I have tried to find specific info on this scenario with no luck
The situation is this;
Off grid power system 12v battery store runs 12v lights etc. and an inverter for things difficult to run or convert.
I have a relay which energizes from the generator output and disconnects the inverter (so I'm not trying to push two unsynchronized ac signals in the same circuit)
The theory seems good, but I have managed to fry two inverters and I think this is due to a quick on/off switching of the relay as the generator powers down causing spikes and knocking out the output stages of the inverter.
It would be good to isolate the inverter completely, i.e 12v input side and ac output side- but I feel like a debouncing type circuit is needed on both relays
Any ideas/suggestions as to the simplest (and cheapest!) way of sorting this would be greatly appreciated.
I am looking to isolate an inverter when power from a generator is applied- debounce relay
debouncegeneratorinverterpower supplyrelay
Related Solutions
Your inverter can't handle more than 15V, but the panel may put out a higher voltage in bright sunlight. Therefore you must find a way to drop the voltage. You want a cheap solution, but does it have to be lossless?
Since you are using the power directly from the solar panel with no storage, it doesn't matter if dropping the voltage results in a power loss (since you won't be using that extra power anyway). What does matter is that the regulator doesn't introduce extra loss when the panel is producing the minimum voltage required to power the inverter.
A series or shunt linear regulator that simply wastes the excess power produced by the panel could still have 100% efficiency when it is really needed. The obvious solution is to simply put a high power 15V Zener diode across the panel, but how much power will it have to handle and can it be done 'inexpensively'?
Here's the IV curves for a typical '12V' 130W solar panel. First thing to note is that it puts out 22V open circuit (at 25°c) and about 17V at the maximum power point.
Your panel might actually put out 12V at maximum power and 17V open circuit, though that would be an unusual spec for a '12V' panel. But let's assume it is correct and your panel produces an IV curve similar to the green line on the graph.
Your panel can produce 400W in full sunlight, but your inverter is only drawing about 190W (150W at 80% efficiency). So the Zener will have to absorb about half the power produced by the panel, ie. ~200W. That's going need several large transistors on big heatsinks.
The other alternative is a series regulator. This drops the excess voltage at the current being drawn by the inverter (~190W/15V = 13A). A good low resistance FET could pass this current with virtually no loss at lower panel voltage, and only has to dissipate about 26W in full sunlight (assuming the panel puts out 17V at 13A). This circuit should be considerably cheaper to make than the 200W shunt regulator.
Or you could just use an off-the-shelf switching regulator designed to deliver 12V at 16A or higher. This may have a slightly higher minimum voltage drop, but will the difference be significant?
The panel may only have to produce a fraction of a volt more to compensate for voltage drop in the regulator. But light intensity and temperature variations have a much greater effect on panel output. The difference between the panel producing just enough power to run the inverter and not enough power is so small that it will hardly be noticed, so 99% of the time the extra loss in the regulator will be nothing to worry about.
If you are considering connecting anything that might send power even accidentally into the mains network you want to be on your best electrical behaviour.
Generally what are used is two switches or contractors that are mechanically (and or electrically) interlocked so that your and the utility supply cannot ever connect to each other simultaneously. It prevents many dangers.
Check out the following picture search and see if something looks interesting. There will be DIY, off the shelf (OTS) and custom (PLC if you like) systems to do what you want.
https://www.google.com/search?q=automatic+mains+backup+relay&tbm=isch
Best Answer
I believe that what you might really need is a changeover contact then neither grid/AC nor inverter output can be connected together. When AC resumes the relay, in sequence, disconnects the inverter and milli seconds later reconnects the AC to the load.
There may also be another problem that you haven't considered and that is when you disconnect the inverter output when it's supplying a high current to the load. This may cause some back emf (due to the inverter output filter) and this may "hurt" the inverter.
Anyway, it's worth trying the changeover relay and if it still gives problems then you'll have to find a way of ramping down the inverter output voltage to a safe level before disconecting and reconnecting the AC.