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Forum Index : Solar : High Voltage DC solar Inverter
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| Davo99 Guru  Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
I'm sure I know and have seen the answer to this before but probably having a senior moment.... For the last week or so. Very bad time of the year for my thinking but trying to take my mind off things anyway. I'm wanting to know if there is an Inverter that I could in time of mains outage, where I could take the DC feed off the GTI and put it into a Battery inverter? I take it I would need some batteries and it wouldn't work Direct.... unless it was a multi thousand dollar unit which I could not justify. My arrays run usually 8 panels/ 280V +_ so I was thinking of something that I could just connect the DC from the GTI to the " Backup" inverter without having to change the series string to a parallel one. I'm thinking this would be a handy backup as I could use minimal Batteries and just run off the solar input with the batteries as " Ballast" for the inverter..... If such a thing does indeed exist or there is a way to hack one? Is there any sort of battery Charger that could be plugged into DC input even if it is an AC unit? Tested some LED Transformers and they seem to run pretty well off 60-80V DC. Other amateur Question, Is it possible to have takeoff from a string series so it is, say, 1-2 panels in parallel while still being in series? I was thinking something like using Y connectors so the panels were linked in series but would have a 2nd take off between them? Thinking take off between 3 Phase but not sure how it would go with DC? My guess is no but can't get my head round this atm either. :0( |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Dave, anything is possible, but.... A commercial system will not come cheap, and neither will a suitably large battery. How about a standby generator, something you could put together or repair yourself quite easily, and if its made from resurrected junk, may not be that costly to do either. In my case, the main objection to doing that was noise, but I have overcome that. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
I have generators Tony but I was thinking mainly to use some of the solar I have for something like the fridges if the power went out. Just thought it might be good to be able to use the resources I have in the solar for a backup system. It's hardly a need, just a would do if I could do without too much cost and trouble. I always loved the sound of engines but I'm getting a bit over them now too. I have mainly Diesels and even the watercooleds can make bit of a racket. It's the intake noise that's the problem not the exhaust. The aircooleds are noisy thorough the block. I have a Lister clone but not really happy with it and would like to get a genuine. The other one I really like is the little water cooled Kubota 3 Cyl inline engines. Very compact, reliable and relatively quiet but worth an arm and a leg when you can find them used. I was going to pull down some of my smaller, non ideal located panels but I might leave a string up and re wire them for my 24V Inverter. Even if the voltage out the panels is a bit high, won't be that much, and the inverter will drag them down. I though I had seen an inverter or a way of using multi hundred volt arrays for doing battery charging but maybe not. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
The main problem will be your refrigerator/freezer. I originally did a power audit when I first became interested in this off grid lark. I read the rating plate on my refrigerator and it said, running power 127 watts, 330 watts defrost, and that is what I measured. What I was not expecting was a 19 amp start up surge for the first half cycle, quickly dropping back to about half an amp one second later. That caused me all kinds of grief with the 500 watt inverter I had built, planning to just run the refrigerator directly off several solar panels as a backup. Even with a suitable inverter, the fridge would just pull the solar voltage down to nothing, and would not start up. It really needs a battery, and the system keeps growing in size, cost, and complexity, just to run a refrigerator when the grid is down. Realistically, for the maybe one day a year the grid goes down for a few hours, its just not worth the trouble and expense of a solar based system. Its either a proper full off grid system or just have a generator, a jerry can of fuel, and an extension cord. Something you can fire up in about half an hour should save all your frozen goodies, and provide some lighting, and electronic entertainment. I went off grid because it was an interesting challenge, but overall I am way behind financially. But its been fun, and instructive, and well worth the cost. Cheers, Tony. |
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| nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458 |
If you have 280VDC, you need a boost converter to make 340-350VDC. Then use an H bridge with 4 IGBT's and LC filter to make 230-240VAC. I think you can drive that with those EG8010 chips or poida's nanoverter board. Or instead you can also make AC straight from that 280VDc and use a transformer to step up the voltage. Would be a nice project but you need a lot of 350V (400-450V) bus capacitors to handle the surge currents like Warpspeed is saying. And when there is not enough sun you would need a dcdc converter that's feeding your 350VDC bus from a battery. If you want to do it right you will need to spent some money... Edited 2020-07-08 18:46 by nickskethisniks |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Capacitors are just not practical. You need to source maybe 20 amps peak without pulling down the ac and dc voltages too much over about one second. If you average it out to say 10 amps at 230v that is 2.3Kw of required stored energy. To sustain that for one second is 2,300 Joules (watt seconds). If you allow for voltage sag that means your energy storage needs to be perhaps ten times that amount, falling to 90% of stored energy during the start up surge. 23,000 Joules is a LOT. Energy stored in a capacitor in Joules, is 1/2 x Farads x voltage squared. Farads = 2 x 23,000 divided by voltage squared (340 x 340v) = 0.198 Farads. That is 198,000 uF worth of electrolytics all charged up to 340v dc. You also need an inverter capable of 20 amps ac short time surge. Something decent, say 3,300Uf 400v might cost about $50 to $100 per piece. And you may need about sixty of them. ($3K to $6K worth of electrolytics). A battery would store a lot more energy for a lot less dollars. