[TCML] X-ray cable best practices for feeder cable
sparktron01 at comcast.net
sparktron01 at comcast.net
Mon Jan 5 20:24:32 MST 2009
I am now in the world of "Power Engineering", and I absolutely
concur with DC; I've seen evidence of it in Medium Voltage
(>600V, <15kV) systems.
One of our companies clients had a bad substation ground at
2.4kV L-L to a 1000 HP motor. Interesting to see a 45 deg
"crucifix" burned around a bonding bushing on a 600A
feeder conduit entering the MV MCC. MCC/Motor Ground
missing to substation ground, EXCEPT for MV cable shields
(not there from original installation, not our companies
installation); length of "burn/arc" flashing at least 3" long.
So this situation is EXACTLY like using a shielded X-Ray
cable in Tesla service.
On a delta system (no ground) with a "capacitive" single phase
arcing fault to ground, transient over voltages can exceed
8X V applied. If you doubt, check the IEEE "Buff" Technical
reference for details. I trust we all understand the implications
of applying ~1kVAC to a 120VAC branch circuit....
Proper grounding is important in house wiring, but MUST
be installed correctly at MV; a fire, catastrophic equipment
damage (even collateral to unrelated systems), or electrocution
will result if improperly installed.
Imagine what would happen if a plant technician
had his bare hand on 1000HP motor while running, and a single
phase (or capacitive gradient) current occurred. At a minimum
a severe shock, or more likely death would have occurred.
I have made it a personal rule to never touch high horsepower
medium voltage motors while running for this reason...
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA USA
-------------- Original message ----------------------
From: "DC Cox" <resonance at wildblue.net>
> If you should elect to use X-Ray cable, I suggest putting a pair of 1" dia.
> brass ball sphere gaps directly across the pole or potential transformer.
> If resonances should occur, the sparking at the sphere gap will indicate it
> and suggest a correction to your circuit.
> >From what other members on the list have said, my case may have been an
> isolated situation that occured with my 60 nF pri capacitance being
> back into the pole xmfr and establishing some type of unwanted resonance. I
> remember the sparks firing entirely across a 125 kV breakdown rated busing
> on the pole xmfr to the case.
> Never again for me! Now I always use 150 kV rated cable with a single 6 AWG
> conductor, (never X-ray cable with a grounded outer shield), ground one
> bushing, and then run the other bushing thru my protective circuit to the
> coil's sparkgap. For a ground I use some green 10 AWG cable, and, of
> course, a separate 2 ought fine stranded welding cable for the high
> frequency tank circuit and also to my dedicated external building ground.
> I'm also a strong advocate of using a protective filter, even with a robust
> pole xmfr. Some experimenters don't go thru this trouble, but I think this
> is poor
> engineering practice. A pole or PT transformer is usually an investment,
> and I think a good investment is worth protection.
> Jim, with your cable, just strip off the outer grounded shield, use the
> cable, and then run a separate 8 AWG ground wire over to the spark gap.
> This comes from the case of the xmfr to which the other HV bushing is
> grounded. Then, you don't risk any resonances.
> Most power engineers are well aware of nasty unwanted resonances in their
> transmission systems, and they rate second only to lightning for damages
> Dr. Resonance
> On Mon, Jan 5, 2009 at 5:27 PM, Jim Mora <wavetuner at gmail.com> wrote:
> > Hello Fellow List Members,
> > I am building two cabinets for a pig or my 7500KVA PT @ 18Kv. There are
> > lots
> > of choices to make in terms of the coil HV design. My 19" control rack will
> > be 240V/60 and be at house ground of course. 240-280v will go out to the
> > transformer cabinet isolated LV windings that will be configured as
> > follows:
> > H2 grounded to the cabinet and RF isolated ground. The inner turn of the
> > primary and bottom of the 12" coil will be joined to the RF ground
> > (something I haven't done before). The primary will be only 1" from the
> > secondary; but this setup should allow for no Pri/Sec flashover as one of
> > its advantages. K will be kept at around .12 .
> > Additionally the HV supply cabinet will have a 50 ohm 3KW resistor bank
> > (BIG) resistors - 110 ohm 20"x2.5"x (2) in parallel. This will be tied to
> > H1
> > and go to a 4" 100 turn air coil inductor and out to the TC per Dr. R's
> > experience to reduce the speed of transients to the primo PT via inductive
> > reactance/impedance blumline effect.
> > Here we revisit a long,old thread: I have 60 feet of primo, terminated
> > 100KV
> > x-ray cable. As in turns out the capacitance of the cable is ~ 50pf per
> > foot. This by fate this turns out to .003uf and is resonate at very close
> > to
> > my TC frequency. Ok, so is that maximum power transfer or a formula for PT
> > resonance rise failure?
> > I neglected to say there will be ~19KV set horn gaps at both the PT and the
> > TC cap. I would like to use a X-ray screw jack at the HV cabinet to supply
> > RF ground and aforementioned HV. I like the engineered safety and balance
> > of
> > the cable. If the cable was say half as long, (.0015uf) I would use these
> > receptacles on both ends.
> > It is evident I should cut the cable in half and likely strip back the TC
> > side for quick connection via threaded brass draw pull knobs at the TC.
> > This is not meant to begin an old debate but to cause some calculated
> > opinions on the design. After I use up my mongo old iron, I will want to go
> > to a rat holed VTCC and a SS coil in the future. I guess I like the clearly
> > intimidating appearance of big iron which more obviously demands respect
> > ;-)
> > I'll post a picture of my 3p Nikkie missile transformer which has 18"
> > insulators and is absolutely a Frankenstein-like tube PS if anyone doubts
> > that. This baby is DC via (6) 8020 tube rectifiers a 400H filter choke ;-)
> > Does anyone know the blumline calaculations with a known capacitance? No
> > one
> > seems to speak to the impedance of X-ray cable in 60hz so that correct
> > termination can be had sans all the reflected back garbage.
> > Happy New to All,
> > Jim Mora
> > _______________________________________________
> > Tesla mailing list
> > Tesla at www.pupman.com
> > http://www.pupman.com/mailman/listinfo/tesla
> Tesla mailing list
> Tesla at www.pupman.com
More information about the Tesla