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Forum Index : Solar : High Line voltage from backfeed.
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| George65 Guru  Joined: 18/09/2017 Location: AustraliaPosts: 308 |
Would like some suggestions and Ideas about a problem I am having with my solar backfeed. I have a couple of inverters in the shed which is about 25M away from the house. One inverter has 5.5 Kw of panels and a 5 Kw inverter and feeds through 2.5mm cable. The other is a 2KW and is connected through 1.5MM cable. I am having a problem with the voltage going high at the inverter end of the circuit. Today the big inverter would trip out at 3.5KW with the line voltage hitting 265 Before it disconnected. The little one would get to about 1800W before the same thing happened. This has happened before but for the last couple of weeks they have been OK. I had the thing feeding 4800W last week and the voltage never went over 260. The house circuits rarely go over 250V even through they are on the same phases. I tried running the Ducted AC today and that had no difference on the voltage at the inverters. I ended up running a 500W flood light on the same circuit as the 5Kw inverter to drop the voltage which it did and was running a 150W light off the 2 KW but it was still tripping out. Wasteful but getting 3 KW on the main inverter after " Losses" is a lot better than getting none. As the afternoon progressed I was able to turn the light off and the voltage stayed acceptable. Where I believe the shed power comes from the junction on the house I took the cover off and saw some tarnished looking wires Joined with acorn connectors. I could feel no warmth from the box before I opened it which I though may show a bad connection. I am puzzled by the fact the line voltage was going high a few weeks back, last week was perfectly happy more than a KW above what it was tripping out on today and I have changed or touched nothing. I ran a lead down to the house and got a slightly better result off the small inverter but given the lead is only 1mm and 25M run, I didn't expect much. I tried a cable of 6 and 2.5MM joined the other day and that didn't help much either. When I change my other inverters and run them through a socket attached to the house, I can pump 3KW and the line voltage, the same phase as connected to the shed, never goes over 250. It it possible the connectors in the junction box are not making good contact or would that cause them to get too hot if that was the issue? Given the 2.5MM cable has handled nearly 5 KW, I can't see why it is going so high which is resistance like voltage drop. I would think it was just the phase voltage but even with another inverter pumping up to 3 KW into it, it never goes over 250 and is usually a tad lower. With Just the 5kw shed inverter pumping into it ( and tripping out) the voltage of the same phase at the house is usually under 245 even when the inverter is a whole 20V higher. Same goes for the other circuit with less than 2KW through 1.5mm cable tripping out all the time. Would soldering the connections at the junction box help? I'm wondering if the string voltage has any influence in the inverters output to AC? I'm running strings that are about 320V open circuit and drop to about 270 under load. Depending on time of day. Both inverters list 360V as their optimal input so I'm certainly not over taxing them. Ideas? Suggestions? I want to put another 4KW on the end of the house but I'm going to run 4mm cable for that and do a separate circuit. I an thinking of pulling through the existing cable to the shed and running one lot of 4mm up there, probably 3 phase but don't want to go to the trouble and expense if there is something I'm over looking here and going to get the same result. The fact I ran a cable that was 2.5mm and 6mm joined and didn't get much better results has me worried. |
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| Mulver Senior Member  Joined: 27/02/2017 Location: AustraliaPosts: 160 |
Hey George. I think your cables are too small. In one of my inverter manuals it states minimum cable cross sections. This is to avoid disconnections caused by high impedance etc etc. 3kw. 4mm2 cable 3,6 to 4.2kw. 6mm2 cable. 5 to 6kw 10mm2 cable. I've taken a photo I'll load it up tomorrow. Edit. When you parallelled the 6mm2 with the 2.5mm. Did you run an active and netutral in 6mm ? |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Yep undersized, here is a calculator. Given that you are running it back through your house wiring you are at risk of burning down the house. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Mulver Senior Member Joined: 27/02/2017 Location: AustraliaPosts: 160 |
Reading that again, could you clarify if you have both inverters pumping into your shed circuit ? If so. What size is the feed from your supply to your shed ? |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
I second Madness's comment- you are horribly undersized and if there are dodgy connections as well, you are at serious risk of an overheated connection causing a fire I would advise turning off the inverters until you get the wiring sorted.... The calculator will give you the correct sized cable (the cable rating will have to be derated according to multibundling like a 3 phase cable and whether its covered by insulation or in conduit, and inverters should never ever be coupled to the existing house wiring, they should have their own breaker and independent feed An electrician will know the correct size and wiring requirements....  |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