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Seems Like the answer is basically a NO! :0) I wasn't against battery's, For a short term solution I have plenty of cars, Tractors etc I could pull them out of. just not looking to invest big money for an ongoing need I don't have. I have a 2Kw, 24V Good quality inverter. It did run an older full size fridge, Not sure how it would go with the new Geewizz one with all the bells and whistles. On one hand I would like to be prepared for a longer outage using solar but as you point out Tony, for the time needed...... Last outage we had last year I did pretty much what you said, Got out the genny and fired it up and ran some leads for The fridges, TV and lights. Showed off by putting on a heap of lights while everyone else was in the dark. :0) And thinking of that, was at Night so solar would have been useless. I would like to find a 5-10 Kw Gen head. been looking for one at a decent price for ages. Matter of fact, finding 2 bearing gen head at any price is not easy. Couple of tool places have them listed on their site but unavailable when you go to order one. Have plenty of engines, Nothing to couple them to. Got a couple of big Induction motors but we know the problems with them as Gennys. I would like something a bit better than the Chinese 2 bearing screamers but again, with the outages we get I will probably never wear one out in my lifetime. |
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| mab1 Senior Member  Joined: 10/02/2015 Location: United KingdomPosts: 209 |
If your 2kw 24v inverter is the low frequency type, pure sine (like the ozinverter), then you could connect your existing GTI's output to the output of your 24v inverter and the solar array would charge the battery through the inverter ( and run the fridge, etc). The issue then is that if you're not using the power the solar is producing (2kw ish?) all that power is going into the battery through the inverter so you would need a suitably sized dump load & controller, and of course your 2kw inverter will be working on or over it's limit. I suspect that sourcing the dump load controller 'off the shelf' would be expensive, but if you can do the electronics to make it yourself may be viable - if your 24v inverter is suitable. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
An Oz type transformer inverter definitely solves half the problem. A 2Kw+ Oz inverter would easily handle the types of momentary surge currents demanded by a fridge/freezer when starting up. No problem there. The difficulty then becomes supplying the inverter with sufficient dc current during the start up surge. It sure isn't going to come direct from the solar panels. It really keeps coming back to requiring a battery. A standby generator will have a flywheel, possibly a really large flywheel, and that solves the surge problem too. Cheers, Tony. |
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| mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 209 |
yes he would need a battery - he says he has plenty of vehicles to pull batteries out of. A starter battery pair (24v) can easily supply the startup surge via the inverter, and a 2kw inverter ought to be able to handle a fridge startup surge unless it's a real cheap'n'nasty. Although in the real world there could be an issue of voltage sag under startup load causing the GTI to cut out on grid undervoltage error - would need to be a good inverter for that. Then the solar recharges the battery via the GTI so the starter batteries are not deep-cycled. But he would need an automatic means of dealing with the power from the panels that isn't used - starter batteries can't soak up 2kw for long. And if his existing inverter isn't the low frequency tranny type then it's probably not going to like being connected to a GTI. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
My Inverter is the electronic not the tranny type BUT, would not the GTI need to be running in the first place? That would require the Mains to be present would it not which would remove the need to have the Inverter and batteries in the first place being this would be for mains failure backup. That reminds me of something I have seen.... Using a generator to supply the GTI with a frequency and voltage the inverter wants to see to Kick in. Same deal though, you have to be using MORE power than the GTI can supply otherwise it tries to push it back to the genny which it won't like one bit. This would be OK if you were trying to run a water heater or some constant load but if the heater thermo Kicked out or a fridge motor stopped..... Not going to be good. May be possible to run something through a DPDT relay to kick in a dump load but that's more or less permanent or impractical wiring for occasional use. The parallel Panels with the batteries on the 24V Inverter might be the way to go or use a genny in the shorter term. |
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| nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458 |
Another thought, if your current inverter suits your needs. You could look for a simple switch mode battery charger and then feed direct dc from the panels to it. It probably charges your battery without any issues. I used to do this a few years back for a year. I used such charger 36V and I adjusted the winding ratio to match my 240V panel/ 24V battery voltage. (I removed some secondary windings and adjusted the output voltage trimmer.) It was a 20A Zivan charger. It was no problem if there was little sunshine, then there was little power going in the battery. The nice thing was that it had current and voltage limit. Edited 2020-07-10 04:06 by nickskethisniks |
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| mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 209 |
No, it's like your using a genny to supply the GTI - only your using an inverter to supply it - but as it's an electronic type inverter it isn't a good idea to back feed it. As an aside i know of someone who did try running a lister generator with an SMA sunny boy GTI (which has frequency control):- with load > PV output it was fine or course; with load backed off the lister backed off until the genny was driving the engine - still fine; and then with the load reduced further the genny started to speed up as the lister couldn't hold it back, then as the frequency rose above 50Hz the SMA GTI backed off it's output until GTI output matched load - so it can work. Of course it probably helps that the old lister has a big flywheel to keep things stable. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Would this work without modification? I thought of something like that but I am still not sure how to discern what will run off DC and what won't? Is there any risk of Damage connecting AC appliance to DC or will they just not work without being damaged if incompatible? |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
