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Forum Index : Solar : High Line voltage from backfeed.
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| Mulver Senior Member  Joined: 27/02/2017 Location: AustraliaPosts: 160 |
Just for the sake of clarity a clearer picture  |
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| George65 Guru  Joined: 18/09/2017 Location: AustraliaPosts: 308 |
10mm seems rather overkill for a current of 5 Kw. I guess that's deliberate to cover their backsides. Seems odd they don't specify run length either. I'll see what my sparky mate says. He likes overkill with everything so what's good for him is good for me. 100M roll of twin and earth 10mm cable, $418. " " 4 Core + earth, 10mm, $693. Ouch! Don't look like I'll be putting 3 phase up there in 10mm! Probably better running 6mm 3 phase and split the array into 2 inverters as I have now and feed each phase with <4kw. |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
I have bought very heavy gauge 3 phase wire from the scrap metal dealer a few times for around $1 per metre, nothing wrong with it. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
[quote]10mm seems rather overkill for a current of 5 Kw.[/quote] No it isn't, its actually undersized for the distances you are talking about. The voltage drop will be a lot more than the 3% allowed for in the rules. Cheers, Tony. |
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| Mulver Senior Member Joined: 27/02/2017 Location: AustraliaPosts: 160 |
Yep. 50 metres is a long way. 16mm2 keeps your voltage drop nice and low. Start ringing around scrap metal yards! On the topic of cables. My other inverter manual states dc panels cables to be kept under 30 m. What's everyone's experience on this? This calculator. Here Tells me I can have a string run of 100m at 360v dc 3% loss on 4mm2 cable. Is a longer dc run better in George's case ? |
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| George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308 |
Good idea with the scrap yards. My experiences in buying from them has not been good but then again, I have never tried the local ones out here and everything is Different in Australia, rather than Sydney. The DC run is short, About 5M for the furthest array. Just enough to bring the power down off the roof to the shed wall. Could not make it any closer unless I screwed the inverter under the roof. No where else to put it! :0) This is very confusing..... According to the calculator, I can go 75m with the wire size from the panels (2.5mm) at the voltage of the string ( 320V open) at the amps the panels put out (8A). I have 3 Strings on the shed all doing about the same output and all wired and fused separately. If I combined them onto one circuit, calculator says I could do a 40M run on just 4mm. I have been reading that a lot of electrical engineering is going back to DC transmission but I always thought the whole Idea of AC in the early days was because DC dropped off too much over longer runs. In any case, having those inverters closer to the fuse box is not desirable. The inverters in the shed are protected from the weather and out of sight. Also plenty of room for the sub board with the breakers etc. I'm already trying to figure what to do with the array I want to put on the house as far as siting the inverter etc. I was of the opinion that the closer it was to the panels the better. Seems that's not true within wide paramaters. That's convenient for me because I rather put the inverter on the other end to where the panels will be because that leaves the walls free around the entertaining area. That end already has the water heater, AC unit, drains etc. Didn't really want switches and inverters hanging off the wall looking industrial around where everyone sits and I have a TV and sound system to put out there as well as an outside Kitchen. The wall space is limited with the windows. It is covered though so I'll have to look at some sort of enclosure I can put the inverters in on the other end. Outdoor or not, I'd like to protect everything from the weather and keep it out of the sun which will hit on that side from about 1 Pm on. Maybe a meter box will be OK with some fans in the bottom and 90o PVC elbows at the top on the sides facing downwards. Maybe an upside down U shape at the top extended up would be better for convection? Already have some thermal switches I bought to put on the inverters to force cool them. Maybe I could get real efficent and have a flexible duct I could put through the window there for a really different co-gen system. There is a LOT of heat that comes off those inverters when they are running. Wonder if there is a spec so as to calculate on a 5Kw inverter what amount of heat they are producing? I have a fan blowing on the 5KW in the shed now and the air that comes off it seems much more than any little fan heater I have ever seen. Thanks for all the input and suggestions Gents. Learning a lot here and getting some good info and ideas. |
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| hotwater Senior Member Joined: 29/08/2017 Location: United StatesPosts: 120 |
High voltage transmission is limited by the arc over voltage. DC power is the same as RMS. Peak Voltage is 1.4 times RMS. Therefore you can transmit two times the power with DC with the same power lines. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Probably, because the voltage will be a lot higher, and the current a lot lower than transferring the same power at only 230v. We are also talking real watts not VAs which can be higher than watts. An extra panel would more than compensate for any voltage drops, provided the inverter can withstand the extra voltage. Cheers, Tony. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
You can calculate the heat, just see what the efficiency is ie 92% so 8% is being turned into heat. 5KW X 8% = 400W There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308 |
When you put it that way with realistic numbers, it does not seem much and certainly nothing like the amount of heat the inverters SEEM to produce! 4-500W probably wouldn't be worth the trouble of ducting the air. One of those things I guess where something seems a lot more than it really is. On that, I have been thinking of winter and heating. As I can not back feed one leg of the 3 phase AC (or at least come up with a way to do it as yet) I'm thinking that the best/ cheapest solution for winter heating for us might be to just use the fan heaters like we had before. The power circuits are on the meters I can backfeed so at least through the day when it's not quite as cold, using the resistance heaters will probably be cheaper than paying for the one phase I can't supplement the AC uses. Given the much better insulation in this place, running one of the fan heaters all day might be enough to keep the chill off the place. At night I'll probably need more heat so can kick in the AC. Really want to hurry up and get all this sorted because when winter comes, I'll have less solar generated power to play with and need to run the inverters flat out. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
You can run unbalanced loads in a three phase system, no problem there, the meter just adds the three measured currents to record total power. Should be possible to feed back power on only one phase of a three phase meter and the meter should respond accordingly. Cheers, Tony. |
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| George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308 |
Extra panels are my " Key to solar efficiency". There are so many things where trying to get more out of panels like tracking, orientation, tilt etc never gives the bang for the buck just adding a couple more panels to an array does. I have the position of a 12 panel string will handle a lot of things far from ideal and still out do a 10 panel string tuned to perfection..... especially on cloudy days where all the efficiency techniques just go out the window. I was crunching the numbers a couple of weeks back about on putting panels on the west side of the house where I have loads of space as against on the north side where it is much more limited. The efficiency of the west side seemed rather poor till I worked out that an extra 2 more panels on that sized array would give me the same sort of output over the course of a year as what the North aspect was. Given what I have been buying panels for, that would be about an $80 max cost. |
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