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Re: [TCML] large DC coil power supply

You asked about 480v hazards on a generator.
The stock answer is " it depends ". :)

The problem with any generator is rotational
Inertia.  If a fault occurs worst case between
Generator windings and line side of first OCPD
Times up to 250 ms to fully shut down a
1 megawatt Caterpillar diesel genset, and that
Includes "scamming" the field.  These 
Controls won't be available on a Rental Unit.
Bottom line is in most cases even with full
Differential protection relaying, afh levels on
Genders are usually 3, 4, or Dangerous
( no PPE exists, energized work prohibited)

Dave S.

On Aug 15, 2010, at 7:00 PM, jimlux <jimlux@xxxxxxxxxxxxx> wrote:

> 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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