She'll be right mate. 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 |
Didn't parallel them, I joined them to make one long cable. Thanks for the calculator. Played with it a bit and I'm still not getting what it says I should which brings me back to the connector box. Not sure If I'll have to go to 4mm or 6. Probably better to go 6 and be done with it. It would be close to 50M from the fuse box to the inverters I suppose. Not going to burn the house down Mad. As I tried to explain, the voltage is still quite low at the house. The resistance between the shed and the house connection is preventing rated current let alone over current. Then there is the small point the loads in the house would be taking the current out once it gets that far, IF it got that far which it's not which is the whole problem I'm trying to solve! Only thing that could possibly burn would be the buried wiring and I'm not even getting Rated current into that let alone over doing it. Maybe I should be more worried about when I try to fire up the plasma cutter and the compressor. This is after all voltage drop in reverse. |
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| George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308 |
There are 2 Circuits, one for power, one for light. The 5KW inverter is on the power circuit which is 2.5mm. The 2Kw inverter is on the light circuit which is 1.5mm If the total run from the fuse box tot he shed is 50M maybe a little less, (close enough,) what would the max amp draw be I should be able to get in the shed with 2.5mm cable? From whats being said here, seems it may be under rated for even a 2400W socket. I was of the belief a standard Circuit should be good for 3600W total, 2400W at any given outlet. IF the connections in the box are not right, would that mean they would get hot or is it possible they would just not conduct? ...... which I thought would make them hot. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
The rise in voltage means there is too much resistance which in turn makes heat, get enough heat and things get real bad. The Inverters should connect to the switchboard, the wiring in the house is not designed to have 5KW run through it. If it did go wrong the insurance will see it as an easy out for them to not pay. The wire in the house should be limited to the right size breaker so if it is overloaded the breaker trips before it overheats. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
You series the two lengths together and expected to get less voltage drop? I'm backing away from this one.... My advice- get a professional in- as required by law (there are regulations for a reason- this is the reason) |
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| George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308 |
I put 2 lengths of substantially larger diameter wire than what I was using together to go to the same point and yes, I did expect less resistance and therefore less voltage buildup being the reverse of Voltage drop in this case. What would you have expected? I should have remembered that electrical discussions always have a lot of varying opinion but the fact is, on the wire size I do have, every chart and calculator I can find is telling me I should be able to get more amps through it than what I am for the length I have. Might try pulling the junction box apart tomorrow and soldering the connections. I may not be able to get 5KW with what I have and I'll talk to sparky mate over the weekend about up sizing the wiring and putting in a dedicated new circuit, but the fact remains I should be able to get more though atm than what I am. If I can't get 3600W down the line which is it's rating, I'm not going to be able to get it back up either and that's the real problem. |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
The fact is you dont know what you are doing, what you are doing is illegal, and will certainly void any household insurance you have, and sounds as if it is having major issues Those are the facts This is my last post in this thread |
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| George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308 |
I resized several of the installed breakers, to a lower amperage. One was stupidly oversized. It looked like it had been there since the place was built, how that got passed I have no idea. Nothing in this place needs a 40A breaker. Put in a 16 instead and haven't tripped it yet. I also put in some combo RCD's. The shed wasn't on the main RCD board and although I could have put it on it, I preferred to put it on it's own. Same for the lighting circuit. I know they don't normally have RCD's but the power is the same and just as deadly. Put that on the main RCD. One that really got me was the Bio cycle. No RCD and a 20A breaker. The thing is full of water, if ever there is a good spot for an RCD it would have to be there! It runs an air and water pump so why a 20A breaker I have no idea. Put a 10A on that with a built in RCD and haven't had a problem yet. It's not a big pump so not like it's pulling 20X it's run amperage on startup. I also have the inverters running through Conservative rated breakers in the sub board I made up for them. Did that all in 4mm wire so there is no problem with getting the power through there. Bugger to wire up though. The smallish breakers are still enough for what the inverter can push out, just, and that's the way I like them. My 5 KW inverter was specced on the unit to have a 32A breaker. How the hell that Adds up I have no idea! Why would you want something capable of 20A flat out on a 32A breaker?? Haven't tripped it yet so 20A can't be too small. I may not know everything about electricity but I know enough to protect myself and stop the place going up in smoke even if I do get something further upstream wrong. My other house had 18 Circuits and could pull more power than the wires coming from the pole were rated for. I sure as hell would not have had this problem there. Looks like I'm going to have to uprate this place the same. |
