Whatt Voltage/cable, 220 metres from array to house?

[WhattsDC]

Will need cable(s) to house from nearest barn roof 220metres distant. Area for 20 panels, initially. 10 additional panels might become feasible. Want more than 8kW. Think 12kW would be max. feasible with extended building (using panels now on sale).
Expecting DC transmission, underground, but need to avoid problems, not make them. Plenty of series/parallel options available, so need to find optimum transmission voltage/cable size.
Still looking for good reference sources. Found no system designers yet.
Hoping that readers will find time to make suggestions, based on theory or practice.
Thank you for any help.

 

[binky]

you won't like the cost of the cable! based on single phase AC back to house, you are looking at 50mm SWA for 12kW or 35mm SWA for 8kw at roughly £16 per meter for the 35mm that's £3,500 .

If you have a 3 phase supply then cable drops to something like 16mm, then that's still £11/m for 4 core SWA or £2520.

That's based on a 3% voltage drop, for soalr you are supoossed to work with 1% voltage drop, so cable sizes should be larger than those above.

Burying DC isn't a great idea either, you have to keep it dry, or it gets soggy and fails after 5 years.

It's probably cheaper to build a ground mount system closer to the house.

 

[WhattsDC]

Thank you for researching and replying, Binky.
Not enough sun nearer house, according to PVGIS but we
have not given up on the project, yet.
Hope we could use 2core with Armour and glands as protective conductor, also hope to find cable with waterproof outer sheathing.
There is still the puzzle of what voltage to select for DC transmission to an inverter at the house. I am continuing search for info. on that topic (including inverter input voltage spec).
Thanks again….

 

[stuboy]

To minimize losses you need to transmit your power at the highest voltage possible (losses are squared with resistance!), for a solar array that means seriesing all the panels up as high as the inverter input will allow. See specs of your chosen inverter, lets say 1000v DC! So for an 8000W array that would be 8A at 1000v. Now as Binky alludes to burying VH DC cables is no laughing matter. I would think you would be best off burying some sort of pipe, my favourite would be 4" drain pipe and suspending the wires apart inside it. All of this is non trivial and cannot be done without due care and attention. 250V DC is bad enough when you get to 1000V things start getting even more "fun"! With 6mm solar cable over 220m that would be about 0.75ohms so that 48W loss in the cable, and a 1% voltage drop around 10V, so its doable just needs to be done by someone that knows what they are doing, deffo not a job for a cowboy!
Cheers
Stu

 

[binky]

Thank you for researching and replying, Binky.
Not enough sun nearer house, according to PVGIS but we
have not given up on the project, yet.
Hope we could use 2core with Armour and glands as protective conductor, also hope to find cable with waterproof outer sheathing.
There is still the puzzle of what voltage to select for DC transmission to an inverter at the house. I am continuing search for info. on that topic (including inverter input voltage spec).
Thanks again….

Cable ratings apply to DC and AC system, so the answer is SWA which can be buried direct in the ground.

I take it you have shade issues near the house?

 

[WhattsDC]

To minimize losses you need to transmit your power at the highest voltage possible (losses are squared with resistance!), for a solar array that means seriesing all the panels up as high as the inverter input will allow. See specs of your chosen inverter, lets say 1000v DC! So for an 8000W array that would be 8A at 1000v. Now as Binky alludes to burying VH DC cables is no laughing matter. I would think you would be best off burying some sort of pipe, my favourite would be 4" drain pipe and suspending the wires apart inside it. All of this is non trivial and cannot be done without due care and attention. 250V DC is bad enough when you get to 1000V things start getting even more "fun"! With 6mm solar cable over 220m that would be about 0.75ohms so that 48W loss in the cable, and a 1% voltage drop around 10V, so its doable just needs to be done by someone that knows what they are doing, deffo not a job for a cowboy!
Cheers
Stu

Hello Stu, thank you for replying with plenty info.
Have been hoping to find online ref. source(s) explaining arc issues as voltage increases (no luck yet). Inverted commas for “fun” seem most appropriate (not much fun for isolator contacts?).
Bit of a juggling act to deal with String size, cable size/spec. and inverter spec.
We will now budget for duct, not direct burial, for cable. Also will be finding full spec for cables, eg. DC voltage and impedance.
Suspending cable sounds quite a challenge!

 

[binky]

Hello Stu, thank you for replying with plenty info.
Have been hoping to find online ref. source(s) explaining arc issues as voltage increases (no luck yet). Inverted commas for “fun” seem most appropriate (not much fun for isolator contacts?).
Bit of a juggling act to deal with String size, cable size/spec. and inverter spec.
We will now budget for duct, not direct burial, for cable. Also will be finding full spec for cables, eg. DC voltage and impedance.
Suspending cable sounds quite a challenge!

You can use something like water pipe for the duct, just don't use land drain, coz it's got loads of holes in...

 

[WhattsDC]

Cable ratings apply to DC and AC system, so the answer is SWA which can be buried direct in the ground.

I take it you have shade issues near the house?

Hello Binky, yes, serious shading, very serious to make us think about 220metres of cabling.
Stu has sent interesting message, too.
Thinking about duct and SWA.
Also cable resistance for DC!
Will stick strictly to manufacturer’s spec.
Thank you for help!

 

[WhattsDC]

You can use something like water pipe for the duct, just don't use land drain, coz it's got loads of holes in...

…good reminder! some lying around…to avoid! Know a man who stocks coloured services ducting as well as drain…
Will need careful installation to avoid low spots where water might accumulate in case of ingress.
Wonder what Utility engineers do? Maybe they reckon that water will generally find a way in therefore main design principle is to let it out more easily??
Thanks again…

 

[binky]

…good reminder! some lying around…to avoid! Know a man who stocks coloured services ducting as well as drain…
Will need careful installation to avoid low spots where water might accumulate in case of ingress.
Wonder what Utility engineers do? Maybe they reckon that water will generally find a way in therefore main design principle is to let it out more easily??
Thanks again…

If you are using SWA it's a non-issue anyway. Trouble occurs when people run DC cable in the ground.

 

[WhattsDC]

If you are using SWA it's a non-issue anyway. Trouble occurs when people run DC cable in the ground.

Thank you, really appreciate all the help.