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Forum Index : Solar : RS485 Switching For High  Dc Voltage Inverter Input 400V DC

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Old Seagull Man

Joined: 21/12/2019
Location: Australia
Posts: 18
Posted: 03:20pm 11 Mar 2020
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We have our own home grown solar. Best described as additions to what was on the house. Like most it started with a 1.5kw system with some 180 watt panels. And some got added etc.

Now the place has three inverters, house, caravan, and my shed.
Each inverter has its own modbus energy meter.AS all the inverters are older Single MPPT, non comms type. read very basic.

Our son has written us quite a good monitoring program. That shows us import, export, consumption.Daily, weekly and monthly data.

The problem is that on a good day, we can generate to much power. Export to much.
On the days when there is no air-con, or much cooking etc.

At the moment we simply walk down the  flights to he garage and tour off the dc switch to one of the inverters. And all is well.

What I am think to do is turn offf the dc supply to a one or two of the inverters, with a contactor, or ssr that i can control by rs485 (read son writes more software)but i haven't found anything that will stand up to switching 350-400volts DC at 10-15 amps. so im a bit stuck.

And am hoping some off your have gone down this road before and can offer guidance.
I have thought of using a 4 pole 415v contactor and looping the positive thought the first 3 poles, and the negative through the last. Thinking there might be enough air gap in the 3 sections of the contactor to stop the arking and have it turn off more than once before bursting into flames.
I know that a ssr in the mains connection to the inverter would be much simpler, but the inverter would have to completely restart every time it was throttled. And wonder if turning the inverter on and off 20 times a day would do it any good. Cutting the DC would just make it nite time for a while.

So if any one has any thoughts or ideas, suggestions, or just wants to say, hay dumbbell, why didn't you do this, feel free.

PS sorry i can be a bit long winded.

Joined: 03/06/2019
Location: Australia
Posts: 494
Posted: 09:14pm 11 Mar 2020
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Hi Andrew,

You will get a lot better answers than I can get off others but couple of things come to mind here.

First of all it's so refreshing to read someone saying they have a home grown system rather than on so many other forums, One in Oz based popular one particular, where all the participants whom I believe would struggle to change a light bulb or tap washer, Piss and moan endlessly about rules and regulations. I swear a lot of these people that don't call in a sparky to change a light bulb, spend 10 Min putting on PPE  and have paramedics on standby before they attempt it.

My solar is all DIY as well and from what I read on a lot of forums, It's a damn sight better than a lot of the " Professionally" installed setups that people post problems about every day.  Mine works and that's it.

I take it exporting too much means like me, you still have the spinny meters and don't want to go into negative usage. If that is the case, then just turn the inverters off for a week and then you can play catch up for another week or two till they do. That's what I do.  I know when a read is due ( today as it turns out) and just make sure the week before I'm above the last read and that's it. Embarrassingly, for summer with all my panels, I'm behind for this read being summer which I should have walked in.  

Between all the smoke where I couldn't see to the other side of the street, the ash and dust covering everything like a brown snow storm, stinking hot days we had for weeks and now barely a clear day for about 6 weeks, going to cost me about $50 More than I would have liked to have got the reading down to.  Ah well.  Without the solar I have no doubt our bill would have been over $2k.

I have all my solar arrays on double breakers switching BOTH poles and I have switched them off dozens of times when the inverters are running flat out and never had problems. I always do it arse about and switch the Feed ( DC) before I switch the  AC side even though every time I do it I think I did that the wrong way.

I believe if you used the contactor and did 2 Loops on each POLE that would be perfectly adequate.My inverters are 3.5 to 5 KW and all work the same.

The other idea off the top of my head and it's early for me to be doing any thinking,
is to not switch the power OFF completely, but to use a DPDT and switch it back to the other inverter.  In my way of explaining with what I have Observed, I think of DC as having like Inertia. Once it's going it does not like to stop.  I have also played a bit with DPDT relays and if when you switch the DC you give it somewhere else to go, it's happy. Bit like the difference between trying to stop a runaway train and switching the points to another line where it can go and cause no problems.

I'm thinking if you Switched the DC back to the supply Line for the other remaining inverter(s), that would quench any arcing because it has a path to go to on those contacts on the relay. Switching back to the supply would basically ground both poles and there would be no reason to arc. Unless the feed from the other arrays was quite a bit different ( and then I don't see a great problem) I think this would work fine.

What I played with illustrated if you try to switch the DC off it will turn into a Plasma Cutter.  It you give it something else to do, the arc you get due to the speed of the relay Switching is no more than switching a heavy AC load.

Myself, I'd stick to mechanical relays for this.  Good SSRs are going to be expensive as I have found out and the cheaper ones are bit of a lucky dip from every thing I have read and recent experience.
SSR's a would be better if you wanted to switch the things on and off 3 times a second, for what you are talking about I would suggest mechanical relays/ Contactors would be best.

One more idea....

Is your water heater electric and if so, how is it powered? I took mine off the off peak and put it on the normal Mains which the solar feeds. I don't have smartarse meters so your setup may be different.

I have mine on a Voltage monitoring relay which detects when the solar pushes the mains voltage in the house up enough to signify it's kicking out decent power.
The relay detects this and uses said power to heat the water. Ends up we just heat in the day with free solar power than in the night with Purchased off peak.

Depending on how much power you need to burn off if you have TOU type metering  ( AGGGHHH!) the other thing would be to leave the inverters running and switch an AC Dump load.  Maybe a 44 or 2 with some water heater elements that are switched in with the arduinos when the inverters are exporting. Think of it Lister cooling style.
You could also have a fan blowing on them so they didn't boil as quickly OR, as I have done, get a couple or however many you need, of those fan heaters which will pull about 2KW ea or can be switched down to 1KW and run them.

You can buy them for as little as $14 in Kmart and target or your Op shop should have a few. If your problem is just in summer like me, what about a little AC to air condition the shed?  That would be pretty upmarket!  :0)

I like Boiling water in an urn I have. Best week killer out there but even on my block, I do run out of weeds pretty quick.
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