OK, makes sense. I tried that many years ago and learned the hard way but I didn't have enough load applied either. That is interesting. Sounds much like an IMAG would behave... Which is an interesting thought in itself..... IF a motor was brought up to speed, the GTI connected and then the driving engine was disconnected, would the motor be able to supply frequency to keep the GTI engaged? I doubt the motor would be able to hold Voltage and frequency parameters tight enough for the GTI but in theroy.....?? I spose there are many different types of gen heads but I know the old listers had a Diode pack in them. I would have thought in order to drive the engine the current would have to flow backwards which the diodes would prevent but there is probably other things at play i'm forgetting.... as usual. Perhaps it was a Start O matic with the motor and the generator in the one head which is another possibility. As generators, the Listers are renowned for NOT being smooth at least in the smaller 3-6 Hp range. Despite the big flywheels, the compression of the engines and then the power pulse causes what is called " lister Flicker" which shows up as the pulsing of ( incandescent) light bulbs. It can be hard to get rid of. People have tried cap banks and putting 2 lots of flywheels on engines but it's usually reduced not eliminated. The larger engines, the 8 Hp and up and the twins are usual OK as they have the stover flywheels which seem to eliminate the problem beyond their greater weight. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
LED lighting should eliminate "Lister flicker". Led lights dim noticeably below about 200v ac, but above that they give a constant light output due to the constant current regulator. So as long as your Lister can stay up in the 230v to 240v region, it can be a bit wobbly but there should be no discernible light flicker. Another random thought... These low cost Chinese MIG welders may be of some interest to us. MIG welding is a constant voltage process, unlike stick or TIG which is a constant current process. With a MIG, the welding current is controlled by adjusting wire speed. Anyhow, these cheapy Chinese e-bay Mig welders should produce a fairly constant (if noisy) dc output voltage around the typical useful welding range of 15v to 45v dc. What needs to be confirmed, is that these welders work over the international ac mains supply voltage of 80v to 265v without having to set or adjust anything. If that is true, they may also run off a straight dc input. An 80v ac supply (minimum spec for Japan) would be rectified to produce around 110 v dc. A 265v ac supply (max spec for Oz) would produce about 375v dc. If these welders can run straight of a dc input, as I suspect may be possible, a solar input of between 110v dc and 375v dc might make it go. That could be useful  Now the welder will be rated for intermittent Chinese welding amps, which as we know are a lot smaller than good full sized Australian amps. So we might have to seriously de-rate a 200 amp welder when powering an inverter. But on the other hand, to run a refrigerator or freezer, the average load current on the welder and the inverter would be quite low. But if the game is fair, the welder might rise to the job and supply a 200 amps surge current during compressor starting. So I am thinking..... Cheapy Chinese MIG welder + capacitor bank (to clean up the dc a bit) + an Oz type transformer inverter may just be able to run a fridge from straight solar. Wishful thinking ? Don't really know, but I think we might be in there with a chance  Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Just so happens I have the opportunity to avail myself of a good Italian MIG that has blown the wire feed Board. Father wanted a new toy which is half the size of this old monster so no longer wants it. I'm going to see if I can replace the wire feed control with a Chinese PWM Board to restore it to working condition but if not, It's a treasure trove of useful components. If it does work, I still have My old MIG I replaced which still works but is pretty worn in the wire feed. I also wanted something that could do TIG and the right special came up at the right time so... The old one is still the newer style so I'll see what I can get out of it. Should have a decent transformer as it's not young but I'll have a look. One things for sure, the new MIGS are very Different to the old ones and it's very interesting how things can be re purposed. |
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| mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 209 |
bringing back an old topic, but I saw this on fieldlines forum and thought it was sort of what you wanted (hope the mods don't mind me referencing another forum):- ideally you'd need 350-400v dc for 230v of course and probably some extra caps to start a fridge, but... |
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| mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 209 |
Uh oh, i balls'd up the link in my last post and i get a 'fatal error' (though i'm still alive) when i tried to edit it. link still seems to work though. sorry i missed this before. I can't remember what lister it was other than 2cyl. As for using an IMAG? well I use one as a hydroelectric genny and I doubt you could keep the frequency within GTI tolerance myself Edited 2020-07-29 08:14 by mab1 |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Thanks for thinking of this. I do appreciate it and don't want to sound like a Bitch but I wouldn't urinate on that site if it was on fire. Apart from the greenwashed and obsessed mods, Pretty sure the site owner is an alco the way he goes off on rants and completely and repeatedly misunderstands the obvious and goes off at people only to remove the comments later without apology. Would not give them the benefit of adding to their ( failing) visitor numbers. Again, I do appreciate your thoughts and efforts in trying to help though. Yes, I agree, they are extremely touch at the best of times and the inverters have very low tolerance of frequency shift. I'm thinking the welder Idea may be the best. I haven't got to looking at that yet but will have to. I hope I can fix it, if not, it will still be very useful. |
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