AC and DC in same trench?

[joshuabardwell]

I think that you missed 3 zeros at the wave length at 60 Hz. It should be 16000 ft.

Uhhh... Yeah. Duh. Sorry about that. I accidentally input the frequency as MHz, not Hz, and then was so busy typing my answer I didn't sanity-check my math.

In that case, the length of the antenna or cable is most certainly less than 1/2 the wavelength, and the formula for calculating the near/far field changes. The near-field is the region within a radius (r << λ), while the far-field is the region for which (r >> 2λ).

 

[Redneck in training]

Irregardless of all the fine answers here, code requires you to have a disconnect at the solar array for the DC run. Because it's DC, these disconnects run around $1000 for a 5000 W array.... Does that answer your question?

You should have your inverter(s) at the array, and make your home run to the meter in 240V AC. The inverter(s) will have the DC switch built in, for your required cut off at the array.

It all depends. If the distance between the arrays and the AC is long then it is advantageous to place the inverter away from the arrays. 100A fusible DC disconnect is about $250 at CivicSolar.

 

[joshuabardwell]

It all depends. If the distance between the arrays and the AC is long then it is advantageous to place the inverter away from the arrays.

Can you say why that is? I always thought that AC was better than DC over long runs because it had less loss.

 

[mjncad]

Metal conduit for above ground runs, and PVC conduit for underground runs.

 

[tcartwri]

It all depends. If the distance between the arrays and the AC is long then it is advantageous to place the inverter away from the arrays. 100A fusible DC disconnect is about $250 at CivicSolar.

Ya, I looked into that a little bit more last night. It's amazing what 18 months will do in the market when there is a demand. These switches were not easily available when I built my system, and they were crazy expensive. It looks like you can run AC or DC now as your home run.

 

[BeezFun]

It all depends. If the distance between the arrays and the AC is long then it is advantageous to place the inverter away from the arrays. 100A fusible DC disconnect is about $250 at CivicSolar.

Yes, you're exactly right. It's about 300' from the arrays to my meter. I had to put the arrays out in the pasture to get to sunlight, our house is in a forest. That also makes it easier for mounting, I can put them right on the ground rather than post mounted. We're in a cold climate so I need to put the batteries in the basement, so it's advantageous all around to run the DC back to the house. Plus it's 600v and the AC is 220v, so there's some power savings. I've been watching your thread with interest, glad you finally got your loan. I have to start researching that.

 

[Redneck in training]

Can you say why that is? I always thought that AC was better than DC over long runs because it had less loss.

1.) DC has lesser losses than AC. It has only leakage loss and wire heating loss. AC has capacitive and inductive loss in addition to leakage and wire heating. In OP context losses are only secondary consideration anyway.
2.) Since the cable carries 600 V it is much cheaper than 220V due to about 40% of current.
3.) The inverters (if grid tie) have maximum voltage limited to 264 V AC. So the 300 ft long cable (for AC) would have to have small voltage drop (typically 2-3%. DC cable can be sized for the amps while AC cable has to be sized for maximum voltage drop and that is usually significantly more expensive.
4.) If the system is off grid then the inverter might be also the charge controller and should be close to the batteries.

 

[Redneck in training]

Yes, you're exactly right. It's about 300' from the arrays to my meter. I had to put the arrays out in the pasture to get to sunlight, our house is in a forest. That also makes it easier for mounting, I can put them right on the ground rather than post mounted. We're in a cold climate so I need to put the batteries in the basement, so it's advantageous all around to run the DC back to the house. Plus it's 600v and the AC is 220v, so there's some power savings. I've been watching your thread with interest, glad you finally got your loan. I have to start researching that.

Hi BeezFun,
Do you have any pictures and can you post technical data? I am specifically interested how you mounted the panels. Did you do it DIY or did you hire somebody?
We intend installing 24kW grid tie system in next few weeks. We should close on a loan tomorrow and order all the stuff shortly after.

 

[joshuabardwell]

1.) DC has lesser losses than AC. It has only leakage loss and wire heating loss. AC has capacitive and inductive loss in addition to leakage and wire heating. In OP context losses are only secondary consideration anyway.
2.) Since the cable carries 600 V it is much cheaper than 220V due to about 40% of current.
3.) The inverters (if grid tie) have maximum voltage limited to 264 V AC. So the 300 ft long cable (for AC) would have to have small voltage drop (typically 2-3%. DC cable can be sized for the amps while AC cable has to be sized for maximum voltage drop and that is usually significantly more expensive.
4.) If the system is off grid then the inverter might be also the charge controller and should be close to the batteries.

Thanks for that. Regarding point 2: the inverse relationship between voltage and current, and its effect on wire sizing is always counter-intuitive to me. Intuitively, it seems like higher voltage lines should be heavier.

The context in which I was thinking of my original statement about AC being better over distance than DC was the power grid as a whole. Edison promoted DC, but AC won out at least in part because of the much higher transmission losses of DC. I have since done some reading and have come to learn that was because DC step-down transformers didn't exist or weren't economical, and that DC could have been competitive if it had been possible to have high-voltage transmission lines.

 

[Redneck in training]

There are few very long DC high voltage lines around the world. I saw one somewhere in Russia from hydro plant in the middle of nowhere to big city. The turbines produced three phase AC that run electric motor running high voltage DC generator feeding two wire DC line. At the other end was DC motor running AC generator. DC line doesn't suffer skin effect as high voltage AC line so it can carry the same current as AC but at significantly lower voltage. DC lines are cheaper to build but are not popular due to issues with the AC/DC/AC conversion.