An idea i had!

[philwzen]

hi everyone, this is just an idea i had a while ago. not really had a chance to test it but what i have done looks promising. Basically to do with power transmission, trying to get away from having heavy exspensive cables from power source to batteries/controller. i live off grid , have done for years mainly solar but a small 600w windmill also. thinking about rearanging my equipment for optimum placement gave me a problem as controls and batteries would be approx equidistant from panels and wind mill. id need a lot (130 meters+) of heavy duty cable for the 12v side!!! particulary as the windmill is 3 phase ac output . this got me to thinking , we all know you want to have high voltage to travel long distance to minimise cable loss. so how about inverting at the panels and at the mill sending it down the line then drop down to 12v again rectify and regulate for controllers and charging. I know people will say loss in inverter but try this ! get cheap chinese inverter . only use the first half of the circuit which chops the dc at a high frequency and transforms it up to around 250v, dont bother with second stage 50hz chopper as we actually want a high freq ac for transformer so it will be smaller, (higher freq ac needs smaller transformer for same power apparently!).this voltage can then be fed down the line or even transformed higher,(this is as far as ive got testing the idea) we may run into problems from RF but i doubt it as its only about 5khz i think, if so perhaps would need to use a coax cable after all its not different from a high power radio transmitter if not just bell wire . at the batteries use same ratio transformer to drop down then rectify it and away you go! for the windmill i would 3 phase rectify to get single dc source to invert up. sounds mad but i think it would work, and save a fortune in cableing! please any comments. its just an idea for people to play with.
 regards phil. in the jungle Fiji!.

[Frank S]

You didn't mention your array size but I think if it is much over 1800w I would forget about 12v and go with 24
 if over say 4 kw I think I will be connecting my bank at 48v when I upgrade my system because the option of going with an inverter that has 220/ plus split phase 120 will fit my needs better
  Given a choice there wouldn't be any split phase at all in my stuff
 You mentioned 50hz so I will consider that your AC voltage is 220 or 240 already
 As for your wind gen can you connect your coils to a higher voltage by going series wound?

[boB]

Well, you could attach a micro inverter to each module and run that AC back to another sine-wave bi-directional
inverter  near the batteries and use AC coupling to charge them.  Or you could use a high voltage charger
controller like the Xantrex XW 80-600  or almost any other semi-high input voltage CC.  Midnite Solar 250/200
For really LONG wire runs, the 600V CC is probably a good way to go.

boB

[OperaHouse]

Don't think for a second that you wouldn't have losses sending high frequency AC over distance.  You are better off sending the DC from the inverter and converting it at the other end. 

[Bruce S]

Hello Phil in Fiji, welcome to the forum !
Here's my number crunching for this, and some thoughts.
Your idea of stepping up the DC is a good one and has been done before. an idea that was posted (and I can't find on the old forum ) is to use the 1/2 section of stepping the DC side up to the 250DC. Use this as transmission for lower losses.
HOWEVER these voltages of DC CAN BE LETHAL !! plus you haven't said how much you plan on trying to send down the wire or the wire size you plan on using to go 400 feet. THIS will make a big difference. At these distances either AC at 250V or DC at 250V the losses are pretty much the same, using a 12AWG wire will net you about the same losses, going with a good 1AWG would net you less loss.
I've done a few test trials  of small sized (300 watt inverters) using one to step up to about 177VDC at battery end then stepping it back down at other end for eventual 120VAC output; they worked out nicely. I'd still have it up, but I now have a place for my panels that is less then 50 feet away so not longer necessary.
I still have to say, at these voltages DC can and will reach out to play tag  and needs every bit of respect it deserves and more. IT can be lethal !!! 

YOU HAVE BEEN WARNED

[Mary B]

High frequency transformers are hard to locate too.

[philwzen]

thanks bruce nice to know its not completely crasy idea!  ive only got 1400W of panels and 600W windmill, system has to be 12v as house is all 12v! suggestions to use high voltage controlllers are not an option here sorry.  i have tested using a 1kw cheapy inverter and basically  taken the power off the inverter after the first stage chopper and transformer before the first rectifiers so its high freq high volt ac , about 240V and i think around 5khz, this travels fine. for the  battery end im going dismantle another inverter and use the transformers in 'reverse'' to drop down to 12v ish again then rectify.  For the windmill i have considerred just a 3phase transformer setup at each end of the line . but size canbe a problem , mill is nearly 100ft up a tree!!!, thats why i like the high freq stuff as transformer size gets physically smaller .
 

[joestue]

keep in mind the inductance of the transmission line at high frequency.

you might be interested in the 4 phase boost converter i built, using only 80 cent pfc controllers without a hypervisor.
http://www.fieldlines.com/index.php/topic,147868.0.html