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Old 02-15-13, 11:08 AM   #1
Xringer
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Default Using RF coax cable as DC Solar PV cable

Warning, if you are not trained to handle high DC voltages,
please do not attempt any of the experiments discussed below.
The DC voltage from series-configured solar panels can seriously burn and/or kill you.



At work, I've had success using RF coaxial cable as a high voltage DC cable.

Because of the small diameter of the center conductors of coax cables,
the are not suitable for high current applications.

Specs for the cable I've been testing (RG-214U) shows 6 ohms/km in the center conductor,
and 3.1 ohms/km on outer (shielding) conductors.
(RG-8u is more common and has similar specs).

My goal was to test surplus RG-214U cable for currents under 10A,
so the power losses at 100 feet would be minimal. (Around 0.2 to 0.4 ohms).
I'm using a negative ground system. + voltage is on the center pin.
RG-214U is direct burial cable. But beware of using old network cable, underground.
Since some network RG8 type cables have tiny pin-holes in the plastic jacket. Not waterproof.

The aim of these tests is to confirm the utility of the Coax PV power method of power transfer.
If this method is found safe & effective, for use in temporary/emergency installations,
it could be useful to provide power for communications equipment,
using existing (and available) RF coax cables.

~~~


By using panels in series, peak amperage would always remain near the
max amperes rating for a single panel.
So, using four panels in series increases the voltage, but not the amperage.
Power Loss= Amps squared x Resistance.
For 8A panels, using a 100 run, the losses should be less than 5 or 6 watts.


What worried me is the connections. Crimping and soldering large terminals
at the ends of the cables works okay. But, I wanted to use standard 'N' type coax connectors
(since I also use them for Ham Radio).

Some Type N connectors are difficult to install, but sometimes,
you can get cables with factory installed connectors..

The center pin of N connectors is tiny. But, I've only had one get blasted.
I think it was installed off-center or it wasn't really gold plated.

The water heater experiment, uses N connectors at both ends (100')
and no heat (losses) can be detected from the connectors.

Typical high power is 600 to 800 watts. With voltages between 60 & 100vdc,
with currents under 8.5 peak amps.

The cable (left) is connected to a N type Bulkhead or chassis connector,
which is wired to the water heater load. (Scan shows 45deg F).


This is the outdoor side. PV comes in the bottom wires.
The + side is switched and fused before going to the center conductor of a very short coax cable..
Which is connected to a double female N bulkhead.


Warning, just like the MC-4 or other Solar panel connectors,
these can NEVER be unplugged or plugged in while the sun is on the panels..
The power will arc weld/melt the center pin. It will have to be replaced.

This flexible cable has an OD is 0.405" and can be quickly buried a few inches underground,
just by parting the sod with a spade blade.

So far, my results have been good. I've also been using this same setup
on my 500W back-up system without any problems.

Of course, since this is a temporary Ham Radio related research project, (I hold an Extra Class FFC license)
and can be quickly uninstalled, I'm not worried about meeting code requirements for buried cables etc.

Edit:
I have not yet tested the low-cost version of this method, which will use the PL-259 / SO-239 type connectors.
These are a bit easier to install and have a larger center pin, which might make it less lossy.
I might try these (cheaper) connectors this summer..

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Old 02-15-13, 12:24 PM   #2
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Why not just use regular wire? Is coax cheaper, or did you just have it on hand?
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Old 02-15-13, 12:36 PM   #3
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Could a surge protector help protect the system during plugging unplugging ? or is it not surge, only flow ?
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Old 02-15-13, 01:25 PM   #4
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Quote:
Originally Posted by Daox View Post
Why not just use regular wire? Is coax cheaper, or did you just have it on hand?
I had a three 100 ft sections with N connectors, purchased cheap at a Ham fea-market.

IIRC you can find 100 ft sections (w/ PL-259 connectors) on Ebay for 20 to 30 bucks.
Much cheaper without connectors.

I wanted to use this type of direct burial cable for my testing, but other cable could be used.
I've actually used regular 120Vac extension cords for short tests.
They are easy to use, and the connectors are pretty cheap.
'N' connectors are very water resistant, which helps on the outdoor side.