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| George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308 |
I think that would apply to many people here and what they have done with DIY Projects. The very fact a person is not qualified renders them as not knowing what they are doing. And to be blunt, I couldn't give a damn about what is legal or not. Technically it's illegal to change your own light bulbs or tap washers but unless you call and electrician in to do that for you, I'd say we have all done illegal things around our homes. Don't even get me started on what I have seen " Authourised" people do and what I have found and had to fix after them. I would certainly not call it a major issue, been working for months just not at full power as I'd like. The self protection kicks in as it is supposed to, rendering the issue a non event and certainly not unsafe. No, that's just your Opinion. You aren't required to post anything and frankly, what you did post was of no help and far more for your benefit than mine. Sorry this seems to have upset you so much and my attitude is not what you want it to be. Perhaps it would have been better never to have posted in the thread at all given the consternation it seems to have brought you. I was asking for ideas on what might be causing the variance in the problem, not moral or personal judgements and brow beating. If you are one of those people that prefers to do everything 100% by the book, good on you. I won't get upset about that and you don't have to worry or give a damn if my house burns down or I BBQ myself to a crisp because it certainly won't affect you or your family. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
George I used to work with a guy who wired his house himself and then got a sparky to inspect it, now I am not saying there is anything wrong with that I have done the same. The sparky made him redo every connection as he had soldered them together thinking that would be better. 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 |
Yeah I had an idea Soldering on mains cable was not right. I'll skip that one then. Thanks for the heads up. |
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| Mulver Senior Member Joined: 27/02/2017 Location: AustraliaPosts: 160 |
 Sorry its a bit blurry! Regardless you cant punch 5kw up a 2.5mm2 cable. I suggest a nice big fat cable from your switch board to maybe an inverter sub board in your shed. The most id sneek on a 2.5mm2 supply 25m away would be a 1.5kw inverter. For example I have a 2.5mm2 supplying my rain water tank pump 20 m away and id like to sneek a few pannels on top of the big tank. |
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| George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308 |
Yep, Will have to put in a new HD circuit. Was pretty much the idea all along so I can get 3 phase up there. Just wanted what I have to work properly in the mean time. This still brings me back to the fact the individual outlets in the shed should be rated to 2400W. If pushing 2400W down is not safe/ whatever, then logically it is just as unsafe ( or moreso) to pull the power back up. If I can't push it back, how is it legal to have outlets ( and plenty of them) up there that have to be rated to 2400W? My plasma cutter on it's own will go 10A. The circuit is supposed to be rated to 15A. I haven't changed any of the existing wiring, just plugged into it. Pushing the power back would be better than plugging in a load because the amps with voltage drop will be more where the higher volts pushing back will be less amps. Once again, something is wrong here. Either the wiring as the place was built is wrong, I CAN push more amps than what is being advised, or there is something wrong with the wiring if it IS correct. I did put the individual strings on breakers so I'll only run at half power today although it's going to be hot so might run the AC to absorb some of the current. Meter read is next week and I'm in credit on one phase So can use that over the next 4 35o+ days. I'LL go meter the AC while I'm at it today to see what that pulls per leg. I want to set up some sort of diversion to offset the draw on the leg with the 3 phase meter while I'm upgrading everything. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Suggest you buy a copy of AS-3000 the Australian electrical standards. Its not just the law, its based on very sound and sensible engineering principles, common sense, and over a hundred years of practical experience. If something is not allowed, its for very good reasons. Cheers, Tony. |
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Ralph2k6 Senior Member Joined: 24/09/2017 Location: AustraliaPosts: 129 |
They're about to release a new edition of AS3000, so suggest waiting a couple more weeks. Also George pushing voltage back against the supply is MUCH harder to do than using power on home appliances. Cable sizing is a big deal, as you already may know you get voltage rise instead of voltage drop which will trip out the inverter. Ralph |
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