Inverter/mains switching

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I want to run a number of mains powered appliances from a 12V battery. The battery is charged from solar panels, and I have a pure sine inverter running from the battery.

Now I want to switch automatically between mains power from the grid and mains power from the inverter. That is: take power from the inverter when battery can deliver, otherwise from the grid. So I need a switch. Requirements are:

Must break before connect, but connect so fast that appliances will not discover.

I am considering a good old mechanical relay or back-to-back power mosfets optically controlled.

Other ideas are welcome. I already have an AVR, controlling loads from the battery depending on state of charge, it can easily control inverter and switch too.

Peter

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Consider a grid tie inverter. It will automatically offset the grid power with whatever PV power is available, and avoids arcing across contacts when switching inductive loads.

Backup if the grid goes down is not in your specification, but there are battery backup grid tie inverters as well. They would require moving your load from the mains to the inverter panel.

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They had a shoebox full of 100 amp latching relays at Skycraft. Still have a bunch.

Imagecraft compiler user

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dak664 wrote:
Consider a grid tie inverter. It will automatically offset the grid power with whatever PV power is available, and avoids arcing across contacts when switching inductive loads.

Backup if the grid goes down is not in your specification, but there are battery backup grid tie inverters as well. They would require moving your load from the mains to the inverter panel.

My solar panels are just about 500W peak. I will probably ad a litte more later. But I have to pay some fees for grid tie, so it will not pay off to do so, unless I will add many more panels.

My original purpose was lighting in a Building that was not tied to the grid.

I am just trying to make most of the watts useful.

Peter

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You want an automatic transfer switch. Basically a dpdt relay.

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Kartman wrote:
You want an automatic transfer switch. Basically a dpdt relay.

I was thinking something electronical, but a 230V dpdt relay may be much simpler. If the inverters 230V pulls the relay, then it will automatically fall and change to grid power whenever the inverter is turned off. And vice versa. Good idea.

Peter

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I assume that you can't control the inverter to be in phase with the grid, so you can't make a real hot swap, so the next best would be two solid state relays, but a normal mechanic relay should be ok.
I have two 195w solar panels I plan to do the same with.
Where in Denmark do you live? (I'm close to Aalborg)

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I have 120W solar cells on my roof, connected it to 2x 100AmpH batteries and run seperate wires to a couple of places in the house. As car appliances and camping stuff are well buyable here I charge my cell phones/tablet navigation device and batteries(digicam and AA) from that. 2 rooms in the house also have lighting installed so I run that all from 12V and no longer from 230V.
I did that as a couple of years back I was experimenting and then we had a massive power surge(for our standards being 16hours without electricity is really long, specially as nothing special happened). A couple of neighbors had solar panels fitted with normal inverters and it turned out that they has no mains at all, despite it being a sunny day. It seems that here it is not allowed to use an inverter that can keep mains UP when it fails. So I decided to make a separate power system. I have bought a couple of inverters that can give me 220V when need to, but during tests I have seen that they are not full sine so need to either buy better ones or make a filter for it to run my fridge on....

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Make sure your wiring is properly fused. The fuse is sized according to the wire so no smoke can happen.

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wire is 4mm2 and fuses are 15amp max for the car outlets and 3amp for all other stuff connected. even a 15Amp fuse between the solar panels and the batteries.
all connections to/from the batteries and to the solar panels are fused.

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Fused at the source of course and the batteries well ventilated? Just want to be sure you have a safe setup. I've seen the effects of when things go wrong.

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30cm wire from battery to fuse box. from there to the rest of the system.

batteries are on the attic which is a large open room. The window there does not close air tight and if the wind is on it you can feel it so there is ventilation.
also I have build a protection circuit that cuts solar power when it goes over 14,5V so the batteries should be save from venting anyway, specially as I do not have the power to charge to fast (2x100amp battery and max charge is 10amp so should be safe from venting under normal charging condition) I have seen/heard that this can lead to gas forming in the battery when it is full, but then the battery voltage rises to 16V, so I took 14,5V as limit.
use car batteries btw so max voltage would be 13,8V for full battery, add little voltage to be able to keep charging.
I have seen that while charging the voltage stays at about 13,5V for along time and then rises when the battery is full. Had first thought of taking that as limit, (dv/dt) but being sunlight dependant (and we are holland, the land of water, nice clouds and rain) I took some margin on that. I have done a lot of measurements to see where the max level would be and I have seen that voltage goes to 13V while really charging, then goes to 14V and stays there a little when the battery seems to be almost full and then rises quickly to 16V, so I took the moment I start over charging to make sure my battery is always full-full and then switch OFF.
I have as said made an avr measurement device that switches controls 2 relays(one for each panel being 60W each) when at 14,2V it switches off 1 panel and normally at 14,5V the second. If a single measuremnt goes over 14,7V I immediately switch OFF both panels.
this I tested and works. I have a meter hooked-up (just a analog panel meter) in the bed-room to the 12V so I can always see what the actual battery voltage is. As my avr system has a display I also have that updated with a "I am alive" timer so I can see that it has not crashed, plus it shows the ADC measurement. So I thought my set-up was pretty fail safe. but suggestions are always welcome. (the avr also runs from the solar cells so free of cost ;) )

regards

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I'd say at a guess you'll be fine with the batteries. With a large area , any potential gas liberated will be diluted so you don't get an explosive buildup. There's a formula in a iec standard i read years ago regarding batteries and their enclosures. Even the 'sealed' lead acid batteries can liberate gas so you don't put them in sealed enclosures either.

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Kartman,
if you can dig up that standard or its number I would love to take a peek at that to see if I missed out on anything.

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sparrow2 wrote:
I assume that you can't control the inverter to be in phase with the grid, so you can't make a real hot swap, so the next best would be two solid state relays, but a normal mechanic relay should be ok.
I have two 195w solar panels I plan to do the same with.
Where in Denmark do you live? (I'm close to Aalborg)

I live near Copenhagen.

My inverter is a high quality Victron, specified to be safe from shortcuts, inductive loads, overvoltage and so on. You can even put severel of them in parallel, and they will automatically sync each other. So maybe contact with the grid is no problem, but I want to be safe and avoid contact.

Peter

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This is not a difficult problem, and I'm sure that the solutions presented will work fine.

But, if you have an electrical fire, and they find this kind of stuff connected to your breaker box, you could have some trouble.

They make switchover devices for use with emergency generators. I'm certain that some are automatic. They will have all of the required approvals. You can leave it hooked up when you sell your house.

hj