 30-50kVA range. I'm not sure if they are set up to allow the casual rental customer to adjust the excitation.

Good point though, because my load is likely pretty bad power factor.

But, 2 or 3 transformers is a "big" package.. (but then, a 25kW coil is a
big pile o' stuff anyway), and they're oil filled which makes transportation
a bit more of a hassle.  Do you put them in separate road cases? or just one
big 1000 pounder?
1-1000 lb case with dry type transformer shown above would work.

But, from a "buying surplus" standpoint, I think the oil filled units would be cheaper than dry. The dry has a lot more iron and copper for the same rating, which raises the scrap value. I'm just starting to look into this, so haven't started calling around. Might be a wash.

But maybe there *is* some sort of industry that uses up 20kV 1 amp DC power
supplies?  (yep.. buy 20 1kW class inverter microwave ovens, work out the
non-trivial primary isolation issues, and run them in series/parallel)
Perhaps industrial microwave generators in the 50-100kW range?

Or older 10kW-20kW class TV transmitters with a big klystron (today, most of these are solid state). I have a 30kV,300mA supply for an E-beam system, but it's not trivially retrofittable for lower voltage/higher current (8020A rectifier tubes, for one thing)

And then, once you've got your DC power.. how do you get it to the coil?
It's fairly obvious that the power supply is big and bulky, and you're not
necessarily going to want to stash it under the coil.. so that means running
a few amps at 20kV around.. RG213 coax should do nicely for that.
Resonant DC PS would require charging inductor, de-Qing diode stack
and would provide up to 40kVDC.  Don't know if '213 would hold off that.

Push the diode/inductor downstream of the power supply. that keeps the voltages lower. HV cable is probably the least of my problems.. there's lots of stuff around rated 50kV.

tnx for the ideas

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