[TCML] large DC coil power supply

[jimlux <jimlux@xxxxxxxxxxxxx>]

I'm looking at various ideas for a power supply for a large-ish DC spark 
gap coil (let's say, 25kW)... It's an art project, so it's not an income 
producing widget where you can trade off customer costs. It needs to be 
portable/transportable without taking days to set up.  The basic 
environment for discussion purposes is a stage or performance setup.. 
coil on the stage, big heavy stuff (generator, transformers, etc) 
offstage, separated by some dozens of feet.

This is sort of like the power supply for Electrum, but Electrum was 
designed for permanent installation, and so some of the design decisions 
might be different.  Technology has also marched forward.

Various issues and alternatives for discussion:

How to get prime power in.. You're not going to use 16 gauge cheap 
extension cords here.

if you want to run off 240 single phase, that's 100A.. bigger than one 
normally plugs into the old dryer socket.

In show business, at this kind of power level, you start running 4 wire 
banded (4 separate wires with what are essentially fancy banana plugs on 
the end)


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?

My gut feel is that 240:14400 transformers are more common (cheaper) 
than 480:14400. And 240V (or 208V) generators are going to be more 
common too (and so cheaper to rent.. in the $100/weekend category, it 
appears).  If you put the HV power stuff near the generator, then the 
fact that you're running BIG wires for the lower primary voltage won't 
be as big a deal, cost wise.  (and if someone is *paying* you to run 
this thing, and they want to put the generator elsewhere, then they can 
*pay* for the rental of the longer wiring)

You've got to get to HV somehow, and the simple way is just to get a 
couple or three pole transformers and run them (open) delta.   25 kW @ 
15kV is a couple amps, well within solid state rectifier territory, 
especially in oil.

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?

OR... what about switching power supplies.. Big three phase rectifier 
off the prime power, driving a big power supply (hmm.. but you'd 
probably not be able to find such a power supply *surplus/cheap* off the 
 shelf, unlike you could find pole transformers)  And designing and 
building such a thing is a huge project in itself. (compared to a fairly 
easy 6 pulse rectifier with 3 transformers or a 12 pulse with 6, which 
one could lash up in a weekend, if you had all the parts)

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)

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.

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