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Old 02-15-13, 01:45 PM   #5
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Quote:
Originally Posted by ecomodded View Post
Could a surge protector help protect the system during plugging unplugging ? or is it not surge, only flow ?
There isn't any device, that I know of that will inhibit arcing of connectors.
If there was, they would be used on PV cables.

If plugging in, the instant first contact is made (or very close to contact),
a large current will melt the (small) point-of-contact,
causing the contact surface to blow open or melt into a dead short.
If the voltage is 60 or more volts, an arc will form (like an arc welder),
melting & pitting the surfaces of both sides of the contact.

The same thing can happen when unplugging under power.
That last (tiny) point of contact melts, starts an arc, that damages the contact surfaces.

If you had enough panels, I'll bet you could arc weld steel..
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Old 02-15-13, 02:15 PM   #6
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Quote:
Originally Posted by ecomodded View Post
Could a surge protector help protect the system during plugging unplugging ? or is it not surge, only flow ?
That wouldn't help. The reality is that all you need to do is block the sun from them so they can't produce power. Cover the entire surface of the panel with a black garbage bag, cardboard, or something like that would do the trick while you plug or unplug the panel.

Actually nevermind. With panels in series you would need to cover the whole set.
Also since this isn't UL listed, it isn't to code and can't be integrated to the grid unless the panels and interconnects are UL listed.

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Old 02-15-13, 02:57 PM   #7
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When I'm out messing with the panels on a sunny day, I'll take a sheet of cardboard
with me to block off the bottom of one of the panels..
That way, I can use the cut-off toggle switch to kill the power out, without toasting the switch contacts..
It's actually much safer to just do the work after the sun goes down..

I was thinking about one of these Contactors for controlling the power going into the hotwater heater element..
White Rodgers 120 114751 6 Coil 24V DC 120 904 Heavy Duty Contactor New | eBay
But, maybe a big MOSFET would be better.. Only need to handle 10A max..

This is an Off-Grid experiment. Any inverter that can take 122 volts is out of my price range anyways..
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Old 02-15-13, 03:33 PM   #8
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Wow - this discussion takes me way, way back to the '60s. I knew well that RG-8 could handle 2,000 watts with ease and had friends that were trying to wire up a "shack" for late night CW contests (ham radio stuff where you are up all night).

The RG-8 was already in the ground - in fact in conduit (to a defunct antenna tower), so we used it to pipe out 120 V ac. I think it was about 150-200 feet.

We tested it by putting on 1200 W electric space heaters until we saw a voltage drop. If I recall, we put on 4-5 of them before we saw any voltage drop. The cable was not even warm.

We also hooked up an intercom system from the house to the "shack" that used the old wired intercoms on AC systems.

Later on - and in an arena that was completely different I recalled this and used RG-8 to multiplex signals when we used burst communication in very short pulses. The pulse (20 msec) was about 20-30 kW in ERP and only used once or twice a day. The burst was also in Rg-8 coax that was used on a 240 V system drawing about 40 amps. No code inspectors were present, not even thought of . . .

Coax is used in a lot of undeveloped countries for household power as it is readily available and cheap. Some people even use it to wire their homes. The center conductor looks about like AWG #10 or so and the braid must have the equivalent cross section.

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Old 02-15-13, 04:09 PM   #9
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Wouldn't a circuit breaker solve the problem of a arc at disconnect ?

I think I got it ..
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Old 02-15-13, 04:38 PM   #10
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Quote:
Originally Posted by ecomodded View Post
Wouldn't a circuit breaker solve the problem of a arc at disconnect ?

I think I got it ..
Is that labeled as a 220v~ 3,000 amps??
That's not really going to work with a 0.66 megawatt load is it?

What uses that much power??

~~~

An AC~ breaker that had large self-wiping contacts and
a good air-gap when open might to the job on DC.
You could try it.

On any kind of switch for DC power switching,
one thing you want is speed. It's going to arc anyways,
but if you minimize the time it's arcing, the contacts will last longer.

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