Re: [TCML] large DC coil power supply

[jimlux <jimlux@xxxxxxxxxxxxx>]

David Sharpe wrote:
> Jim
>
> Since I am working with switchgear on a daily basis, I can give a little
> guidance from the power side...
>
> <SNIP>
>
> > 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.
>
> http://www.gedigitalenergy.com/products/specs/3CPT3-60-30.pdf


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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