How to combine several AC power sources

[zainka]

Hi

Consider this.
We have three integrated solar power units, which gives 220V AC as output voltage, with the following "specs".
2x 220V 50Hz giving a maximum of 100W
1x 220V 50Hz giving a maximum of 200W

They are localized in different spots but we want to combine them into a single 500W 220V AC grid distributing the voltage over some distance (100-300 m). I know there needs to be a "synchronize" somewhere to combine AC lines but this is really not my field and I need some input to gain the information needed. Somewhere to start.

This will be like creating our own private local grid with several power plants feeding the same network and in the future there might be added other sources like hydro power plants or windmills.

So how do we attack this problem, where can i read up on the theory and who sells suitable equipment????


Thanks in advance

Breg
Vidar

 

[nikhilmahasvar]

Hi

Consider this.
We have three integrated solar power units, which gives 220V AC as output voltage, with the following "specs".
2x 220V 50Hz giving a maximum of 100W
1x 220V 50Hz giving a maximum of 200W

They are localized in different spots but we want to combine them into a single 500W 220V AC grid distributing the voltage over some distance (100-300 m). I know there needs to be a "synchronize" somewhere to combine AC lines but this is really not my field and I need some input to gain the information needed. Somewhere to start.

This will be like creating our own private local grid with several power plants feeding the same network and in the future there might be added other sources like hydro power plants or windmills.

So how do we attack this problem, where can i read up on the theory and who sells suitable equipment????


Thanks in advance

Breg
Vidar

You can visit microchip website and search for grid tied solar inverter there is a sample design which includes source code, circuits , BOM and every thing you require.

 

[FvM]

Grid tied inverters are required to shut down their operation if the grid fails (as a safety feature). Thus they are usually not suited for stand-alone operation. I also understand, that you have ready-made inverter devices, so the question is targetting to knowing their specification or possible options to modify them.

Assumed the inverter operation can be changed between grid tied and stand-alone (island) operation, one could be set up for stand-alone as a master and the others for gried-tied as slaves.

 

[zainka]

Hi

I was actually just about to comment the problem with non standalone inverters wile you wrote about it, FvM, so I had to re read a bit before posting. Thing is that the inverter from Microchip and a few others I now have been reading about has this "Islanding protection" feature which effectively will shut down the inverter when main grid is down effectively creating a chicken and hen problem since there are no main grid in the first place . As you say one do need an option to set one of the inverter to island-operation.

Actually, in my case I could have all running in Island mode since the grid is fully controllable as it is not connected to the big grid.

However, your assumption that the solar panels is equipped with an inverter is kinda wrong since there are only an dc/ac converter in the current setup and i guess it has to be replaced with and inverter design I realize now if there aint any pure ac inverter synchronizing already existing 220VAC sources.

You could think of it in a different way. What if I had 3 x 12v DC to 220v AC converters like these little fellows feeding from a PB battery each. How could one possible connect them together creating a local grid (assuming they are to far away that one could simply connect the batteries in parallel and use one inverter)? Just connecting them I guess would lead to a few devestating problems . There needs to some sort of a synchronizer sensing the zero crossing of a chosen main inverter...


EDIT: Added for reference this thread at CR4 : CR4 - Thread: Grid-Tie Inverter Design

 

[FvM]

The most simple method would to synchronize the inverter's signal generation circuits directly. If the inverters are mounted at different locations, synchronization over the power line can be reasonable however.

 

[BradtheRad]

I have several years' experience with PV panels, inverters, and batteries.

As you know, getting 3 waveforms to synch is a major hurdle. I'm not acquainted with equipment that does this, but I imagine a synch signal to the slave inverters (sent by wire or radio) could drive their inner workings so they synch to the millisecond (or microsecond).

It will require specialized inverters of course.

And they'll have to match peak voltage, RMS voltage, etc., to the millisecond. Voltages will all have to match at the combining point, after the power has traveled through known and unknown cable resistances.

To accomplish matching will require that you be able to custom adjust output settings at each inverter.

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You say you wish to send AC power over several hundred feet. Consider stepping the voltage up 2 or 3 times through a transformer (or autoformer).

The purpose is to step down the CURRENT so that you can use smaller cable and save some money perhaps. In that case you'll lose less power (due to I_squared x R) by running lower current.

Then you'll step the V back down at the end of the wire run with an identical autoformer in reverse. Don't know for sure if it will reduce your net cost.

You won't need the first autoformer if you were to use synchronized inverters which step up battery voltage to high V in the first place.

-------------------------------------

Which brings us to the easiest way to do it, theoretically speaking.

Wire all the PV panels in series. Total the voltages and send it through long wire runs in the form of high voltage DC. Again this has the advantage of small current flow. On the order of 1/2 amp. You wouldn't need any inverters or synchronizing circuitry.

You'll have to decide whether it's feasible to wire all 3 PV installations in one long series string. It could work only if your PV panels all output the same current, and if they all receive identical sunlight. Because you must not put large current devices inline with small current devices.

Or else you would parallel the 3 PV installations. This requires that they produce matching voltages. It doesn't require they produce matching currents. However this is hard to ensure. If one of the 3 drops in voltage then you can have problems with current backflow.

Once it's all combined you could send 1/2 amp through a single 16 gauge wire for 1000 feet and it would only be 4 ohms. (That's if you use the earth as your return wire.) Your power losses will be minimal.

So you'd need only a single chopper-stepdown-converter to deliver 220 VAC at the usage point.